Round Island splitjaw snakes

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Adult female Casarea dussumieri

Not to be confused with the unfortunate Round Island burrowing boa (Bolyeria multocarinata), last seen in 1975, the Round Island keel-scaled boa is still with us, although just barely. In 1996, less than 250 adult individuals remained alive, although recent captive breeding efforts have raised that to about 1000. Together, Casarea and Bolyeria made up the strange and intriguing family Bolyeriidae. Although they're sometimes called 'boas', they are distinct from the Boidae, or true boas, in not having any vestiges of a pelvic girdle. In fact, they are more closely related to the advanced snakes (Caenophidia) than to the true boas, although their phylogenetic relationship to other snakes is not quite certain. Some have advocated calling them 'splitjaw snakes' instead of boas. Due to their remote range (and the extirpations they suffered), few herpetologists have been lucky enough to see a living specimen, especially a wild one. I hope to cover what is known about these two species in this short article.

Mauritius and surrounding islets

Round Island is a herpetologically interesting volcanic islet, 151 ha (just over half a square mile) in size, located approximately 22.5 km NNE of Mauritius, in the Indian Ocean east of Madagascar. Round Island and Mauritius are part of the Mascarene archipelago, which formed between 35 and 2 million years ago as a result of the Réunion hotspot. Before the 16th century, no humans inhabited the islands, which were covered in unique tropical moist broadleaf forest. All of the Mascarene flora and fauna arrived by oversea dispersal, possibly using prehistoric islands of the Mascarene plateau, now submerged by the sea, as 'stepping stones'. Round Island is also home to an endemic skink (Leiolopisma telfairii) and an endemic day gecko (Phelsuma guentheri). Both snakes used to be found on Mauritius and other nearby islets, from which they were first extirpated. Microhabitats include fallen palm fronds and the burrows of nesting pelagic birds such as shearwaters. Captive breeding efforts are hindered by the fact that these two endangered lizards constitute the sole natural prey of Casarea, so they must be enticed to eat mice in captivity, which is more easily said than done. A few parasites of the taxon have been described, mostly by Peter Daszak of the EcoHealth Alliance.

What certainly hasn't helped clarify their taxonomy is that relatively few specimens and tissue samples are available for study. Twenty-eight genes have been sequenced for Casarea, but (unsurprisingly) none for Bolyeria, which is represented in museums by only seven specimens. Scientists are understandably reluctant to collect fresh Casarea specimens for study (DNA is far easier to sequence from fresh tissue), and the number of snakes in captivity is relatively few. Only in 2005 did we learn, posthumously, that Bolyeria probably laid eggs, rather than give live birth like many boas.

Casarea dussumieri in captivity

The skull of Casarea, which was described using high-resolution X-ray computed tomography by Masiano & Reippel (2007), is unique in having the maxilla subdivided into two movably jointed parts. That's right - the maxilla - the upper jaw. Snakes are renowned for the highly kinetic skulls, but no other snakes (or vertebrates, for that matter) have a kinetic maxilla. This jaw and its associated musculature are the basis for classifying Casarea and Bolyeria in a family of their own. Other lizard-eating snakes have analogous adaptations for grasping their hard-bodied prey, but no group takes this adaptation to such extremes as the bolyeriids. But think - on an island with no mammals and few birds, with little else but lizards to eat, selection is stronger than anywhere else for adaptations to saurophagy.

High-resolution X-ray computed tomography image of Casarea skull

Juvenile Casarea

Round Island keel-scaled boas reach 1 to 1.5 meters in length. Color changes ontogenetically (with age). According to observations made in captivity, Casarea are primarily nocturnal. Donald McAlpine, a researcher at the Jersey Wildlife Preservation Trust, where captive C. dussumeri are bred and studied, published a paper in 1983 showing data that captive specimens changed color every day, from light at night to dark during the day, reminiscent of another island-living boa, the Hog Island race of Boa constrictor from Cayos Cochinos in Honduras. Physiological color change in snakes has also been documented in the southeast Asian snake Enhydris gyii. Whether the purpose is cryptic, thermoregulatory, or something else entirely, we can only speculate. McAlpine ended his paper with the statement: "Hopefully this interesting phenomenon will be examined before Casarea becomes extinct." McAlpine's paper has been cited only twice, and as far as I can tell no research on this topic has been done since. An expedition to look for Bolyeria in 2001 was unsuccessful.

Painting of Bolyeria - no photos of living animals are known

Update 11/2/2013: I have just learned of an effort by the Mauritius Reptile Recovery Programme to reintroduce Casarea dussumieri to Gunner's Quoin, a nearby island where invasive rats and rabbits have been extirpated and to which the boa’s key prey item, the Telfair’s skink, was reintroduced from Round Island in 2007. In October of 2012, six months after this article was published, 60 C. dussumieri were released on Gunner's Quoin after 150 years of absence. If I learn the results of efforts to monitor this new population, I will post them here.

Update 3/2/2018: Thirteen new juveniles and one adult recapture were found on Gunner's Quoin in November 2015, and 28 snakes (17 of which were not recaptures) were found on Round Island in the same year. Today, conservation biologist Aurelie Hector at the Mauritian Wildlife Foundation & EDGE of Existence/Zoological Society of London/Fondation Segré is surveying the growing Round Island population of Casarea dussumieri monthly and educating children about the snakes. She observed possible predation attempt on a tropicbird egg, adding to our knowledge of the ecology of these snakes. She has also documented growth of some of the individuals released on Gunner's Quoin in 2012, as well as thriving Telfair's Skink populations, on which the snakes feed (sometimes successfully, sometimes not). Data analysis is on-going, e.g. new update here.

ACKNOWLEDGMENTS

Thanks to photographers Richard Gibson, Gregory Guida, Mike Pingleton, Drymarchon32, and Jim.

REFERENCES

Bauer A, Günther R, 2004. On a newly identified specimen of the extinct bolyeriid snake Bolyeria multocarinata. Herpetozoa 17:179-181.

Cundall D, Irish FJ, 1989. The function of the intramaxillary joint in the Round Island boa, Casarea dussumieri. Journal of Zoology 217:569-598.

Frazzetta T, 1971. Notes upon the jaw musculature of the Bolyerine snake, Casarea dussumieri. Journal of  Herpetology 5:61-63.

Hallermann J, Glaw F, 2005. Evidence for oviparity in the extinct bolyeriid snake Bolyeria multocarinata (Boie, 1827). Herpetozoa 19:82-85.

Korsós Z, Trócsányi B, 2002. Herpetofauna of Round Island, Mauritius. Biota 3:77-84.

Korsós Z, Trócsányi B, 2006. The enigmatic Round Island burrowing boa (Bolyeria multocarinata): survival in the wild remains unconfirmed. African Herp News 40:2-7.

Maisano JA, Rieppel O, 2007. The skull of the Round Island boa, Casarea dussumieri Schlegel, based on high-resolution X-ray computed tomography. Journal of Morphology 268:371-384.

McAlpine DF, 1983. Correlated physiological color change and activity patterns in an Indian Ocean Boa (Casarea dussumeri). Journal of Herpetology 17:198-201.

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.