Amphibians and reptiles are treated as a unit primarily because that usually is the way they are studied. Collectively, they are informally called herps, and those who study them are herpetologists. As expectable in the desert, the amphibian fauna is relatively depauperate, but the reptilian fauna is diverse. A checklist of amphibians and reptiles of the El Paso-Juarez Border Region is available on the Laboratory for Environmental Biology pages. More useful for information other than just presence are sections on the amphibians and the reptiles of the Chihuahuan Desert; many of these have photographs, maps of distribution, and some natural history information. Other information of various of the species is available on the Centennial Museum's Desert Diary pages on amphibians and reptiles.
Two orders of the class Amphibia occur: the Urodela (sometimes listed as the Caudata) and the Anura. The former are salamanders and the latter toads and frogs. The salamanders are represented by one species, Ambystoma tigrinum, the Tiger Salamander. As with the other desert amphibians, these salamanders need water for breeding; the eggs are laid in water and the larvae develop in water until they metamorphose into the adult form. Other than in the breeding season, these salamanders may move some distance from water, utilizing rodent burrows and other microhabitats to prevent desiccation.
The Anura is more diverse, with six families occurring in the desert region. The Bufonidae (true toads) has six species represented; the Hylidae (cricket frogs, tree frogs) has two; the Leptodactylidae (leptodactylid frogs), two; the Microhylidae (narrowmouth frogs), one; the Pelobatidae (spadefoot toads), three; and the Ranidae (true frogs), four. Photographs and basic information can be found at the Centennial Museum's Chihuahuan Desert Amphibian pages. The Animal Diversity Web has a wealth of information on a wide assortment of amphibians.
With a minimum of waterproofing, amphibians need to protect themselves from desiccation. Some anurans, such as members of the Ranidae, do so by remaining near water so they can easily plop into it when their skin begins to dry (or when predators show up). Dispersal of such species usually is limited to times of good summer rains. A number of the desert amphibians, though, rely on microhabitats and behaviors that minimize loss of water. The spadefoot toads, for example, spend a good part of their lives underground, usually coming to the surface only during the rainy season to feed and breed. Breeding is at ephemeral ponds and development may be very rapid, completing metamorphosis before the ponds become dry again.
The terrestrial toads form very loud breeding choruses about the edge of the temporary ponds; each species has its own distinctive call, allowing males and females of the same species to find each other (and to distinguish members of their own species in mixed choruses).
Two orders of reptiles occur in the Chihuahuan Desert: Testudines (turtles) and Squamata (lizards and snakes). Turtles are an ancient group, with most enclosed by an external shell except for varying proportions of shelless areas in the vicinity of the limbs and anterior and posterior parts of the body. The shell allows considerable, but not complete, protection from predators. The box turtles have the capacity to close the shell entirely. Most turtles are aquatic, which protects them from most terrestrial predators. Reliance on water, however, leaves them vulnerable to environmental changes that eradicate streams and ponds. As the Chihuahuan Desert water resources are overexploided, streams and ponds tend to go dry with disastrous results for turtles as for fishes and other aquatic organisms.
Most of the turtles are associated with water, but two are not. The Bolson Tortoise (Gopherus flavomarginatus) is one of four large tortoises of North America and is limited geographically to the Bolsón de Mapimí of the Chihuahuan Desert. The Western Box Turtle also is primarily terrestrial. However, a relative, the Coahuilan Box Turtle, limited to Cuatro Ciénegas, is not, and is the only box turtle that is primarily aquatic.
The aquatic turtles belong to three families: Emydidae (which also includes the terrestrial Western Box Turtle), Kinosternidae (the mud turtles), and the Trionychidae (softshelled turtles). The latter have the shell much reduced and covered with leathery tissue.
The order Squamata includes the snakes and lizards, which are divided into two suborders (Sauria and Serpentes, respectively). The difference is not the presence or absence of legs, as often assumed. Indeed, there are legless lizards. Differences in the skull include reductions in snakes beyond those of lizards along with (usually) an increase in the movability of many of the skull bones in snakes. This loose construction allows engulfing of prey larger than the normal head size of a snake. Snakes also lack eyelids—thus the "hypnotic gaze" so often attributed to them. Most lizards and all snakes are predators.
Seven families of lizards are recorded from the Chihuahuan Desert region (see the checklist). The geckos are of interest for (among other attributes) being able to cling to vertical (or even overhanging) surfaces. Only recently have scientists learned how this is done: not by sticky pads, but by millions of hair-like structures on the toes, each split into hundreds of tips, each of which is only about 200 billionth of a millimeter wide. This scale allows van der Waals forces (intermolecular attractions) to act; multiplied by the millions of "hairs", the individually weak attractions are easily sufficient to allow the gecko acrobatics.
The Gila Monster is one of only two poisonous lizards (the other being a close relative farther south in Mexico into northern Central America). It barely gets into the Chihuahuan Desert in the northwestern portion near the Arizona/New Mexico border; most of its range is in the Sonoran Desert.
The Teiidae is an interesting family of lizards in part because some species of the genus Cnemidophorus (our species are considered by some to be in the genus Aspidoscelis) consist solely of females. The eggs develop parthenogenetically (without fertilization). See the Desert Diary account of the Whiptail Lizards.
The suborder Serpentes in the Chihuahuan Desert has about three-quarters of its species within the very successful family Colubridae. These are generally harmless snakes (though capable of drawing blood with sharp teeth). A few are rear-fanged snakes that have grooved teeth toward the back of the jaws that help guide venom into their prey; they are not considered dangerous to humans. Many people seem to think that every snake is deadly, and the desert has a number of venomous species capable of harming humans; such snakes are in the minority, however.
The family Elapidae is represented by the coral snakes; the cobras of the Old World belong to the same family. Two species barely reach into the margins of the Chihuahuan Desert, the Eastern Coral Snake on the eastern fringes, the Western Coral Snake in the northwest where the desert extends toward the Sonoran Desert. These snakes are relatively small, not equipped with the large fangs of the pit vipers. They also tend to be shy and reclusive, seldom seen and seldom aggressive. However, their venom includes a powerful neurotoxin and a bite should be considered as life-threatening and immediate medical aid sought.
The more familiar and more often encountered venomous snakes in the region are pit vipers, family Viperidae. The family in the Chihuahuan Desert includes the Copperhead,
Massasauga,
and five rattlesnakes (the Massasauga usually is
considered a rattlesnake, also). The name comes from the presence of a pit between the eyes and nostrils that contains receptors for infrared radiation (heat). Warm-blooded prey can be quite
accurately located at close range in complete darkness by this organ. The venom is primarily for capture of prey. The animal is struck by fangs that act as hypodermic needles, injecting venom into
the prey. The usual result is the prey animal leaving the scene, only to be overcome by the venom at some point. The snake tracks the victim by olfaction, hanging back until the prey is overcome
and unlikely to inflict damage to the snake. The tongue is used as in olfaction, picking up scent molecules from the air and inserting the forked end of the tongue into a sensory structure on the
roof of the mouth (Jacobson's organ). As with all snakes, prey items are swallowed whole.
Head of Crotalus molossus. The red rectangle surrounds the pit. The black structure to its right, coming out of the front of the head, is the tongue "tasting" the air. Photograph by A.H. Harris.
Although the death rate by rattlesnake bites is low, death can occur, as can tissue damage potentially causing loss of digits. Medical attention should be sought. Treatment is usually by anti-venom if symptoms indicate that venom has entered the body (some percentage of strikes are without venom injection, but assumptions should not be made).
Last Update: 26 Jun 2006