Splitjaw snakes

(Bolyeriidae)

Class Reptilia

Order Squamata

Family Bolyeriidae

Thumbnail description
A monotypic family consisting of a species of small, slender snakes characterized by a divided and hinged maxilla (upper jaw bone)

Size
Maximum size approaches 4 ft (1.3 m); largest specimen caught in the wild, collected by the Jersey Wildlife Preservation Trust in 1977, weighed 17 oz (510 g)

Number of genera, species
1 genus; 1 species

Habitat
Found throughout Round Island, most often in or near the remnants of forest

Conservation status
Endangered

Distribution
Restricted to Round Island, a small island north of Mauritius in the Indian Ocean

Evolution and systematics

Two genera of bolyeriid snakes survived into the late twentieth century on Mauritius and several closely associated small islands. This family is believed to be the modern descendant of an early lineage of macrostomatan snakes that diverged from the alethinophidians at the end of the Cretaceous. It is distinguished by a character that is unique among all vertebrate animals: the maxilla is divided into separate anterior and posterior parts by a movable joint. The joint is located beneath the eye; it allows the front of the upper jaw to bend up or down independently of the rear portion of the jaw.

For many years the Bolyeriidae family was classified as a subfamily within the Boidae. For that reason bolyeriid snakes most often were referred to as "boas." It now is recognized that the Bolyeriidae is a unique lineage not closely allied with the Boidae. In recognition of the uniqueness of the modified maxilla of the Bolyeriidae, the snakes in this family are identified as "splitjaw snakes." The sole member of the Bolyeriidae to survive into the twenty-first century is the keel-scaled splitjaw, Casarea dussumieri.

The smooth-scaled splitjaw, Bolyeria multocarinata, is the second bolyeriid species known to science. One specimen, identified by a unique scar and believed to be the last survivor of the species, was found in faunal surveys of Round Island in the 1960s and early 1970s. It was last seen in 1974, and the species is considered extinct.

No fossils of this family exist. Other common names of the splitjaw snake include Round Island boa, splitjaw boa, and keel-scaled boa. No subfamilies are recognized.

The taxonomy of the keel-scaled splitjaw is Casarea dussumieri Schlegel, 1837, Round Island, Mauritius.

Physical characteristics

The splitjaw snake has a long and flat head that is distinct from the neck. The eye has an elliptical pupil. The snake is slender and has a long tail that equals or exceeds 25% of the total body length. The dorsal body scales are hexagonal, with little overlap. There is no tracheal lung, and the left lung is small and poorly developed, more similar to the condition of colubrid snakes than that of boid snakes. Vestiges of the pelvic bones and cloacal spurs are absent.

Distribution

The Bolyeriidae was endemic to Mauritius and several nearby small islands including Flat Island, Gunner Quoin, and Round Island. The Bolyeriidae are the only snakes native to this region. Today the sole surviving species is restricted to Round Island, a 374-acre (151-ha) island of volcanic origin that lies approximately 13 mi (20 km) north and slightly west of Mauritius.

Habitat

Round Island was once a verdant island, forested with a hardwood scrub plant community that included many endemic and unique species of palms. Introduction of goats and rabbits in the early nineteenth century caused rampant ecological damage, including the near total lost of the forest. The loss of the dominant plant community resulted in dramatic erosion; it is estimated that 90% of the soil was lost, and the island is now crisscrossed with deep gullies. The flora and fauna of Round Island rank among the world's most threatened.

Round Island acted as a final refuge for many Mauritian plants and animals, because rats were never introduced. The plight of Round Island came to the attention of conservationists in the 1970s, in part through the efforts of Gerald Durrell and the Jersey Wildlife Preservation Trust in the United Kingdom. Goats were removed from the island in 1979, and rabbits were exterminated in 1986, protecting some of the remaining plant species from grazing. Keel-scaled splitjaws are found throughout the island, but they are associated strongly with the sparse remaining forest.

Behavior

The splitjaw snake has been encountered at all times of the day, but it is primarily nocturnal. Most often the species is found on the ground, but it has been seen in vegetation up to 8 ft (2.5 m) above the ground. It is believed to shelter in humid burrows.

Feeding ecology and diet

This species is a visually oriented predator of lizards. These snakes have been observed feeding in daylight on a day gecko, Phelsuma ornata; the Round Island skink, Leiolopisma telfairii; and another skink, Gongylomorphus bojerii. It is believed that the snake employs ambush-hunting techniques during the day and forages actively at night. The keel-scaled splitjaw employs an unusual method to stalk lizards at night. The snake raises its head several inches (centimeters) above the substrate and, while continually flicking its tongue, slowly approaches its prey using rectilinear motion. In captivity, adult splitjaws accept mice as prey; the species is not known to feed on mammals in nature.

Reproductive biology

The splitjaw snake is oviparous (egg-laying). Nothing is known about its reproduction in the wild. The species was first bred in captivity in 1982 at the Jersey Wildlife Preservation Trust. In the following years, the species has been bred in numbers in captivity. It has been determined that copulations most likely to result in fertile eggs occur between March and July, with oviposition then taking place between May and October. Clutch size varies from three to 11 eggs.

Conservation status

The species is considered Endangered by the IUCN. In the late 1980s, surveys of Round Island indicated that the population of splitjaws there numbered several hundred animals and the species was apparently stable and holding its own. Between 1977 and 1984, 11 specimens were taken from Round Island to the Jersey Wildlife Preservation Trust in the United Kingdom. It was not until two young were hatched there in 1982 that it was realized that the snake is oviparous. From this nucleus, there is now a stable, self-sustaining captive population that is distributed among several zoos. This provides a buffer against the extinction of the species, should something go wrong on Round Island. Keel-scaled splitjaws breed well in captivity, but hatchlings have proved very difficult to raise; they rarely feed voluntarily and must be force-fed until they are large enough to feed on young mice.

Because of the presence of rats and other introduced animals on the large island of Mauritius, it is unlikely that keel-scaled splitjaws will be reestablished there. Efforts are underway, however, to remove introduced species from the other small islands included in the original range of the splitjaw snake, and there are plans to reintroduce endemic animals and plants, including splitjaws, when the habitats are recovered sufficiently.

Significance to humans

None known.

Resources

Books

Day, David. The Doomsday Book of Animals. London: London Editions Limited, 1981.

Greene, Harry W. Snakes: The Evolution of Mystery in Nature. Berkeley: University of California Press, 1997.

Periodicals

Bloxam, Q. M. C., and S. J. Tonge. "The Round Island Boa Casarea dussumieri Breeding Program at the Jersey Wildlife Preservation Trust." Dodo, Journal of the Jersey Wildlife Preservation Trust 23 (1986): 101–107.

Cundall, David, and Frances Irish. "Aspects of Locomotor and Feeding Behavior in the Round Island Boa Casarea dussumieri." Dodo, Journal of the Jersey Wildlife Preservation Trust 23 (1986): 108–111.

Frazzetta, T. H. "From Hopeful Monsters to Bolyerine Snakes?" American Naturalist 104, no. 935 (January–February 1970): 55–72.

Guibe, J. "Les Serpents de Madagascar." Mémoires de L'Institut Scientifique de Madagascar série A., tome 12 (1958): 190–260.

Kluge, Arnold G. "Boine Snake Phylogeny and Research Cycles." Miscellaneous Publications, Museum of Zoology, University of Michigan, no. 178 (1991): 1–58.

McAlpine, Donald F. "Activity Patterns of the Keel-Scaled Boa (Casarea dussumieri) at the Jersey Wildlife Preservation Trust." Dodo, Journal of the Jersey Wildlife Preservation Trust 18 (1981): 74–78.

McDowell, S. B. "A Catalogue of the Snakes of New Guinea and the Solomons, with Special Reference to Those in the Bernice P. Bishop Museum. Part 2. Anilioidae and Pythonidae." Journal of Herpetology 9, no. 1 (1975): 1–79.

David G. Barker, MS

Tracy M. Barker, MS