Mole salamanders

(Ambystomatidae)

Class Amphibia

Order Caudata

Suborder Salamandroidea

Family Ambystomatidae

Thumbnail description
These are medium to large, stocky salamanders, generally with both aquatic larval and terrestrial metamorphosed stages; often boldly patterned as adults, with well-developed costal grooves (successive vertical grooves on the sides of the body)

Size
3.5–13.8 in (90–350 mm) in total length

Number of genera, species
1 genus; 33 species

Habitat
Ambystomatids inhabit woodlands and grasslands, including semi-arid pine and juniper woodland with vernal pools, ponds, or occasionally streams for breeding; they are absent from arid deserts within their range

Conservation status
Critically Endangered: 1 species; Vulnerable: 3 species

Distribution
North America from southern Canada to the mountains bordering the Mexican Plateau in central Mexico

Evolution and systematics

Ambystomatids have had a complex taxonomic history, with up to three subfamilies and six genera recognized. The current understanding of the family removes both Dicamptodon and Rhyacotriton from the ambystomatids and subsumes the old generic names Rhyacosiredon, Bathysiredon, and Siredon into the single genus Ambystoma. All species that occur in Mexico (about half of the total in the genus) are close relatives of the tiger salamander (Ambystoma tigrinum) and generally are placed in the tiger salamander complex based on molecular and morphologic evidence. Amazingly, these Mexican species originally were placed in four genera, reflecting their diverse ecological and morphologic characteristics. Molecular and fossil evidence suggest that the members of the tiger salamander complex are a recently derived adaptive radiation of salamanders. However, the other members of the family are an old, deeply differentiated set of species that have a fossil record extending from the Lower Oligocene (30 million years ago) through the Pleistocene. No subfamilies are recognized.

Physical characteristics

Ambystomatids are small to large heavy-bodied salamanders with broad heads, small protuberant eyes, well-developed costal grooves, and a long, laterally flattened tail. Many species are brightly colored as metamorphosed adults, with yellow, orange, or silver spots, bars, and frosted patterns on a black background. In some species large poison glands are found on the head and along the body, and transformed adults of all species are distasteful to predators. Lungs are present in all metamorphosed animals. The tiger salamander, at 13.8 in (350 mm) in total length, is one of the largest terrestrial salamanders in the world.

All ambystomatids also have an aquatic larval stage, characterized by filamentous external gills, a large tail-body fin, and small eyes that lack moveable eyelids. In many species of the tiger salamander complex in Mexico and the United States, metamorphosis from the larval stage never occurs, and adults breed and remain in the larval phase throughout their lives. This fascinating life history pattern goes by many names, but we use

the term paedomorphosis. Some Mexican species are obligatorily paedomorphic (individuals never go through a full metamorphosis in nature), whereas others from Mexico and the United States are facultative paedomorphs, with some individuals metamorphosing and others failing to do so. The most famous case of an obligate paedomorph is the Mexican axolotl, Ambystoma mexicanum. Recent genetic work suggests that metamorphosis is controlled by one or a few genes and that paedomorphosis has evolved several times within the family. Facultative paedomorphosis also is seen in A. gracile and A. talpoideum.

Distribution

Ambystomatids are distributed from southern Canada south through the Mexican Plateau to just south of Mexico City. Within that range, they occur in most habitats, except the arid deserts of the Great Basin, the southwestern United States, and the central deserts of the Mexican Plateau.

Habitat

Ambystomatid salamanders occur in two primary kinds of habitat. The tiger salamander complex (18 species currently recognized) represent grassland species that shun woodlands but can survive in relatively dry grassland habitat. Paedomorphic species also are found in large, permanent lakes in central Mexico, as long as they are free of predatory fish. The remaining members of the family (15 species) are woodland species found primarily in the eastern and central United States and Canada (13 species), with two representatives restricted to western North America (A. gracile and A. macrodactylum). Most species spend the vast majority of the year in underground rodent burrows and emerge only on rainy nights to feed or migrate to breeding sites, where they may remain for several weeks.

Behavior

Two key types of behavior of ambystomatid salamanders are their antipredator responses and their migratory movements. Many metamorphosed ambystomatids assume characteristic defensive postures and actions when confronted by predators, including the head-down stance of A. talpoideum and A. gracile and tail lashing (seen in most transforming species). Both behaviors present a potential predator with parts of the body that are heavily laden with poison-secreting glands.

Ambystomatid salamanders are also famous for their migration to breeding ponds. In some species, literally hundreds of animals may migrate on a single rainy night to a breeding site, presenting a spectacular display of salamanders crossing the landscape. In other species, migrations are more protracted and may take many weeks to complete. Generally, males migrate before females and remain in the pond longer.

Feeding ecology and diet

Like all salamanders, ambystomatids are strictly predators both as larvae and as adults. They consume a wide variety of invertebrate and vertebrate prey, ranging from insects, earthworms, and crustaceans to frog tadpoles and even baby rodents.

Reproductive biology

Most ambystomatids breed in the winter or spring, although montane forms breed in the summer. Terrestrial adults move into vernal pools, ponds, or, more rarely, streams to breed; two species mate and lay eggs on land. Courtship is relatively simple, with males often competing for the opportunity to mate with females. Sperm transfer is via a spermatophore, a packet of sperm on a cone-shaped protienaceous base that is deposited on the substrate and picked up by a female. One male may deposit more than 30 spermatophores during a single night. Inseminated females lay their eggs either singly, attached to the pond bottom or vegetation, or in large clusters. Larvae spend several months to several years in the water before metamorphosing and assuming a terrestrial lifestyle.

Conservation status

The IUCN lists one species (Ambystoma lermaense) as Critically Endangered and three species (A. californiense, A. cingulatum, and A. mexicanum) as Vulnerable. In addition, many other Mexican species are suspected of being Endangered. Primary threats are terrestrial and aquatic habitat loss, introduced predatory fish that consume the larvae, and, possibly, a chitrid fungal disease. Paedomorphic species are particularly susceptible, because they often occur in a single lake, where introduced fish, pollution, or draining can endanger an entire species.

Significance to humans

Although they are consumed widely in Mexico, ambystomatids are generally of little direct significance to humans. Like many amphibians, they are considered an important indicator of overall environmental quality.

Species accounts

List of Species

Flatwoods salamander

Ambystoma cingulatum

taxonomy

Ambystoma cingulatum Cope, 1868, Grahamville, South Carolina, United States. Some authors recognize two subspecies, and others recognize none.

other common names

English: Reticulated salamander.

physical characteristics

This small species grows to only 5.3 in (135 mm) in length. The head is relatively narrow. The flatwoods salamander is dark gray to black, with grayish or silvery lines or flecks that form a reticulate or frosted pattern on the back.

distribution

This salamander is restricted to the southeastern United States, where it ranges across northern Florida and southern Alabama east through southern Georgia and the extreme southern part of South Carolina.

habitat

The flatwoods salamander inhabits seasonally wet pine flat-woods with vernal pools. Originally it was associated with a unique community of longleaf pine/wire grass, but much of this habitat is now replaced by slash pine plantations.

behavior

This species spends most of the year in underground crayfish burrows or tunnels left by dead roots. The larval period lasts about four months, from about January to April.

feeding ecology and diet

Larvae of this species feed on small zooplankton and other invertebrates. After metamorphosis, adults spend most of their lives in underground burrows, where they feed on earthworms and small invertebrates.

reproductive biology

This is one of two species of Ambystoma (the other is A. opacum) that court and lay eggs on land. Females lay eggs in dry pond bottoms; the embryos develop and hatch when the ponds fill after heavy rains. This strategy is thought to give young larvae a head start over potential predators that might exclude them from ponds if they hatched later in the season.

conservation status

The flatwoods salamander is listed as Vulnerable by the IUCN and as Endangered under the U.S. Endangered Species Act. It is extirpated from Alabama, and its remaining stronghold is in Florida.

significance to humans

None known.

Northwestern salamander

Ambystoma gracile

taxonomy

Siredon gracilis Baird, 1859, Cascade Mountains, near latitude 44° north, Oregon, United States. Two subspecies generally are recognized, although scientific evidence for this is weak.

other common names

None known.

physical characteristics

These fairly large salamanders grow to 8.7 in (220 mm) in length. They are uniformly brown or black and breed both as metamorphs and paedomorphs. Metamorphosed adults have extensive poison glands in the parotoid region and on the base of the tail; when disturbed, they often secrete a white, sticky, toxic secretion.

distribution

The northwestern salamander occurs in wet fir and redwood forests of northwestern North America from Sonoma County, California, United States, to British Columbia, Canada.

habitat

Paedomorphs of this species are most common in permanent lakes at higher elevation, whereas metamorphs tend to occur in conifer forests at lower elevations. Unlike many other ambystomatids, paedomorphs can coexist with predatory fish by shifting their activity patterns and becoming nocturnal.

behavior

Individuals of this species spend most of their adult life in underground burrows, although they may be found on the surface during rains. When these salamanders are disturbed, they assume a rigid posture with the tail partially raised and secrete a white toxic liquid from the parotoid region of the head and from the upper ridge of the tail.

feeding ecology and diet

Larval northwestern salamanders feed on zooplankton, a wide variety of aquatic invertebrates, and frog tadpoles; larger individuals take larger prey. Terrestrial adults presumably feed on earthworms and other invertebrates.

reproductive biology

Reproductive maturity is reached in two to several years, depending on elevation. Populations vary in terms of metamorphosis/paedomorphosis; it is not known whether the two types interbreed.

conservation status

No obvious declines have been documented, although there may be evidence that populations are reduced in logged or secondary-growth forests.

significance to humans

None known.

Mexican axolotl

Ambystoma mexicanum

taxonomy

Gyrinus mexicanus Shaw, 1789, Mexico. Ambystoma mexicanum is a member of the tiger salamander complex. It was known for many years as Siredon mexicanum.

other common names

German: Axolotl; Spanish: Ajolote.

physical characteristics

This is a large paedomorphic species that occasionally metamorphoses in captivity but apparently is an obligate paedomorph in the wild. Adults are dark brown, often with faint black reticulations. Captive strains are available in a variety of colors, including white, gold, black, and albino.

distribution

This species is known only from Lake Xochimilco (the "Floating Gardens") and associated canals and springs immediately southeast of Mexico City, Mexico. This area has been highly modified by human activities for centuries, but the salamanders remain in moderate numbers.

habitat

Because it is an obligate paedomorph, this species occurs only in permanent aquatic habitats. In the vicinity of Lake Xochimilco, it is found frequently near vegetation.

behavior

This species has not been well studied in the wild. These salamanders apparently feed and grow throughout the year and are able to coexist with introduced carp and other fish.

feeding ecology and diet

Like many other members of the tiger salamander complex in Mexico, the axolotl was probably the top aquatic predator in its habitat before fish were introduced. They feed on insects, snails, worms, tadpoles, and small fish.

reproductive biology

In the wild, most Mexican members of the tiger salamander complex, including the Mexican axolotl, breed between November and February. In captivity they breed during most months of the year, except June, July, and August.

conservation status

Listed as Vulnerable by the IUCN, the axolotl is protected under CITES from international trade and is protected in Mexico. This species is the best-studied species of salamander in the world and has been a "model system" in developmental biology for well over 100 years. Although it is endangered in the wild, the axolotl is commonly reared in captivity for the scientific and pet trades. Salamanders with albino, gold, and wild-type color patterns are frequently sold in the pet trade.

significance to humans

The axolotl was an important species to the Aztec cultures that were centered in the valley of Mexico. All of the paedomorphic Mexican species of ambystomatids continue to be exploited locally for food and medicine in central Mexico, and the axolotl is an important study system for developmental biology throughout the world.

Tiger salamander

Ambystoma tigrinum

taxonomy

Salamandra tigrina Green, 1825, Near Moore's town (Moorestown), New Jersey, United States. Up to six subspecies are recognized in the United States. Formerly, the California tiger salamander, Ambystoma californiense, was considered a subspecies of A. tigrinum. Considerable debate remains over whether the species should be considered one or several species.

other common names

English: Mud puppy, water dog; French: Ambystome tigré German: Tigerquerzahnmolch; Spanish: Salamandra tigre.

physical characteristics

This is a large, robust species that grows to 13.8 in (350 mm) in length. Adults vary widely in color pattern, from black with bright yellow spots and bars or indistinct yellow flecks and reticulations to pure brown or black. In the central United States and Rocky Mountains, tiger salamanders may breed as paedomorphs or metamorphs; elsewhere (including A. californiense) they always metamorphose.

distribution

The tiger salamander is the most widely distributed salamander in North America, ranging from southern Canada south, roughly to the border of Mexico and the United States. It is absent from the Appalachian Mountains, the northeastern United States, parts of the southern United States, and the Great Basin, including the Mojave Desert. In California it is replaced by the California tiger salamander, A. californiense.

habitat

Primarily a grassland-associated species, the tiger salamander is found in prairie and open, dry woodland habitats. It ranges from sea level to an elevation of more than 11,000 ft (3350 m).

behavior

Adults of this species spend virtually all of their lives in underground rodent burrows. They emerge and migrate to breeding ponds during spring rains and sometimes can be found on the surface at night during heavy rains.

feeding ecology and diet

A fearsome predator, the tiger salamander is a feeding generalist. As larvae, they eat prey ranging from tiny zooplankton to tadpoles and even each other. In captivity they consume prey almost as large as they are. On land they eat all kinds of invertebrates and small vertebrate prey.

reproductive biology

Like many other members of the tiger salamander complex and the northwestern salamander group, many tiger salamander populations vary in terms of metamorphosis, with both metamorphs and paedomorphs coexisting in permanent or semipermanent water bodies.

conservation status

In many areas tiger salamanders are abundant and under no obvious threats, whereas in other regions they are Endangered. The Sonoran tiger salamander, A. t. stebbinsi is listed as Endangered under the U.S. Endangered Species Act, and the related California tiger salamander (A. californiense) is listed as Vulnerable by the IUCN. Across its range the tiger salamander cannot coexist with predatory fish, and the introduction of bass, catfish, and other species poses a threat to these salamanders.

significance to humans

Larval tiger salamanders are often the top predator in the vernal pools and ponds where they live, and they are therefore an important part of many aquatic ecosystems. In many parts of the United States, tiger salamander larvae are commercially valuable as fish bait. Particularly in the southwestern United States, these "water dogs" are sold in large numbers.

Resources

Books

Bishop, Sherman C. Handbook of Salamanders: The Salamanders of the United States, of Canada, and of Lower California. Ithaca, NY: Comstock, 1994.

Duellman, William E., and Linda Trueb. Biology of Amphibians. New York: McGraw-Hill Book Co., 1986.

Frost, Darrel R. Amphibian Species of the World: A Taxonomic and Geographical Reference. Lawrence, KS: Allen Press and Association of Systematics Collections, 1985.

Petranka, James W. Salamanders of the United States and Canada. Washington, DC: Smithsonian Institution Press, 1998.

Shaffer, H. Bradley. "Ambystoma gracile." In Status and Conservation of U.S. Amphibians, edited by M. J. Lannoo. Vol. 2, Species Accounts. Berkeley, CA: University of California Press, 2001.

Periodicals

Brodie Jr., E. D., and L. S. Gibson. "Defensive Behavior and Skin Glands of the Northwestern Salamander, Ambystoma gracile." Herpetologica 25 (1969): 187–194.

Collins, J. P., J. B. Mitton, and B. A. Pierce. "Ambystoma tigrinum: A Multispecies Conglomerate?" Copeia no. 4 (1980): 938–941.

Eagleson, G. W. "A Comparison of the Life Histories and Growth Patterns of Populations of the Salamander Ambystoma gracile (Baird) from Permanent Low-Altitude and Montane Lakes." Canadian Journal of Zoology 54 (1976): 2098–2111.

Means, D. Bruce, John G. Palis, and Mary Baggett. "Effects of Slash Pine Silviculture on a Florida Population of Flatwoods Salamander." Conservation Biology 10, no. 2 (1996): 426–437.

Shaffer, H. Bradley. "Natural History, Ecology, and Evolution of the Mexican 'Axolotls.'" Axolotl Newsletter 18 (1989): 5–11.

——. "Systematics of Model Organisms: The Laboratory Axolotl, Ambystoma mexicanum." Systematic Biology 42, no. 4 (1993): 508–522.

——, and M. L. McKnight. "The Polytypic Species Revisited: Genetic Differentiation and Molecular Phylogenetics of the Tiger Salamander, Ambystoma tigrinum (Amphibia: Caudata), Complex." Evolution 50, no. 1 (1996): 417–433.

Taylor, J. "Orientation and Flight Behavior of a Neotenic Salamander (Ambystoma gracile) in Oregon." American Midland Naturalist 109 (1983): 40–49.

Titus, T. A. "Genetic Variation in Two Subspecies of Ambystoma gracile Baird (Caudata: Ambystomatidae)." Journal of Herpetology 24, no. 1 (1990): 107–111.

H. Bradley Shaffer, PhD