 | Kingdom, Animalia. Phylum,Chordata, |
The great blue heron is replaced in the Old World by the very similar gray heron (Ardea cinerea), which differs in being somewhat smaller (90–98 cm (35–39 in)), with a pale gray neck and legs, lacking the browner colors that great blue heron has there. It forms a superspecies with this and also with the cocoi heron from South America, which differs in having more extensive black on the head, and a white breast and neck.
- A. h. Herodias Linnaeus, 1758, most of North America, except as below
- A. h. fannini Chapman, 1901, the Pacific Northwest from southern Alaska south to Washington; coastal
- A. h. wardi Ridgway, 1882, Kansas and Oklahoma to northern Florida, sightings in southeastern Georgia
- A. h. occidentalis Audubon, 1835, southern Florida, Caribbean islands, formerly known as a separate species, the great white heron
- A. h. cognata Bangs, 1903, Galápagos Islands
Description[edit]
It is the largest North American heron and, among all extant herons, it is surpassed only by the goliath heron (Ardea goliath) and the white-bellied heron (Ardea insignis). It has head-to-tail length of 91–137 cm (36–54 in), a wingspan of 167–201 cm (66–79 in), a height of 115–138 cm (45–54 in), and a weight of 1.82–3.6 kg (4.0–7.9 lb).[5][6][7][8] In British Columbia, adult males averaged 2.48 kg (5.5 lb) and adult females 2.11 kg (4.7 lb).[9] In Nova Scotia and New England, adult herons of both sexes averaged 2.23 kg (4.9 lb),[10] while in Oregon, both sexes averaged 2.09 kg (4.6 lb)[11] Thus, great blue herons are roughly twice as heavy as great egrets (Ardea alba), although only slightly taller than them, but they can weigh about half as much as a large goliath heron.[12] Notable features of great blue herons include slaty (gray with a slight azure blue) flight feathers, red-brown thighs, and a paired red-brown and black stripe up the flanks; the neck is rusty-gray, with black and white streaking down the front; the head is paler, with a nearly white face, and a pair of black or slate plumes runs from just above the eye to the back of the head. The feathers on the lower neck are long and plume-like; it also has plumes on the lower back at the start of the breeding season. The bill is dull yellowish, becoming orange briefly at the start of the breeding season, and the lower legs are gray, also becoming orangey at the start of the breeding season. Immature birds are duller in color, with a dull blackish-gray crown, and the flank pattern is only weakly defined; they have no plumes, and the bill is dull gray-yellow.[4][13][14] Among standard measurements, the wing chord is 43–49.2 cm (16.9–19.4 in), the tail is 15.2–19.5 cm (6.0–7.7 in), the culmen is 12.3–15.2 cm (4.8–6.0 in), and the tarsus is 15.7–21 cm (6.2–8.3 in).[15][16]
The heron's stride is around 22 cm (8.7 in), almost in a straight line. Two of the three front toes are generally closer together. In a track, the front toes, as well as the back, often show the small talons.[17]
The subspecies differ only slightly in size and plumage tone, with the exception of subspecies A. h. occidentalis, which also has a distinct white morph, known as the great white heron (not to be confused with the great egret, for which "great white heron" was once a common name). It is found only in south Florida and some parts of the Caribbean. The great white heron differs from other great blues in bill morphology, head plume length, and in having a total lack of pigment in its plumage. It averages somewhat larger than the sympatric race A. h. wardi and may be the largest race in the species. In a survey of A. h. occidentalis in Florida, males were found to average 3.02 kg (6.7 lb) and females average 2.57 kg (5.7 lb), with a range for both sexes of 2 to 3.39 kg (4.4 to 7.5 lb).[5] This is mainly found near salt water, and was long thought to be a separate species. Birds intermediate between the normal morph and the white morph are known as Würdemann's heron; these birds resemble a "normal" great blue with a white head.
The theory that great white herons may be a separate species (A. occidentalis) from great blue heron has again been given some support by David Sibley.[18]
Similar species[edit]
The "great white heron" could be confused with great egret, but is larger, with yellow legs as opposed to the great egret's black legs. The reddish egret (Egretta rufescens) and little blue heron (Egretta caerulea) could be mistaken for the great blue heron, but are much smaller, and lack white on the head and yellow in the bill. In the southern reaches of its range, the great blue sometimes overlaps in range with the closely related and similarly sized cocoi heron (A. cocoi). The cocoi is distinguished by a striking white neck and solid black crown, but the duller juveniles are more easily confused. More superficially similar is the slightly smaller gray heron, which may sometimes vagrate to the northern coasts of North America. The gray heron (which occupies the same ecological niche inEurasia as the great blue heron) has very similar plumage, but has a solidly soft-gray neck. Erroneously, the great blue heron is sometimes referred to as a "crane".
Distribution and habitat[edit]
The great blue heron is found throughout most of North America, as far north as Alaska and the southern Canadian provinces. However, they are only present year-round in parts of Canada which have warmer winters, including coastal British Columbia and parts of its interior such as the warmer Okanagan Valley, as well as most of the Maritime provinces of eastern Canada. The range extends south through Florida, Mexico, and the Caribbean to South America. Birds east of the Rocky Mountains in the northern part of their range are migratory and winter in Central America or northern South America. From the southern United States southwards, and on the Pacific coast, they are year-round residents.[4] However, their hardiness is such that individuals often remain through cold northern winters, as well, so long as fish-bearing waters remain unfrozen (which may be the case only in flowing water such as streams, creeks, and rivers).
The great blue heron can adapt to almost any wetland habitat in its range. It may be found in numbers in fresh and saltwater marshes, mangrove swamps, flooded meadows, lake edges, or shorelines. It is quite adaptable and may be seen in heavily developed areas as long as they hold bodies of fish-bearing water.
Great blue herons rarely venture far from bodies of water, but are occasionally seen flying over upland areas. They usually nest in trees or bushes near water's edge, often on islands (which minimizes the potential for predation) or partially isolated spots.[19]
Behavior[edit]
Eating a small fish, the main prey
The primary food for great blue heron is small fish, though it is also known to opportunistically feed on a wide range of shrimp, crabs, aquatic insects, rodents, and other small mammals, amphibians, reptiles, and birds. Primary prey is variable based on availability and abundance. In Nova Scotia, 98% of the diet was flounders.[10] In British Columbia, the primary prey species are sticklebacks, gunnels, sculpins, and perch.[21] California herons were found to live mostly on sculpin, bass, perch, flounder, and top smelt.[22] Nonpiscine prey is rarely quantitatively important, though one study in Idaho showed that from 24 to 40% of the diet was made up of voles.[23]
Herons locate their food by sight and usually swallow it whole. They have been known to choke on prey that is too large.[24][25] It is generally a solitary feeder. Individuals usually forage while standing in water, but also feed in fields or drop from the air, or a perch, into water. Mice are occasionally preyed on in upland areas far from the species' typical aquatic environment.[19] Occasionally, loose feeding flocks form and may be beneficial since they are able to locate schools of fish more easily.[19] As large wading birds, great blue herons are capable of feeding in deeper waters, thus are able to harvest from niche areas not open to most other heron species. Typically, the great blue heron feeds in shallow waters, usually less than 50 cm (20 in) deep,[19] or at the water's edge during both the night and the day, but especially around dawn and dusk. The most commonly employed hunting technique of the species is wading slowly with its long legs through shallow water and quickly spearing fish or frogs with its long, sharp bill. Although usually ponderous in movements, the great blue heron is adaptable in its fishing methods. Feeding behaviors variably have consisted of standing in one place, probing, pecking, walking at slow speeds, moving quickly, flying short distances and alighting, hovering over water and picking up prey, diving headfirst into the water, alighting on water feet-first, jumping from perches feet-first, and swimming or floating on the surface of the water.[19]
Breeding[edit]
This species usually breeds in colonies, in trees close to lakes or other wetlands. Adults generally return to the colony site after winter from December (in warmer climes such as California and Florida) to March (in cooler areas such as Canada). Usually, colonies include only great blue herons, though sometimes they nest alongside other species of herons. These groups are called a heronry (a more specific term than "rookery"). The size of these colonies may be large, ranging between five and 500 nests per colony, with an average around 160 nests per colony. A heronry is usually relatively close, usually within 4 to 5 km (2.5 to 3.1 mi), to ideal feeding spots.[19]Heronry sites are usually difficult to reach on foot (e.g., islands, trees in swamps, high branches, etc.) to protect from potential mammalian predators. Trees of any type are used when available. When not, herons may nest on the ground, sagebrush, cacti, channel markers, artificial platforms, beaver mounds, and duck blinds. Other waterbirds (especially smaller herons) and, occasionally, even fish and mammal-eating raptors may nest amongst colonies.[26][27] Although nests are often reused for many years and herons are socially monogamous within a single breeding season, individuals usually choose new mates each year.[28] Males arrive at colonies first and settle on nests, where they court females; most males choose a different nest each year.[28] Great blue herons build a bulky stick nest. Nests are usually around 50 cm (20 in) across when first constructed, but can grow to more than 120 cm (47 in) in width and 90 cm (35 in) deep with repeated use and additional construction.[29] If the nest is abandoned or destroyed, the female may lay a replacement clutch. Reproduction is negatively affected by human disturbance, particularly during the beginning of nesting. Repeated human intrusion into nesting areas often results in nest failure, with abandonment of eggs or chicks. However, Vancouver B.C. Canada's Stanley Park has had a healthy colony for some years right near its main entrance and tennis courts adjacent to English Bay and not far from Lost Lagoon. The park's colony has had as many as 140 nests. The Stanley Park Colony can be observed at their interactive web cam at http://vancouver.ca/parks-recreation-culture/heron-cam.aspx
The female lays three to six pale blue eggs. Eggs can measure from 50.7 to 76.5 mm (2.00 to 3.01 in) in length and 29 to 50.5 mm (1.14 to 1.99 in) in width, though the smallest eggs in the above sample may have been consider "runt eggs" too small to produce viable young. Egg weight ranges from 61 to 80 g (2.2 to 2.8 oz).[30] One brood is raised each year. First broods are laid generally from March to April.[31][32] Eggs are usually laid at two-day intervals, incubated around 27 days, and hatch asynchronously over a period of several days.[28] Males incubate for about 10.5 hours of each day, while females usually incubate for the remainder of each day and the night, with eggs left without incubation for about 6 minutes of each hour.[28] The first chick to hatch usually becomes more experienced in food handling and aggressive interactions with siblings, so often grows more quickly than the other chicks.[33] Both parents feed the young at the nest by regurgitating food. Parent birds have been shown to consume up to four times as much food when they are feeding young chicks (about 4300 kJ/day) than when laying or incubating eggs (about 1200 kJ/day).[28] By the time they are 45 days old, the young weigh 86% of the adult's mass.[34] After about 55 days at the northern edge of the range (Alberta) and 80 days at the southern edge of the range (California), young herons take their first flight.[28] They return to the nest to be fed for about another 3 weeks, following adults back from foraging grounds, and are likely to gradually disperse away from their original nest over the course of the ensuing winter.[28] Young herons are not as successful at fish capture as adults, as strike rates are similar, but capture rates are about half that of adults during the first 2 months after fledging.[28]
Predation[edit]
Predators of eggs and nestlings include turkey vultures (Cathartes aura), common ravens (Corvus corax), and American crows (Corvus brachyrhynchos). Red-tailed hawks (Buteo jamaicensis), American black bears (Ursus americanus), and raccoons (Procyon lotor) are known to take larger nestlings or fledglings and, in the latter predator, many eggs.[9][35][36][37] Adult herons, due to their size, have few natural predators, but a few of the larger avian predators have been known to kill both young and adults, including bald eagles(Haliaeetus leucocephalus) (the only predator known to attack great blue herons at every stage of their lifecycle from in the egg to adulthood), golden eagles (Aquila chrysaetos) and, less frequently, great horned owls (Bubo virginianus) and Harris's hawks(Parabuteo unicinctus).[38][39][40][41][42] An occasional adult or, more likely, an unsteady fledgling may be snatched by an American alligator (Alligator mississippiensis) or an American crocodile (Crocodylus acutus). Using its considerable size and dagger-like bill, a full-grown heron can be a formidable foe to a predator. In one instance, during an act of attempted predation by a golden eagle, a heron was able to mortally wound the eagle although it succumbed to injuries sustained in the fight.[43] When predation on an adult or chick occurs at a breeding colony, the colony can sometimes be abandoned by the other birds. The primary source of disturbance and breeding failures at heronries is human activities, mostly through human recreation or habitat destruction, as well as by egg-collectors and hunters.[21][44]
Frogs are a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (Ancient Greek an-, without + oura, tail). The oldest fossil "proto-frog" appeared in the early Triassic ofMadagascar, but molecular clock dating suggests their origins may extend further back to the Permian, 265 million years ago. Frogs are widely distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is in tropical rainforests. There are approximately 4,800 recorded species, accounting for over 85% of extant amphibian species. They are also one of the five most diverse vertebrateorders.
The body plan of an adult frog is generally characterized by a stout body, protruding eyes, cleft tongue, limbs folded underneath, and the absence of a tail in adults. Besides living in fresh water and on dry land, the adults of some species are adapted for living underground or in trees. The skin of the frog is glandular, with secretions ranging from distasteful to toxic. Warty species of frog tend to be called toads but the distinction between frogs and toads is based on informal naming conventions concentrating on the warts rather than taxonomy or evolutionary history; some toads are more closely related to frogs than to other toads. Frogs' skins vary in colour from well-camouflaged dappled brown, grey and green to vivid patterns of bright red or yellow and black to advertise toxicity and warn off predators.
Frogs typically lay their eggs in water. The eggs hatch into aquatic larvae called tadpoles that have tails and internal gills. They have highly specialized rasping mouth parts suitable for herbivorous, omnivorous orplanktivorous diets. The life cycle is completed when they metamorphose into adults. A few species deposit eggs on land or bypass the tadpole stage. Adult frogs generally have a carnivorous diet consisting of small invertebrates, but omnivorous species exist and a few feed on fruit. Frogs are extremely efficient at converting what they eat into body mass. They are an important food source for predators and part of the food webdynamics of many of the world's ecosystems. The skin is semi-permeable, making them susceptible to dehydration, so they either live in moist places or have special adaptations to deal with dry habitats. Frogs produce a wide range of vocalizations, particularly in their breeding season, and exhibit many different kinds of complex behaviours to attract mates, to fend off predators and to generally survive.
Frogs are valued as food by humans and also have many cultural roles in literature, symbolism and religion. Frog populations have declined significantly since the 1950s. More than one third of species are considered to be threatened with extinction and over one hundred and twenty are believed to have become extinct since the 1980s.[1] The number of malformations among frogs is on the rise and an emerging fungal disease,chytridiomycosis, has spread around the world. Conservation biologists are working to understand the causes of these problems and to resolve them.
Etymology and taxonomy
The use of the common names "frog" and "toad" has no taxonomic justification. From a classification perspective, all members of the order Anura are frogs, but only members of the family Bufonidae are considered "true toads". The use of the term "frog" in common names usually refers to species that are aquatic or semi-aquatic and have smooth, moist skins; the term "toad" generally refers to species that are terrestrial with dry, warty skins.[4][5] There are numerous exceptions to this rule. The European fire-bellied toad (Bombina bombina) has a slightly warty skin and prefers a watery habitat[6] whereas the Panamanian golden frog (Atelopus zeteki) is in the toad family Bufonidae and has a smooth skin.[7]
The Anura include all modern frogs and any fossil species that fit within the anuran definition. The characteristics of anuran adults include: 9 or fewer presacral vertebrae, the presence of a urostyle formed of fused vertebrae, no tail, a long and forward-sloping ilium, shorter fore limbs than hind limbs, radius and ulna fused, tibia and fibula fused, elongated ankle bones, absence of a prefrontal bone, presence of a hyoid plate, a lower jaw without teeth (with the exception of Gastrotheca guentheri) consisting of three pairs of bones (angulosplenial, dentary, and mentomeckelian, with the last pair being absent in Pipoidea),[8] an unsupported tongue, lymph spaces underneath the skin, and a muscle, the protractor lentis, attached to the lens of the eye.[9] The anuran larva or tadpole has a single central respiratory spiracle and mouthparts consisting of keratinous beaks and denticles.[9]
Frogs and toads are broadly classified into three suborders: Archaeobatrachia, which includes four families of primitive frogs; Mesobatrachia, which includes five families of more evolutionary intermediate frogs; and Neobatrachia, by far the largest group, which contains the remaining 24 families of modern frogs, including most common species throughout the world. The Neobatrachia suborder is further divided into the two superfamilies Hyloidea and Ranoidea.[10] This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle, and the morphology of tadpoles. While this classification is largely accepted, relationships among families of frogs are still debated.[11]
Some species of anurans hybridize readily. For instance, the edible frog (Pelophylax esculentus) is a hybrid between the pool frog (P. lessonae) and the marsh frog (P. ridibundus).[12] The fire-bellied toads Bombina bombinaand B. variegata are similar in forming hybrids. These are less fertile than their parents, giving rise to a hybrid zone where the hybrids are prevalent.[13]
Evolution
The origins and evolutionary relationships between the three main groups of amphibians are hotly debated. A molecular phylogeny based on rDNA analysis dating from 2005 suggests that salamanders and caecilians are more closely related to each other than they are to frogs and the divergence of the three groups took place in the Paleozoic or early Mesozoic before the breakup of the supercontinent Pangaea and soon after their divergence from the lobe-finned fishes. This would help account for the relative scarcity of amphibian fossils from the period before the groups split.[14] Another molecular phylogenetic analysis conducted about the same time concluded that lissamphibians first appeared about 330 million years ago and that thetemnospondyl-origin hypothesis is more credible than other theories. The neobatrachians seemed to have originated in Africa/India, the salamanders in East Asia and the caecilians in tropical Pangaea.[15] Other researchers, while agreeing with the main thrust of this study, questioned the choice of calibration points used to synchronise the data. They proposed that the date of lissamphibian diversification should be placed in the Permian, rather less than 300 million years ago, a date in better agreement with the palaeontological data.[16] A further study in 2011 using both extinct and living taxa sampled for morphological, as well as molecular data, came to the conclusion that Lissamphibia is monophyletic and that it should be nested within Lepospondyli rather than within Temnospondyli. The study postulated that Lissamphibia originated no earlier than the late Carboniferous, some 290 to 305 million years ago. The split between Anura and Caudata was estimated as taking place 292 million years ago, rather later than most molecular studies suggest, with the caecilians splitting off 239 million years ago.[17]
In 2008, Gerobatrachus hottoni, a temnospondyl with many frog- and salamander-like characteristics, was discovered in Texas. It dated back 290 million years and was hailed as a missing link, a stem batrachian close to the common ancestor of frogs and salamanders, consistent with the widely accepted hypothesis that frogs and salamanders are more closely related to each other (forming a clade called Batrachia) than they are to caecilians.[18][19] However, others have suggested that Gerobatrachus hottoni was only a dissorophoid temnospondyl unrelated to extant amphibians.[20]
Salientia (Latin salere (salio), "to jump") is the name of the total group that includes modern frogs in the order Anura as well as their close fossil relatives, the "proto-frogs" or "stem-frogs". The common features possessed by these proto-frogs include 14 presacral vertebrae (modern frogs have eight or 9), a long and forward-sloping ilium in the pelvis, the presence of a frontoparietal bone, and a lower jaw without teeth. The earliest known amphibians that were more closely related to frogs than to salamanders are Triadobatrachus massinoti, from the early Triassic period of Madagascar (about 250 million years ago), and Czatkobatrachus polonicus, from the Early Triassic of Poland (about the same age as Triadobatrachus).[21] The skull of Triadobatrachus is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern frogs. These include a longer body with more vertebrae. The tail has separate vertebrae unlike the fused urostyle or coccyx in modern frogs. The tibia and fibula bones are also separate, making it probable that Triadobatrachus was not an efficient leaper.[21]
The earliest known "true frogs" that fall into the anuran lineage proper all lived in the early Jurassic period.[4][22] One such early frog species, Prosalirus bitis, was discovered in 1995 in the Kayenta Formation of Arizona and dates back to the Early Jurassic epoch (199.6 to 175 million years ago), making Prosalirus somewhat more recent than Triadobatrachus.[23] Like the latter, Prosalirus did not have greatly enlarged legs, but had the typical three-pronged pelvic structure of modern frogs. Unlike Triadobatrachus, Prosalirus had already lost nearly all of its tail[24] and was well adapted for jumping.[25] Another Early Jurassic frog is Vieraella herbsti, which is known only from dorsal and ventral impressions of a single animal and was estimated to be 33 mm (1.3 in) from snout to vent. Notobatrachus degiustoi from the middle Jurassic is slightly younger, about 155–170 million years old. The main evolutionary changes in this species involved the shortening of the body and the loss of the tail. The evolution of modern Anura likely was complete by the Jurassic period. Since then, evolutionary changes in chromosome numbers have taken place about 20 times faster in mammals than in frogs, which means speciation is occurring more rapidly in mammals.[26]
Frog fossils have been found on all continents except Antarctica, but biogeographic evidence suggests they also inhabited Antarctica in an earlier era when the climate was warmer.[27]
| [show]Frog classification |
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A cladogram showing the relationships of the different families of frogs in the clade Anura can be seen in the table above. This diagram, in the form of a tree, shows how each frog family is related to other families, with each node representing a point of common ancestry. It is based on Frost et al. (2006),[28] Heinicke et al. (2009)[29] and Pyron and Wiens (2011).[30]
Morphology and physiology
Skeleton of Pelophylax esculentus showing bones of the head, vertebral column, ribs, pectoral and pelvic girdles, and limbs.
Frogs have no tail, except as larvae, and most have long hind legs, elongated ankle bones, webbed toes, no claws, large eyes, and a smooth or warty skin. They have short vertebral columns, with no more than 10 free vertebrae and fused tailbones (urostyle or coccyx).[31] Like other amphibians, oxygen can pass through their highly permeable skins. This unique feature allows them to remain in places without access to the air, respiring through their skins.[32] The ribs are poorly developed, so the lungs are filled by buccal pumping and a frog deprived of its lungs can maintain its body functions without them.[32] For the skin to serve as a respiratory organ, it must remain moist. This makes frogs susceptible to various substances they may encounter in the environment, some of which may be toxic and can dissolve in the water film and be passed into their bloodstream. This may be one of the causes of the worldwide decline in frog populations.[33][34][35][36]
Frogs range in size from the recently discovered 7.7-millimetre (0.30 in) Paedophryne amauensis of Papua New Guinea[37] to the 300-millimetre (12 in) goliath frog (Conraua goliath) of Cameroon. The skin hangs loosely on the body because of the lack of loose connective tissue. Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from translucent to opaque. They have a tympanum on each side of their heads which is involved in hearing and, in some species, is covered by skin. True toads completely lack teeth, but most frogs have them, specifically pedicellate teeth in which the crown is separated from the root by fibrous tissue. These are on the edge of the upper jaw and vomerine teeth are also on the roof of their mouths. No teeth are in the lower jaw and frogs usually swallow their food whole. The teeth are mainly used to grip the prey and keep it in place till swallowed, a process assisted by retracting the eyes into the head.[38] The African bullfrog (Pyxicephalus), which preys on relatively large animals such as mice and other frogs, has cone shaped bony projections called odontoid processes at the front of the lower jaw which function like teeth.[3]
A bullfrog skeleton, showing elongated limb bones and extra joints. Red marks indicate bones which have been substantially elongated in frogs and joints which have become mobile. Blue indicates joints and bones which have not been modified or only somewhat elongated.
Feet and legs
The structure of the feet and legs varies greatly among frog species, depending in part on whether they live primarily on the ground, in water, in trees or in burrows. Frogs must be able to move quickly through their environment to catch prey and escape predators, and numerous adaptations help them to do so. Most frogs are either proficient at jumping or are descended from ancestors that were, with much of the musculoskeletal morphology modified for this purpose. The tibia, fibula, and tarsals have been fused into a single, strong bone, as have the radius and ulna in the fore limbs (which must absorb the impact on landing). The metatarsals have become elongated to add to the leg length and allow the frog to push against the ground for a longer period on take-off. The illium has elongated and formed a mobile joint with the sacrum which, in specialist jumpers such as ranids and hylids, functions as an additional limb joint to further power the leaps. The tail vertebrae have fused into a urostyle which is retracted inside the pelvis. This enables the force to be transferred from the legs to the body during a leap.[31]
Webbed hind foot of common frog
(Rana temporaria)
The muscular system has been similarly modified. The hind limbs of ancestral frogs presumably contained pairs of muscles which would act in opposition (one muscle to flex the knee, a different muscle to extend it), as is seen in most other limbed animals. However, in modern frogs, almost all muscles have been modified to contribute to the action of jumping, with only a few small muscles remaining to bring the limb back to the starting position and maintain posture. The muscles have also been greatly enlarged, with the main leg muscles accounting for over 17% of the total mass of the frog.[39]
Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time the species spends in the water.[40] The completely aquatic African dwarf frog(Hymenochirus sp.) has fully webbed toes, whereas those of White's tree frog (Litoria caerulea), an arboreal species, are only a quarter or half webbed.[41]
Arboreal frogs have pads located on the ends of their toes to help grip vertical surfaces. These are not suction pads, the surface consisting instead of columnar cells with flat tops with small gaps between them lubricated by mucous glands. When the frog applies pressure, the cells adhere to irregularities on the surface and the grip is maintained through surface tension. This allows the frog to climb on smooth surfaces, but the system does not function efficiently when the pads are excessively wet.[42]
In many arboreal frogs, a small "intercalary structure" on each toe increases the surface area touching the substrate. Furthermore, since hopping through trees can be dangerous, many arboreal frogs have hip joints to allow both hopping and walking. Some frogs that live high in trees even possess an elaborate degree of webbing between their toes. This allows the frogs to "parachute" or make a controlled glide from one position in the canopy to another.[43]
Ground-dwelling frogs generally lack the adaptations of aquatic and arboreal frogs. Most have smaller toe pads, if any, and little webbing. Some burrowing frogs such as Couch's spadefoot (Scaphiopus couchii) have a flap-like toe extension on the hind feet, a keratinised tubercle often referred to as a spade, that helps them to burrow.[44]
Sometimes during the tadpole stage, one of the developing rear legs is eaten by a predator such as a dragonfly nymph. In some cases, the full leg still grows, but in others it does not, although the frog may still live out its normal lifespan with only three limbs. Occasionally, a parasitic flatworm (Ribeiroia ondatrae) digs into the rear of a tadpole, causing a rearrangement of the limb bud cells and the frog develops an extra leg or two.[45]
Skin
A frog's skin is protective, has a respiratory function, can absorb water and helps control body temperature. It has many glands, particularly on the head and back, which often exude distasteful and toxic substances. The secretion is often sticky and helps keep the skin moist, protects against the entry of moulds and bacteria, and make the animal slippery and more able to escape from predators.[46] The skin is shed every few weeks. It usually splits down the middle of the back and across the belly, and the frog pulls its arms and legs free. The sloughed skin is then worked towards the head where it is quickly eaten.[47]
Being cold-blooded, frogs have to adopt suitable behaviour patterns to regulate their temperature. To warm up, they can move into the sun or onto a warm surface; if they overheat, they can move into the shade or adopt a stance that exposes the minimum area of skin to the air. This posture is also used to prevent water loss and involves the frog squatting close to the substrate with its hands and feet tucked under its chin and body.[48] The colour of a frog's skin is used for thermoregulation. In cool damp conditions, the colour will be darker than on a hot dry day. The grey foam-nest tree frog (Chiromantis xerampelina) is even able to turn white to minimize the chance of overheating.[49]
Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area, but the permeability of a frog's skin can also result in water loss. Glands located all over the body exude mucus which helps keep the skin moist and reduces evaporation. Some glands on the hands and chest of males are specialized to produce sticky secretions to aid in amplexus. Similar glands in tree frogs produce a glue-like substance on the adhesive discs of the feet. Some arboreal frogs reduce water loss by having a waterproof layer of skin, and several South American species coat their skin with a waxy secretion. Others frogs have adopted behaviours to conserve water, including becoming nocturnal and resting in a water-conserving position. Some frogs may also rest in large groups with each frog pressed against its neighbours. This reduces the amount of skin exposed to the air or a dry surface, and thus reduces water loss.[48] Woodhouse's toad (Bufo woodhousii), if given access to water after confinement in a dry location, sits in the shallows to rehydrate.[50] The male hairy frog (Trichobatrachus robustus) has dermal papillae projecting from its lower back and thighs, giving it a bristly appearance. They contain blood vessels and are thought to increase the area of the skin available for respiration.[51]
Some species have bony plates embedded in their skin, a trait that appears to have evolved independently several times.[52] In certain other species, the skin at the top of the head is compacted and the connective tissue of the dermis is co-ossified with the bones of the skull (exostosis).[53][54]
Camouflage is a common defensive mechanism in frogs. Most camouflaged frogs are nocturnal; during the day, they seek out a position where they can blend into the background and remain undetected. Some frogs have the ability to change colour, but this is usually restricted to a small range of colours. For example, White's tree frog (Litoria caerulea) varies between pale green and dull brown according to the temperature, and the Pacific tree frog (Pseudacris regilla) has green and brown morphs, plain or spotted, and changes colour depending on the time of year and general background colour.[55] Features such as warts and skin folds are usually on ground-dwelling frogs, for whom smooth skin would not provide such effective camouflage. Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract.[32]
Pouched frog (Assa darlingtoni) camouflaged against leaf litter
Respiration and circulation
The skin of a frog is permeable to oxygen and carbon dioxide, as well as to water. There are blood vessels near the surface of the skin and when a frog is underwater, oxygen diffuses directly into the blood. When not submerged, a frog breathes by a process known as buccal pumping. Its lungs are similar to those of humans, but the chest muscles are not involved in respiration, and no ribs or diaphragm exist to help move air in and out. Instead, it puffs out its throat and draws air in through the nostrils, which in many species can then be closed by valves. When the floor of the mouth is compressed, air is forced into the lungs.[56] The fully aquatic Bornean flat-headed frog (Barbourula kalimantanensis) is the first frog known to lack lungs entirely.[57]
Frogs have three-chambered hearts, a feature they share with lizards.[58] Oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enter the heart through separate atria. When these chambers contract, the two blood streams pass into a common ventricle before being pumped via a spiral valve to the appropriate vessel, the aorta for oxygenated blood and pulmonary artery for deoxygenated blood. The ventricle is partially divided into narrow cavities which minimizes the mixing of the two types of blood. These features enable frogs to have a higher metabolic rate and be more active than would otherwise be possible.[58]
Some species of frog have adaptations that allow them to survive in oxygen deficient water. The Lake Titicaca frog (Telmatobius culeus) is one such species and has wrinkly skin that increases its surface area to enhance gas exchange. It normally makes no use of its rudimentary lungs but will sometimes raise and lower its body rhythmically while on the lake bed to increase the flow of water around it.[59]
Anatomical model of a dissected frog: 1 Right atrium, 2 Lungs, 3 Aorta, 4 Egg mass, 5 Colon, 6 Left atrium, 7 Ventricle, 8 Stomach, 9 Liver, 10 Gallbladder, 11 Small intestine, 12 Cloaca
Digestion and excretion
Frogs have maxillary teeth along their upper jaw which are used to hold food before it is swallowed. These teeth are very weak, and cannot be used to chew or catch and harm agile prey. Instead, the frog uses its sticky, cleft tongue to catch flies and other small moving prey. The tongue normally lies coiled in the mouth, free at the back and attached to the mandible at the front. It can be shot out and retracted at great speed.[40] Some frogs have no tongue and just stuff food into their mouths with their hands.[40] The eyes assist in the swallowing of food as they can be retracted through holes in the skull and help push food down the throat.[40] The food then moves through the oesophagus into the stomach where digestive enzymes are added and it is churned up. It then proceeds to the small intestine (duodenum and ileum) where most digestion occurs. Pancreatic juice from the pancreas, and bile, produced by the liver and stored in the gallbladder, are secreted into the small intestine, where the fluids digest the food and the nutrients are absorbed. The food residue passes into the large intestine where excess water is removed and the wastes are passed out through the cloaca.[60]
Although adapted to terrestrial life, frogs resemble freshwater fish in their inability to conserve body water effectively. When they are on land, much water is lost by evaporation from the skin. The excretory system is similar to that of mammals and there are two kidneys that remove nitrogenous products from the blood. Frogs produce large quantities of dilute urine in order to flush out toxic products from the kidney tubules.[61] The nitrogen is excreted as ammonia by tadpoles and aquatic frogs but mainly as urea, a less toxic product, by most terrestrial adults. A few species of tree frog with little access to water excrete the even less toxic uric acid.[61] The urine passes along paired ureters to the urinary bladder from which it is vented periodically into the cloaca. All bodily wastes exit the body through the cloaca which terminates in a cloacal vent.[62]
Reproductive system
In the male frog, the two testes are attached to the kidneys and semen passes into the kidneys through fine tubes called efferent ducts. It then travels on through the ureters, which are consequently known as urinogenital ducts. There is no penis, and sperm is ejected from the cloaca directly onto the eggs as the female lays them. The ovaries of the female frog are beside the kidneys and the eggs pass down a pair of oviducts and through the cloaca to the exterior.[62]
When frogs mate, the male climbs on the back of the female and wraps his fore limbs round her body, either behind the front legs or just in front of the hind legs. This position is called amplexus and may be held for several days.[63] The male frog has certain hormone-dependent secondary sexual characteristics. These include the development of special pads on his thumbs in the breeding season, to give him a firm hold.[64] The grip of the male frog during amplexus stimulates the female to release eggs, usually wrapped in jelly, as spawn.[62] In many species the male is smaller and slimmer than the female. Males have vocal cords and make a range of croaks, particularly in the breeding season, and in some species they also have vocal sacs to amplify the sound.[62]
Nervous system
The frog has a highly developed nervous system that consists of a brain, spinal cord and nerves. Many parts of the frog's brain correspond with those of humans. It consists of two olfactory lobes, two cerebral hemispheres, a pineal body, two optic lobes, a cerebellum and a medulla oblongata. Muscular coordination and posture are controlled by the cerebellum, and the medulla oblongata regulates respiration, digestion and other automatic functions.[62] The relative size of the cerebrum in frogs is much smaller than it is in humans. Frogs have ten pairs of cranial nerves which pass information from the outside directly to the brain, and ten pairs of spinal nerves which pass information from the extremities to the brain through the spinal cord.[62] By contrast, all amniotes (mammals, birds and reptiles) have twelve pairs of cranial nerves.[65]
Close-up of frog's head showing eye, nostril, mouth and tympanum.
Sight
The eyes of most frogs are located on either side of the head near the top and project outwards as hemispherical bulges. They provide binocular vision over a field of 100° to the front and a total visual field of almost 360°.[66]They may be the only part of an otherwise submerged frog to protrude from the water. Each eye has closable upper and lower lids and a nictitating membrane which provides further protection, especially when the frog is swimming.[67] Members of the aquatic family Pipidae have the eyes located at the top of the head, a position better suited for detecting prey in the water above.[66] The irises come in a range of colours and the pupils in a range of shapes. The common toad (Bufo bufo) has golden irises and horizontal slit-like pupils, the red-eyed tree frog (Agalychnis callidryas) has vertical slit pupils, the poison dart frog has dark irises, the fire-bellied toad (Bombina spp.) has triangular pupils and the tomato frog (Dyscophus spp.) has circular ones. The irises of the southern toad (Anaxyrus terrestris) are patterned so as to blend in with the surrounding camouflaged skin.[67]
The distant vision of a frog is better than its near vision. Calling frogs will quickly become silent when they see an intruder or even a moving shadow but the closer an object is, the less well it is seen.[67] When a frog shoots out its tongue to catch an insect it is reacting to a small moving object that it cannot see well and must line it up precisely beforehand because it shuts its eyes as the tongue is extended.[40] Whether a frog sees in colour is debatable but it has been shown that it responds positively to blue light, perhaps because that colour is associated with bodies of water that can provide refuge when the frog feels threatened.[68]
Hearing
Frogs can hear both in the air and below water. They do not have external ears; the eardrums (tympanic membranes) are directly exposed or may be covered by a layer of skin and are visible as a circular area just behind the eye. The size and distance apart of the eardrums is related to the frequency and wavelength at which the frog calls. In some species such as the bullfrog, the size of the tympanum indicates the sex of the frog; males have tympani that are larger than their eyes while in females, the eyes and tympani are much the same size.[69] A noise causes the tympanum to vibrate and the sound is transmitted to the middle and inner ear. The middle ear contains semicircular canals which help control balance and orientation. In the inner ear, the auditory hair cells are arranged in two areas of the cochlea, the basilar papilla and the amphibian papilla. The former detects high frequencies and the latter low frequencies.[70] Because the cochlea is short, frogs use electrical tuning to extend their range of audible frequencies and help discriminate different sounds.[71] This arrangement enables detection of the territorial and breeding calls of their conspecifics. In some species that inhabit arid regions, the sound of thunder or heavy rain may arouse them from a dormant state.[70] A frog may be startled by an unexpected noise but it will not usually take any action until it has located the source of the sound by sight.[69]
Call
The call or croak of a frog is unique to its species. Frogs create this sound by passing air through the larynx in the throat. In most calling frogs, the sound is amplified by one or more vocal sacs, membranes of skin under the throat or on the corner of the mouth, that distend during the amplification of the call. Some frog calls are so loud that they can be heard up to a mile away.[72]
Frogs in the genera Heleioporus and Neobatrachus lack vocal sacs but can still produce a loud call. Their buccal cavity is enlarged and dome-shaped, acting as a resonance chamber that amplifies the sound. Species of frog that lack vocal sacs and that do not have a loud call tend to inhabit areas close to constantly noisy, flowing water. They need to use an alternative means to communicate. The coastal tailed frog (Ascaphus truei) lives in mountain streams in North America and does not vocalize.[73]
The main reason for calling is to allow male frogs to attract a mate. Males may call individually or there may be a chorus of sound where numerous males have converged on breeding sites. Females of many frog species, such as the common tree frog (Polypedates leucomystax), reply to the male calls, which acts to reinforce reproductive activity in a breeding colony.[74] Female frogs prefer males that produce sounds of greater intensity and lower frequency, attributes that stand out in a crowd. The rationale for this is thought to be that by demonstrating his prowess, the male shows his fitness to produce superior offspring.[75]
A different call is emitted by a male frog or unreceptive female when mounted by another male. This is a distinct chirruping sound and is accompanied by a vibration of the body.[76] Tree frogs and some non-aquatic species have a rain call that they make on the basis of humidity cues prior to a shower.[76] Many species also have a territorial call that is used to drive away other males. All of these calls are emitted with the mouth of the frog closed.[76] A distress call, emitted by some frogs when they are in danger, is produced with the mouth open resulting in a higher-pitched call. It is typically used when the frog has been grabbed by a predator and may serve to distract or disorientate the attacker so that it releases the frog.[76]
Torpor
During extreme conditions, some frogs enter a state of torpor and remain inactive for months. In colder regions, many species of frog hibernate in winter. Those that live on land such as the American toad (Bufo americanus) dig a burrow and make a hibernaculum in which to lie dormant. Others, less proficient at digging, find a crevice or bury themselves in dead leaves. Aquatic species such as the American bullfrog (Rana catesbeiana) normally sink to the bottom of the pond where they lie, semi-immersed in mud but still able to access the oxygen dissolved in the water. Their metabolism slows down and they live on their energy reserves. Some frogs can even survive being frozen. Ice crystals form under the skin and in the body cavity but the essential organs are protected from freezing by a high concentration of glucose. An apparently lifeless, frozen frog can resume respiration and the heart beat can restart when conditions warm up.[80]
At the other extreme, the striped burrowing frog (Cyclorana alboguttata) regularly aestivates during the hot, dry season in Australia, surviving in a dormant state without access to food and water for nine or ten months of the year. It burrows underground and curls up inside a protective cocoon formed by its shed skin. Researchers at the University of Queensland have found that during aestivation, the metabolism of the frog is altered and the operational efficiency of the mitochondria is increased. This means that the limited amount of energy available to the comatose frog is used in a more efficient manner. This survival mechanism is only useful to animals that remain completely unconscious for an extended period of time and whose energy requirements are low because they are cold-blooded and have no need to generate heat.[81] Other research showed that, to provide these energy requirements, muscles atrophy, but hind limb muscles are preferentially unaffected.[82] Frogs have been found to have upper critical temperatures of around 41 degrees Celsius.[83]
Locomotion
- Jumping
Frogs are generally recognized as exceptional jumpers and, relative to their size, the best jumpers of all vertebrates.[84] The striped rocket frog, Litoria nasuta, can leap over 2 metres (6 ft 7 in), a distance that is more than fifty times its body length of 5.5 centimetres (2.2 in).[85] There are tremendous differences between species in jumping capability. Within a species, jump distance increases with increasing size, but relative jumping distance (body-lengths jumped) decreases. The Indian skipper frog (Euphlyctis cyanophlyctis) has the ability to leap out of the water from a position floating on the surface.[86] The tiny northern cricket frog (Acris crepitans) can "skitter" across the surface of a pond with a series of short rapid jumps.[87]
Slow-motion photography shows that the muscles have passive flexibility. They are first stretched while the frog is still in the crouched position, then they are contracted before being stretched again to launch the frog into the air. The fore legs are folded against the chest and the hind legs remain in the extended, streamlined position for the duration of the jump.[39] In some extremely capable jumpers, such as the Cuban tree frog (Osteopilus septentrionalis) and the northern leopard frog (Rana pipiens), the peak power exerted during a jump can exceed that which the muscle is theoretically capable of producing. When the muscles contract, the energy is first transferred into the stretched tendon which is wrapped around the ankle bone. Then the muscles stretch again at the same time as the tendon releases its energy like a catapult to produce a powerful acceleration beyond the limits of muscle-powered acceleration.[88] A similar mechanism has been documented in locusts and grasshoppers.[89]
- Walking and running
Frogs in the families Bufonidae, Rhinophrynidae, and Microhylidae have short back legs and tend to walk rather than jump.[90] When they try to move rapidly, they speed up the rate of movement of their limbs or resort to an ungainly hopping gait. The Great Plains narrow-mouthed toad (Gastrophryne olivacea) has been described as having a gait that is "a combination of running and short hops that are usually only an inch or two in length".[91] In an experiment, Fowler's toad (Bufo fowleri) was placed on a treadmill which was turned at varying speeds. By measuring the toad's uptake of oxygen it was found that hopping was an inefficient use of resources during sustained locomotion but was a useful strategy during short bursts of high-intensity activity.[92]
The red-legged running frog (Kassina maculata) has short, slim hind limbs unsuited to jumping. It can move fast by using a running gait in which the two hind legs are used alternately. Slow-motion photography shows, unlike a horse that can trot or gallop, the frog's gait remained similar at slow, medium, and fast speeds.[93] This species can also climb trees and shrubs, and does so at night to catch insects.[94] The Indian skipper frog (Euphlyctis cyanophlyctis) has broad feet and can run across the surface of the water for several metres (yards).[87]
- Swimming
Frogs that live in or visit water have adaptations that improve their swimming abilities. The hind limbs are heavily muscled and strong. The webbing between the toes of the hind feet increases the area of the foot and helps propel the frog powerfully through the water. Members of the family Pipidae are wholly aquatic and show the most marked specialization. They have inflexible vertebral columns, flattened, streamlined bodies, lateral linesystems, and powerful hind limbs with large webbed feet.[95] Tadpoles mostly have large tail fins which provide thrust when the tail is moved from side to side.[96]
- Burrowing
Some frogs have become adapted for burrowing and a life underground. They tend to have rounded bodies, short limbs, small heads with bulging eyes, and hind feet adapted for excavation. An extreme example of this is thepurple frog (Nasikabatrachus sahyadrensis) from southern India which feeds on termites and spends almost its whole life underground. It emerges briefly during the monsoon to mate and breed in temporary pools. It has a tiny head with a pointed snout and a plump, rounded body. Because of this fossorial existence, it was first described in 2003, being new to the scientific community at that time, although previously known to local people.[97]
Purple frog (Nasikabatrachus sahyadrensis).
The spadefoot toads of North America are also adapted to underground life. The Plains spadefoot toad (Spea bombifrons) is typical and has a flap of keratinised bone attached to one of the metatarsals of the hind feet which it uses to dig itself backwards into the ground. As it digs, the toad wriggles its hips from side to side to sink into the loose soil. It has a shallow burrow in the summer from which it emerges at night to forage. In winter, it digs much deeper and has been recorded at a depth of 4.5 m (15 ft).[98] The tunnel is filled with soil and the toad hibernates in a small chamber at the end. During this time, urea accumulates in its tissues and water is drawn in from the surrounding damp soil by osmosis to supply the toad's needs.[98]Spadefoot toads are "explosive breeders", all emerging from their burrows at the same time and converging on temporary pools, attracted to one of these by the calling of the first male to find a suitable breeding location.[99]
The burrowing frogs of Australia have a rather different lifestyle. The western spotted frog (Heleioporus albopunctatus) digs a burrow beside a river or in the bed of an ephemeral stream and regularly emerges to forage. Mating takes place and eggs are laid in a foam nest inside the burrow. The eggs partially develop there, but do not hatch until they are submerged following heavy rainfall. The tadpoles then swim out into the open water and rapidly complete their development.[100] Madagascan burrowing frogs are less fossorial and mostly bury themselves in leaf litter. One of these, the green burrowing frog (Scaphiophryne marmorata), has a flattened head with a short snout and well-developed metatarsal tubercles on its hind feet to help with excavation. It also has greatly enlarged terminal discs on its fore feet that help it to clamber around in bushes.[101] It breeds in temporary pools that form after rains.[102]
- Climbing
Tree frogs live high in the canopy, where they scramble around on the branches, twigs, and leaves, sometimes never coming down to earth. The "true" tree frogs belong to the family Hylidae, but members of other frog families have independently adopted an arboreal habit, a case of convergent evolution. These include the glass frogs (Centrolenidae), the bush frogs (Hyperoliidae), some of the narrow-mouthed frogs (Microhylidae), and the shrub frogs (Rhacophoridae).[90] Most tree frogs are under 10 cm (4 in) in length, with long legs and long toes with adhesive pads on the tips. The surface of the toe pads is formed from a closely packed layer of flat-topped, hexagonal epidermal cells separated by grooves into which glands secrete mucus. These toe pads, moistened by the mucus, provide the grip on any wet or dry surface, including glass. The forces involved include boundary friction of the toe pad epidermis on the surface and also surface tension and viscosity.[103] Tree frogs are very acrobatic and can catch insects while hanging by one toe from a twig or clutching onto the blade of a windswept reed.[104] Some members of the subfamily Phyllomedusinae have opposable toes on their feet. The reticulated leaf frog (Phyllomedusa ayeaye) has a single opposed digit on each fore foot and two opposed digits on its hind feet. This allows it to grasp the stems of bushes as it clambers around in its riverside habitat.[105]
- Gliding
During the evolutionary history of the frog, several different groups have independently taken to the air.[106] Some frogs in the tropical rainforest are specially adapted for gliding from tree to tree or parachuting to the forest floor. Typical of them is Wallace's flying frog (Rhacophorus nigropalmatus) from Malaysia and Borneo. It has large feet with the fingertips expanded into flat adhesive discs and the digits fully webbed. Flaps of skin occur on the lateral margins of the limbs and across the tail region. With the digits splayed, the limbs outstretched, and these flaps spread, it can glide considerable distances, but is unable to undertake powered flight.[107] It can alter its direction of travel and navigate distances of up to 15 m (49 ft) between trees.[108]
Life history
Like other amphibians, the life cycle of a frog normally starts in water with an egg that hatches into a limbless larva with gills, commonly known as a tadpole. After further growth, during which it develops limbs and lungs, the tadpole undergoes metamorphosis in which its appearance and internal organs are rearranged. After this it is able to leave the water as a miniature, air-breathing frog.
Reproduction
Two main types of reproduction occur in frogs, prolonged breeding and explosive breeding. In the former, adopted by the majority of species, adult frogs at certain times of year assemble at a pond, lake or stream to breed. Many frogs return to the bodies of water in which they developed as larvae. This often results in annual migrations involving thousands of individuals. In explosive breeders, mature adult frogs arrive at breeding sites in response to certain trigger factors such as rainfall occurring in an arid area. In these frogs, mating and spawning take place promptly and the speed of larval growth is rapid in order to make use of the ephemeral pools before they dry up.[109]
Among prolonged breeders, males usually arrive at the breeding site first and remain there for some time whereas females tend to arrive later and depart soon after they have spawned. This means that males outnumber females at the water's edge and defend territories from which they expel other males. They advertise their presence by calling, often alternating their croaks with neighbouring frogs. Larger, stronger males tend to have deeper calls and maintain higher quality territories. Females select their mates at least partly on the basis of the depth of their voice.[110] In some species there are satellite males who have no territory and do not call. They may intercept females that are approaching a calling male or take over a vacated territory. Calling is an energy-sapping activity. Sometimes the two roles are reversed and a calling male gives up its territory and becomes a satellite.[109]
In explosive breeders, the first male that finds a suitable breeding location, such as a temporary pool, calls loudly and other frogs of both sexes converge on the pool. Explosive breeders tend to call in unison creating a chorus that can be heard from far away. The spadefoot toads (Scaphiopus spp.) of North America fall into this category. Mate selection and courtship is not as important as speed in reproduction. In some years, suitable conditions may not occur and the frogs may go for two or more years without breeding.[109] Some female New Mexico spadefoot toads (Spea multiplicata) only spawn half of the available eggs at a time, perhaps retaining some in case a better reproductive opportunity arises later.[111]
Life cycle
Eggs / frogspawn
Frogs' embryos are typically surrounded by several layers of gelatinous material. When several eggs are clumped together, they are collectively known as frogspawn. The jelly provides support and protection while allowing the passage of oxygen, carbon dioxide and ammonia. It absorbs moisture and swells on contact with water. After fertilization, the innermost portion liquifies to allow free movement of the developing embryo. In certain species, such as the Northern red-legged frog (Rana aurora) and the wood frog (Rana sylvatica), symbiotic unicellular green algae are present in the gelatinous material. It is thought that these may benefit the developing larvae by providing them with extra oxygen through photosynthesis.[115] Most eggs are black or dark brown and this has the advantage of absorbing warmth from the sun which the insulating capsule retains. The interior of globular egg clusters of the wood frog (Rana sylvatica) has been found to be up to 6 °C (11 °F) warmer than the surrounding water and this speeds up the development of the larvae.[116]
The shape and size of the egg mass is characteristic of the species. Ranids tend to produce globular clusters containing large numbers of eggs whereas bufonids produce long, cylindrical strings. The tiny yellow-striped pygmy eleuth (Eleutherodactylus limbatus) lays eggs singly, burying them in moist soil.[117] The smoky jungle frog (Leptodactylus pentadactylus) makes a nest of foam in a hollow. The eggs hatch when the nest is flooded, or the tadpoles may complete their development in the foam if flooding does not occur.[118] The red-eyed treefrog (Agalychnis callidryas) deposits its eggs on a leaf above a pool and when they hatch, the larvae fall into the water below.[119] The larvae developing in the eggs can detect vibrations caused by nearby predatory wasps or snakes, and will hatch early to avoid being eaten.[120] In general, the length of the egg stage depends on the species and the environmental conditions. Aquatic eggs normally hatch within one week when the capsule splits as a result of enzymes released by the developing larvae.[121]
Tadpoles
The larvae that emerge from the eggs, known as tadpoles (or occasionally polliwogs), typically have oval bodies and long, vertically flattened tails. As a general rule, free-living larvae are fully aquatic, but at least one species (Nannophrys ceylonensis) has semiterrestrial tadpoles which live among wet rocks.[122][123] Tadpoles lack eyelids and have cartilaginous skeletons, lateral line systems, gills for respiration (external gills at first, internal gills later), and vertically flattened tails they use for swimming.[96]
From early in its development, a gill pouch covers the tadpole's gills and front legs. The lungs soon start to develop and are used as an accessory breathing organ. Some species go through metamorphosis while still inside the egg and hatch directly into small frogs. Tadpoles lack true teeth, but the jaws in most species have two elongated, parallel rows of small, keratinized structures called keradonts in their upper jaws. Their lower jaws usually have three rows of keradonts surrounded by a horny beak, but the number of rows can vary and the exact arrangements of mouth parts provide a means for species identification.[121] In the Pipidae, with the exception ofHymenochirus, the tadpoles have paired anterior barbels, which make them resemble small catfish.[95] Their tails are stiffened by a notochord, but does not contain any bony or cartilaginous elements except for a few vertebrae at the base which forms the urostyle during metamorphosis. This has been suggested as an adaptation to their lifestyles; because the transformation into frogs happens very fast, the tail is made of soft tissue only, as bone and cartilage take a much longer time to be broken down and absorbed. The tail fin and tip is fragile and will easily tear, which is seen as an adaptation to escape from predators which tries to grasp them by the tail.[124]
Tadpoles are typically herbivorous, feeding mostly on algae, including diatoms filtered from the water through the gills. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles, and fish. The Cuban tree frog (Osteopilus septentrionalis) is one of a number of species in which the tadpoles can be cannibalistic. Tadpoles that develop legs early may be eaten by the others, so late developers may have better long-term survival prospects.[125]
Tadpoles are highly vulnerable to being eaten by fish, newts, predatory diving beetles, and birds, such as kingfishers. Some tadpoles, including those of the cane toad (Bufo marinus), are poisonous. The tadpole stage may be as short as a week in explosive breeders or it may last through one or more winters followed by metamorphosis in the spring.[126]
Metamorphosis
At the end of the tadpole stage, a frog undergoes metamorphosis in which its body makes a sudden transition into the adult form. This metamorphosis typically lasts only 24 hours, and is initiated by production of the hormone thyroxine. This causes different tissues to develop in different ways. The principal changes that take place include the development of the lungs and the disappearance of the gills and gill pouch, making the front legs visible. The lower jaw transforms into the big mandible of the carnivorous adult, and the long, spiral gut of the herbivorous tadpole is replaced by the typical short gut of a predator.[121] The nervous system becomes adapted for hearing and stereoscopic vision, and for new methods of locomotion and feeding.[121] The eyes are repositioned higher up on the head and the eyelids and associated glands are formed. The eardrum, middle ear, and inner ear are developed. The skin becomes thicker and tougher, the lateral line system is lost, and skin glands are developed.[121] The final stage is the disappearance of the tail, but this takes place rather later, the tissue being used to produce a spurt of growth in the limbs.[127] Frogs are at their most vulnerable to predators when they are undergoing metamorphosis. At this time, the tail is being lost and locomotion by means of limbs is only just becoming established.[90]
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Larva of the common frog Rana temporaria a day before metamorphosis
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Metamorphosis stage with deforming jaws, large eyes, and remains of gill pouch
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Young frog with a stumpy tail, metamorphosis nearly complete
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Adults
After metamorphosis, young adults may disperse into terrestrial habitats or continue to live in water. Almost all frog species are carnivorous as adults, preying on invertebrates, including arthropods, worms, snails, and slugs. A few of the larger ones may eat other frogs, small mammals, and fish. Some frogs use their sticky tongues to catch fast-moving prey, while others push food into their mouths with their hands. A few species also eat plant matter; the tree frog Xenohyla truncata is partly herbivorous, its diet including a large proportion of fruit,[128] Leptodactylus mystaceus has been found to eat plants,[129][130] and folivory occurs in Euphlyctis hexadactylus, with plants constituting 79.5% of its diet by volume.[131] Adult frogs are themselves attacked by many predators. The northern leopard frog (Rana pipiens) is eaten by herons, hawks, fish, large salamanders, snakes, raccoons, skunks,mink, bullfrogs, and other animals.[132]
A trophic pyramid showing frogs as primary predators.
Frogs are primary predators and an important part of the food web. Being cold-blooded, they make efficient use of the food they eat with little energy being used for metabolic processes, while the rest is transformed into biomass. They are themselves eaten by secondary predators and are the primary terrestrial consumers of invertebrates, most of which feed on plants. By reducing herbivory, they play a part in increasing the growth of plants and are thus part of a delicately balanced ecosystem.[133]
Little is known about the longevity of frogs and toads in the wild, but some can live for many years. Skeletochronology is a method of examining bones to determine age. Using this method, the ages of mountain yellow-legged frogs (Rana muscosa) were studied, the phalanges of the toes showing seasonal lines where growth slows in winter. The oldest frogs had ten bands, so their age was believed to be 14 years, including the four-year tadpole stage.[134] Captive frogs and toads have been recorded as living for up to 40 years, an age achieved by a European common toad (Bufo bufo). The cane toad (Bufo marinus) has been known to survive 24 years in captivity, and the American bullfrog (Rana catesbeiana) 14 years.[135] Frogs from temperate climates hibernate during the winter, and four species are known to be able to withstand freezing during this time, including the wood frog (Rana sylvatica).[136]
Parental care
Although care of offspring is poorly understood in frogs, up to an estimated 20% of amphibian species may care for their young in some way.[137] The evolution of parental care in frogs is driven primarily by the size of the water body in which they breed. Those that breed in smaller water bodies tend to have greater and more complex parental care behaviour.[138] Because predation of eggs and larvae is high in large water bodies, some frog species started to lay their eggs on land. Once this happened, the desiccating terrestrial environment demands that one or both parents keep them moist to ensure their survival.[139] The subsequent need to transport hatched tadpoles to a water body required an even more intense form of parental care.[138]
In small pools, predators are mostly absent and competition between tadpoles becomes the variable that constrains their survival. Certain frog species avoid this competition by making use of smaller phytotelmata (water-filled leaf axils or small woody cavities) as sites for depositing a few tadpoles.[140] While these smaller rearing sites are free from competition, they also lack sufficient nutrients to support a tadpole without parental assistance. Frog species that changed from the use of larger to smaller phytotelmata have evolved a strategy of providing their offspring with nutritive but unfertilized eggs.[138] The female strawberry poison-dart frog (Oophaga pumilio) lays her eggs on the forest floor. The male frog guards them from predation and carries water in his cloaca to keep them moist. When they hatch, the female moves the tadpoles on her back to a water-holding bromeliad or other similar water body, depositing just one in each location. She visits them regularly and feeds them by laying one or two unfertilized eggs in the phytotelma, continuing to do this until the young are large enough to undergo metamorphosis.[141] The granular poison frog (Oophaga granulifera) looks after its tadpoles in a similar way.[142]
Many other diverse forms of parental care are seen in frogs. The tiny male Colostethus subpunctatus stands guard over his egg cluster, laid under a stone or log. When the eggs hatch, he transports the tadpoles on his back to a temporary pool, where he partially immerses himself in the water and one or more tadpoles drop off. He then moves on to another pool.[143] The male common midwife toad (Alytes obstetricans) carries the eggs around with him attached to his hind legs. He keeps them damp in dry weather by immersing himself in a pond, and prevents them from getting too wet in soggy vegetation by raising his hindquarters. After three to six weeks, he travels to a pond and the eggs hatch into tadpoles.[144] The tungara frog (Physalaemus pustulosus) builds a floating nest from foam to protect its eggs from predation. The foam is made from proteins and lectins, and seems to have antimicrobial properties.[145] Several pairs of frogs may form a colonial nest on a previously built raft. The eggs are laid in the centre, followed by alternate layers of foam and eggs, finishing with a foam capping.[146]
Some frogs protect their offspring inside their own bodies. Both male and female pouched frogs (Assa darlingtoni) guard their eggs, which are laid on the ground. When the eggs hatch, the male lubricates his body with the jelly surrounding them and immerses himself in the egg mass. The tadpoles wriggle into skin pouches on his side, where they develop until they metamorphose into juvenile frogs.[147] The female gastric-brooding frog(Rheobatrachus sp.) from Australia, now probably extinct, swallows her fertilized eggs, which then develop inside her stomach. She ceases to feed and stops secreting stomach acid. The tadpoles rely on the yolks of the eggs for nourishment. After six or seven weeks, they are ready for metamorphosis. The mother regurgitates the tiny frogs, which hop away from her mouth.[148] The female Darwin's frog (Rhinoderma darwinii) from Chile lays up to 40 eggs on the ground, where they are guarded by the male. When the tadpoles are about to hatch, they are engulfed by the male, which carries them around inside his much-enlarged vocal sac. Here they are immersed in a frothy, viscous liquid that contains some nourishment to supplement what they obtain from the yolks of the eggs. They remain in the sac for seven to ten weeks before undergoing metamorphosis, after which they move into the male's mouth and emerge.[149]
Defence
At first sight, frogs seem rather defenceless because of their small size, slow movement, thin skin, and lack of defensive structures, such as spines, claws or teeth. Many use camouflage to avoid detection, the skin often being spotted or streaked in neutral colours that allow a stationary frog to merge into its surroundings. Some can make prodigious leaps, often into water, that help them to evade potential attackers, while many have other defensive adaptations and strategies.[109]
The skin of many frogs contains mild toxic substances called bufotoxins to make them unpalatable to potential predators. Most toads and some frogs have large poison glands, the parotoid glands, located on the sides of their heads behind the eyes and other glands elsewhere on their bodies. These glands secrete mucus and a range of toxins that make frogs slippery to hold and distasteful or poisonous. If the noxious effect is immediate, the predator may cease its action and the frog may escape. If the effect develops more slowly, the predator may learn to avoid that species in future.[150] Poisonous frogs tend to advertise their toxicity with bright colours, an adaptive strategy known as aposematism. The poison dart frogs in the family Dendrobatidae do this. They are typically red, orange, or yellow, often with contrasting black markings on their bodies. Allobates zaparo is not poisonous, but mimics the appearance of two different toxic species with which it shares a common range in an effort to deceive predators.[151] Other species, such as the European fire-bellied toad (Bombina bombina), have their warning colour underneath. They "flash" this when attacked, adopting a pose that exposes the vivid colouring on their bellies.[6]
A common toad adopting a defensive stance.
Some frogs, such as the poison dart frogs, are especially toxic. The native people of South America extract poison from these frogs to apply to their weapons for hunting,[152] although few species are toxic enough to be used for this purpose. At least two non-poisonous frog species in tropical America (Eleutherodactylus gaigei and Lithodytes lineatus) mimic the colouration of dart poison frogs for self-protection.[153][154] Some frogs obtain poisons from the ants and other arthropods they eat.[155] Others, such as the Australian corroboree frogs (Pseudophryne corroboree and Pseudophryne pengilleyi), can synthesize the alkaloids themselves.[156] The chemicals involved may be irritants, hallucinogens, convulsants, nerve poisons orvasoconstrictors. Many predators of frogs have become adapted to tolerate high levels of these poisons, but other creatures, including humans who handle the frogs, may be severely affected.[157]
Some frogs use bluff or deception. The European common toad (Bufo bufo) adopts a characteristic stance when attacked, inflating its body and standing with its hindquarters raised and its head lowered.[158] The bullfrog (Rana catesbeiana) crouches down with eyes closed and head tipped forward when threatened. This places the parotoid glands in the most effective position, the other glands on its back begin to ooze noxious secretions and the most vulnerable parts of its body are protected.[109] Another tactic used by some frogs is to "scream", the sudden loud noise tending to startle the predator. The gray tree frog (Hyla versicolor) makes an explosive sound that sometimes repels the shrew Blarina brevicauda.[109] Although toads are avoided by many predators, the common garter snake (Thamnophis sirtalis) regularly feeds on them. The strategy employed by juvenile American toads (Bufo americanus) on being approached by a snake is to crouch down and remain immobile. This is usually successful, with the snake passing by and the toad remaining undetected. If it is encountered by the snake's head, however, the toad hops away before crouching defensively.[159]
Distribution and conservation status
Frogs live on all the continents except Antarctica, but they are not present on certain islands, especially those far away from continental land masses.[160][161] Many species are isolated in restricted ranges by changes of climate or inhospitable territory, such as stretches of sea, mountain ridges, deserts, forest clearance, road construction, or other man-made barriers.[162] Usually, a greater diversity of frogs occurs in tropical areas than in temperate regions, such as Europe.[163] Some frogs inhabit arid areas, such as deserts, and rely on specific adaptations to survive. Members of the Australian genus Cyclorana bury themselves underground where they create a water-impervious cocoon in which to aestivate during dry periods. Once it rains, they emerge, find a temporary pool, and breed. Egg and tadpole development is very fast in comparison to those of most other frogs, so breeding can be completed before the pond dries up.[164] Some frog species are adapted to a cold environment. The wood frog (Rana sylvatica), whose habitat extends into the Arctic Circle, buries itself in the ground during winter. Although much of its body freezes during this time, it maintains a high concentration of glucose in its vital organs, which protects them from damage.[40]
In 2006, of 4,035 species of amphibians that depend on water during some lifecycle stage, 1,356 (33.6%) were considered to be threatened. This is likely to be an underestimate because it excludes 1,427 species for which evidence was insufficient to assess their status.[165] Frog populations have declined dramatically since the 1950s. More than one-third of frog species are considered to be threatened with extinction, and more than 120 species are believed to have become extinct since the 1980s.[1] Among these species are the gastric-brooding frogs of Australia and the golden toad of Costa Rica. The latter is of particular concern to scientists because it inhabited the pristine Monteverde Cloud Forest Reserve and its population crashed in 1987, along with about 20 other frog species in the area. This could not be linked directly to human activities, such as deforestation, and was outside the range of normal fluctuations in population size.[166] Elsewhere, habitat loss is a significant cause of frog population decline, as are pollutants, climate change, increased UVB radiation, and the introduction of non-native predators and competitors.[167] A Canadian study conducted in 2006 suggested heavy traffic in their environment was a larger threat to frog populations than was habitat loss.[168] Emerging infectious diseases, including chytridiomycosis and ranavirus, are also devastating populations.[169][170]
Many environmental scientists believe amphibians, including frogs, are good biological indicators of broader ecosystem health because of their intermediate positions in food chains, their permeable skins, and typically biphasic lives (aquatic larvae and terrestrial adults).[171] It appears that species with both aquatic eggs and larvae are most affected by the decline, while those with direct development are the most resistant.[172]
Frog mutations and genetic defects have increased since the 1990s. These often include missing legs or extra legs. Various causes have been identified or hypothesized, including an increase in ultraviolet radiation affecting the spawn on the surface of ponds, chemical contamination from pesticides and fertilizers, and parasites such as the trematode Ribeiroia ondatrae. Probably all these are involved in a complex way as stressors, environmental factors contributing to rates of disease, and vulnerability to attack by parasites. Malformations impair mobility and the individuals may not survive to adulthood. An increase in the number of frogs eaten by birds may actually increase the likelihood of parasitism of other frogs, because the trematode's complex lifecycle includes the ramshorn snail and several intermediate hosts such as birds.[173][174]
In a few cases, captive breeding programs have been established and have largely been successful.[175][176] In 2007, the application of certain probiotic bacteria was reported to protect amphibians from chytridiomycosis.[177]One current project, the Panama Amphibian Rescue and Conservation Project, has subsequently been developed to rescue species at risk of this disease in eastern Panama, and to develop field applications for probiotic therapy.[178][179] The World Association of Zoos and Aquariums named 2008 as the "Year of the Frog" in order to draw attention to the conservation issues faced by them.[180]
The cane toad (Bufo marinus) is a very adaptable species native to South and Central America. In the 1930s, it was introduced into Puerto Rico, and later various other islands in the Pacific and Caribbean region, as abiological pest control agent.[181] In 1935, 3000 toads were liberated in the sugar cane fields of Queensland, Australia, in an attempt to control cane beetles such as Dermolepida albohirtum, the larvae of which damage and kill the canes. Initial results in many of these countries were positive, but it later became apparent that the toads upset the ecological balance in their new environments. They bred freely, competed with native frog species, ate bees and other harmless native invertebrates, had few predators in their adopted habitats, and poisoned pets, carnivorous birds, and mammals. In many of these countries, they are now regarded both as pests and invasive species, and scientists are looking for a biological method to control them.[182]
Uses
Culinary
French cuisses de grenouille.
Frog legs are eaten by humans in many parts of the world. French cuisses de grenouille or frog legs dish is a traditional dish particularly served in the region of the Dombes (département of Ain). The dish is also common in French-speaking parts of Louisiana, particularly the Cajun areas of Southern Louisiana as well as New Orleans, United States. In Asia, frog legs are consumed in China, Vietnam, Thailand and Indonesia. Chinese edible frogand pig frogs are farmed and consumed on a large scale in some areas of China. Frog legs are part of Chinese Sichuan and Cantonese cuisine. In Indonesia, frog-leg soup is known as swikee or swike. Indonesia is the world's largest exporter of frog meat, exporting more than 5,000 tonnes of frog meat each year, mostly to France, Belgium and Luxembourg.[183]
Originally, they were supplied from local wild populations, but overexploitation led to a diminution in the supply. This resulted in the development of frog farming and a global trade in frogs.[184] The main importing countries are France, Belgium, Luxembourg, and the United States, while the chief exporting nations are Indonesia and China.[184] The annual global trade in the American bullfrog (Rana catesbeiana), mostly farmed in China, varies between 1200 and 2400 tonnes.[185]
Coon, possum, partridges, prairie hen, and frogs were among the fare Mark Twain recorded as part of American cuisine.[186]
Scientific research
Frogs are used for dissections in high school and university anatomy classes, often first being injected with coloured substances to enhance contrasts among the biological systems. This practice is declining due to animal welfare concerns, and "digital frogs" are now available for virtual dissection.[187]
Frogs are used in cloning research and other branches of embryology. Although alternative pregnancy tests have been developed, biologists continue to use Xenopus as a model organism in developmental biology because their embryos are large and easy to manipulate, they are readily obtainable, and can easily be kept in the laboratory.[192] Xenopus laevis is increasingly being displaced by its smaller relative, Xenopus tropicalis, which reaches its reproductive age in five months rather than the one to two years for X. laevis,[193] thus facilitating faster studies across generations. The genome of X. tropicalis is being sequenced.[194]
Pharmaceutical
Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid epibatidine, a painkiller 200 times more potent than morphine is made by some species of poison dart frogs, although it can also cause death by lung paralysis. Other chemicals isolated from the skins of frogs may offer resistance to HIV infection.[195] Dart poisons are under active investigation for their potential as therapeutic drugs.[196]
Exudations from the skin of the golden poison frog (Phyllobates terribilis) are traditionally used by native Colombians to poison the darts they use for hunting. The tip of the projectile is rubbed over the back of the frog and the dart is launched from a blowgun. The combination of the two alkaloid toxins batrachotoxin and homobatrachotoxin is so powerful, one frog contains enough poison to kill an estimated 22,000 mice.[199] Two other species, theKokoe poison dart frog (Phyllobates aurotaenia) and the black-legged dart frog (Phyllobates bicolor) are also used for this purpose. These are less toxic and less abundant than the golden poison frog. They are impaled on pointed sticks and may be heated over a fire to maximise the quantity of poison that can be transferred to the dart.[199]
Cultural beliefs
Frogs feature prominently in folklore, fairy tales, and popular culture. They tend to be portrayed as benign, ugly, and clumsy, but with hidden talents. Examples include Michigan J. Frog, "The Frog Prince", and Kermit the Frog. The Warner Brothers cartoon One Froggy Evening features Michigan J. Frog, that will only dance and sing for the demolition worker who opens his time capsule, but will not perform in public.[200] "The Frog Prince" is a fairy tale about a frog that turns into a handsome prince after he has rescued a princess's golden ball and she has taken him into her palace.[201] Kermit the Frog is a conscientious and disciplined character from The Muppet Show andSesame Street; while openly friendly and greatly talented, he is often portrayed as cringing at the fanciful behavior of more flamboyant characters.[202]
Toads have a more sinister reputation. It was believed in European folklore that they were associated with witches as their familiar spirits and had magical powers. The toxic secretions from their skin was used in brewing evil potions, but was also put to use to create magical cures for human and livestock ailments. They were associated with the devil; in John Milton's Paradise Lost, Satan was depicted as a toad pouring poison into Eve's ear.[203]
The Moche people of ancient Peru worshipped animals, and often depicted frogs in their art.[204] In Panama, local legend held that good fortune would come to anyone who spotted a Panamanian golden frog. Some believed when one of these frogs died, it would turn into a golden talisman known as a huaca. Today, despite being extinct in the wild, Panamanian golden frogs remain an important cultural symbol and are illustrated on decorative cloth molas made by the Kuna people. They also appear as part of the inlaid design on a new overpass in Panama City, on T-shirts, and even on lottery tickets.[205]
Living snakes are found on every continent except Antarctica, and on most smaller land masses; exceptions include some large islands, such as Ireland, Iceland, Greenland, the Hawaiian archipelago, and the islands of New Zealand, and many small islands of the Atlantic and central Pacific oceans. [3] Additionally, sea snakes are widespread throughout the Indian and Pacific Oceans. More than 20 families are currently recognized, comprising about 500 genera and about 3,400 species. [4][5] They range in size from the tiny, 10.4 cm-long thread snake[6] to the reticulated python of 6.95 meters (22.8 ft) in length. [7] The fossil species Titanoboa cerrejonensis was 12.8 meters (42 ft) long. [8] Snakes are thought to have evolved from either burrowing or aquatic lizards, perhaps during the Jurassic period, with the earliest known fossils dating to between 143 and 167 Ma ago. [9] The diversity of modern snakes appeared during the Paleocene period ( c 66 to 56 Ma ago). The oldest preserved descriptions of snakes can be found in the Brooklyn Papyrus.
Most species are nonvenomous and those that have venom use it primarily to kill and subdue prey rather than for self-defense. Some possess venom potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by constriction.
Etymology
The English word snake comes from Old English snaca, itself from Proto-Germanic *snak-an- (cf. Germanic Schnake "ring snake", Swedish snok "grass snake"), from Proto-Indo-European root *(s)nēg-o- "to crawl", "to creep", which also gave sneak as well as Sanskrit nāgá "snake". [10] The word ousted adder, as adder went on to narrow in meaning, though in Old English næddre was the general word for snake. [11] The other term, serpent, is from French, ultimately from Indo-European *serp- (to creep), [12] which also gave Ancient Greek hérpō (ἕρπω) "I crawl".
Evolution
| A phylogenetic overview of the extant groups |
|
|
| Note: the tree only indicates relationships, not evolutionary branching times.[13] |
The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile making fossilization uncommon. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the Cretaceous period. [14] The earliest known true snake fossils (members of the crown group Serpentes) come from the marine simoliophiids, the oldest of which is the Late Cretaceous ( Cenomanian age) Haasiophis terrasanctus, [1] dated to between 112 and 94 million years old. [15]
Front limbs are nonexistent in all known snakes. This is caused by the evolution of Hox genes, controlling limb morphogenesis. The axial skeleton of the snakes’ common ancestor, like most other tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic), and caudal (tail) vertebrae. Early in snake evolution, the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant. As a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the atlas, axis, and 1–3 neck vertebrae). In other words, most of a snake's skeleton is an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. Neck, lumbar and pelvic vertebrae are very reduced in number (only 2–10 lumbar and pelvic vertebrae are present), while only a short tail remains of the caudal vertebrae. However, the tail is still long enough to be of important use in many species, and is modified in some aquatic and tree-dwelling species.
Modern snakes greatly diversified during the Paleocene. This occurred alongside the adaptive radiation of mammals, following the extinction of (non-avian) dinosaurs. The colubrids, one of the more common snake groups, became particularly diverse due to preying on rodents, an especially successful mammal group.
Origins
There is fossil evidence to suggest that snakes may have evolved from burrowing lizards, such as the varanids (or a similar group) during the Cretaceous Period. [19] An early fossil snake relative, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was fully terrestrial. [20] One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo (though it also is semiaquatic). [21] Subterranean species evolved bodies streamlined for burrowing, and eventually lost their limbs. [21]According to this hypothesis, features such as the transparent, fused eyelids ( brille) and loss of external ears evolved to cope with fossorial difficulties, such as scratched corneas and dirt in the ears. [19][21] Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lacked a direct connection to the vertebrae. These include fossil species like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash. [18]
Fossil of Archaeophis proavus.
This hypothesis was strengthened in 2015 by the discovery of a 113m year-old fossil of a four-legged snake in Brazil that has been named Tetrapodophis amplectus. It has many snake-like features, is adapted for burrowing and its stomach indicates that it was preying on other animals. [22] It is currently uncertain if Tetrapodophis is a snake or another species, in the squamate order, as a snake-like body has independently evolved at least 26 times. Tetrapodophis does not have distinctive snake features in its spine and skull. [23][24]
An alternative hypothesis, based on morphology, suggests the ancestors of snakes were related to mosasaurs—extinct aquatic reptiles from the Cretaceous—which in turn are thought to have derived from varanid lizards. [17]According to this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), and the external ears were lost through disuse in an aquatic environment. This ultimately led to an animal similar to today's sea snakes. In the Late Cretaceous, snakes recolonized land, and continued to diversify into today's snakes. Fossilized snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis; particularly so, as they are older than the terrestrial Najash rionegrina. Similar skull structure, reduced or absent limbs, and other anatomical features found in both mosasaurs and snakes lead to a positive cladistical correlation, although some of these features are shared with varanids. [citation needed]
Genetic studies in recent years have indicated snakes are not as closely related to monitor lizards as was once believed—and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, more evidence links mosasaurs to snakes than to varanids. Fragmented remains found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may potentially refute either hypothesis. [citation needed]
Distribution
Taxonomy
The two infraorders of Serpentes are: Alethinophidia and Scolecophidia. [4] This separation is based on morphological characteristics and mitochondrial DNA sequence similarity. Alethinophidia is sometimes split into Henophidia and Caenophidia, with the latter consisting of "colubroid" snakes ( colubrids, vipers, elapids, hydrophiids, and atractaspids) and acrochordids, while the other alethinophidian families comprise Henophidia. [27] While not extant today, the Madtsoiidae, a family of giant, primitive, python-like snakes, was around until 50,000 years ago in Australia, represented by genera such as Wonambi.
There are numerous debates in the systematics within the group. For instance, many sources classify Boidae and Pythonidae as one family, while some keep the Elapidae and Hydrophiidae (sea snakes) separate for practical reasons despite their extremely close relation.
Recent molecular studies support the monophyly of the clades of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids ( Cylindrophis, Anomochilus, uropeltines), macrostomatans, booids, boids, pythonids and caenophidians. [13]
Families
| Infraorder Alethinophidia 15 families |
| Family[4] | Taxon author[4] | Genera[4] | Species[4] | Common name | Geographic range[28] |
|---|
| Acrochordidae | Bonaparte, 1831 | 1 | 3 | Wart snakes | Western India and Sri Lanka through tropical Southeast Asia to the Philippines, south through the Indonesian/Malaysian island group to Timor, east through New Guinea to the northern coast of Australia to Mussau Island, the Bismarck Archipelago and Guadalcanal Island in the Solomon Islands. |
| Aniliidae | Stejneger, 1907 | 1 | 1 | False coral snake | Tropical South America. |
| Anomochilidae | Cundall, Wallach, 1993 | 1 | 2 | Dwarf pipe snakes | West Malaysia and on the Indonesian island of Sumatra. |
| Atractaspididae | Günther, 1858 | 12 | 64 | Burrowing asps | Africa and the Middle East.[16][29][30] |
| Boidae | Gray, 1825 | 8 | 43 | Boas | Northern, Central and South America, the Caribbean, southeastern Europe and Asia Minor, Northern, Central and East Africa, Madagascar and Reunion Island, the Arabian Peninsula, Central and southwestern Asia, India and Sri Lanka, the Moluccas and New Guinea through to Melanesia and Samoa. |
| Bolyeriidae | Hoffstetter, 1946 | 2 | 2 | Splitjaw snakes | Mauritius. |
| Colubridae | Oppel, 1811 | 304[5] | 1938[5] | Typical snakes | Widespread on all continents, except Antarctica.[31] |
| Cylindrophiidae | Fitzinger, 1843 | 1 | 8 | Asian pipe snakes | Sri Lanka east through Myanmar, Thailand, Cambodia, Vietnam and the Malay Archipelago to as far east as Aru Islands off the southwestern coast of New Guinea. Also found in southern China (Fujian, Hong Kong and on Hainan Island) and in Laos. |
| Elapidae | Boie, 1827 | 61 | 235 | Elapids | On land, worldwide in tropical and subtropical regions, except in Europe. Sea snakes occur in the Indian Ocean and the Pacific.[32] |
| Loxocemidae | Cope, 1861 | 1 | 1 | Mexican burrowing snake | Along the Pacific versant from Mexico south to Costa Rica. |
| Pythonidae | Fitzinger, 1826 | 8 | 26 | Pythons | Subsaharan Africa, India, Myanmar, southern China, Southeast Asia and from the Philippines southeast through Indonesia to New Guinea and Australia. |
| Tropidophiidae | Brongersma, 1951 | 4 | 22 | Dwarf boas | From southern Mexico and Central America, south to northwestern South America in Colombia, (Amazonian) Ecuador and Peru, as well as in northwestern and southeastern Brazil. Also found in the West Indies. |
| Uropeltidae | Müller, 1832 | 8 | 47 | Shield-tailed snakes | Southern India and Sri Lanka. |
| Viperidae | Oppel, 1811 | 32 | 224 | Vipers | The Americas, Africa and Eurasia. |
| Xenopeltidae | Bonaparte, 1845 | 1 | 2 | Sunbeam snakes | Southeast Asia from the Andaman and Nicobar Islands, east through Myanmar to southern China, Thailand, Laos, Cambodia, Vietnam, the Malay Peninsula and the East Indies to Sulawesi, as well as the Philippines. |
| Infraorder Scolecophidia 3 families |
| Family[4] | Taxon author[4] | Genera[4] | Species[4] | Common name | Geographic range[28] |
|---|
| Anomalepidae | Taylor, 1939 | 4 | 15 | Primitive blind snakes | From southern Central America to northwestern South America. Disjunct populations in northeastern and southeastern South America. |
| Leptotyphlopidae | Stejneger, 1892 | 2 | 87 | Slender blind snakes | Africa, western Asia from Turkey to northwestern India, on Socotra Island, from the southwestern United States south through Mexico and Central to South America, though not in the high Andes. In Pacific South America they occur as far south as southern coastal Peru, and on the Atlantic side as far as Uruguay and Argentina. In the Caribbean they are found on the Bahamas, Hispaniola and the Lesser Antilles. |
| Typhlopidae | Merrem, 1820 | 6 | 203 | Typical blind snakes | Most tropical and many subtropical regions around the world, particularly in Africa, Madagascar, Asia, islands in the Pacific, tropical America and in southeastern Europe. |
Legless lizards
While snakes are limbless reptiles, which evolved from (and are grouped with) lizards, there are many other species of lizards which have lost their limbs independently and superficially look similar to snakes. These include the slow worm and glass snake.
Biology
Size
At the other end of the scale, the smallest extant snake is Leptotyphlops carlae, with a length of about 10.4 cm (4.1 in). [6] Most snakes are fairly small animals, approximately 1 m (3.3 ft) in length. [34]
Perception
Thermographic image of a snake eating a mouse.
Pit vipers, pythons, and some boas have infrared-sensitive receptors in deep grooves on the snout, which allow them to "see" the radiated heat of warm-blooded prey. In pit vipers, the grooves are located between the nostril and the eye in a large "pit" on each side of the head. Other infrared-sensitive snakes have multiple, smaller labial pits lining the upper lip, just below the nostrils. [35]
Snakes use smell to track their prey. They smell by using their forked tongues to collect airborne particles, then passing them to the vomeronasal organ or Jacobson's organ in the mouth for examination. [35] The fork in the tongue gives snakes a sort of directional sense of smell and taste simultaneously. [35] They keep their tongues constantly in motion, sampling particles from the air, ground, and water, analyzing the chemicals found, and determining the presence of prey or predators in the local environment. In water-dwelling snakes, such as the anaconda, the tongue functions efficiently underwater. [35]
The underside is very sensitive to vibration. This allows snakes to be able to sense approaching animals by detecting faint vibrations in the ground. [35]
Snake vision varies greatly, from only being able to distinguish light from dark to keen eyesight, but the main trend is that their vision is adequate although not sharp, and allows them to track movements. [36] Generally, vision is best in arboreal snakes and weakest in burrowing snakes. Some snakes, such as the Asian vine snake (genus Ahaetulla), have binocular vision, with both eyes capable of focusing on the same point. Most snakes focus by moving the lens back and forth in relation to the retina, while in the other amniote groups, the lens is stretched. Many nocturnal snakes have slit pupils while diurnal snakes have round pupils.
Skin
The skin of a snake is covered in scales. Contrary to the popular notion of snakes being slimy because of possible confusion of snakes with worms, snakeskin has a smooth, dry texture. Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, keeled, or granular. The eyelids of a snake are transparent "spectacle" scales, which remain permanently closed, also known as brille.
The shedding of scales is called ecdysis (or in normal usage, molting or sloughing). In the case of snakes, the complete outer layer of skin is shed in one layer. [37] Snake scales are not discrete, but extensions of the epidermis—hence they are not shed separately but as a complete outer layer during each molt, akin to a sock being turned inside out.
The shape and number of scales on the head, back, and belly are often characteristic and used for taxonomic purposes. Scales are named mainly according to their positions on the body. In "advanced" ( Caenophidian) snakes, the broad belly scales and rows of dorsal scales correspond to the vertebrae, allowing scientists to count the vertebrae without dissection.
Snakes' eyes are covered by their clear scales (the brille) rather than movable eyelids. Their eyes are always open, and for sleeping, the retina can be closed or the face buried among the folds of the body.
Molting
A snake shedding its skin.
Molting serves a number of functions. Firstly, the old and worn skin is replaced; secondly, it helps get rid of parasites such as mites and ticks. Renewal of the skin by moulting is supposed to allow growth in some animals such as insects; however, this has been disputed in the case of snakes. [39]
Molting occurs periodically throughout the snake's life. Before a molt, the snake stops eating and often hides or moves to a safe place. Just before shedding, the skin becomes dull and dry looking and the eyes become cloudy or blue-colored. The inner surface of the old skin liquefies. This causes the old skin to separate from the new skin beneath it. After a few days, the eyes clear and the snake "crawls" out of its old skin. The old skin breaks near the mouth and the snake wriggles out, aided by rubbing against rough surfaces. In many cases, the cast skin peels backward over the body from head to tail in one piece, like pulling a sock off inside-out. A new, larger, brighter layer of skin has formed underneath. [40]
An older snake may shed its skin only once or twice a year. But a younger snake, still growing, may shed up to four times a year. [40] The discarded skin gives a perfect imprint of the scale pattern, and it is usually possible to identify the snake if the discarded skin is reasonably intact. This periodic renewal has led to the snake being a symbol of healing and medicine, as pictured in the Rod of Asclepius. [41]
Scale counts can sometimes be used to tell the sex of a snake when the species is not distinctly sexually dimorphic. A probe is inserted into the cloaca until it can go no further. The probe is marked at the point where it stops, removed, and compared to the subcaudal depth by laying it alongside the scales. [42] The scalation count determines whether the snake is a male or female as hemipenes of a male will probe to a different depth (usually longer) than the cloaca of a female. [42][clarification needed]
Skeleton
When compared, the skeletons of snakes are radically different from those of most other reptiles (such as the turtle, right), being made up almost entirely of an extended ribcage.
The skeleton of most snakes consists solely of the skull, hyoid, vertebral column, and ribs, though henophidian snakes retain vestiges of the pelvis and rear limbs.
The skull of the snake consists of a solid and complete neurocranium, to which many of the other bones are only loosely attached, particularly the highly mobile jaw bones, which facilitate manipulation and ingestion of large prey items. The left and right sides of the lower jaw are joined only by a flexible ligament at the anterior tips, allowing them to separate widely, while the posterior end of the lower jaw bones articulate with a quadrate bone, allowing further mobility. The bones of the mandible and quadrate bones can also pick up ground borne vibrations. [43] Because the sides of the jaw can move independently of one another, snakes resting their jaws on a surface have sensitive stereo hearing which can detect the position of prey. The jaw-quadrate-stapes pathway is capable of detecting vibrations on the angstrom scale, despite the absence of an outer ear and the ossicle mechanism of impedance matching used in other vertebrates to receive vibrations from the air. [44][45]
The hyoid is a small bone located posterior and ventral to the skull, in the 'neck' region, which serves as an attachment for muscles of the snake's tongue, as it does in all other tetrapods.
The vertebral column consists of anywhere between 200 and 400 (or more) vertebrae. Tail vertebrae are comparatively few in number (often less than 20% of the total) and lack ribs, while body vertebrae each have two ribs articulating with them. The vertebrae have projections that allow for strong muscle attachment enabling locomotion without limbs.
Autotomy of the tail, a feature found in some lizards is absent in most snakes. [46] Caudal autotomy in snakes is rare and is intervertebral, unlike that in lizards, which is intravertebral—that is, the break happens along a predefined fracture plane present on a vertebra. [47][48]
In some snakes, most notably boas and pythons, there are vestiges of the hindlimbs in the form of a pair of pelvic spurs. These small, claw-like protrusions on each side of the cloaca are the external portion of the vestigial hindlimb skeleton, which includes the remains of an ilium and femur.
Internal organs
Anatomy of a snake. 1 esophagus, 2 trachea, 3 tracheal lungs, 4 rudimentary left lung, 5 right lung, 6 heart, 7 liver, 8 stomach, 9 air sac, 10 gallbladder, 11 pancreas, 12 spleen, 13 intestine, 14 testicles, 15 kidneys.
The snake's heart is encased in a sac, called the pericardium, located at the bifurcation of the bronchi. The heart is able to move around, however, owing to the lack of a diaphragm. This adjustment protects the heart from potential damage when large ingested prey is passed through the esophagus. The spleen is attached to the gall bladder and pancreas and filters the blood. The thymus gland is located in fatty tissue above the heart and is responsible for the generation of immune cells in the blood. The cardiovascular system of snakes is also unique for the presence of a renal portal system in which the blood from the snake's tail passes through the kidneys before returning to the heart. [50]
The vestigial left lung is often small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin. [50] In the majority of species, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion that does not function in gas exchange. [50] This 'saccular lung' is used for hydrostatic purposes to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species. [50] Many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other. [50]
Venom
Cobras, vipers, and closely related species use venom to immobilize or kill their prey. The venom is modified saliva, delivered through fangs. [16]:243 The fangs of 'advanced' venomous snakes like viperids and elapids are hollow to inject venom more effectively, while the fangs of rear-fanged snakes such as the boomslang merely have a groove on the posterior edge to channel venom into the wound. Snake venoms are often prey specific—their role in self-defense is secondary. [16]:243
Venom, like all salivary secretions, is a predigestant that initiates the breakdown of food into soluble compounds, facilitating proper digestion. Even nonvenomous snake bites (like any animal bite) will cause tissue damage. [16]:209
Certain birds, mammals, and other snakes (such as kingsnakes) that prey on venomous snakes have developed resistance and even immunity to certain venoms. [16]:243 Venomous snakes include three families of snakes, and do not constitute a formal classification group used in taxonomy.
The colloquial term "poisonous snake" is generally an incorrect label for snakes. A poison is inhaled or ingested, whereas venom produced by snakes is injected into its victim via fangs. [51] There are, however, two exceptions: Rhabdophis sequesters toxins from the toads it eats, then secretes them from nuchal glands to ward off predators, and a small unusual population of garter snakes in the U.S. state of Oregon retains enough toxins in their livers from the newts they eat to be effectively poisonous to small local predators (such as crows and foxes). [52]
Snake venoms are complex mixtures of proteins, and are stored in venom glands at the back of the head. [52] In all venomous snakes, these glands open through ducts into grooved or hollow teeth in the upper jaw. [16]:243[51]These proteins can potentially be a mix of neurotoxins (which attack the nervous system), hemotoxins (which attack the circulatory system), cytotoxins, bungarotoxins and many other toxins that affect the body in different ways. [51] Almost all snake venom contains hyaluronidase, an enzyme that ensures rapid diffusion of the venom. [16]:243
Venomous snakes that use hemotoxins usually have fangs in the front of their mouths, making it easier for them to inject the venom into their victims. [51] Some snakes that use neurotoxins (such as the mangrove snake) have fangs in the back of their mouths, with the fangs curled backwards. [53] This makes it difficult both for the snake to use its venom and for scientists to milk them. [51] Elapids, however, such as cobras and kraits are proteroglyphous—they possess hollow fangs that cannot be erected toward the front of their mouths, and cannot "stab" like a viper. They must actually bite the victim. [16]:242
It has recently been suggested that all snakes may be venomous to a certain degree, with harmless snakes having weak venom and no fangs. [54] Most snakes currently labelled "nonvenomous" would still be considered harmless according to this theory, as they either lack a venom delivery method or are incapable of delivering enough to endanger a human. This theory postulates that snakes may have evolved from a common lizard ancestor that was venomous—and that venomous lizards like the gila monster, beaded lizard, monitor lizards, and the now-extinct mosasaurs may also have derived from it. They share this venom clade with various other saurian species.
There is a third family containing the opistoglyphous (rear-fanged) snakes (as well as the majority of other snake species):
Reproduction
Although a wide range of reproductive modes are used by snakes, all snakes employ internal fertilization. This is accomplished by means of paired, forked hemipenes, which are stored, inverted, in the male's tail. [55] The hemipenes are often grooved, hooked, or spined in order to grip the walls of the female's cloaca. [55]
Most species of snakes lay eggs which they abandon shortly after laying. However, a few species (such as the king cobra) actually construct nests and stay in the vicinity of the hatchlings after incubation. [55] Most pythons coil around their egg-clutches and remain with them until they hatch. [56] A female python will not leave the eggs, except to occasionally bask in the sun or drink water. She will even "shiver" to generate heat to incubate the eggs. [56]
The Garter snake has been studied for sexual selection
Sexual selection in snakes is demonstrated by the three thousand species that each use different tactics in acquiring mates. [59] Ritual combat between males for the females they want to mate with includes topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and forcing it downward. It is common for neck biting to occur while the snakes are entwined. [60]
Facultative parthenogenesis
Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization. Agkistrodon contortrix (copperhead) and Agkistrodon piscivorus (cotton mouth) can reproduce by facultative parthenogenesis. That is, they are capable of switching from a sexual mode of reproduction to an asexual mode. [61] The type of parthenogenesis that likely occurs is automixis with terminal fusion, a process in which two terminal products from the same meiosis fuse to form a diploid zygote. This process leads to genome-wide homozygosity, expression of deleterious recessive alleles and often to developmental abnormalities. Both captive-born and wild-born A. contortrix and A. piscivorus appear to be capable of this form of parthenogenesis. [61]
Reproduction in squamate reptiles is almost exclusively sexual. Males ordinarily have a ZZ pair of sex-determining chromosomes, and females a ZW pair. However, the Colombian Rainbow boa, Epicrates maurus can also reproduce by facultative parthenogenesis resulting in production of WW female progeny. [62] The WW females are likely produced by terminal automixis.
Behavior
Winter dormancy
In regions where winters are colder than snakes can tolerate while remaining active, local species will brumate. Unlike hibernation, in which mammals are actually asleep, brumating reptiles are awake but inactive. Individual snakes may brumate in burrows, under rock piles, or inside fallen trees, or snakes may aggregate in large numbers at hibernacula.
Feeding and diet
All snakes are strictly carnivorous, eating small animals including lizards, frogs, other snakes, small mammals, birds, eggs, fish, snails or insects. [16][3][17][63] Because snakes cannot bite or tear their food to pieces, they must swallow prey whole. The body size of a snake has a major influence on its eating habits. Smaller snakes eat smaller prey. Juvenile pythons might start out feeding on lizards or mice and graduate to small deer or antelope as an adult, for example.
The snake's jaw is a complex structure. Contrary to the popular belief that snakes can dislocate their jaws, snakes have a very flexible lower jaw, the two halves of which are not rigidly attached, and numerous other joints in their skull (see snake skull), allowing them to open their mouths wide enough to swallow their prey whole, even if it is larger in diameter than the snake itself. [63] For example, the African egg-eating snake has flexible jaws adapted for eating eggs much larger than the diameter of its head. [16]:81 This snake has no teeth, but does have bony protrusions on the inside edge of its spine, which it uses to break shells when it eats eggs. [16]:81
While the majority of snakes eat a variety of prey animals, there is some specialization by some species. King cobras and the Australian bandy-bandy consume other snakes. Pareas iwesakii and other snail-eating colubrids of subfamily Pareatinae have more teeth on the right side of their mouths than on the left, as the shells of their prey usually spiral clockwise [16]:184[64]
Some snakes have a venomous bite, which they use to kill their prey before eating it. [63][65] Other snakes kill their prey by constriction. [63] Still others swallow their prey whole and alive. [16]:81[63]
After eating, snakes become dormant while the process of digestion takes place. [42] Digestion is an intense activity, especially after consumption of large prey. In species that feed only sporadically, the entire intestine enters a reduced state between meals to conserve energy. The digestive system is then 'up-regulated' to full capacity within 48 hours of prey consumption. Being ectothermic ("cold-blooded"), the surrounding temperature plays a large role in snake digestion. The ideal temperature for snakes to digest is 30 °C (86 °F). So much metabolic energy is involved in a snake's digestion that in the Mexican rattlesnake ( Crotalus durissus), surface body temperature increases by as much as 1.2 °C (2.2 °F) during the digestive process. [66] Because of this, a snake disturbed after having eaten recently will often regurgitate its prey to be able to escape the perceived threat. When undisturbed, the digestive process is highly efficient, with the snake's digestive enzymes dissolving and absorbing everything but the prey's hair (or feathers) and claws, which are excreted along with waste.
Locomotion
The lack of limbs does not impede the movement of snakes. They have developed several different modes of locomotion to deal with particular environments. Unlike the gaits of limbed animals, which form a continuum, each mode of snake locomotion is discrete and distinct from the others; transitions between modes are abrupt. [67][68]
Lateral undulation
Crawling prints of a snake
Lateral undulation is the sole mode of aquatic locomotion, and the most common mode of terrestrial locomotion. [68] In this mode, the body of the snake alternately flexes to the left and right, resulting in a series of rearward-moving "waves". [67] While this movement appears rapid, snakes have rarely been documented moving faster than two body-lengths per second, often much less. [69] This mode of movement has the same net cost of transport (calories burned per meter moved) as running in lizards of the same mass. [70]
Terrestrial lateral undulation is the most common mode of terrestrial locomotion for most snake species. [67] In this mode, the posteriorly moving waves push against contact points in the environment, such as rocks, twigs, irregularities in the soil, etc. [67] Each of these environmental objects, in turn, generates a reaction force directed forward and towards the midline of the snake, resulting in forward thrust while the lateral components cancel out. [71] The speed of this movement depends upon the density of push-points in the environment, with a medium density of about 8 [clarification needed] along the snake's length being ideal. [69] The wave speed is precisely the same as the snake speed, and as a result, every point on the snake's body follows the path of the point ahead of it, allowing snakes to move through very dense vegetation and small openings. [71]
When swimming, the waves become larger as they move down the snake's body, and the wave travels backwards faster than the snake moves forwards. [72] Thrust is generated by pushing their body against the water, resulting in the observed slip. In spite of overall similarities, studies show that the pattern of muscle activation is different in aquatic versus terrestrial lateral undulation, which justifies calling them separate modes. [73] All snakes can laterally undulate forward (with backward-moving waves), but only sea snakes have been observed reversing the motion (moving backwards with forward-moving waves). [67]
Sidewinding
Most often employed by colubroid snakes ( colubrids, elapids, and vipers) when the snake must move in an environment that lacks irregularities to push against (rendering lateral undulation impossible), such as a slick mud flat, or a sand dune, sidewinding is a modified form of lateral undulation in which all of the body segments oriented in one direction remain in contact with the ground, while the other segments are lifted up, resulting in a peculiar "rolling" motion. [74][75] This mode of locomotion overcomes the slippery nature of sand or mud by pushing off with only static portions on the body, thereby minimizing slipping. [74] The static nature of the contact points can be shown from the tracks of a sidewinding snake, which show each belly scale imprint, without any smearing. This mode of locomotion has very low caloric cost, less than ⅓ of the cost for a lizard to move the same distance. [70]Contrary to popular belief, there is no evidence that sidewinding is associated with the sand being hot. [74]
Concertina
When push-points are absent, but there is not enough space to use sidewinding because of lateral constraints, such as in tunnels, snakes rely on concertina locomotion. [67][75] In this mode, the snake braces the posterior portion of its body against the tunnel wall while the front of the snake extends and straightens. [74] The front portion then flexes and forms an anchor point, and the posterior is straightened and pulled forwards. This mode of locomotion is slow and very demanding, up to seven times the cost of laterally undulating over the same distance. [70] This high cost is due to the repeated stops and starts of portions of the body as well as the necessity of using active muscular effort to brace against the tunnel walls.
Arboreal
The movement of snakes in arboreal habitats has only recently been studied. [76] While on tree branches, snakes use several modes of locomotion depending on species and bark texture. [76] In general, snakes will use a modified form of concertina locomotion on smooth branches, but will laterally undulate if contact points are available. [76] Snakes move faster on small branches and when contact points are present, in contrast to limbed animals, which do better on large branches with little 'clutter'. [76]
Gliding snakes ( Chrysopelea) of Southeast Asia launch themselves from branch tips, spreading their ribs and laterally undulating as they glide between trees. [74][77][78] These snakes can perform a controlled glide for hundreds of feet depending upon launch altitude and can even turn in midair. [74][77]
Rectilinear
The slowest mode of snake locomotion is rectilinear locomotion, which is also the only one where the snake does not need to bend its body laterally, though it may do so when turning. [79] In this mode, the belly scales are lifted and pulled forward before being placed down and the body pulled over them. Waves of movement and stasis pass posteriorly, resulting in a series of ripples in the skin. [79] The ribs of the snake do not move in this mode of locomotion and this method is most often used by large pythons, boas, and vipers when stalking prey across open ground as the snake's movements are subtle and harder to detect by their prey in this manner. [74]
Interactions with humans
Most common symptoms of any kind of snake bite envenomation. [80][81]Furthermore, there is vast variation in symptoms between bites from different types of snakes. [80]
Bite
Vipera berus, one fang in glove with a small venom stain, the other still in place.
Snakes do not ordinarily prey on humans. Unless startled or injured, most snakes prefer to avoid contact and will not attack humans. With the exception of large constrictors, nonvenomous snakes are not a threat to humans. The bite of a nonvenomous snake is usually harmless; their teeth are not designed for tearing or inflicting a deep puncture wound, but rather grabbing and holding. Although the possibility of infection and tissue damage is present in the bite of a nonvenomous snake, venomous snakes present far greater hazard to humans. [16]:209 The World Health Organisation lists snakebite under the "other neglected conditions" category. [82]
Documented deaths resulting from snake bites are uncommon. Nonfatal bites from venomous snakes may result in the need for amputation of a limb or part thereof. Of the roughly 725 species of venomous snakes worldwide, only 250 are able to kill a human with one bite. Australia averages only one fatal snake bite per year. In India, 250,000 snakebites are recorded in a single year, with as many as 50,000 recorded initial deaths. [83]
The treatment for a snakebite is as variable as the bite itself. The most common and effective method is through antivenom (or antivenin), a serum made from the venom of the snake. Some antivenom is species-specific (monovalent) while some is made for use with multiple species in mind (polyvalent). In the United States for example, all species of venomous snakes are pit vipers, with the exception of the coral snake. To produce antivenom, a mixture of the venoms of the different species of rattlesnakes, copperheads, and cottonmouths is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted from the immunized horse. The serum is separated and further purified and freeze-dried. It is reconstituted with sterile water and becomes antivenom. For this reason, people who are allergic to horses are more likely to suffer an allergic reaction to antivenom. [84]Antivenom for the more dangerous species (such as mambas, taipans, and cobras) is made in a similar manner in India, South Africa, and Australia, although these antivenoms are species-specific.
Snake charmers
An Indian cobra in a basket with a snake charmer. These snakes are perhaps the most common subjects of snake charmings.
In some parts of the world, especially in India, snake charming is a roadside show performed by a charmer. In such a show, the snake charmer carries a basket that contains a snake that he seemingly charms by playing tunes from his flutelike musical instrument, to which the snake responds. [85] Snakes lack external ears, though they do have internal ears, and respond to the movement of the flute, not the actual noise. [85][86]
The Wildlife Protection Act of 1972 in India technically proscribes snake charming on grounds of reducing animal cruelty. Other snake charmers also have a snake and mongoose show, where both the animals have a mock fight; however, this is not very common, as the snakes, as well as the mongooses, may be seriously injured or killed. Snake charming as a profession is dying out in India because of competition from modern forms of entertainment and environment laws proscribing the practice. [85]
Trapping
The Irulas tribe of Andhra Pradesh and Tamil Nadu in India have been hunter-gatherers in the hot, dry plains forests, and have practiced the art of snake catching for generations. They have a vast knowledge of snakes in the field. They generally catch the snakes with the help of a simple stick. Earlier, the Irulas caught thousands of snakes for the snake-skin industry. After the complete ban of the snake-skin industry in India and protection of all snakes under the Indian Wildlife (Protection) Act 1972, they formed the Irula Snake Catcher's Cooperative and switched to catching snakes for removal of venom, releasing them in the wild after four extractions. The venom so collected is used for producing life-saving antivenom, biomedical research and for other medicinal products. [87] The Irulas are also known to eat some of the snakes they catch and are very useful in rat extermination in the villages.
Consumption
A "海豹蛇" ("sea-leopard snake", supposedly Enhydris bocourti) occupies a place of honor among the live delicacies waiting to meet their consumers outside of a Guangzhourestaurant.
Snake meat, in a Taipei restaurant
While not commonly thought of as food in most cultures, in some cultures, the consumption of snakes is acceptable, or even considered a delicacy, prized for its alleged pharmaceutical effect of warming the heart. Snake soup of Cantonese cuisine is consumed by local people in autumn, to warm up their body. Western cultures document the consumption of snakes under extreme circumstances of hunger. [88] Cooked rattlesnake meat is an exception, which is commonly consumed in parts of the Midwestern United States. In Asian countries such as China, Taiwan, Thailand, Indonesia, Vietnam and Cambodia, drinking the blood of snakes—particularly the cobra—is believed to increase sexual virility. [89] The blood is drained while the cobra is still alive when possible, and is usually mixed with some form of liquor to improve the taste. [89]
In some Asian countries, the use of snakes in alcohol is also accepted. In such cases, the body of a snake or several snakes is left to steep in a jar or container of liquor. It is claimed that this makes the liquor stronger (as well as more expensive). One example of this is the Habu snake sometimes placed in the Okinawan liquor Awamori also known as "Habu Sake". [90]
Pets
In the Western world, some snakes (especially docile species such as the ball python and corn snake) are kept as pets. To meet this demand a captive breeding industry has developed. Snakes bred in captivity tend to make better pets and are considered preferable to wild caught specimens. [92] Snakes can be very low maintenance pets, especially compared to more traditional species. They require minimal space, as most common species do not exceed five feet (1.5 m) in length. Pet snakes can be fed relatively infrequently, usually once every 5 to 14 days. Certain snakes have a lifespan of more than 40 years if given proper care.
Symbolism
In Egyptian history, the snake occupies a primary role with the Nile cobra adorning the crown of the pharaoh in ancient times. It was worshipped as one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide ( Cleopatra).
In Greek mythology snakes are often associated with deadly and dangerous antagonists, but this is not to say that snakes are symbolic of evil; in fact, snakes are a chthonic symbol, roughly translated as 'earthbound'. The nine-headed Lernaean Hydra that Hercules defeated and the three Gorgon sisters are children of Gaia, the earth. [93] Medusa was one of the three Gorgon sisters who Perseus defeated. [93] Medusa is described as a hideous mortal, with snakes instead of hair and the power to turn men to stone with her gaze. [93] After killing her, Perseus gave her head to Athena who fixed it to her shield called the Aegis. [93] The Titans are also depicted in art with snakes instead of legs and feet for the same reason—they are children of Gaia and Uranus, so they are bound to the earth. [citation needed]
The legendary account of the foundation of Thebes mentioned a monster snake guarding the spring from which the new settlement was to draw its water. In fighting and killing the snake, the companions of the founder Cadmus all perished - leading to the term " Cadmean victory" (i.e. a victory involving one's own ruin). [citation needed]
India is often called the land of snakes and is steeped in tradition regarding snakes. [94] Snakes are worshipped as gods even today with many women pouring milk on snake pits (despite snakes' aversion for milk). [94] The cobra is seen on the neck of Shiva and Vishnu is depicted often as sleeping on a seven-headed snake or within the coils of a serpent. [95] There are also several temples in India solely for cobras sometimes called Nagraj (King of Snakes) and it is believed that snakes are symbols of fertility. There is a Hindu festival called Nag Panchami each year on which day snakes are venerated and prayed to. See also Nāga. [citation needed]
In India there is another mythology about snakes. Commonly known in Hindi as " Ichchhadhari" snakes. Such snakes can take the form of any living creature, but prefer human form. These mythical snakes possess a valuable gem called "Mani", which is more brilliant than diamond. There are many stories in India about greedy people trying to possess this gem and ending up getting killed. [citation needed]
The ouroboros is a symbol associated with many different religions and customs, and is claimed to be related to alchemy. The ouroboros or uroboros is a snake eating its own tail in a clock-wise direction (from the head to the tail) in the shape of a circle, representing the cycle of life, death and rebirth, leading to immortality. [citation needed]
Many ancient Peruvian cultures worshipped nature. [96] They emphasized animals and often depicted snakes in their art. [97]
Religion
Snakes are a part of Hindu worship. A festival, Nag Panchami, in which participants worship either images of or live Nāgas (cobras) is celebrated every year. Most images of Lord Shiva depict snake around his neck. Puranas have various stories associated with snakes. In the Puranas, Shesha is said to hold all the planets of the Universe on his hoods and to constantly sing the glories of Vishnu from all his mouths. He is sometimes referred to as "Ananta-Shesha", which means "Endless Shesha". Other notable snakes in Hinduism are Ananta, Vasuki, Taxak, Karkotaka and Pingala. The term Nāga is used to refer to entities that take the form of large snakes in Hinduism and Buddhism.
Snakes have also been widely revered, such as in ancient Greece, where the serpent was seen as a healer. Asclepius carried a serpent wound around his wand, a symbol seen today on many ambulances.
In Judaism, the snake of brass is also a symbol of healing, of one's life being saved from imminent death. [99]
In some parts of Christianity, Christ's redemptive work is compared to saving one's life through beholding the Nehushtan (serpent of brass). [100] Snake handlers use snakes as an integral part of church worship in order to exhibit their faith in divine protection. However, more commonly in Christianity, the serpent has been seen as a representative of evil and sly plotting, which can be seen in the description in Genesis chapter 3 of a snake in the Garden of Edentempting Eve. [101] Saint Patrick is reputed to have expelled all snakes from Ireland while converting the country to Christianity in the 5th century, thus explaining the absence of snakes there.
In Christianity and Judaism, the snake makes its infamous appearance in the first book of the Bible when a serpent appears before the first couple Adam and Eve and tempts them with the forbidden fruit from the Tree of Knowledge. [101] The snake returns in Exodus when Moses, as a sign of God's power, turns his staff into a snake and when Moses made the Nehushtan, a bronze snake on a pole that when looked at cured the people of bites from the snakes that plagued them in the desert. The serpent makes its final appearance symbolizing Satan in the Book of Revelation: "And he laid hold on the dragon the old serpent, which is the devil and Satan, and bound him for a thousand years." [102]
Ballcourt marker from the Postclassic site of Mixco Viejoin Guatemala. This sculpture depicts Kukulkan, jaws agape, with the head of a human warrior emerging from his maw. [103]
Medicine
The cytotoxic effect of snake venom is being researched as a potential treatment for cancers. [104]
nails that respire using a lung belong to the group Pulmonata, while those with gills form a polyphyletic group; in other words, snails with gills form a number of taxonomic groups that are not necessarily more closely related to each other than they are related to some other groups. Both snails that have lungs and snails that have gills have diversified so widely over geological time that a few species with gills can be found on land and numerous species with lungs can be found in freshwater. Even a few marine species have lungs.
Snails can be found in a very wide range of environments, including ditches, deserts, and the abyssal depths of the sea. Although land snails may be more familiar to laymen, marine snails constitute the majority of snail species, and have much greater diversity and a greater biomass. Numerous kinds of snail can also be found in fresh water.
Most snails have thousands of microscopic tooth-like structures located on a banded ribbon-like tongue called a radula. The radula works like a file, ripping food into small pieces. Many snails are herbivorous, eating plants or rasping algae from surfaces with their radulae, though a few land species and many marine species are omnivores or predatory carnivores.
Several species of the genus Achatina and related genera are known as giant African land snails; some grow to 15 in (38 cm) from snout to tail, and weigh 1 kg (2 lb). [1] The largest living species of sea snail is Syrinx aruanus; its shell can measure up to 90 cm (35 in) in length, and the whole animal with the shell can weigh up to 18 kg (40 lb).
Snail moving on a wet ground
The snail Lymnaea makes decisions by using only two types of neuron: one deciding whether the snail is hungry, and the other deciding whether there is food in the vicinity. [2]
Types of snails by habitat[edit]
Gastropods that lacks a conspicuous shell are commonly called slugs rather than snails. Some species of slug have a red shell, some have only an internal vestige that serves mainly as a calcium repository, and others have no shell at all. Other than that there is little morphological difference between slugs and snails. There are however important differences in habitats and behavior.
A shell-less animal is much more maneuverable and compressible, so even quite large land slugs can take advantage of habitats or retreats with very little space, retreats that would be inaccessible to a similar-sized snail. Slugs squeeze themselves into confined spaces such as under loose bark on trees or under stone slabs, logs or wooden boards lying on the ground. In such retreats they are in less danger from either predators or desiccation, and often those also are suitable places for laying their eggs.
Slugs as a group are far from monophyletic; biologically speaking "slug" is a term of convenience with little taxonomic significance. The reduction or loss of the shell has evolved many times independently within several very different lineages of gastropods. The various taxa of land and sea gastropods with slug morphology occur within numerous higher taxonomic groups of shelled species; such independent slug taxa are not in general closely related to one another.
Human relevance[edit]
Land snails are known as an agricultural and garden pest but some species are an edible delicacy and occasionally household pets.
In agriculture[edit]
There are a variety of snail-control measures that gardeners and farmers use in an attempt to reduce damage to valuable plants. Traditional pesticides are still used, as are many less toxic control options such as concentrated garlic or wormwood solutions. Copper metal is also a snail repellent, and thus a copper band around the trunk of a tree will prevent snails from climbing up and reaching the foliage and fruit. Placing crushed egg shells on the soil around garden plants can also deter snails from coming to the plants.
The decollate snail ( Rumina decollata) will capture and eat garden snails, and because of this it has sometimes been introduced as a biological pest control agent. However, this is not without problems, as the decollate snail is just as likely to attack and devour other gastropods that may represent a valuable part of the native fauna of the region.
As food[edit]
In French cuisine, edible snails are served for instance in Escargot à la Bourguignonne. The practice of rearing snails for food is known as heliciculture. For purposes of cultivation, the snails are kept in a dark place in a wired cage with dry straw or dry wood. Coppiced wine-grape vines are often used for this purpose. During the rainy period the snails come out of hibernation and release most of their mucus onto the dry wood/straw. The snails are then prepared for cooking. Their texture when cooked is slightly chewy.
As well as being relished as gourmet food, several species of land snails provide an easily harvested source of protein to many people in poor communities around the world. Many land snails are valuable because they can feed on a wide range of agricultural wastes, such as shed leaves in banana plantations. In some countries, giant African land snails are produced commercially for food.
Land snails, freshwater snails and sea snails are all eaten in a number of countries (principally Spain, Philippines, Morocco, Nigeria, Algeria, Cameroon, France, Italy, Portugal, Greece, Bulgaria, Belgium, Vietnam, Laos, Cambodia, Cyprus, Ghana, Malta, Terai of Nepal, southwestern China, Northeast India states such as Manipur, Tripura and parts of the United States). In certain parts of the world, snails are fried. For example, in Indonesia, they are fried as satay, a dish known as sate kakul. The eggs of certain snail species are eaten in a fashion similar to the way caviar is eaten.
In Bulgaria snails are traditionally cooked in an oven with rice or fried in a pan with vegetable oil and red paprika powder. Before they are used for those dishes however, they are thoroughly boiled in hot water (for up to 90 minutes) and manually extracted from their shells. The two species most commonly used for food in the country are Helix lucorum and Helix pomatia.
Famine food[edit]
Snails and slug species that are not normally eaten in certain areas have occasionally been used as famine food in historical times. Variants of the following event have occurred in Europe from time to time:
- In a popular publication quoted below occurs the following notice of a well-known land mollusk, in connection with a traditionary story of the plague, which has long had general currency in Scotland: ‘In the woodlands, the more formidable black nude slug, theArion or Limax, will also be often encountered. It is a huge voracious creature, herbivorous, feeding, to Barbara’s astonishment, on tender plants; fruits, as strawberries, apples; and even turnips and mushrooms; appearing morning and evening, or after rain; suffering severely in its concealment in long droughts, and remaining torpid in winter. The gray field slug (Limax agrestis) is actually recommended to be swallowed by consumptive patients! In the town of Dundee there exists a strange traditionary story of the plague, connected with the conversion, from dire necessity of the Arion ater, or black slug, to a use similar to that which the luxurious Romans are said to have made of the great apple-snail. Two young and blooming maidens lived together at that dread time, likeBessie Bell and Mary Gray, in a remote cottage on the steep (indeed almost perpendicular) ascent of the Bonnetmaker’s Hill. Deprived of friends or support by the pestilence that walked at noonday, they still retained their good looks and healthful aspect, even when the famine had succeeded to the plague. The jaundiced eyes of the famine-wasted wretches around them were instantly turned towards the poor girls, who appeared to thrive so well whilst others were famishing. They were unhesitatingly accused of witchcraft, and had nearly fallen a prey to that terrible charge; for betwixt themselves they had sworn never to tell in words by what means they were supported, ashamed as they felt of the resource to which they had been driven; and resolved, if possible, to escape the anticipated derision of their neighbours on its disclosure. It was only when about to be dragged before their stern inquisitors, that one of the girls, drawing aside the covering of a great barrel which stood in a corner of their domicile, discovered, without violating her oath, that the youthful pair had been driven to the desperate necessity of collecting and preserving for food large quantities of these Limacinae, which they ultimately acknowledged to have proved to them generous and even agreeable sustenance. To the credit of the times of George Wishart—a glimpse of pre-reforming enlightenment—the explanation sufficed; the young women escaped with their lives, and were even applauded for their prudence.[3]
Cosmetic[edit]
Skin creams derived from Helix aspersa snails are sold for use on wrinkles, scars, dry skin, and acne. A research study suggested that secretions produced under stress by Helix aspersa might facilitate regeneration of wounded tissue. [4]
Cultural depictions[edit]
Symbolism[edit]
Divination and other religious uses[edit]
Snails were widely noted and used in divination. [5] The Greek poet Hesiod wrote that snails signified the time to harvest by climbing the stalks, while the Aztec moon god Tecciztecatl bore a snail shell on his back. This symbolised rebirth; the snail's penchant for appearing and disappearing was analogised with the moon. [7]
Love darts and Cupid[edit]
Metaphor[edit]
In contemporary speech, the expression "a snail's pace" is often used to describe a slow, inefficient process. The phrase " snail mail" is used to mean regular postal service delivery of paper messages as opposed to the delivery of email, which can be virtually instantaneous.
In Indonesia mythology[edit]
Dewi Sekartaji as Keong Emas
Keong Emas ( Javanese and Indonesian for Golden Snail) is a popular Javanese folklore about a princess magically transformed and contained in a golden snail shell. The folklore is a part of popular Javanese Panji cycletelling the stories about the prince Panji Asmoro Bangun (also known as Raden Inu Kertapati) and his consort, princess Dewi Sekartaji (also known as Dewi Chandra Kirana).
Textiles[edit]
Certain varieties of snails, notably the family Muricidae, produce a secretion that is a color-fast natural dye. The ancient Tyrian purple was made in this way as were other purple and blue dyes. [9][10][11] The extreme expense of extracting this secretion is sufficient quantities limited its use to the very wealthy. It is such dyes as these that led to certain shades of purple and blue being associated with royalty and wealth. [12]
See also[edit]
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