MOLE-LIMBED WORMLIZARDS: Bipedidae

PHYSICAL CHARACTERISTICS

The three species of mole-limbed wormlizards in this family are sometimes confused with earthworms, but they have scales and front legs. They are one of four families that fall into the group known as wormlizards or amphisbaenians (am-fizz-BAY-nee-ens). In all amphisbaenians, small rectangular scales form circular rings around their long thin bodies. A worm has rings around its body, too, but it has no scales and lacks most of the other features of wormlizards. Mole-limbed wormlizards, like other amphisbaenians, have one large tooth in the middle of the upper jaw, a thick and strong skull, small and sometimes invisible eyes, and a forked tongue. They do not, however, have ear holes or eyelids, like most lizards do. The mole-limbed wormlizards are different from other wormlizards, because they have a pair of small but strong front legs right behind the short rounded head. In addition, one of their clawed fingers has an extra bony piece, compared to the fingers of other reptiles. Mole-limbed wormlizards use their strong front legs, and probably this extra finger bone, to help them dig. Some scientists believe that, because the mole-limbed wormlizards have front legs, they are probably the most primitive of all the amphisbaenians. Other scientists disagree. These questions will no doubt continue, since no one has yet found a single fossil of any member of this family. Although mole-limbed wormlizards do not have hind legs, the skeleton still has some bits of hip bone and a tiny nub of thigh bone.

Mole-limbed wormlizards grow to 4.5 to 9.4 inches (11.5 to 24 centimeters) long and at the middle of the body are about 0.27 to 0.39 inches (7 to 10 millimeters) across. Only one-tenth to one-fifth of the body length is tail. The body is very bland-looking with no pattern and is colored pale pink, sometimes with a slightly orange tint. Individuals occasionally have a whiter belly. This animal sheds its skin (actually just the outer layer of skin) once in a while. When it sheds, the skin layer comes off in a single piece, just like it does in most snakes.

GEOGRAPHIC RANGE

The three species of mole-limbed wormlizard, or ajolote (ah-joe-LOW-tay) as they are often called, live in western Mexico. Depending on the species, they may make their homes in Baja California, Guerro, or Michoacán.

HABITAT

Mole-limbed wormlizards are found along the coast in deserts and dry shrubby areas, in dried streambeds, or in the shoreline soils of streams and rivers. They usually remain in their underground burrows but sometimes crawl above ground, especially at night.

WHAT DO THEY EAT?

In many cases, scientists learn about the diets of animals by watching them eat. When the animal eats at night or underground, however, their food habits can remain a mystery. This is true of the mole-limbed wormlizards. Instead of trying to catch a peek of one of these hard-to-find animals dining in the wild, scientists sometimes collect their droppings. Droppings, also called feces (FEE-sees), contain clues to the animal's diet, such as bits of food that the animal could not digest. If the wormlizard ate an insect, for example, the droppings might contain a little piece of the insect's leg. In addition, scientists sometimes find a dead animal in the wild and cut open its stomach to see what is inside. Through these two methods, they can learn what even the shyest of animals eat.

DIET

Mole-limbed wormlizards are like many other underground-living, or fossorial (foss-OR-ee-ul), animals in that their diet is something of a mystery. Scientists have not watched them feed but have occasionally caught them and looked at what was in their stomachs. From these scraps of partially digested food, they have learned that the mole-limbed wormlizards will eat ants, termites, grubs, and other invertebrates (in-VER-teh-brehts), which are animals without backbones. These species find their food underground or beneath logs, rotting leaves, and other things that cover the ground by following chemicals trails that the invertebrates leave behind. Mole-limbed wormlizards pick up these chemicals with the tongue. The tongue then places the chemical odors on a little opening, or duct, on the roof of the mouth that connects to a special organ. This organ, called a Jacobson's organ, helps them smell the chemicals.

BEHAVIOR AND REPRODUCTION

These three species spend most of their time in the underground tunnels that they dig. They dig their tunnels with their front legs and with their heads, typically starting new tunnels with their legs and then switching to their heads to make them longer and deeper. When they are digging with their heads, they lay the front legs along the sides of the body. Their tunnels can wander through the soil, sometimes opening underneath rocks or logs at the surface, scooting along less than an inch (2.5 centimeters) underground, or dropping down to almost 8 inches (20 centimeters) deep. At night, they may leave their tunnels and crawl about above ground, but they rarely venture out during the daytime. By living underground, they avoid most predators. If a predator (PREH-duh-ter) does manage to capture one, the mole-limbed wormlizard is able to drop its tail. Unlike many other lizards, however, it does not regrow its tail.

Females of all three species lay eggs, usually one to four at a time. Some may only have young every other year. Females in two of the three species lay their eggs in January, and the eggs hatch three months later. Females of the third species lay their eggs in July, and the eggs hatch two months later.

MOLE-LIMBED WORMLIZARDS AND PEOPLE

People rarely see these animals. Occasionally, a person may turn over a rock or log and see a mole-limbed wormlizard for a few seconds until it quickly slinks back into its tunnel and disappears. Although people rarely think about them, the wormlizards may be helpful to humans because they eat termites and other so-called pest animals.

CONSERVATION STATUS

Scientists still have much to learn about these animals; however, they are not now considered endangered or threatened.

TWO-LEGGED WORMLIZARD (Bipes biporus): SPECIES ACCOUNT

Physical characteristics: Colored very pale pink or orangish-pink, and sometimes with a whitish belly, the two-legged wormlizard has two front legs, each with five claws. Adults can reach 7.5 to 8.3 inches (19 to 21 centimeters) long, including a short tail. The tail looks much like the rest of the body but actually begins at the vent, a slit-like opening on the underside of the animal. In this species, the tail is about one-tenth as long as the rest of the body. In other words, a 7.5-inch-long (19-centimeter-long) wormlizard has a tail about 0.75 inches (1.9 centimeters) long. It is a thin animal, and at the middle of its body, it only measures about one-quarter of an inch (6 to 7 millimeters) across.

Geographic range: It makes its home along the western side of the Baja California peninsula in Mexico.

Habitat: The two-legged wormlizard lives underground in sandy soils usually around the roots of certain shrubs called mesquite (mess-KEET). Their tunnels are usually very shallow—less than an inch (2.5 centimeters) deep—but they sometimes drop to about 6 inches (15 centimeters) under the surface.

Diet: They search underground for ants, termites, and the larvae (LAR-vee) of insects to eat. Larvae are newly hatched insects that usually have soft bodies. Grubs, for example, are the larvae of beetles. At night, they also look for food, including insects and spiders, above ground.

Behavior and reproduction: These animals stay in their shallow tunnels most of the time. In the mornings, they tend to move up to shallower tunnels, then go deeper as the day warms up. Scientists believe that they also search for warm or cool spots underground by moving into the open where the sun beats down to heat up the sand, or under chillier shady areas beneath shrubs or trees. They will leave their tunnels and come up to the surface sometimes, especially at night, to hunt for invertebrates. They are not speedy, graceful animals. Rather, they move slowly and clumsily, sometimes swinging around their front legs in an overhand swimming type of motion. Like other members of this family, the two-legged wormlizard can drop its tail if it is attacked. They squeeze muscles around a weak spot in the tail bone, and the tail drops off. The wound heals, but the worm lizard cannot grow a new tail.

The females lay one to four eggs in July, which is a very dry time in their habitat. The eggs hatch about two months later, just as the rainy season starts and food for the young becomes more plentiful. In the summer after the females reach their fourth birthday, they are old enough to have young of their own. Some scientists think that females may only have young once every other year. Only more research will say for sure.

Two-legged wormlizards and people: People rarely see one of these wormlizards, unless they happen to turn over a rock, a pile of leaves, or some other hiding spot where one is lying. The wormlizard usually responds by quickly slipping into a nearby tunnel and disappearing.

Conservation status: This species is not considered endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Burnie, David, and Don Wilson, eds. The Definitive Visual Guide to the World's Wildlife. New York: DK Publishing, 2001.

Gans, C. Biomechanics: An Approach to Vertebrate Biology. Philadelphia: J. B. Lippincott Company, 1974.

Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians. New York: Facts on File, 1986.

Mattison, Chris. Lizards of the World. New York: Facts on File, 1989.

Schwenk, K. Feeding: Form, Function, and Evolution in Tetrapod Vertebrates. San Diego: Academic Press, 2000.

Vanzolini, P. E. Evolution, Adaptation and Distribution of the Amphisbaenid Lizards (Sauria: Amphisbaenidae). Ph.D. diss. Harvard University, 1951.

Web sites:

"Family Bipedidae (two-legged worm lizards)." Animal Diversity Web. University of Michigan Museum of Zoology. http://animaldiversity.ummz.umich.edu/site/accounts/classification/Bipedidae.html (accessed on December 1, 2004).