Biology of salamanders
Salamanders in North America
Salamanders and humans
Resources
Salamanders and newts are about 500 species of aquatic or amphibious animals in the order Caudata (sometimes known as the Urodela). Salamanders have an ancient fossil lineage, extending back to the Upper Jurassic period, more than 140 million years ago.
Like other amphibians, salamanders have a complex life cycle, the stages of which are egg, larva, and adult. The morphology, physiology, and ecology of salamanders in their different stages are very different, and the transitional process involves a complex metamorphosis.
Salamanders are most abundant in the temperate regions of the Northern Hemisphere, with fewer species occurring elsewhere. The greatest number of species of salamanders occurs in the eastern United States, and to a lesser degree in eastern China.
Biology of salamanders
Species of salamanders display a wide range of body plans and life histories. The smallest salamander is an unnamed species of Thorius from Mexico; mature males of which have a total body length of only 1 in (2.5 cm). The world’s largest living salamanders can be as long as 5.5 ft (1.7 m). These are the giant Asiatic salamanders, Andrias davidianus and A. japonicus, which can achieve body weights of 88 lb (40 kg) or more. One individual giant Japanese salamander (A. japoni-cus) lived for an extraordinary 55 years in captivity.
Adult salamanders have four relatively small, similar-sized walking legs, and a long tail. The skeletal structure of living salamanders is relatively little modified from their geologically ancient relatives, and among the tetrapod vertebrates is considered to be relatively primitive.
Some species of salamanders have lost key elements of the skeleton during their evolution. For example, species within the salamander family Sirenidae have lost their limbs, and are eel-like in appearance. Remarkably, the numbers and shapes of limb bones are not necessarily the same within some salamander species, as is the case of the red-backed salamander (Plethodon cinereus) of North America. Within the same population, individuals of this species can have varying numbers of limb bones, and these structures can vary significantly in size, shape, and degree of calcification.
Salamanders have a protrusile tongue, used for feeding and sensory purposes. Many species are very brightly colored, usually to warn predators of the poisonous nature of the skin of these animals. Some salamanders secrete a chemical known as tetrodotoxin from their skin glands. This is one of the most poisonous substances known, and it can easily kill predators that are intolerant of the chemical, as most are.
Salamanders vary greatly in their reproductive biology. They typically have internal fertilization, meaning the ova are fertilized by male sperm within the reproductive tract of the female. During the breeding season, male salamanders of many species deposit packets of sperm, known as spermatophores, on the surface of aquatic sediment or debris. The male salamander then manipulates a female to pass over the spermatophores, which are picked up by the slightly prehensile lips of her cloaca, and stored in a special, internal structure known as a spermatheca. The sperm then fertilize the ova as they are laid by the female, producing fertile zygotes. These are then laid as single eggs encased in a protective jelly, or sometimes as a larger egg mass that can contain several or many eggs within a jelly matrix.
Hatched larvae of typical salamanders look rather similar to the adults, but they are fully aquatic animals, with gill slits and external gills, a large head, teeth, a flattened tail used for swimming, and initially they lack legs. The metamorphosis to the adult form involves the loss of the external gills, the growth of
legs, and the development of internal lungs, which, together with the moist skin of the body, act in the exchange of respiratory gases. Adult salamanders also have eyelids that can close.
Salamanders in the family Plethodontidae show direct development. For example, the aquatic larval state of the fully terrestrial red-backed salamander occurs within the egg. What hatches from the egg is a miniature replica of the adult salamander. The red-backed salamander lacks lungs, so that all gas exchange occurs across the moist skin of the body and mouth.
The female of the European salamander, Salamandra atra, retains the eggs within her body. There they develop through the larval stage, so that the young are born as miniature adults.
Some salamanders do not have a terrestrial adult stage, and become sexually mature even though they still retain many characteristics of the larval stage. This phenomenon is known as neoteny, and occurs in species such as the mudpuppy (Necturus maculosus) of central and eastern North America. Neoteny also occurs in the axolotyl (Ambystoma mexicanum), a rare species found in Mexico, in which the breeding adults have external gills, a large head, a flattened tail, and other typically larval traits. The axolotyl is a common species in laboratories where developmental biology is studied, and sometimes individuals of this species will undergo metamorphosis and develop more typical, adult characteristics. Often, particular populations of other species in the genus Ambystoma will display neoteny, for example the tiger salamander (A. tigrinum), common in small lakes and ponds over much of North America.
The red-spotted newt (Notophthalmus viridescens) of North America has two distinct, adult stages. The stage that follows from transformation of the aquatic larva is known as the red eft. This is a bright-red colored, adult form that wanders widely for several years in forests, especially on moist nights. The red eft eventually returns to an aquatic habitat, adopts a yellowish color, and becomes a breeding adult.
Salamanders with a terrestrial adult stage generally have a keen ability to home back to the vicinity of their natal or home pond. One study done in California found that red-bellied newts (Taricha rivularis) were capable of returning to their native stream over a distance of 5 mi (8 km), within only one year.
Salamanders in North America
Most of the 112 species of North America salamanders occur in the Appalachian region. In terms of species richness of salamanders, no other part of the Earth compares with Appalachia. However, salamanders also occur over most of the rest of North America, in moist habitats ranging from boreal to subtropical.
The mudpuppies and waterdogs are five species of aquatic, neotenous salamanders in the family Proteidae, occurring in eastern North America. The most widespread and abundant species is the mud-puppy (Necturus maculosus).
The hellbender (Cryptobranchus alleganiensis) is the only North American representative of the Cryptobranchidae, the family of giant salamanders. The hellbender is an impressively large animal, which can reach a body length of 2.5 ft (74 cm). Hellbenders live in streams and rivers. The hellbender is one of the relatively few salamanders that does not have internal fertilization of its eggs. The male hellbender deposits sperm over the ova after they are laid, so that external fertilization takes place.
Amphiumas (family Amphiumidae) are long, eel-like, aquatic creatures with tiny legs, that live in streams, swamps, and other wet places in the extreme southeastern United States. Amphiumas are vicious animals when disturbed, and can inflict a painful bite. The most widespread of the three North American species is the two-toed amphiuma (Amphiuma means) of Florida and parts of coastal Georgia and the Carolinas. The three-toed amphiuma (Amphiuma tridactylum) can achieve a body length of about 3 ft (1 m), and is the longest amphibian in North America.
Sirens (family Sirenidae) are also long and slender, aquatic salamanders. Sirens have diminutive fore-limbs, and they lack hind limbs. These animals are aquatic, and they retain gills and other larval characters as adults. Mating of sirens has not been observed, but it is believed that they have external fertilization. There are three species of sirens in North America, the most widespread of which is the lesser siren (Siren intermedia), occurring in the drainage of the Mississippi River and in the southeastern states. The greater siren (S. lacertina) of the southeastern coastal plain can be as long as 3 ft (95 cm).
The mole salamanders (family Ambystomidae) are terrestrial as adults, commonly burrowing into moist ground or rotting wood. The largest of the 9 North American species is the tiger salamander (Ambystoma tigrinum), measuring up to 12 in (30 cm) in length. This is a widespread species, occurring over most of the United States, parts of southern Canada, and into northern Mexico. Other relatively widespread mole salamanders are the spotted salamander (A. maculatum) of the eastern United States and southeastern Canada, the marbled salamander (A. opacum) of the southeastern states, and the blue-spotted salamander (A. laterale) of northeastern North America.
The four species of Pacific giant salamanders (family Dicamptodontidae) inhabit the Pacific Northwest from northern California to British Columbia. The California giant salamander (Dicamptodon ensatus) is a fairly large species, with a length of up to 12 in (30 cm). The four species of torrent salamanders (family Rhyacotritonidae) also occur in the Pacific Northwest from northern California to Washington state. Their common name is misleading, since these small, yellowish green salamanders are most often found in seeps, springs, or gravelly habitats beside fast-moving streams.
There are at least 77 species of lungless salamanders (family Plethodontidae) in North America. The red-backed salamander (Plethodon cinereus) is a common and widespread species in the northeastern United States and southeastern Canada. The ensatina salamander (Ensatina eschscholtzi) occurs in subalpine conifer forests of the humid west coast.
There are six species of newts (family Salamandridae) in North America. The eastern newt (Notophthalmus viridescens) is widespread in the east. Initially transformed adults usually leave their natal pond to wander in moist forests for several years as the red-eft stage. The eft eventually returns to an aquatic habitat where it transforms into a sexually mature adult, and it spends the rest of its life in this stage. Some races of eastern newts do not have the red eft stage. The most widespread of the western newts is the rough-skinned newt (Taricha granulosa), occurring in or near various types of still-water aquatic habitats of the humid west coast.
Salamanders and humans
Other than a few species that are sometimes kept as unusual pets, salamanders have little direct economic value. However, salamanders are ecologically important in some natural communities, in part because they are productive animals that may be fed upon by a wide range of other animals. In addition, salamanders are interesting creatures, with great intrinsic value.
Considering these direct and indirect values of salamanders, it is very unfortunate that so many species are threatened by population declines, and even extinction. The most important threat to salamanders is the conversion of their natural habitats, such as mature forests, into other types of ecosystems, such as agricultural fields, residential developments, and clear-cuts and other types of harvested forests. These converted ecosystems do not provide adequate habitat for many species of salamanders, and sometimes for none at all. It is critically important that a sufficient area of natural forest and other native habitat types be provided to sustain populations of species of salamanders, and other native wild life.
On January 31, 2000, the U.S. Fish and Wildlife Service, Division of Endangered Species, listed the following 11 species of North American salamanders as being endangered:
- Barton Springs salamander (Eurycea sosorum). First listed: April 30, 1997. Historic range: Texas
- Cheat Mountain salamander (Plethodon nettingi). First listed: August 18, 1989. Historic range: West Virginia
- California tiger salamander [Ambystoma califor-niense (A. tigrinum c.)]. First listed: January 19, 2000. Historic range: California
- Desert slender salamander (Batrachoseps aridus). First listed: June 4, 1973. Historic range: California
- Flatwoods salamander (Ambystoma cingulatum). First listed: April 1, 1999. Historic range: Alabama, Florida, Georgia, South Carolina
- Red Hills salamander (Phaeognathus hubrichti). First listed: December 3, 1976. Historic range: Alabama
- San Marcos salamander (Eurycea nana). First listed: July 14, 1980. Historic range: Texas
- Santa Cruz long-toed salamander (Ambystoma mac-rodactylum croceum). First listed: March 11, 1967. Historic range: California
- Shenandoah salamander (Plethodon shenandoah). First listed: August 18,1989. Historic range: Virginia
- Sonoran tiger salamander (Ambystoma tigrinum steb-binsi). First listed: January 6, 1997. Historic range: Arizona, Mexico
KEY TERMS
Complex life cycle— A life marked by several radical transformations in anatomy, physiology, and ecology.
Neoteny— The retardation of typical development processes, so that sexual maturity occurs in animals that retain many juvenile characteristics.
- Texas Blind Salamander (Typhlomolge rathbuni). First listed: March 11, 1967. Historic range: Texas
Resources
BOOKS
Conant, Roger, et al. A Field Guide to Reptiles and Amphibians of Eastern and Central North America. Boston: Houghton Mifflin, 1998.
Halliday, T.R., and K. Adler. The New Encyclopedia of Reptiles and Amphibians. New York: Oxford University Press, 2002.
Harris, C. Leon. Concepts in Zoology. 2nd ed. New York: HarperCollins, 1996.
Hofrichter, Robert. Amphibians: The World of Frogs, Toads, Salamanders and Newts. Toronto: Firefly Books, 2000.
Petranka, James W. Salamanders of the United States and Canada. Washington, DC: Smithsonian Institution Press, 1998.
Zug, George R., Laurie J. Vitt, and Janalee P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. 2nd ed. New York: Academic Press, 2001.
PERIODICALS
Petranka, James W. “Effectiveness of Removal Sampling for Determining Salamander Dens.” Journal of Herpetology 35, no.1 (2001): 36-44.
Taylor, Barbara E., David E. Scott, and J. Whitfield Gibbons. “Catastrophic Reproductive Failure, Terrestrial Survival, and Persistence of the Marbled Salamander.” Conservation Biology 20 (June 2006): 792-801.
Bill Freedman Randall Frost
