Overview
The Round Island Boa (Casarea dussumieri), or Keel-scaled Boa, is an extraordinary, evolutionarily unique serpent native exclusively to the tiny, storm-battered volcanic islet of Round Island off the coast of Mauritius. The sole surviving member of the archaic family Bolyeriidae, this slender predator represents a monumental evolutionary anomaly: it possesses a flexible intramaxillary joint in its upper jaw, allowing the front and rear portions of its maxilla to flex independently—an anatomical marvel found in no other living vertebrate on Earth.
Biology and Physical Characteristics
Casarea dussumieri has a slender, muscular torso covered in keeled, rough scales that provide gripping friction on slippery volcanic basalt rocks. Its dorsal coloration displays remarkable 24-hour metachrosis (color change): during daytime resting hours, the snake is a dark charcoal-grey or olive-brown; at night when hunting, its skin lightens to a pale sandy grey or cream with faint dark blotches. Its anatomical crown jewel is its hinged upper jaw, where an intramaxillary joint allows the anterior maxilla to flex downward, functioning like a set of biological pliers to grip the smooth, hard carapaces of wriggling skinks.
Behavior and Ecology
Primarily nocturnal and terrestrial, the Round Island Boa stalks through native palm fronds and volcanic scree, actively hunting endemic day geckos (Phelsuma) and giant skinks (Leiolopisma telfairii). Unlike true boas (Boidae) which give birth to live young, Casarea dussumieri is oviparous, laying clutches of soft-shelled eggs in subterranean rock cavities or inside decomposing leaf bases of native Hurricane Palms. Incubation lasts approximately 90 days, with hatchlings displaying vivid neon orange or bright pinkish-red coloration that gradually shifts to adult grey.
Conservation and Human Impact
Historically imperiled when introduced goats and rabbits destroyed Round Island’s vegetation, the snake faced impending extinction in the 1970s. Following the complete eradication of invasive herbivores by the Mauritian Wildlife Foundation and Durrell Wildlife Conservation Trust, the native palm savanna rebounded dramatically, allowing wild boa populations to recover from fewer than 250 individuals to over 1,500 today. The species was downlisted from Critically Endangered to Vulnerable in a historic island restoration triumph.
Interesting Factoid
The Round Island Boa is the only vertebrate animal on Earth with a ‘split’ upper jaw: its top jaw has an internal hinge joint that lets the front teeth bend downward to firmly clamp onto slippery lizards like a pair of locking pliers.
Last updated: October 2026