Sunbeam snakes

(Xenopeltidae)

Class Reptilia

Order Squamata

Suborder Serpentes

Family Xenopeltidae

Thumbnail description
Common semifossorial snakes with smooth, shiny, iridescent scales. Although dark brown on top and pale gray or yellow beneath, the scales reflect the colors of the spectrum, hence their common name, sunbeam snakes

Size
2–3 ft (approximately 1 m)

Number of genera, species
1 genus; 2 species

Habitat
Agricultural and settled areas and along the edges of forests

Conservation status
Not threatened

Distribution
Southeast Asia

Evolution and systematics

Xenopeltids appear to be a relict lineage basal to the Macrostomata, although some recent authors have included the family within Macrostomata on the assumption that these snakes possess macrostomatan properties. The two recognized species of Xenopeltis are very similar to each other but differ in a number of structural features from other snakes. Current phylogenetic analyses place the family between aniliids and booids (pythonids, boids, and related snakes). No subfamilies are recognized.

Physical characteristics

Adult Xenopeltis have bodies that appear slightly flattened, but otherwise similar to pipe snakes. The dorsal half of the body is a dark, purplish brown, while the ventral half is white, light gray, or light yellow. On the head the yellow extends onto the upper labial scales. The young have a distinct light ring around the rear of the head and anterior neck. The head is wedge-shaped and flattened. The tail comprises about a tenth of the total length and has paired subcaudal scales. Scales on the body are large, typically in 15 rows throughout the length of the trunk, head plates are reduced in number and large, and the eye is small. These snakes have a number of unusual anatomical features, including teeth on the premaxilla, mobile attachment of teeth (hinged teeth) on all of the toothed bones, a left lung about half the length of the right lung, no pelvic vestiges, a palate

tightly attached to the snout, but a snout and facial region (maxilla, prefrontals) that can move up and down on the braincase, and an extraordinary mobile toothed process on the dentary bone that extends backward more than half the length of the lower jaw. Their quadrate is short and vertically oriented and is attached dorsally to a supratemporal partially embedded in the bones roofing the ear. Unlike macrostomatan snakes, the anterior tips of the lower jaw in Xenopeltis are tightly bound, as are the maxillae to the premaxilla. The lower jaw also has long, splintlike coronoids and a mobile intramandibular joint.

Distribution

From Myanmar south to the Nicobar and Andaman Islands, east through Thailand, Laos, Cambodia, Vietnam, and southern China, south through Malaysia and Indonesia east to Borneo, Java, Sulawesi, and the southwestern Philippine Islands.

Habitat

These snakes are common in agricultural and settled areas and along the edges of forests, but either rare or more difficult to find in deep forest. Most specimens are found in litter, under trash, or in disturbed areas.

Behavior

When not burrowing, xenopeltids move rapidly and nervously with their head against the substrate and with rapid tongue flicking. If touched, they jerk stiffly but rapidly in unpredictable directions. They have also been reported to vibrate their tail rapidly in a manner similar to rattlesnakes.

Feeding ecology and diet

Most information available is known to apply only to the common sunbeam snake (Xenopeltis unicolor). Stomach contents of wild-caught snakes contain predominantly lizards (particularly skinks), snakes, and frogs, although small mammals and birds have also been recorded. In captivity, sunbeam snakes will usually eat mice, including adult mice that they kill by constriction. Prey capture in captivity is unusual in appearing accidental and undirected, although snakes will pursue mice. Swallowing is extremely rapid despite the limited mobility of the upper jaws.

Reproductive biology

Sunbeam snakes are oviparous. However, remarkably little is known, and captive breeding has not been reported. Females have been found with up to 17 eggs.

Conservation status

Not threatened.

Significance to humans

None known.

Resources

Books

Campden-Main, S. M. A Field Guide to the Snakes of South Vietnam. Washington, DC: Smithsonian Institution, 1970.

Cox, M. J. The Snakes of Thailand and Their Husbandry. Malabar, FL: Krieger Publishing Company, 1991.

Deuve, J. Serpents du Laos. Paris: ORSTOM, 1970.

Zug, G. R., L. J. Vitt, and J. P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles, 2nd ed. San Diego: Academic Press, 2001.

Periodicals

Frazzetta, T. H. "Adaptations and Significance of the Cranial Feeding Apparatus of the Sunbeam Snake (Xenopeltis unicolor): Part 1. Anatomy of the Skull." Journal of Morphology 239 (1999): 27–43.

David Cundall, PhD