Amphibians

The term amphibian is derived from the Greek word amphibios, which means “to live two lives.” Most amphibians spend the first part of their lives as aquatic, gill-breathing larvae and then transform into terrestrial adults. The larval stage can be as short as a few weeks or as long as several years. Completion of the larval stage is triggered by hormonal events that initiate some dramatic developmental processes, collectively termed metamorphosis.

As adults, most amphibians seek out aquatic environments in which to deposit their eggs. These can range from fast-flowing mountain streams to ephemeral roadside ditches. Most male frogs have species-specific mating calls that serve both to attract females and to prevent interbreeding. Frogs reproduce by external fertilization: the male typically grasps the female and encourages her to deposit her eggs, which he promptly fertilizes. Normally, both parents abandon the eggs, but some variations of this pattern exist. In contrast, most salamanders practice internal fertilization, accomplished after the male has performed a type of species-specific courtship dance that culminates with the deposition of a packet of sperm cells, called a spermatophore. The female squats on the spermatophore, transferring the spermatozoa to a specialized holding structure called a spermatheca. The spermatozoa can be used to fertilize her eggs up to several months after mating.

Evolution of Amphibians

Amphibians were the first vertebrates to possess adaptations that allowed them to spend considerable periods of time out of the water. The earliest fossil amphibians, members of the order Ichthyostegalia, appear in the geologic record during the Devonian period, about 320 million years ago. Among experts, there is a general consensus that the ancestor of the amphibians is to be found in the primitive, lobe-finned fish (class Osteichthyes, subclass Sarcopterygii). This conclusion is based on a detailed analysis of the comparative anatomy of hard body parts that fossilized, such as the vertebrae, shoulder girdles, teeth, and skulls. Characteristics that the first amphibians shared with their fish ancestors include internal nasal openings (nares); a strange, hinged skull; and a distinctive tooth structure, in which the enamel is folded into intricate patterns.

The environmental conditions that led to the abandonment of aquatic habitats in favor of a more terrestrial existence remain a major topic of discussion. One scenario envisions the early amphibian ancestors in an environment that was gradually becoming more and more arid. To survive, it would have been advantageous to be able to crawl on land for short distances to escape drying pools in favor of more permanent bodies of water. Those that could migrate would have survived in higher numbers than those that lacked this adaptation. A second scenario suggests that heavy predation pressures from the jawed, carnivorous fish that are known to have been abundant in the shallow freshwater lakes of the time may have been selected for individuals that could leave the water, even if only briefly. A third scenario depicts competition for food as strong in the aquatic environments and much weaker on land, where several groups of invertebrates were known to be abundant.

Taxonomy of Fossil Amphibians

The taxonomy of fossil amphibians is confusing because of a series of problems. First, on the whole, the range of morphological variations within the group has been conservative. Striking differences of taxonomic value (which would have to fossilize) are not numerous. Second, many of the skeletal elements that are important in determining the relationships of living amphibians are composed of cartilage rather than bone, and this material does not fossilize well. Because of this limitation, bony elements such as the vertebrae and skull have played a major role in determining amphibian phylogeny. The molecular techniques that have greatly assisted modern taxonomists, such as electrophoresis, immunology, karyotyping, and deoxyribonucleic acid (DNA) sequencing, cannot be used on fossilized materials.

The class Amphibia is further divided into orders. The number of orders recognized by various authorities ranges from eight to thirteen. All but three of these are extinct. One extinct order is Ichthyostegalia, which includes the earliest recorded fossil amphibians. Most were small, with elongated bodies and weakly developed limbs. Many were almost assuredly aquatic, but at least some were capable of spending extended periods of time out of the water. By the time of the Devonian period, when they first appear in the fossil record, they were already diverse, with several different genera and species present.

Another extinct group is the order Temnospondyli, which was abundant during the late Permian period and persisted until the end of the Triassic period. Most in this order were of moderate size (0.5 to 1.0 meter in body length), with low, stout profiles and flattened skulls. Some were highly aquatic and had short, weak appendages. One group, the clade Trematosauria, was marine, and they are apparently the only amphibians to have been successful at invading the oceans.

The order Anthracosauria appears in the fossil record during the Carboniferous period and was extinct by the end of the Permian. Common names of members of this group include the seymouriamorphs and the embolomeres. This order contains a mixture of terrestrial and aquatic amphibians. From an evolutionary standpoint, Anthracosauria is important because it gave rise to the ancestors of reptiles.

Members of the order Aistopoda were eel-like, aquatic amphibians with elongated, limbless bodies. They are characterized by a large number of vertebrae—more than one hundred—that are clearly divisible into cervical, trunk, and caudal regions.

The order Nectridea consists of fully aquatic, salamander-like amphibians that persisted during the Carboniferous period. Appendages were weak or absent, and most fossils indicate body forms that were flattened dorsiventrally. They probably persisted by slowly crawling about the bottoms of ponds and lakes, where they preyed on unwitting animals that crossed their paths.

Amphibians of the order Microsauria were a diverse group of elongate, weak-limbed amphibians. Fossils of this group are fairly abundant in habitats that were swamplike during the Carboniferous period. The remarkable physical similarities between some microsaurs and some of the earliest reptiles are considered by most authorities to be the result of convergence of body form rather than an indication of a true evolutionary relationship.

The order Proanura consists of a single froglike fossil that dates from the Triassic period on the island of Madagascar. The skull is distinctly froglike, but a tail is present, and the hind limbs have not been modified for jumping.

The taxonomy of the living members of the class Amphibia is still under considerable debate. A central question revolves around whether all currently living amphibians are of monophyletic origin, meaning they derive from a single common ancestor, or whether they are of polyphyletic origin, meaning they arose independently from two or more separate stocks of fish. Based on the presence of unique features, such as pedicellate teeth, a distinctive part of the inner ear, and specialized eye receptors, most experts have concluded that all living amphibians are monophyletic and can be grouped in the superorder Lissamphibia. As recognized, living amphibians are placed in the orders Caudata, Anura, and Gymnophiona.

Salamanders

Salamanders are grouped in the order Caudata. They are distributed over temperate parts of Europe, Asia, and North and South America. There are approximately 550 recognized species, grouped into eight different families. Eastern North America has the greatest overall diversity, with seven of the eight described families represented there. One family, Plethodontidae, invaded South America and underwent a period of such tremendous speciation that approximately 60 percent of the living species of salamanders are members of this family.

Almost everyone is quick to recognize a frog based on its appearance, but the same cannot be said for salamanders. The families Proteidae, Cryptobranchidae, Sirenidae, and Amphiumidae are entirely aquatic. The family Cryptobranchidae is disjunctly distributed and occurs today only in eastern Asia and eastern North America. It contains the genus Andrias, which includes the largest living salamanders—one species, Andrias davidianus, has been known to reach lengths as great as 1.8 meters. Members of the family Proteidae are isolated in the twenty-first century in Europe and eastern North America. Commonly called “water dogs,” they are frequently dissected in comparative anatomy classes. Members of the families Sirenidae and Amphiumidae are restricted to the southeastern United States, where they are called sirens and amphiumas, respectively. Sirens have external gills and two front legs, while amphiumas have minute front and back legs and lack external gills.

The family Ambystomatidae is entirely North American in its distribution. Most species are highly secretive and are only encountered under objects or intercepted as they migrate to breeding ponds during spring rains.

The family Hynobildae is exclusively Asian in its present distribution. Reproduction in this family is considered primitive, in that the female lays eggs that are enclosed in a loose sac and are subsequently fertilized externally by the male.

The family Salamandridae is widely distributed in Europe, North Africa, Asia, and North America, with the greatest diversity in the Eastern Hemisphere. Many species have developed highly toxic skin secretions to protect themselves from predators. Many species advertise their toxicity by being very brightly and distinctly colored, a phenomenon known as aposematic coloration, while members of other, less toxic families also display these color patterns for protection from predators, a process called mimicry.

Members of the family Plethodontidae all share the unique feature of being lungless; respiration is accomplished by diffusion across their moist skins. Many species have abandoned laying eggs in water in favor of damp, terrestrial nests. Females guard the eggs until they hatch as miniatures of the adult, having completed their abbreviated metamorphosis while still in the egg.

Caecilians

The order Gymnophiona consists of a highly specialized group of wormlike, limbless amphibians called caecilians. They inhabit tropical regions of North and South America, Asia, and Africa. Most are terrestrial burrowers and are rarely observed. Some primitive forms have dermal scales embedded in their skin. All caecilians possess a unique sensory organ called a tentacle. Fertilization is internal, and male caecilians possess a copulatory organ that is derived from the cloaca. In the majority of caecilian species, the females retain their eggs in the oviduct and give birth to fully developed young. Fossil caecilians are extremely rare. In 2024, researchers reported that a species of caecilian had been discovered that produces a milk-like substance to feed its young in a process similar to that of a mammal.

Frogs

The order Anura is composed of tailless amphibians called frogs. Their hind legs are typically modified for jumping, their presacral vertebrae are usually eight in number, and their postsacral vertebrae are fused to form a coccyx. Frogs occur on all continents except Antarctica but reach their greatest diversity in the tropics of South America, Africa, and Asia. They have successfully invaded deserts, rivers, cold mountain streams, and arboreal vegetation. Several families have undergone tremendous adaptive radiation in the tropics, so that in the twenty-first century, almost 80 percent of living amphibians are anurans.

Families that are widely distributed include Bufonidae (toads), Hylidae (tree frogs), Microhylidae (mostly small frogs), and Ranidae (true frogs). These families are almost worldwide in their geographic distributions, although Australia lacks Bufonidae, Ranidae, and Microhylidae and Africa lacks Hylidae. Toads often have dry, warty skins containing numerous glands that produce noxious, protective secretions. Tree frogs have expanded disks on the tips of their toes that have allowed them to occupy arboreal habitats unavailable to many other families.

The families Leptodactylidae, Brachycephalidae, Rhinodermatidae, Pseudidae, Centrolenidae, and Dendrobatidae reach their greatest abundance in Central and South America. The leptodactylids are a diverse assemblage of nearly seven hundred species. Many of these lay eggs in specially constructed foam nests. Other species have taken this a step further and deposit their eggs in damp, terrestrial locations, thereby avoiding aquatic predators almost completely. The dendrobatids are often small, brightly colored frogs that have been given the common name of poison arrow frogs because of their extremely toxic skin secretions, which have been used by some Indigenous peoples to poison the tips of hunting arrows. Members of the family Pseudidae are unique in producing very large tadpoles that metamorphose into rather small frogs. The rhinodermatids consist of only two species, but one is unique in that the larvae do not feed and are carried in the mouth of the adult until they complete metamorphosis.

The Discoglossidae are found mostly in Europe, and Pelodytidae are native to southwestern Europe and western Asia. The midwife toad (Alytes obstetricians), a discoglossid, has an unusual reproductive mode: after fertilizing the eggs, the male cements them on his back and carries them to and from the water until they are ready to hatch.

The Rhacophoridae are a moderate-sized family of about 180 species, distributed over southern Africa and southeast Asia. Most members have expanded terminal digits, and some even have extensive webbing between their toes, which allows them to glide between arboreal perches. Diverse reproductive tactics occur in this family. Several species use water-filled tree holes in which to deposit their eggs.

The family Myobatrachidae is a diverse group of about ninety-nine species that occur in Australia and New Zealand. One species, Rheobatrachus silus, has a unique reproductive mode that includes brooding eggs in the stomach of the female.

The family Leiopelmatidae is a small group of frogs that are disjunctly distributed in western North America and New Zealand. The tailed frog (Ascaphus truei) is the only frog to possess an intromittent organ that is used to transfer sperm to the female for internal fertilization. This organ apparently evolved in response to the swift, cold streams in which the frog lives.

Bases of Characterization

As a group, amphibians are easier to characterize by the morphological features that they lack than by the unique characteristics that they possess. Missing are the scales that cover fish and reptiles (although these are not closely related structures), as well as the hair and feathers associated with mammals and birds. Amphibians’ skin is relatively thin and contains numerous glands. Large amounts of water can be lost or gained via the epidermis. In many forms, the skin serves as a major organ for respiration. Amphibians are ectothermic, which means they do not have internal physiological mechanisms for maintaining a constant body temperature. The circulatory system is closed, and the heart is composed of three chambers, two atria and one ventricle.

The taxonomic relationships of salamanders are based primarily on the arrangement of bones of the skull, which of these bones possess teeth, and the shape of the centrum of the vertebrae. Living forms are further compared by the manner of reproduction. In general, salamanders have been relatively conservative, and characteristics such as the number of chromosomes have not proved especially useful in determining phylogenetic relationships. However, modern molecular techniques, such as electrophoresis, immunology, and the use of restriction enzymes, are adding to the understanding of selected groups. With these techniques, it has been possible to show that several forms that were indistinguishable based on morphological data are in fact genetically isolated from one another and are actually distinct sibling species.

The taxonomic relationships of frogs are also based largely on differences in bony anatomy. Skull morphology, the shape of the vertebral centrum and its manner of development, and the arrangement of the bones that make up the pectoral girdle are important diagnostic characteristics. In living forms, molecular techniques are also shedding new light on relationships. The number of chromosomes is more variable and has more value as a diagnostic tool in frogs than in salamanders. The morphology of the larvae is another important taxonomic tool.

The Importance of Amphibians

There are about eight thousand recognized species of living amphibians. This number represents only a small fraction of the number of species that have been present on the earth over the past 350 million years. In many habitats, amphibians still represent a major portion of the biomass, and because ecologists often relate a group’s “worth” to its biomass, amphibians can be considered major members of many terrestrial communities, often serving as keystone species for their ecological niches. Amphibians often exhibit traits such as low mobility, fidelity to breeding sites, and species-specific behaviors that are sought by ecologists and animal behaviorists for their studies. As a whole, salamanders and frogs represent some of the most thoroughly studied vertebrates.

Areas to which amphibians have contributed a significant portion of current knowledge include the evolution of mating systems, sexual selection, reproductive isolation mechanisms, niche partitioning, and community structure. Embryologists have long used amphibians to gain a basic understanding of complex development processes.

One area of extreme interest is the decline of many populations of frogs and salamanders over large geographic areas, notably in western North America. Loss of breeding sites through habitat modification, acid rain, and competition from exotic species have all contributed to their demise. Another likely cause is chytridiomycosis, an infectious and deadly skin disease caused by the chytrid fungus Batrachochytrium dendrobatidis. The disease was first discovered in frogs in Queensland, Australia, in 1993 and was first described by Lee Berger et al. in 1998. The specific fungus was identified by Joyce E. Longcore, Allan P. Pessier, and Donald K. Nichols in 1999.

Besides disease, climate change and habitat loss from agriculture, logging, and other forms of human activity were identified as major causes of amphibian population loss by the early twenty-first century. In 2023, researchers presented evidence in the journal Nature that found that of nearly every known species of amphibian on Earth, 41 percent were threatened with extinction in the short-term or long-term future. However, at the same time, about 155 new species of amphibians were being discovered each year.

Principal Terms

Adaptive Radiation: Rapid speciation that occurs as the result of a particular group being able to exploit a new resource

Convergence: A phenomenon in which two forms that are not closely related evolve structures that appear similar

Disjunct Distribution: A geographic distribution pattern in which two closely related groups are separated by large areas that are devoid of either group

Metamorphosis: The complex developmental process of morphological change in which larval amphibians are transformed into adults

Neoteny: The retention of larval features by adults, a process that has played a major role in the evolution of amphibians

Phylogeny: The determination of the evolutionary history of a particular group of organisms

Bibliography

"Amphibians: Different Types, Definition, Photos, and More." A-Z Animals, 28 Oct. 2022, a-z-animals.com/animals/amphibians. Accessed 2 July 2023.

Ballinger, Royce E., and John D. Lynch. How to Know the Amphibians and Reptiles. Brown, 1983.

Berger, Lee, et al. "Chytridiomycosis Causes Amphibian Mortality Associated with Population Declines in the Rain Forests of Australia and Central America." Proceedings of the National Academy of Sciences, vol. 95, no. 15, 1998, pp. 9031–9036. doi.org/10.1073/pnas.95.15.9031.

Brumfiel, Geoff. "Researchers Have Found an Amphibian that Makes Milk for Its Babies." NPR, 7 Mar. 2024, www.npr.org/2024/03/07/1236408925/researchers-have-found-an-amphibian-that-makes-milk-for-its-babies. Accessed 27 Mar. 2024.

Conant, Roger, and Joseph T. Collins. A Field Guide to Reptiles and Amphibians of Eastern and Central North America. 3rd ed. Houghton, 1998.

Duellman, William E. Amphibian Species of the World: Additions and Corrections. U of Kansas, 1993.

Duellman, William E., and Linda Trueb. Biology of Amphibians. 2nd ed. John Hopkins UP, 1994.

Frost, Darrel R. Amphibian Species of the World: A Taxonomic and Geographical Reference. Allen, 1985.

Green, David M., and Stanley K. Sessions, eds. Amphibian Cytogenetics and Evolution. Academic, 1991.

Heatwole, Harold, and John W. Wilkinson, eds. Amphibian Biology. vol. 11 pt. 3. Pelagic, 2015.

Longcore, Joyce E., Allan P. Pessier, and Donald K. Nichols. "Batrachochytrium dendrobatidis gen. et sp. nov., a Chytrid Pathogenic to Amphibians." Mycologia, vol. 91, no. 2, 1999, pp. 219–27. doi:10.2307/3761366.

Pough, F. Harvey, et al. Herpetology. 4th ed. Prentice, 2016.

Rott, Nathan. "Scientists Looked at Nearly Every Known Amphibian Type. They're Not Doing Great." NPR, 4 Oct. 2023, www.npr.org/2023/10/04/1203120401/scientists-looked-at-nearly-every-known-amphibian-type-theyre-not-doing-great. Accessed 27 Mar. 2024.

Vitt, Laurie J., and Janalee P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. 4th ed. Academic, 2014.

Voyles, Jamie, Erica B. Rosenblum, and Lee Berger. "Interactions between Batrachochytrium dendrobatidis and Its Amphibian Hosts: A Review of Pathogenesis and Immunity." Microbes and Infection, vol. 13, no. 1, 2011, pp. 25–32. doi.org/10.1016/j.micinf.2010.09.015.