The American Museum of Natural History. Members of this order are eel -like aquatic salamanders with much reduced forelimbs and no hind limbs. Despite this, the eggs are laid singly, a behaviour not conducive for external fertilisation. The hearts of amphibians have three chambers, two atria and one ventricle. Reptiles have twelve cranial nerve pairs.
Their larvae feed on glandular secretions and develop within the female's oviduct, often for long periods. Other amphibians, but not caecilians, are ovoviviparous. The eggs are retained in or on the parent's body, but the larvae subsist on the yolks of their eggs and receive no nourishment from the adult. The larvae emerge at varying stages of their growth, either before or after metamorphosis, according to their species. Frog larvae are known as tadpoles and typically have oval bodies and long, vertically flattened tails with fins.
The free-living larvae are normally fully aquatic, but the tadpoles of some species such as Nannophrys ceylonensis are semi-terrestrial and live among wet rocks. The lungs develop early and are used as accessory breathing organs, the tadpoles rising to the water surface to gulp air.
Some species complete their development inside the egg and hatch directly into small frogs. These larvae do not have gills but instead have specialised areas of skin through which respiration takes place. While tadpoles do not have true teeth, in most species, the jaws have long, parallel rows of small keratinized structures called keradonts surrounded by a horny beak. Iodine and T4 over stimulate the spectacular apoptosis [programmed cell death] of the cells of the larval gills, tail and fins also stimulate the evolution of nervous systems transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting.
In fact, tadpoles developing in ponds and streams are typically herbivorous. Pond tadpoles tend to have deep bodies, large caudal fins and small mouths; they swim in the quiet waters feeding on growing or loose fragments of vegetation.
Stream dwellers mostly have larger mouths, shallow bodies and caudal fins; they attach themselves to plants and stones and feed on the surface films of algae and bacteria. They have a relatively long, spiral-shaped gut to enable them to digest this diet.
Young of the Cuban tree frog Osteopilus septentrionalis can occasionally be cannibalistic , the younger tadpoles attacking a larger, more developed tadpole when it is undergoing metamorphosis. At metamorphosis, rapid changes in the body take place as the lifestyle of the frog changes completely.
The animal develops a large jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, and a tongue is formed. There are associated changes in the neural networks such as development of stereoscopic vision and loss of the lateral line system.
All this can happen in about a day. A few days later, the tail is reabsorbed, due to the higher thyroxine concentration required for this to take place. At hatching, a typical salamander larva has eyes without lids, teeth in both upper and lower jaws, three pairs of feathery external gills, a somewhat laterally flattened body and a long tail with dorsal and ventral fins.
The forelimbs may be partially developed and the hind limbs are rudimentary in pond-living species but may be rather more developed in species that reproduce in moving water. Pond-type larvae often have a pair of balancers, rod-like structures on either side of the head that may prevent the gills from becoming clogged up with sediment.
Some members of the genera Ambystoma and Dicamptodon have larvae that never fully develop into the adult form, but this varies with species and with populations.
The northwestern salamander Ambystoma gracile is one of these and, depending on environmental factors, either remains permanently in the larval state, a condition known as neoteny , or transforms into an adult. The tiger salamander Ambystoma tigrinum also sometimes behaves in this way and may grow particularly large in the process.
The adult tiger salamander is terrestrial, but the larva is aquatic and able to breed while still in the larval state.
When conditions are particularly inhospitable on land, larval breeding may allow continuation of a population that would otherwise die out. There are fifteen species of obligate neotenic salamanders, including species of Necturus , Proteus and Amphiuma , and many examples of facultative ones that adopt this strategy under appropriate environmental circumstances.
Lungless salamanders in the family Plethodontidae are terrestrial and lay a small number of unpigmented eggs in a cluster among damp leaf litter. Each egg has a large yolk sac and the larva feeds on this while it develops inside the egg, emerging fully formed as a juvenile salamander. The female salamander often broods the eggs. In the genus Ensatinas , the female has been observed to coil around them and press her throat area against them, effectively massaging them with a mucous secretion.
In newts and salamanders, metamorphosis is less dramatic than in frogs. This is because the larvae are already carnivorous and continue to feed as predators when they are adults so few changes are needed to their digestive systems. Their lungs are functional early, but the larvae do not make as much use of them as do tadpoles.
Their gills are never covered by gill sacs and are reabsorbed just before the animals leave the water. Other changes include the reduction in size or loss of tail fins, the closure of gill slits, thickening of the skin, the development of eyelids, and certain changes in dentition and tongue structure.
Salamanders are at their most vulnerable at metamorphosis as swimming speeds are reduced and transforming tails are encumbrances on land. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxine. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.
Most terrestrial caecilians that lay eggs do so in burrows or moist places on land near bodies of water. The development of the young of Ichthyophis glutinosus , a species from Sri Lanka, has been much studied. The eel-like larvae hatch out of the eggs and make their way to water. They have three pairs of external red feathery gills, a blunt head with two rudimentary eyes, a lateral line system and a short tail with fins. They swim by undulating their body from side to side.
They are mostly active at night, soon lose their gills and make sorties onto land. By the age of about ten months they have developed a pointed head with sensory tentacles near the mouth and lost their eyes, lateral line systems and tails. The skin thickens, embedded scales develop and the body divides into segments.
By this time, the caecilian has constructed a burrow and is living on land. In the majority of species of caecilians, the young are produced by viviparity. Typhlonectes compressicauda , a species from South America, is typical of these. Up to nine larvae can develop in the oviduct at any one time. They are elongated and have paired sac-like gills, small eyes and specialised scraping teeth. At first, they feed on the yolks of the eggs, but as this source of nourishment declines they begin to rasp at the ciliated epithelial cells that line the oviduct.
This stimulates the secretion of fluids rich in lipids and mucoproteins on which they feed along with scrapings from the oviduct wall. They may increase their length sixfold and be two-fifths as long as their mother before being born.
By this time they have undergone metamorphosis, lost their eyes and gills, developed a thicker skin and mouth tentacles, and reabsorbed their teeth. A permanent set of teeth grow through soon after birth. The ringed caecilian Siphonops annulatus has developed a unique adaptation for the purposes of reproduction. The progeny feed on a skin layer that is specially developed by the adult in a phenomenon known as maternal dermatophagy.
The brood feed as a batch for about seven minutes at intervals of approximately three days which gives the skin an opportunity to regenerate. Meanwhile, they have been observed to ingest fluid exuded from the maternal cloaca. The care of offspring among amphibians has been little studied but, in general, the larger the number of eggs in a batch, the less likely it is that any degree of parental care takes place. Many woodland salamanders lay clutches of eggs under dead logs or stones on land.
The black mountain salamander Desmognathus welteri does this, the mother brooding the eggs and guarding them from predation as the embryos feed on the yolks of their eggs. When fully developed, they break their way out of the egg capsules and disperse as juvenile salamanders. The male then guards the site for the two or three months before the eggs hatch, using body undulations to fan the eggs and increase their supply of oxygen. The male Colostethus subpunctatus , a tiny frog, protects the egg cluster which is hidden under a stone or log.
When the eggs hatch, the male transports the tadpoles on his back, stuck there by a mucous secretion, to a temporary pool where he dips himself into the water and the tadpoles drop off. He keeps them moist and when they are ready to hatch, he visits a pond or ditch and releases the tadpoles. The tadpoles secrete a hormone that inhibits digestion in the mother whilst they develop by consuming their very large yolk supply.
When they hatch, the male carries the tadpoles around in brood pouches on his hind legs. Its eggs are laid on the forest floor and when they hatch, the tadpoles are carried one by one on the back of an adult to a suitable water-filled crevice such as the axil of a leaf or the rosette of a bromeliad.
The female visits the nursery sites regularly and deposits unfertilised eggs in the water and these are consumed by the tadpoles. With a few exceptions, adult amphibians are predators , feeding on virtually anything that moves that they can swallow. The diet mostly consists of small prey that do not move too fast such as beetles, caterpillars, earthworms and spiders. The sirens Siren spp. It projects it with the tip foremost whereas other frogs flick out the rear part first, their tongues being hinged at the front.
Food is mostly selected by sight, even in conditions of dim light. Movement of the prey triggers a feeding response. Frogs have been caught on fish hooks baited with red flannel and green frogs Rana clamitans have been found with stomachs full of elm seeds that they had seen floating past.
This response is mostly secondary because salamanders have been observed to remain stationary near odoriferous prey but only feed if it moves. Cave-dwelling amphibians normally hunt by smell. Some salamanders seem to have learned to recognize immobile prey when it has no smell, even in complete darkness. Amphibians usually swallow food whole but may chew it lightly first to subdue it. The base and crown of these are composed of dentine separated by an uncalcified layer and they are replaced at intervals.
Salamanders, caecilians and some frogs have one or two rows of teeth in both jaws, but some frogs Rana spp. In many amphibians there are also vomerine teeth attached to a facial bone in the roof of the mouth.
The tiger salamander Ambystoma tigrinum is typical of the frogs and salamanders that hide under cover ready to ambush unwary invertebrates. Others amphibians, such as the Bufo spp.
The struggles of the prey and further jaw movements work it inwards and the caecilian usually retreats into its burrow. The subdued prey is gulped down whole. When they are newly hatched, frog larvae feed on the yolk of the egg. When this is exhausted some move on to feed on bacteria, algal crusts, detritus and raspings from submerged plants. Water is drawn in through their mouths, which are usually at the bottom of their heads, and passes through branchial food traps between their mouths and their gills where fine particles are trapped in mucus and filtered out.
Others have specialised mouthparts consisting of a horny beak edged by several rows of labial teeth. They scrape and bite food of many kinds as well as stirring up the bottom sediment, filtering out larger particles with the papillae around their mouths.
Some, such as the spadefoot toads, have strong biting jaws and are carnivorous or even cannibalistic. The calls made by caecilians and salamanders are limited to occasional soft squeaks, grunts or hisses and have not been much studied. A clicking sound sometimes produced by caecilians may be a means of orientation, as in bats, or a form of communication. Most salamanders are considered voiceless, but the California giant salamander Dicamptodon ensatus has vocal cords and can produce a rattling or barking sound.
Some species of salamander emit a quiet squeak or yelp if attacked. Frogs are much more vocal, especially during the breeding season when they use their voices to attract mates.
The presence of a particular species in an area may be more easily discerned by its characteristic call than by a fleeting glimpse of the animal itself. In most species, the sound is produced by expelling air from the lungs over the vocal cords into an air sac or sacs in the throat or at the corner of the mouth. This may distend like a balloon and acts as a resonator, helping to transfer the sound to the atmosphere, or the water at times when the animal is submerged. This call is modified to a quieter courtship call on the approach of a female or to a more aggressive version if a male intruder draws near.
Calling carries the risk of attracting predators and involves the expenditure of much energy. When a frog is attacked, a distress or fright call is emitted, often resembling a scream. Little is known of the territorial behaviour of caecilians, but some frogs and salamanders defend home ranges.
These are usually feeding, breeding or sheltering sites. Males normally exhibit such behaviour though in some species, females and even juveniles are also involved. Although in many frog species, females are larger than males, this is not the case in most species where males are actively involved in territorial defence. Some of these have specific adaptations such as enlarged teeth for biting or spines on the chest, arms or thumbs.
In salamanders, defence of a territory involves adopting an aggressive posture and if necessary attacking the intruder. This may involve snapping, chasing and sometimes biting, occasionally causing the loss of a tail. The behaviour of red back salamanders Plethodon cinereus has been much studied.
Much of their behaviour seemed stereotyped and did not involve any actual contact between individuals. An aggressive posture involved raising the body off the ground and glaring at the opponent who often turned away submissively. If the intruder persisted, a biting lunge was usually launched at either the tail region or the naso-labial grooves.
Damage to either of these areas can reduce the fitness of the rival, either because of the need to regenerate tissue or because it impairs its ability to detect food. In frogs, male territorial behaviour is often observed at breeding locations; calling is both an announcement of ownership of part of this resource and an advertisement call to potential mates. In general, a deeper voice represents a heavier and more powerful individual, and this may be sufficient to prevent intrusion by smaller males.
Much energy is used in the vocalization and it takes a toll on the territory holder who may be displaced by a fitter rival if he tires. There is a tendency for males to tolerate the holders of neighbouring territories while vigorously attacking unknown intruders.
Holders of territories have a "home advantage" and usually come off better in an encounter between two similar-sized frogs. If threats are insufficient, chest to chest tussles may take place. Fighting methods include pushing and shoving, deflating the opponent's vocal sac, seizing him by the head, jumping on his back, biting, chasing, splashing, and ducking him under the water.
Amphibians have soft bodies with thin skins, and lack claws, defensive armour, or spines. Nevertheless, they have evolved various defence mechanisms to keep themselves alive. The first line of defence in salamanders and frogs is the mucous secretion that they produce. This keeps their skin moist and makes them slippery and difficult to grip. The secretion is often sticky and distasteful or toxic. The rough-skinned newt Taricha granulosa from North America and other members of its genus contain the neurotoxin tetrodotoxin TTX , the most toxic non-protein substance known and almost identical to that produced by pufferfish.
Handling the newts does not cause harm, but ingestion of even the most minute amounts of the skin is deadly. In feeding trials, fish, frogs, reptiles, birds and mammals were all found to be susceptible. In locations where both snake and salamander co-exist, the snakes have developed immunity through genetic changes and they feed on the amphibians with impunity. These regions are presented to the attacking animal and their secretions may be foul-tasting or cause various physical or neurological symptoms.
Altogether, over toxins have been isolated from the limited number of amphibian species that have been investigated. Poisonous species often use bright colouring to warn potential predators of their toxicity.
These warning colours tend to be red or yellow combined with black, with the fire salamander Salamandra salamandra being an example. Once a predator has sampled one of these, it is likely to remember the colouration next time it encounters a similar animal. In some species, such as the fire-bellied toad Bombina spp. The frog Allobates zaparo is not poisonous, but mimics the appearance of other toxic species in its locality, a strategy that may deceive predators.
Many amphibians are nocturnal and hide during the day, thereby avoiding diurnal predators that hunt by sight. Other amphibians use camouflage to avoid being detected. They have various colourings such as mottled browns, greys and olives to blend into the background. Some salamanders adopt defensive poses when faced by a potential predator such as the North American northern short-tailed shrew Blarina brevicauda.
Their bodies writhe and they raise and lash their tails which makes it difficult for the predator to avoid contact with their poison-producing granular glands. The tail may have a constriction at its base to allow it to be easily detached.
The tail is regenerated later, but the energy cost to the animal of replacing it is significant. The blackbelly salamander Desmognathus quadramaculatus can bite an attacking common garter snake Thamnophis sirtalis two or three times its size on the head and often manages to escape.
In amphibians, there is evidence of habituation , associative learning through both classical and instrumental learning , and discrimination abilities. In one experiment, when offered live fruit flies Drosophila virilis , salamanders choose the larger of 1 vs 2 and 2 vs 3.
Frogs can distinguish between low numbers 1 vs 2, 2 vs 3, but not 3 vs 4 and large numbers 3 vs 6, 4 vs 8, but not 4 vs 6 of prey. This is irrespective of other characteristics, i. Dramatic declines in amphibian populations, including population crashes and mass localized extinction , have been noted since the late s from locations all over the world, and amphibian declines are thus perceived to be one of the most critical threats to global biodiversity.
In there were believed to be 4, species of amphibians that depended on water at some stage during their life cycle.
Of these, 1, However, many of the causes of amphibian declines are still poorly understood, and are a topic of ongoing discussion. With their complex reproductive needs and permeable skins, amphibians are often considered to be ecological indicators. Any decline in amphibian numbers will affect the patterns of predation. The loss of carnivorous species near the top of the food chain will upset the delicate ecosystem balance and may cause dramatic increases in opportunistic species.
In the Middle East, a growing appetite for eating frog legs and the consequent gathering of them for food was linked to an increase in mosquitoes. The western terrestrial garter snake Thamnophis elegans in California is largely aquatic and depends heavily on two species of frog that are decreasing in numbers, the Yosemite toad Bufo canorus and the mountain yellow-legged frog Rana muscosa , putting the snake's future at risk.
If the snake were to become scarce, this would affect birds of prey and other predators that feed on it. These normally play an important role in controlling the growth of algae and also forage on detritus that accumulates as sediment on the bottom.
A reduction in the number of tadpoles may lead to an overgrowth of algae, resulting in depletion of oxygen in the water when the algae later die and decompose. Aquatic invertebrates and fish might then die and there would be unpredictable ecological consequences.
A global strategy to stem the crisis was released in in the form of the Amphibian Conservation Action Plan. Developed by over eighty leading experts in the field, this call to action details what would be required to curtail amphibian declines and extinctions over the following five years and how much this would cost. The Amphibian Specialist Group of the IUCN is spearheading efforts to implement a comprehensive global strategy for amphibian conservation. From Wikipedia, the free encyclopedia.
A class of ectothermic tetrapods, which typically breed in water. For other uses, see Amphibian disambiguation. List of prehistoric amphibians. Restoration of Eusthenopteron , a fully aquatic lobe-finned fish Bottom: Restoration of Tiktaalik , an advanced tetrapodomorph fish. Sexual selection in amphibians. Decline in amphibian populations. An outline of higher-level classification and survey of taxonomic richness" PDF.
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