Round Island keel-scaled boa  (Casarea dussumieri)

Classified as Endangered (EN - D) on the IUCN Red List 2002 (3), and listed on Appendix I of CITES (4).

The Round Island keel-scaled boa is one of the world's rarest snakes (2). This slender snake may reach up to 1.5 metres in length, the upper surface is generally dark brown whilst the underside is lighter with very dark spots. The body is covered in small, keeled scales that give rise to the species' common name (2).

Found only on Round Island off the north coast of Mauritius in the Indian Ocean (5).

Historically inhabited tropical hardwood forest and palm savannah, but since the introduction of goats and rabbits to the island much of this habitat has been destroyed. As a result of habitat degradation the boa currently persists in degraded palm savannah and shrub layer vegetation (1).

Little is known about the natural ecology and behaviour of this elusive snake. Primarily nocturnal, juveniles and adult males in particular are partially arboreal. Their diet is composed exclusively of lizards, some of which are also endangered species endemic to Round Island (1). The breeding season is thought to begin in April although young have been observed at practically all times of the year. A clutch of up to 12 soft-shelled eggs is laid, perhaps amongst leaf litter or in a hollow palm trunk, and may be attended by the female (1). The incubation period is about 90 days, unusually long for a snake; young are born weighing less than 5 grams and are bright orange in colour (6).

Habitat loss has been rife throughout the Mascarene Islands. Vast tracts of native forest (over 90%) have been cleared to make way for agriculture and on Round Island the introduction of rabbits and goats has further damaged native flora (7); however, these were removed during the 1980’s. Perhaps 500 adults remain (with a total population of approximately 1000 individuals) on the 159-hectare Round Island (1).

Round Island is a protected area and visitors to the island are restricted to scientists and conservationists (5). Rabbits and goats have been eradicated and the native vegetation of the island, much of which is itself endemic and endangered, is beginning to recover. The Government of Mauritius and the Mauritian Wildlife Foundation are also running an intensive habitat restoration programme (1). International trade is prohibited by the listing of this snake on Appendix I of the Convention on International Trade in Endangered Species (4). A captive breeding programme has been established with the aim of safeguarding the future of this rare snake and learning more about its complex biology (2). The Round Island keel-scaled boa is one of two snakes to be found only on this island, belonging to an ancient line that dates back over 60 million years (7). Its relative Bolyeria multocarinata however, has not been seen in the wild since 1975 and is feared to be extinct (2).

View information on this species at the UNEP World Conservation Monitoring Centre.