Amphibian
Amphibians | |
---|---|
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Batrachomorpha |
Class: | Amphibia Gray 1825[2] |
Subclasses | |
(partial list)
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Amphibians are
The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin. They are superficially similar to reptiles like lizards, but unlike reptiles and other amniotes, require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
The earliest amphibians
A fourth group of lissamphibians, the Albanerpetontidae, became extinct around 2 million years ago. The number of known amphibian species is approximately 8,000, of which nearly 90% are frogs. The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amauensis) with a length of just 7.7 mm (0.30 in). The largest living amphibian is the 1.8 m (5 ft 11 in) South China giant salamander (Andrias sligoi), but this is dwarfed by prehistoric temnospondyls such as Mastodonsaurus which could reach up to 6 m (20 ft) in length.[6] The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology.
Classification
The word amphibian is derived from the
- Subclass Lepospondyli† (A potentially polyphyletic Late Paleozoic group of small forms, likely more closely related to amniotes than Lissamphibia)
- Subclass Temnospondyli† (diverse Late Paleozoic and early Mesozoic grade, some of which were large predators)
- Subclass Lissamphibia (all modern amphibians, including frogs, toads, salamanders, newts and caecilians)
- Salientia (frogs, toads and relatives): Early Triassic to present—7,360 current species in 53 families.[9] Modern (crown group) salientians are described via the name Anura.
- Caudata (salamanders, newts and relatives): Late Triassic to present—764 current species in 9 families.[9] Modern (crown group) caudatans are described via the name Urodela.
- Gymnophiona (caecilians and relatives): Late Triassic to present—215 current species in 10 families.[9] The name Apoda is also sometimes used for caecilians.
- Allocaudata† (Albanerpetontidae) Middle Jurassic – Early Pleistocene
These three subclasses do not include all extinct amphibians. Other extinct amphibian groups include Embolomeri (Late Paleozoic large aquatic predators), Seymouriamorpha (semiaquatic to terrestrial Permian forms related to amniotes), among others. Names such as Tetrapoda and Stegocephalia encompass the entirety of amphibian-grade tetrapods, while Reptiliomorpha or Anthracosauria are variably used to describe extinct amphibians more closely related to amniotes than to lissamphibians.
The actual number of species in each group depends on the taxonomic classification followed. The two most common systems are the classification adopted by the website AmphibiaWeb, University of California, Berkeley, and the classification by herpetologist Darrel Frost and the American Museum of Natural History, available as the online reference database "Amphibian Species of the World".[10] The numbers of species cited above follows Frost and the total number of known (living) amphibian species as of March 31, 2019, is exactly 8,000,[11] of which nearly 90% are frogs.[12]
With the
All modern amphibians are included in the subclass Lissamphibia, which is usually considered a
Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. The Lissamphibia are traditionally divided into three orders, but an extinct salamander-like family, the Albanerpetontidae, is now considered part of Lissamphibia alongside the superorder Salientia. Furthermore, Salientia includes all three recent orders plus the Triassic proto-frog, Triadobatrachus.[19]
Evolutionary history
The first major groups of amphibians developed in the
Many examples of species showing transitional features have been discovered. Ichthyostega was one of the first primitive amphibians, with nostrils and more efficient lungs. It had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe-finned fish, Eusthenopteron.[20] Amphibians evolved adaptations that allowed them to stay out of the water for longer periods. Their lungs improved and their skeletons became heavier and stronger, better able to support the weight of their bodies on land. They developed "hands" and "feet" with five or more digits;[22] the skin became more capable of retaining body fluids and resisting desiccation.[21] The fish's hyomandibula bone in the hyoid region behind the gills diminished in size and became the stapes of the amphibian ear, an adaptation necessary for hearing on dry land.[23] An affinity between the amphibians and the teleost fish is the multi-folded structure of the teeth and the paired supra-occipital bones at the back of the head, neither of these features being found elsewhere in the animal kingdom.[24]
At the end of the Devonian period (360 million years ago), the seas, rivers and lakes were teeming with life while the land was the realm of early plants and devoid of vertebrates,
After the Carboniferous rainforest collapse amphibian dominance gave way to reptiles,[25] and amphibians were further devastated by the Permian–Triassic extinction event.[26] During the Triassic Period (252 to 201 million years ago), the reptiles continued to out-compete the amphibians, leading to a reduction in both the amphibians' size and their importance in the biosphere. According to the fossil record, Lissamphibia, which includes all modern amphibians and is the only surviving lineage, may have branched off from the extinct groups Temnospondyli and Lepospondyli at some period between the Late Carboniferous and the Early Triassic. The relative scarcity of fossil evidence precludes precise dating,[21] but the most recent molecular study, based on multilocus sequence typing, suggests a Late Carboniferous/Early Permian origin for extant amphibians.[27]
The origins and evolutionary relationships between the three main groups of amphibians is a matter of debate. A 2005 molecular phylogeny, based on
As they evolved from lunged fish, amphibians had to make certain adaptations for living on land, including the need to develop new means of locomotion. In the water, the sideways thrusts of their tails had propelled them forward, but on land, quite different mechanisms were required. Their vertebral columns, limbs, limb girdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding. Terrestrial adults discarded their lateral line systems and adapted their sensory systems to receive stimuli via the medium of the air. They needed to develop new methods to regulate their body heat to cope with fluctuations in ambient temperature. They developed behaviours suitable for reproduction in a terrestrial environment. Their skins were exposed to harmful ultraviolet rays that had previously been absorbed by the water. The skin changed to become more protective and prevent excessive water loss.[30]
Characteristics
The superclass
Modern amphibians have a simplified anatomy compared to their ancestors due to paedomorphosis, caused by two evolutionary trends: miniaturization and an unusually large genome, which result in a slower growth and development rate compared to other vertebrates.[35][36] Another reason for their size is associated with their rapid metamorphosis, which seems to have evolved only in the ancestors of lissamphibia; in all other known lines the development was much more gradual. Because a remodeling of the feeding apparatus means they do not eat during the metamorphosis, the metamorphosis has to go faster the smaller the individual is, so it happens at an early stage when the larvae are still small. (The largest species of salamanders do not go through a metamorphosis.)[37] Amphibians that lay eggs on land often go through the whole metamorphosis inside the egg. An anamniotic terrestrial egg is less than 1 cm in diameter due to diffusion problems, a size which puts a limit on the amount of posthatching growth.[38]
The smallest amphibian (and vertebrate) in the world is a
Amphibians are ectothermic (cold-blooded) vertebrates that do not maintain their body temperature through internal
Anura
The order
Anura is divided into three suborders that are broadly accepted by the scientific community, but the relationships between some families remain unclear. Future
Caudata
The order
The suborder Cryptobranchoidea contains the primitive salamanders. A number of fossil cryptobranchids have been found, but there are only three living species, the Chinese giant salamander (Andrias davidianus), the Japanese giant salamander (Andrias japonicus) and the hellbender (Cryptobranchus alleganiensis) from North America. These large amphibians retain several larval characteristics in their adult state; gills slits are present and the eyes are unlidded. A unique feature is their ability to feed by suction, depressing either the left side of their lower jaw or the right.[56] The males excavate nests, persuade females to lay their egg strings inside them, and guard them. As well as breathing with lungs, they respire through the many folds in their thin skin, which has capillaries close to the surface.[57]
The suborder
The third suborder,
Gymnophiona
The order
Anatomy and physiology
Skin
The integumentary structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly cornified outer layers, renewed periodically through a moulting process controlled by the pituitary and thyroid glands. Local thickenings (often called warts) are common, such as those found on toads. The outside of the skin is shed periodically mostly in one piece, in contrast to mammals and birds where it is shed in flakes. Amphibians often eat the sloughed skin.[46] Caecilians are unique among amphibians in having mineralized dermal scales embedded in the dermis between the furrows in the skin. The similarity of these to the scales of bony fish is largely superficial. Lizards and some frogs have somewhat similar osteoderms forming bony deposits in the dermis, but this is an example of convergent evolution with similar structures having arisen independently in diverse vertebrate lineages.[63]
Amphibian skin is permeable to water. Gas exchange can take place through the skin (cutaneous respiration) and this allows adult amphibians to respire without rising to the surface of water and to hibernate at the bottom of ponds.[46] To compensate for their thin and delicate skin, amphibians have evolved mucous glands, principally on their heads, backs and tails. The secretions produced by these help keep the skin moist. In addition, most species of amphibian have granular glands that secrete distasteful or poisonous substances. Some amphibian toxins can be lethal to humans while others have little effect.[64] The main poison-producing glands, the parotoids, produce the neurotoxin bufotoxin and are located behind the ears of toads, along the backs of frogs, behind the eyes of salamanders and on the upper surface of caecilians.[65]
The skin colour of amphibians is produced by three layers of pigment cells called chromatophores. These three cell layers consist of the melanophores (occupying the deepest layer), the guanophores (forming an intermediate layer and containing many granules, producing a blue-green colour) and the lipophores (yellow, the most superficial layer). The colour change displayed by many species is initiated by hormones secreted by the pituitary gland. Unlike bony fish, there is no direct control of the pigment cells by the nervous system, and this results in the colour change taking place more slowly than happens in fish. A vividly coloured skin usually indicates that the species is toxic and is a warning sign to predators.[66]
Skeletal system and locomotion
Amphibians have a skeletal system that is structurally
In most amphibians, there are four digits on the fore foot and five on the hind foot, but no claws on either. Some salamanders have fewer digits and the amphiumas are eel-like in appearance with tiny, stubby legs. The sirens are aquatic salamanders with stumpy forelimbs and no hind limbs. The caecilians are limbless. They burrow in the manner of earthworms with zones of muscle contractions moving along the body. On the surface of the ground or in water they move by undulating their body from side to side.[68]
In frogs, the hind legs are larger than the fore legs, especially so in those species that principally move by jumping or swimming. In the walkers and runners the hind limbs are not so large, and the burrowers mostly have short limbs and broad bodies. The feet have adaptations for the way of life, with webbing between the toes for swimming, broad adhesive toe pads for climbing, and keratinised tubercles on the hind feet for digging (frogs usually dig backwards into the soil). In most salamanders, the limbs are short and more or less the same length and project at right angles from the body. Locomotion on land is by walking and the tail often swings from side to side or is used as a prop, particularly when climbing. In their normal gait, only one leg is advanced at a time in the manner adopted by their ancestors, the lobe-finned fish.
Salamanders use their tails in defence and some are prepared to jettison them to save their lives in a process known as autotomy. Certain species in the Plethodontidae have a weak zone at the base of the tail and use this strategy readily. The tail often continues to twitch after separation which may distract the attacker and allow the salamander to escape. Both tails and limbs can be regenerated.[69] Adult frogs are unable to regrow limbs but tadpoles can do so.[68]
Circulatory system
Amphibians have a juvenile stage and an adult stage, and the circulatory systems of the two are distinct. In the juvenile (or tadpole) stage, the circulation is similar to that of a fish; the two-chambered heart pumps the blood through the gills where it is oxygenated, and is spread around the body and back to the heart in a single loop. In the adult stage, amphibians (especially frogs) lose their gills and develop lungs. They have a heart that consists of a single ventricle and two atria. When the ventricle starts contracting, deoxygenated blood is pumped through the pulmonary artery to the lungs. Continued contraction then pumps oxygenated blood around the rest of the body. Mixing of the two bloodstreams is minimized by the anatomy of the chambers.[70]
Nervous and sensory systems
The
Tadpoles retain the lateral line system of their ancestral fishes, but this is lost in terrestrial adult amphibians. Many aquatic salamanders and some caecilians possess
The eyes of tadpoles lack lids, but at metamorphosis, the
Digestive and excretory systems
Many amphibians catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth before seizing the item with their jaws. Some use inertial feeding to help them swallow the prey, repeatedly thrusting their head forward sharply causing the food to move backwards in their mouth by inertia. Most amphibians swallow their prey whole without much chewing so they possess voluminous stomachs. The short oesophagus is lined with cilia that help to move the food to the stomach and mucus produced by glands in the mouth and pharynx eases its passage. The enzyme chitinase produced in the stomach helps digest the chitinous cuticle of arthropod prey.[76]
Amphibians possess a
There are two kidneys located dorsally, near the roof of the body cavity. Their job is to filter the blood of metabolic waste and transport the urine via ureters to the urinary bladder where it is stored before being passed out periodically through the cloacal vent. Larvae and most aquatic adult amphibians excrete the nitrogen as ammonia in large quantities of dilute urine, while terrestrial species, with a greater need to conserve water, excrete the less toxic product urea. Some tree frogs with limited access to water excrete most of their metabolic waste as uric acid.[78]
Urinary bladder
Most aquatic and semi-aquatic amphibians have a membranous skin which allows them to absorb water directly through it. Some semi-aquatic animals also have similarly permeable bladder membrane.[79] As a result, they tend to have high rates of urine production to offset this high water intake, and have urine which is low in dissolved salts. The urinary bladder assists such animals to retain salts. Some aquatic amphibian such as Xenopus do not reabsorb water, to prevent excessive water influx.[80] For land-dwelling amphibians, dehydration results in reduced urine output.[81]
The amphibian bladder is usually highly distensible and among some land-dwelling species of frogs and salamanders may account for between 20% and 50% of their total body weight.[81] Urine flows from the kidneys through the ureters into the bladder and is periodically released from the bladder to the cloaca.[82]
Respiratory system
The lungs in amphibians are primitive compared to those of amniotes, possessing few internal
Reproduction
For the purpose of
Several hundred frog species in
In the tropics, many amphibians breed continuously or at any time of year. In temperate regions, breeding is mostly seasonal, usually in the spring, and is triggered by increasing day length, rising temperatures or rainfall. Experiments have shown the importance of temperature, but the trigger event, especially in arid regions, is often a storm. In anurans, males usually arrive at the breeding sites before females and the vocal chorus they produce may stimulate ovulation in females and the endocrine activity of males that are not yet reproductively active.[87]
In caecilians, fertilisation is internal, the male extruding an intromittent organ, the phallodeum, and inserting it into the female cloaca. The paired Müllerian glands inside the male cloaca secrete a fluid which resembles that produced by mammalian prostate glands and which may transport and nourish the sperm. Fertilisation probably takes place in the oviduct.[88]
The majority of salamanders also engage in
With a few exceptions, frogs use external fertilisation. The male grasps the female tightly with his forelimbs either behind the arms or in front of the back legs, or in the case of
Most frogs can be classified as either prolonged or explosive breeders. Typically, prolonged breeders congregate at a breeding site, the males usually arriving first, calling and setting up territories. Other satellite males remain quietly nearby, waiting for their opportunity to take over a territory. The females arrive sporadically, mate selection takes place and eggs are laid. The females depart and territories may change hands. More females appear and in due course, the breeding season comes to an end. Explosive breeders on the other hand are found where temporary pools appear in dry regions after rainfall. These frogs are typically fossorial species that emerge after heavy rains and congregate at a breeding site. They are attracted there by the calling of the first male to find a suitable place, perhaps a pool that forms in the same place each rainy season. The assembled frogs may call in unison and frenzied activity ensues, the males scrambling to mate with the usually smaller number of females.[89]
There is a direct competition between males to win the attention of the females in salamanders and newts, with elaborate courtship displays to keep the female's attention long enough to get her interested in choosing him to mate with.[91] Some species store sperm through long breeding seasons, as the extra time may allow for interactions with rival sperm.[92]
Life cycle
Most amphibians go through
Eggs
In the egg, the embryo is suspended in perivitelline fluid and surrounded by semi-permeable gelatinous capsules, with the yolk mass providing nutrients. As the larvae hatch, the capsules are dissolved by enzymes secreted from gland at the tip of the snout.[75] The eggs of some salamanders and frogs contain unicellular green algae. These penetrate the jelly envelope after the eggs are laid and may increase the supply of oxygen to the embryo through photosynthesis. They seem to both speed up the development of the larvae and reduce mortality.[95] In the wood frog (Rana sylvatica), the interior of the globular egg cluster has been found to be up to 6 °C (11 °F) warmer than its surroundings, which is an advantage in its cool northern habitat.[96]
The eggs may be deposited singly, in cluster or in long strands. Sites for laying eggs include water, mud, burrows, debris and on plants or under logs or stones.
Larvae
The eggs of amphibians are typically laid in water and hatch into free-living larvae that complete their development in water and later transform into either aquatic or terrestrial adults. In many species of frog and in most lungless salamanders (Plethodontidae), direct development takes place, the larvae growing within the eggs and emerging as miniature adults. Many caecilians and some other amphibians lay their eggs on land, and the newly hatched larvae wriggle or are transported to water bodies. Some caecilians, the alpine salamander (Salamandra atra) and some of the African live-bearing toads (Nectophrynoides spp.) are viviparous. 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.[101] The toad genus Nectophrynoides exhibits all of these developmental patterns among its dozen or so members.[12] Amphibian larvae are known as tadpoles. They have thick, rounded bodies with powerful muscular tails.[78]
Frogs
Unlike in other amphibians, frog tadpoles do not resemble adults.
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.[109][110]
In fact, tadpoles developing in ponds and streams are typically
At metamorphosis, rapid changes in the body take place as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is reabsorbed together with the spiral gut. 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.[111]
Salamanders
At hatching, a typical salamander larva has eyes without lids, teeth in both upper and lower jaws, three pairs of feathery external gills, 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.[113][114] Both of these are able to breed.[115] Some have larvae that never fully develop into the adult form, a condition known as neoteny.[116] Neoteny occurs when the animal's growth rate is very low and is usually linked to adverse conditions such as low water temperatures that may change the response of the tissues to the hormone thyroxine.[117] as well as lack of food. There are fifteen species of obligate neotenic salamanders, including species of Necturus, Proteus and Amphiuma, and many examples of facultative ones, such as the northwestern salamander (Ambystoma gracile) and the tiger salamander (A. tigrinum) that adopt this strategy under appropriate environmental circumstances.[116]
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
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.[119] Adult salamanders often have an aquatic phase in spring and summer, and a land phase in winter. 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.[113]
Caecilians
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. Metamorphosis is gradual. 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.[120]
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.[121][122]
Gills are only necessarily during embryonic development, and in species that give birth the offspring is born after gill degeneration. In egg laying caecilians the gills are either reabsorbed before hatching, or, in species that hatch with gill remnants still present, short lived and only leaves behind a gill slit. For species with scales under their skin, the scales does not form before during metamorphosis.[123]
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.[124]
Parental care
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. Nevertheless, it is estimated that in up to 20% of amphibian species, one or both adults play some role in the care of the young.[125] Those species that breed in smaller water bodies or other specialised habitats tend to have complex patterns of behaviour in the care of their young.[126]
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.[127] The male hellbender, a primitive salamander, excavates an underwater nest and encourages females to lay there. 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.[57]
The male
Genetics and genomics
Amphibians are notable among vertebrates for their diversity of chromosomes and genomes. The karyotypes (chromosomes) have been determined for at least 1,193 (14.5%) of the ≈8,200 known (diploid) species, including 963 anurans, 209 salamanders, and 21 caecilians. Generally, the karyotypes of diploid amphibians are characterized by 20–26 bi-armed chromosomes. Amphibians have also very large genomes compared to other taxa of vertebrates and corresponding variation in genome size (C-value: picograms of DNA in haploid nuclei). The genome sizes range from 0.95 to 11.5 pg in frogs, from 13.89 to 120.56 pg in salamanders, and from 2.94 to 11.78 pg in caecilians.[134]
The large genome sizes have prevented whole-genome sequencing of amphibians although a number of genomes have been published recently. The 1.7GB draft genome of Xenopus tropicalis was the first to be reported for amphibians in 2010.[134] Compared to some salamanders this frog genome is tiny. For instance, the genome of the Mexican axolotl turned out to be 32 Gb, which is more than 10 times larger than the human genome (3GB).[135]
Feeding and diet
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.) often ingest aquatic plant material with the invertebrates on which they feed[136] and a Brazilian tree frog (Xenohyla truncata) includes a large quantity of fruit in its diet.[137] The Mexican burrowing toad (Rhinophrynus dorsalis) has a specially adapted tongue for picking up ants and termites. It projects it with the tip foremost whereas other frogs flick out the rear part first, their tongues being hinged at the front.[138]
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
Amphibians usually swallow food whole but may chew it lightly first to subdue it.
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. toads, actively search for prey, while the Argentine horned frog (Ceratophrys ornata) lures inquisitive prey closer by raising its hind feet over its back and vibrating its yellow toes.[142] Among leaf litter frogs in Panama, frogs that actively hunt prey have narrow mouths and are slim, often brightly coloured and toxic, while ambushers have wide mouths and are broad and well-camouflaged.[143] Caecilians do not flick their tongues, but catch their prey by grabbing it with their slightly backward-pointing teeth. 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.[144]
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.[145]
Vocalization
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
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.[146] The main vocalisation is the male's loud advertisement call which seeks to both encourage a female to approach and discourage other males from intruding on its territory. 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.[147] Other calls include those given by a female in response to the advertisement call and a release call given by a male or female during unwanted attempts at amplexus. When a frog is attacked, a distress or fright call is emitted, often resembling a scream.[148] The usually nocturnal Cuban tree frog (Osteopilus septentrionalis) produces a rain call when there is rainfall during daylight hours.[149]
Territorial behaviour
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.[150]
In salamanders, defence of a
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.[153]
Defence mechanisms
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.
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 warning colouration is on the belly and these animals adopt a defensive pose when attacked, exhibiting their bright colours to the predator. The frog Allobates zaparo is not poisonous, but mimics the appearance of other toxic species in its locality, a strategy that may deceive predators.[162]
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.[163] A few salamanders will autotomise their tails when attacked, sacrificing this part of their anatomy to enable them to escape. 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.[69]
Some frogs and toads inflate themselves to make themselves look large and fierce, and some spadefoot toads (
Cognition
In amphibians, there is evidence of
In one experiment, when offered live fruit flies (Drosophila virilis), salamanders chose 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.e. surface area, volume, weight and movement, although discrimination among large numbers may be based on surface area.[167]
Conservation
Dramatic declines in amphibian populations, including population crashes and mass localized
Food webs and predation
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.
Predators that feed on amphibians are affected by their decline. 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.[172] Meanwhile, in the ponds and lakes, fewer frogs means fewer tadpoles. 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.[173]
Pollution and pesticides
The decline in amphibian and reptile populations has led to an awareness of the effects of pesticides on reptiles and amphibians.
Conservation and protection strategies
The Amphibian Specialist Group of the IUCN is spearheading efforts to implement a comprehensive global strategy for amphibian conservation.[175] Amphibian Ark is an organization that was formed to implement the ex-situ conservation recommendations of this plan, and they have been working with zoos and aquaria around the world, encouraging them to create assurance colonies of threatened amphibians.[175] One such project is the Panama Amphibian Rescue and Conservation Project that built on existing conservation efforts in Panama to create a country-wide response to the threat of chytridiomycosis.[176]
Another measure would be to stop exploitation of frogs for human consumption. In the Middle East, a growing appetite for eating frog legs and the consequent gathering of them for food was already linked to an increase in mosquitoes and thus has direct consequences for human health.[177]
See also
- Amphibian and reptile tunnel
- Cultural depictions of amphibians
- List of amphibians
- List of amphibian genera
- List of threatened reptiles and amphibians of the United States
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{{cite book}}
: CS1 maint: multiple names: authors list (link
Cited texts
- Dorit, R. L.; Walker, W. F.; Barnes, R. D. (1991). Zoology. Saunders College Publishing. ISBN 978-0-03-030504-7.
- ISBN 978-0-691-03281-8.
Further reading
- Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. W. H. Freeman. ISBN 978-0-7167-1822-2.
- Carroll, Robert L. (2009). The Rise of Amphibians: 365 Million Years of Evolution. Johns Hopkins University Press. ISBN 978-0-8018-9140-3.
- Duellman, William E.; Linda Trueb (1994). Biology of Amphibians. Johns Hopkins University Press. ISBN 978-0-8018-4780-6.
- Frost, Darrel R.; Grant, Taran; Faivovich, Julián; Bain, Raoul H.; Haas, Alexander; Haddad, Célio F.B.; De Sá, Rafael O.; Channing, Alan; Wilkinson, Mark; Donnellan, Stephen C.; Raxworthy, Christopher J.; Campbell, Jonathan A.; Blotto, Boris L.; Moler, Paul; Drewes, Robert C.; Nussbaum, Ronald A.; Lynch, John D.; Green, David M.; Wheeler, Ward C. (2006). "The Amphibian Tree of Life". Bulletin of the American Museum of Natural History. 297: 1–291. S2CID 86140137.
- Pounds, J. Alan; Bustamante, Martín R.; Coloma, Luis A.; Consuegra, Jamie A.; Fogden, Michael P. L.; Foster, Pru N.; La Marca, Enrique; Masters, Karen L.; Merino-Viteri, Andrés; Puschendorf, Robert; Ron, Santiago R.; Sánchez-Azofeifa, G. Arturo; Still, Christopher J.; Young, Bruce E. (2006). "Widespread amphibian extinctions from epidemic disease driven by global warming". Nature. 439 (7073): 161–167. S2CID 4430672.
- Stuart, Simon N.; Chanson, Janice S.; Cox, Neil A.; Young, Bruce E.; Rodrigues, Ana S. L.; Fischman, Debra L.; Waller, Robert W. (2004). "Status and trends of amphibian declines and extinctions worldwide". Science. 306 (5702): 1783–1786. S2CID 86238651.
- Stuart, S. N.; Hoffmann, M.; Chanson, J. S.; Cox, N. A.; Berridge, R. J.; Ramani, P.; Young, B. E., eds. (2008). Threatened Amphibians of the World. Published by ISBN 978-84-96553-41-5. Archived from the originalon October 30, 2014. Retrieved October 30, 2014.
External links
- Amphibians – AnimalSpot.net
- ArchéoZooThèque : Amphibians skeletons drawings : available in vector, image and PDF formats
- Amphibian Specialist Group
- Amphibian Ark
- AmphibiaWeb
- Global Amphibian Assessment Archived April 12, 2011, at the Wayback Machine
- Amphibian vocalisations on Archival Sound Recordings Archived May 26, 2010, at the Wayback Machine