Round Island boa

The Round Island boa is the sole living species of the family Bolyeriidae, a group that also includes the extinct Round Island burrowing boa (Bolyeria multocarinata). The family is endemic to the Mascarene Islands and is thought to have diverged from other snakes around 60 million years ago, making it one of the oldest lineages of modern snakes.¹ The species was first described by Schlegel in 1837 as Boa dussumieri, but later placed in its own genus Casarea due to distinct jaw morphology. Unlike most snakes, the Round Island boa has a divided maxilla bone, a feature shared only with its extinct relative, which allows it to manipulate prey more effectively.²

The Round Island boa inhabits the rocky, palm-dominated slopes of Round Island, where it shelters in crevices and under vegetation. It is primarily nocturnal and feeds on small lizards, especially the Round Island skink (Leiolopisma telfairii) and the night gecko (Nactus coindemirensis). The snake is an ambush predator, using its hinged jaws to grasp prey. Its reproductive biology is unusual: females lay eggs, but the eggs have a leathery shell and hatch after about 90 days, which is rare among boas, most of which give live birth.³ The species has a low reproductive rate, with females producing only one clutch every two to three years, making population recovery slow.

The Round Island boa was once on the brink of extinction due to habitat degradation caused by introduced rabbits and goats, which destroyed vegetation and increased erosion. The removal of these herbivores in the 1980s allowed the island's flora to recover, and the boa population has since stabilized, though it remains vulnerable to stochastic events such as cyclones and drought.⁴ The species is legally protected under Mauritian law, and Round Island is a nature reserve. Captive breeding programs have been attempted, but with limited success; the first captive hatching occurred in 1999 at the Durrell Wildlife Conservation Trust, which has been a key partner in conservation efforts. Climate change poses a new threat, as rising sea levels could reduce the island's area.

The Round Island boa is one of the few snake species that exhibits sexual dimorphism in jaw structure, with males having a more elongated maxilla than females, possibly related to dietary differences. The species is also notable for its ability to change color slightly in response to temperature, a trait more common in chameleons than snakes.⁵ Historically, the boa was known to local fishermen, but it was not formally documented by science until the 19th century. The extinct burrowing boa, its closest relative, was last seen in 1975, and its disappearance is attributed to the same habitat loss that nearly doomed the Round Island boa. The species is a flagship for island conservation, symbolizing the recovery of ecosystems after invasive species removal.

Bolyeriidae

A family of snakes endemic to the Mascarene Islands, containing only the Round Island boa and the extinct Round Island burrowing boa.

Maxilla

The upper jaw bone; in the Round Island boa, it is divided into two parts, a unique feature among snakes.

Endemic

Native to a specific geographic area and found nowhere else.