Mole-limbed wormlizards

(Bipedidae)

Class Reptilia

Order Squamata

Suborder Amphisbaenia

Family Bipedidae

Thumbnail description
Elongate, slender, fossorial reptiles with scales arranged in annular rings, short robust forelimbs, hindlimbs absent, no external ear openings, a rounded head, blunt snout, and short tail

Size
4.7–9.4 in (120–240 mm)

Number of genera, species
1 genus; 3 species

Habitat
Arid scrub lands or desert, arroyos, alluvial sediments around river basins

Conservation status
Not listed by the IUCN

Distribution
Bipedids have a restricted distribution in coastal southwestern Mexico and Baja California

Evolution and systematics

Most recent phylogenetic analyses have placed amphisbaenians as one of three suborders of Squamates (the clade that includes snakes, lizards, and amphisbaenians), but their exact placement within that clade is not well understood. The inter-relationships among the four amphisbaenian families are also poorly understood. On the one hand, the retention of fore- limbs in bipedids, in contrast to the completely limbless condition in all other amphisbaenians, suggests that those forms may be the most primitive amphisbaenians. On the other hand, the only amphisbaenian family with a good fossil record is the Rhineuridae, and some of those fossil forms, although limbless, retain different primitive features, such as fully formed, enclosed orbits, suggestive of a potentially basal position for this group within amphisbaenians. Unfortunately, bipedids have no fossil record at all. No subfamilies are recognized.

Physical characteristics

Certain features of bipedids are common to most or all amphisbaenians. These include: reduction or absence of the right lung; an enlarged, medial, premaxillary tooth; the periodic shedding of the skin in a single piece; a heavily ossified and robust skull; absence of eyelids; a forked tongue; and the absence of external ear openings.

The family Bipedidae includes three species that attain a body length of 4.5–9.4 in (115–240 mm) and a body width of 0.27–0.39 in (7–10 mm). The most striking feature of these species is the presence of short, robust forelimbs positioned close to the back of the head, a unique condition among amphisbaenians. Bipedids retain all typical elements of the forelimb, including the humerus, radius, ulna, carpals, metacarpals, and phalanges. In addition, Bipes exhibits a unique condition termed hyperphalangy, which refers to an extra element occurring in the first digit of the hand, one more than is typically found in reptiles. This condition is presumably related to digging functions. The tail of bipedids is very short, as in most amphisbaenians, representing between 10–20% of the entire length of the animal. The eyes are reduced and sometimes covered by a head scale. The head is rounded and blunt. The teeth are conical, slightly recurved, and attached to the jaw in pleurodont fashion. There is no pigment or pattern to the skin, making the external appearance pale pink or flesh-colored. Caudal autotomy occurs at a single autotomy

constriction site, but bipedids do not regenerate tails. Bipedids are also recognized by a number of unique internal anatomical characteristics, which are evident through x rays or dissection. For example, despite the absence of external rear-limbs, bipedids retain elements of the pelvic girdle and a small vestige of the leg bone (the femur) internally. Also, the skull of bipedids is characterized by fusion of the frontal and parietal bones.

Distribution

The genus Bipes is endemic to Mexico, with a relatively narrow distribution in western Mexico. One species, Bipes biporus, occurs throughout the cape region of Baja California, Mexico; B. canaliculatus occurs in Guerrero and Michoacán; and B. tridactylus occurs in coastal Guerrero.

Habitat

Mole-limbed wormlizards are known to occur in loose, sandy, or loamy soils. They are usually found in a narrow range of microhabitats, including those associated with the root systems of shrubs and small trees. Traditionally, Bipes has been thought to be restricted to sandy soils, but recent reports suggest the additional occurrence of Bipes in different microhabitats, including under rocks and rotting logs along gravel slopes and in coarser, rockier soils near river basins. Also, mole-limbed wormlizards have been collected outside of their tunnel systems and are sometimes found on the surface of the soil, especially nocturnally when they emerge from their burrow systems.

Behavior

All bipedid species are fossorial, living in self-constructed tunnel systems that range from 0.78–7.8 in (2–20 cm) below the surface. Bipes uses its digging front limbs to create an initial opening in the soil; once below the surface, it folds the limbs back against the body and uses the head (as all other amphisbaenians do) to burrow through the soil. Rectilinear movements of the body are also used for locomotion within the tunnel.

Often, burrows lead to the soil surface under rocks. When the rocks are lifted, Bipes will retreat down into the burrow system quickly.

Feeding ecology and diet

Direct examination and field studies of feeding in amphisbaenians are almost nonexistent, since these animals are so secretive. Therefore, we must rely mainly on indirect studies and on studies of captive animals. Laboratory studies based on dissections of wild-caught animals and examination of stomach contents indicate that bipedids feed primarily on small invertebrates such as termites, beetle larvae, and ants. Bipes biporus fits the pattern of a generalist predator that exploits prey items found both under the soil and on the soil surface covered by objects such as fallen bark or debris.

For bipedids, chemical cues are the most important means used in locating prey. The forked tongue and the Jacobson's organ allow the detection of chemical odors.

Reproductive biology

All species within this family are oviparous, laying 1–4 eggs per clutch. Eggs are laid in January and hatching occurs in April for Bipes canaliculatus and B. tridactylus. In B. biporus eggs are laid in July and hatchlings appear in September.

Conservation status

No species of Bipedidae is listed by the IUCN.

Significance to humans

Mole-limbed wormlizards are of no economic significance to humans, but they may benefit humans ecologically by feeding on populations of ants and termites and potentially helping to keep these populations in check.

Species accounts

List of Species

Two-legged wormlizard

Bipes biporus

taxonomy

Bipes biporus Cope, 1894, Cape San Lucas, Baja California, Mexico.

other common names

English: Mole lizard; French: Bipédidés; German: Zweifuss-Doppelschleichen; Spanish: Ajolote.

physical characteristics

The two-legged wormlizard has an average body length of 7.5–8.3 in (190–210 mm). Its tail is approximately 10% of total length. It has a midbody diameter of 0.23–0.27 in (6–7 mm). It is typically pink or flesh-colored uniformly, but some specimens are white ventrally. It has five claw-bearing digits on each limb and two preanal pores. It has a caudal autotomy restriction site at a single point

on the tail less than 10 annuli posterior to the cloacal opening. The tail is not regenerated subsequent to autotomization.

distribution

Western half of the Baja California peninsula.

habitat

The two-legged wormlizard inhabits sandy soils. These reptiles can be found by digging to depths up to 6 in (15 cm) below the soil surface. Bipes biporus is often found in association with the roots of mesquite shrubs. It can sometimes be found on the soil surface under debris or rocks, especially at night.

behavior

Bipes biporus are active throughout the year, in any season. They inhabit shallow burrow systems, with an average depth of 0.98 in (2.5 cm) below the soil surface. B. biporus exhibit diurnal vertical movements; studies indicate that they may be found closer to the surface in early morning hours, which is suggestive of thermoregulatory behavior. It is possible that B. biporus also moves horizontally with respect to shade around surface objects such as trees or shrubs in order to thermoregulate. Some data suggest that they are more likely to be found in sunny locations and/or nearer to the surface during early parts of the day.

Bipes biporus leaves it burrow systems at times and travels on the surface of the soil, especially at night. These animals move clumsily on the surface, using a combination of rectilinear locomotion of the body and overhand strokes of the short front limbs.

feeding ecology and diet

Bipes biporus feeds mainly on termites, insect larvae, and ants found below the soil surface. Evidence also exists that Bipes occasionally feeds on spiders and insects, which are surface-active prey, indicating that at least some feeding occurs outside of the tunnel system. Field studies indicate that B. biporus are more likely to be found outside of their tunnel systems during the night.

reproductive biology

Female Bipes biporus become sexually mature once a body size of approximately 7.3 in (185 mm) is achieved, usually at approximately 45 months of age. Females lay eggs in clutches of one to four during the dry season. Hatchlings appear in late September, which is just before the rainy season, ensuring food availability for growing young. Field observations indicate that only about half the adult females are gravid in any given year, which has led to the suggestion that B. biporus are reproductively active every other year.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

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

Schwenk, K. "Feeding in Lepidosaurs." In Feeding: Form, Function, and Evolution in Tetrapod Vertebrates. San Diego: Academic Press, 2000: 175–291.

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

Periodicals

Castañeda, M. R., and T. Alvarez. "Contribucion al conocimiento de la osteologia apendicular de Bipes (Reptilia: Amphisbaenia)." Anales de la Escuela Nacional de Ciencias Biologicas Mexico 17 (1968): 189–206.

Cope, E. D. "On the Genera and Species of Euchirotidae." American Naturalist 28 (1894): 436–437.

Gans, C. "Amphisbaenians—Reptiles Specialized for a Burrowing Existence." Endeavour 28 (1969): 146–151.

——. "The Characteristics and Affinities of the Amphisbaenia." Transactions of the Zoological Society of London 34 (1978): 347–416.

Gans, C., and E. Wever. "The Ear and Hearing in Amphisbaenia (Reptilia)." Journal of Experimental Zoology 179 (1972): 17–34.

——. "The Amphisbaenian Ear: Blanus cinereus and Diplometopon zarudnyi." Proceedings of the National Academy of Sciences 72 (1975): 1487–1490.

Hodges, W. L., and E. Perez-Ramos. "New Localities and Natural History Notes on Bipes canaliculatus in Guerrero, México." Herpetological Review 32, no. 3 (2001): 153–156.

Kearney, M. "The Appendicular Skeleton in Amphisbaenians." Copeia 2002, no. 3 (2002): 719–738.

——. "Diet in the Amphisbaenian Bipes biporus." Journal of Herpetology. In press.

Papenfuss, T. "The Ecology and Systematics of the Amphisbaenian Genus Bipes." Occasional Papers of the California Academy of Sciences 136 (1982): 1–42.

Renous, S., J. P. Gasc, and A. Raynaud. "Comments on the Pelvic Appendicular Vestiges in an Amphisbaenian: Blanus cinereus (Reptilia, Squamata)." Journal of Morphology 209 (1991): 23–28.

Taylor, E. H. "Does the Amphisbaenid Genus Bipes Occur in the United States?" Copeia 1938 (1938): 202.

Zangerl, R. "Contributions to the Osteology of the Postcranial Skeleton of the Amphisbaenidae." American Midland Naturalist 33 (1945): 764–780.

——. "Contributions to the Osteology of the Skull of the Amphisbaenidae." American Midland Naturalist 31 (1944): 417–454.

Maureen Kearney, PhD