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Round Island Keel-scaled Boa

Casarea dussumieri


The Round Island keel-scaled boa is unique amongst all terrestrial vertebrates on Earth, being the only species to have an intramaxilliary joint that can separate and split the anterior and posterior bones of the upper jaw.

It is thought this peculiar adaptation helps the snakes to better hold on to their preferred prey of geckos and skinks. The Round Island keel-scaled boa also has specialised skin cells that allow individuals to change colour over a 24 hour period. They darken during the day, when they are inactive, and are lighter at night when they are more active.

Following the extinction of the Round Island burrowing boa in the 20th Century, the Round Island keel-scaled boa is now the sole surviving representative of its family, Bolyeridae. This group of snakes are not actually true boas, and diverged from all other snakes more than 65 million years ago. That is before the extinction of the dinosaurs!

This species was once distributed across both the mainland and the many islets of Mauritius; however the introduction of mammalian predators such as pigs and rodents led to massive population declines and extirpation on all but one island. Grazing species such as goats and rabbits degraded and destroyed boa habitat and also contributed to the species localised extinction. Round Island was the only island to remain rat-free, which left it as a sanctuary for this species.

Durrell Conservation Trust, along with the Mauritian Wildlife Foundation, has led a restoration programme for the snake, removing all introduced goats and rabbits from Round Island and reintroducing native plants, allowing the natural habitat to regenerate. This conservation work has also helped the lizards of Round Island recover, leading to increased prey availability and therefore population growth for the boa, too.

Encouragingly, a number of boas have recently been translocated to a nearby islet where restoration has also taken place, Gunner’s Quoin, and it is hoped the species will also be able to re-establish to secure a second population for the future of this unique and threatened snake.

  • Order: Squamata
  • Family: Bolyeridae
  • Population: Unknown
  • Trend: unknown
  • Size: 150 cm (?)
  • Weight: Unspecified

EDGE Score

EDGE Score: 6.354592946 (?)
ED Score: 70.8910231 (?)
GE / IUCN Red List (?)
Not Evaluated Data Deficient Least Concern Near Threatened Vulnerable Endangered Critically Endangered Extinct in the Wild Extinct


This species is found on Round Island, Mauritius. It has recently been reintroduced to Gunner’s Quoin, a neighbouring island where the species previously occurred.

Habitat and Ecology

This snake is a semi-arboreal species. The Round Island keel-scaled boa lays clutches of up to 12 soft-shelled eggs in leaf litter or in hollow palm trunks.

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Conservation Actions

For each key category of conservation action, we calculated a conservation attention score based on expert information. In this graph, a higher score means the action is being carried out more intensively over more of the species range. The colour shows how important each action is considered to be for the conservation of this species.

Engaging stakeholders
Addressing threats
Status of knowledge
Management plan
Capacity building
Behaviour change
Awareness raising
  Score: 100 means the activity occurs at high level across more than 75% of the species range
Very Low

Overall Conservation Attention

We combined all of the expert information on conservation actions to calculate an overall conservation attention score for this species. Please help us to reach our goal of establishing dedicated conservation attention at “High” levels for all EDGE species.

Very Low Low Medium High

More information

Recent studies have grouped all possible conservation activities for any species into nine key categories (Washington et. al 2015). For each action, we asked experts for each species to assess the extent to which that action is being carried out and how much of the species’ range that action occurs in. For each action we used these two pieces of information to calculate the conservation attention score per action. A score of 100 means that the action is being carried out to a high level across at least 75% of the species range. We then combined the scores for all actions into an overall conservation attention score for the species. The experts also judged how important each category was to the conservation of that particular species.

This wordcloud illustrates the threats facing this species. The size of each word indicates the extent of a species range that is affected by that threat (larger size means a greater area is affected). The colour of the word indicates how much that threat impacts the species (darker shades of red mean the threat is more severe).

Extreme temperatures Hunting Habitat change Ecosystem changes Extreme weather Droughts Recreation

Threat wordcloud key:

Small area affected
Large area affected
Least severe
Most severe
Severity unknown
Source: EDGE

Download the survival blueprint for this species below. Each survival blueprint is compiled by an EDGE fellow working on the species with input from collaborators and stakeholders. The survival blueprint provides a status review (information on the distribution, protection status, habitat & ecology, threat and stakeholder analysis) and more information on the action programme listed here. 

Vision (30-50 years)

Saving the Endemic Species of Mauritius.

Goal (5-10 years)

To improve the relationship of human and nature on northern Mauritius and the nearby islets, to ensure the survival of endemic species such as the round island keel-scaled boa.


Empowering professionals in the North of Mauritius (Islet dependent professional, Occasional islet visitors and Government staff) to appreciate the ecological opportunities that the islets offer to enhance sustainable livelihoods and to understand why trespassing those islands could make the endemic ecosystem at risk. Critical
Develop regulation guidelines with the different Stakeholders, including government staff, to promote the sustainable use of the northern islets. Critical
Raise awareness of the wider public including coastal villagers and schoolchildren from the north of Mauritius on the best practices respect to adopt when visiting islet to reduce impacts on islet wildlife. High
Inspire youngsters to become the ambassadors of the wildlife to raise the awareness for the protection of nature. High

Aurelie Hector

  • Project name: Conservation status of the Round Island boa and setting future monitoring protocols
  • Project site: Round Island, Mauritius
  • Active: 2017 - 2019
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