Wormlizards

(Amphisbaenidae)

Class Reptilia

Order Squamata

Suborder Amphisbaenia

Family Amphisbaenidae

Thumbnail description
Elongate, slender, limbless, fossorial reptiles with scales arranged in annular rings; eyes greatly reduced or absent; no external ear openings; head either rounded, spatulate, or laterally compressed; and short tails

Size
Amphisbaenids exhibit a huge size range, from tiny worm-like species of 4 in (100 mm) maximum length to some of the largest known amphisbaenians of more than 32 in (800 mm) maximum length in some species; most species fall within the range of 10–16 in (250–400 mm)

Number of genera, species
18 genera; 160 species

Habitat
Amphisbaenids are found in a variety of habitats ranging from lowland rainforests to deciduous forests to arid deserts

Conservation status
No species listed by the IUCN

Distribution
Amphisbaenids occur in Africa, South and Central America, Europe, and the West Indies

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 interrelationships among the four amphisbaenian families are also poorly understood. The Amphisbaenidae has no significant fossil record; a few isolated remains occur in Europe and in Africa. Amphisbaenids are possibly most closely related to trogonophids based on a recent phylogenetic analysis of the group. This is the largest and most heterogeneous amphisbaenian family. No subfamilies are recognized.

Physical characteristics

Certain features of amphisbaenids are common to most or all amphisbaenians. These include: a unique modification of the middle ear in which an elongated structure, the extracolumella, attaches to the stapedial bone of the middle ear extending forward to attach to tissue along the sides of the face and allowing the reception and transmission of vibrations to the inner ear; 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; the absence of eyelids; a forked tongue; and the absence of external ear openings.

The family Amphisbaenidae contains some of the smallest and largest amphisbaenians and even some of the most highly miniaturized reptiles known. One exceptionally tiny amphisbaenid is the African Chirindia, with some species attaining an adult body length of only 4 in (100 mm) and a body diameter of 0.125 in (3 mm). The largest amphisbaenian is probably the South American Amphisbaena alba, which attains an adult body length of over 30 in (800 mm). In general, amphisbaenians show little or no sexual dimorphism in body size. All amphisbaenids are completely limbless, but some retain internal vestiges of the pectoral and pelvic girdles. In some species, a small eye is visible under a translucent head scale, but in others the eye is not visible at all. Amphisbaenids exhibit a variety of cranial shapes (keelheaded, round-headed, shovel-headed) and a generally cylindrical body shape. In some species the snout is conical and in others it is blunt. The tail is always short but may exhibit

a variety of shapes, including rounded and blunt-tipped, pointed, or dorsoventrally flattened. Most amphisbaenid species are capable of autotomizing the tail, but none can regenerate a new tail. All amphisbaenids have pleurodont dentition, with varying numbers of teeth occurring on the tooth-bearing elements of the skull.

Many species are pallid in appearance, presumably related to their subterranean existence. However, some species exhibit striking patterns of coloration, including dark checkerboard patterns on light backgrounds. Still others are a solid brown, yellow, or grey dorsally, with a paler underside.

Beyond those easily recognizable features, amphisbaenids are also characterized by a number of internal conditions, including the presence of pelvic vestiges in all species, the absence of a sternum, and a heavily ossified skull.

Distribution

The family Amphisbaenidae is both the largest and most widespread family of amphisbaenians. Wormlizards occur in tropical and subtropical areas of Central and South America, Africa, Europe, Asia, and the West Indies. Blanus cinereus is the only amphisbaenian occurring in Europe.

Habitat

Wormlizards are known to occur in a variety of habitats, including deserts, tropical rainforests, and woodlands. They are usually found by digging into the soil but may also be found hidden under surface litter or debris, under rocks and logs, or even within ant and termite nests. Wormlizards are sometimes found after heavy rains when the animals are driven from their burrow systems.

Behavior

Because amphisbaenians are fossorial, relatively little is known of their behavior or ecology compared to other reptiles. However, a few insights into their behavior are notable. The family Amphisbaenidae contains the greatest diversity of burrowing specializations, which can be loosely categorized in concert with head shapes. Round-headed species burrow by driving or ramming the head forward. Those species with shovel-shaped heads burrow by driving the head downward, lifting the head up, compressing the soil onto the roof of the tunnel with the flattened snout, and smoothing the tunnel walls with the pectoral region. Keel-headed forms with laterally compressed heads burrow by driving the head forward alternately to the left and right of center and then smoothing the tunnel walls by pressing the sides of the head and trunk against them.

Feeding ecology and diet

Natural diets of amphisbaenians are poorly studied. Some laboratory studies of captive amphisbaenians have led researchers to suggest that amphisbaenians are specialized for feeding on large vertebrate prey, which they apparently handle and eat effectively in captivity. Interestingly, however, all direct studies performed to date on wild-caught animals indicate that amphisbaenians rarely exploit large prey in the wild but feed primarily on small invertebrates instead. The few direct diet studies completed for amphisbaenids indicate a remarkable degree of consistency in diet; all species studied so far appear to feed primarily on such prey items as termites, ants, adult and larval beetles, cockroaches, and lepidopteran larvae. A few irregular occurrences are notable. One study found six specimens of an amphisbaenid species with stomachs filled with fungi, and another study found one specimen with a lizard limb in its stomach and a second specimen containing an entire burrrowing snake.

For amphisbaenids, chemical and auditory cues are the most important means used in locating prey. The uniquely adapted middle ear system allows prey movements to be detected, while the forked tongue and the Jacobson's organ allow the detection of chemical odors. Airborne sounds are picked up and transmitted to the inner ear along the specialized extracolumellar apparatus, which may also amplify the vibrations as well. This unique anatomy is consistent with behavioral studies conducted in laboratory experiments, which suggest that amphisbaenians can hear prey movements through the soil.

Reproductive biology

Information on the reproductive biology of amphisbaenids is extremely limited. From what little we know, amphisbaenids are typically egg-layers, although some species of Loveridgea and Monopeltis bear live young. Clutch sizes average one to four elongate eggs. Wormlizard eggs have been discovered in ant and termite colonies, implying that amphisbaenians may utilize these underground colonies not only as food sources but perhaps as egg-laying chambers as well.

Conservation status

No species of Amphisbaenidae is listed by the IUCN.

Significance to humans

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

White-bellied wormlizard

Amphisbaena alba

taxonomy

Amphisbaena alba Linné, 1758, "America."

other common names

French: Amphisbène blanche; German: Rote Doppelschleiche.

physical characteristics

Amphisbaena alba is the largest South American amphisbaenian, with a total length of 15.7–33.4 in (400–850 mm) and a midbody diameter ranging from 1–2 in (20–25 mm). The tail is approximately 6% of total length. It has smooth, square scales arranged in annular rings that are separated by grooves. The species is uniformly pale-colored,

although some specimens are white ventrally. The head is more or less rounded in shape. Caudal autotomy is absent in this species. Dentition is pleurodont, with conical, sharply pointed, and slightly recurved teeth. It has a large, median premaxillary tooth flanked by six smaller premaxillary teeth. Five teeth occur on the maxilla; seven teeth on the l ower jaw.

distribution

Throughout South America east of the Andes, Panama, and the West Indies.

habitat

The white-bellied wormlizard inhabits forested lowlands, soil, and leaf litter. Amphisbaena alba is rarely seen on the surface.

behavior

Amphisbaena alba is well known to herpetologists for several unusual and interesting behavioral traits, one of which involves a unique ecological relationship with leaf-cutter ants (genus Atta). A. alba apparently follows the pheromone-marked trails of these ants to their nests, where it then takes up residence in one of the nest chambers, feeding primarily on beetle larvae and other arthropods that are also inhabitants of Atta nests.

The defensive behavior of Amphisbaena alba when threatened is also of scientific interest to behavioral biologists. When disturbed, A. alba raises its tail off the ground and waves it, while simultaneously raising its head with an open, gaping mouth. The head and tail are close together, the body held almost in a circle, which almost gives the impression of a twoheaded animal. This stereotyped behavior has led to the common name "two-headed snake" applied by people who live near these animals.

feeding ecology and diet

Only a few studies examining the natural diet of Amphisbaena alba exist. These have documented that A. alba feeds primarily on small arthropods such as beetles, ants, termites, spiders, crickets, and the larvae of various insects. On the other hand, laboratory-based behavioral studies indicate that, when offered larger vertebrate prey items such as rodents, A. alba will prey on them with enthusiasm, even biting and tearing pieces out of them.

reproductive biology

Amphisbaena alba is oviparous, laying eggs in clutches of eight to 16. This is the largest known clutch size for any amphisbaenian and may possibly be related to the large body size of this species. According to some studies, reproduction is seasonal and primarily restricted to the dry season.

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.

Linné, C. V. Systema naturae per regna tria naturae. Editio decima, reformata. Vol. 1. Stockholm, 1758.

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

Colli, G. R., and D. S. Zamboni. "Ecology of the Worm-lizard Amphisbaena alba in the Cerrado of Central Brazil." Copeia 1999: 733–742.

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

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

——. "The Ear and Hearing in Amphisbaenia (Reptilia)." Journal of Experimental Zoology 179 (1972): 17–34.

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

Montero, R., and C. Gans. "The Head Skeleton of Amphisbaena alba." Annals of the Carnegie Museum 68 (1999): 15–80.

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.

Riley, J., J. M. Winch, A. F. Stimson, and R. D. Pope. "The Association of Amphisbaena alba (Reptilia: Amphisbaenia) with the Leaf-cutting Ant Atta cephalotes in Trinidad." Journal of Natural History 20 (1986): 459–470.

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