Overview
The Round Island Boa (Casarea dussumieri), also known as the Round Island keel-scaled boa, is an extraordinary, evolutionarily unique serpent found nowhere else on Earth except on a tiny uninhabited volcanic island off the coast of Mauritius. It is the sole surviving member of the ancient snake family Bolyeriidae, an evolutionary lineage that branched off from all other living boids more than 50 million years ago. Its most celebrated anatomical marvel is a divided, hinged upper jaw—an evolutionary novelty found in no other living vertebrate on Earth.
Adapted to hunt elusive endemic lizards across barren volcanic slopes and native fan-palm canopies, this remarkable snake exhibits physiological color-changing capabilities, shifting from dark charcoal-slate during daylight into a pale greyish-tan at night. Once driven to the edge of extinction with fewer than 250 individuals surviving due to invasive goats and rabbits destroying the island’s vegetation, landmark conservation interventions and invasive species eradication have orchestrated an inspiring population recovery.
Biology and Physical Characteristics
The defining anatomical marvel of Casarea dussumieri is its hinged maxillary bone. The upper jaw is split into anterior and posterior segments connected by an intramaxillary joint, allowing the snake to articulate the front portion of its upper jaw independently. This adaptation acts as an unbreakable mechanical vice designed specifically to grip and swallow hard, smooth-scaled, writhing skinks and geckos without losing hold. Its dorsal scales are heavily keeled, providing superior tactile grip across rough volcanic basalt.
Behavior and Ecology
Round Island boas are predominantly nocturnal and semi-arboreal, spending hot daylight hours coiled within deep basalt crevasses or among the fibrous leaf bases of native Hurricane palms (Dictyosperma album). At night, they emerge to actively hunt sleeping geckos and skinks. Uniquely, the snake undergoes a circadian color shift: its dorsal skin is dark grey-brown by day to absorb thermal solar radiation and camouflage against volcanic basalt, fading into a pale silvery-cream at night to match moonlit palm fronds.
Conservation and Threats
Formerly listed as Critically Endangered, the Round Island Boa was downlisted to Vulnerable by the IUCN following one of the most successful island restoration initiatives in conservation history. The complete eradication of introduced feral rabbits and goats from Round Island permitted native hardwood and palm savannas to regenerate, fostering a dramatic rebound of endemic reptile populations. Boas now number over 1,505 individuals and have been translocated to Gunner’s Quoin.
Interesting Factoid
The Round Island Boa possesses a hinged upper jaw bone found in no other vertebrate on Earth, allowing it to bend the front of its mouth to grip slippery lizards.