Splitjaw Snake: Bolyeriidae

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The splitjaw snake has an upper jaw bone split into front and back halves that are hinged together at a point just below the eye. With this unusual split in the jaw, the bone holding the upper teeth in the front of the mouth can bend up and down, while the bone holding the back teeth can stay in place. No other bird, mammal, reptile, amphibian, or fish has such a strangely jointed jaw. For many years, this snake was considered to be a member of the boa family, but its odd jaw was so unusual that scientists felt it should be in its own family. Despite its listing in its own family, the splitjaw snakes often go by common names that still include the word "boa."

Two members of this family existed in the 20th century, but only one has survived to enter the 21st century. The smooth-scaled splitjaw, also known as the smooth-scaled Round Island boa, is now believed to be extinct. The other species, the keel-scaled splitjaw, still exists today. The main difference between the two snakes is the presence or absence of small ridges, or keels, on the scales. Only the keel-scaled splitjaw has the ridges. In the splitjaws, as in other snakes, the ridges make the skin look a bit dull. Smooth scales, on the other hand, usually give snakes a shiny appearance.

The keel-scaled splitjaw is a thin snake with six-sided, or hexagonal (HEHK-SAE-guh-nuhl), scales running down its back. In many snake species, the back scales overlap, but the splitjaw's back scales barely touch each other, if at all. The snake has a long tail that makes up at least one-quarter of its entire body length. In snakes, the tail begins at the vent, a slitlike opening on the belly side. Its head is wider and flatter than the neck and is quite long, with an often noticeable black stripe behind the eye. Sometimes a white stripe lies alongside the black face stripe. The snake has a catlike pupil, but since its eye color is quite dark, the pupil is usually difficult to see. The upper body is light-to-dark brown, and the cream-colored belly is speckled with brown.

Some snake species have bits and pieces of leftover hip bones. In humans and other walking animals, the hip bones link to the leg bones, but since snakes have no legs, they do not need them. In splitjaws, no bits of hip bone remain. Adult keel-scaled splitjaws generally reach about 4 feet (1.3 meters) in length.


Also known as the Round Island casarea boa, the keel-scaled splitjaw lives only on Round Island, which is located in the Indian Ocean east of Madasgascar and just northwest of the island of Mauritius. At one time, this snake made its home on other small islands near Round Island and on the much-larger Mauritius, but now they live on just the one island. Round Island covers only 374 acres (151 hectares) and was created from lava ejected from a volcano. In the 1960s and 1970s, Round Island also had another species of splitjaw. During that period, observers on the island discovered a smooth-scaled splitjaw and watched the snake over a two-decade period. They were able to identify the snake from sighting to sighting by a distinctive scar on its body. They saw that lone snake on Round Island for the last time in 1975, and no one has ever seen a smooth-scaled splitjaw again. The discovery of the living snake was quite fortunate, because scientists would otherwise have never known of this species. While fossils of many other living and extinct snake species have been found, no one has ever found and identified a fossil from the smooth-scaled splitjaw.


The keel-scaled splitjaw prefers to live in the lush palm-covered rainforest of Round Island. Because much of the rainforest is now gone, however, the snake is surviving among stumps, scraggly bushes, and what few areas of thick forest it can find. The snake stays underground much of the time and therefore relies on proper soil conditions. Unfortunately, humans introduced animals, such as rabbits and goats, to the island. These animals eat plants and have completely wiped out many of the plants native to Round Island. Without the plants and their roots to hold the soil in place, rain can wash away and wind can blow away the soil that makes up the snake's habitat. Now, scientists estimate that 90 percent of the soil has disappeared.


A picky eater, the keel-scaled splitjaw snake eats little other than lizards, especially the day gecko and two types of skink. The splitjaw catches the slender and often-quick lizards during the day by remaining motionless and waiting for a lizard to accidentally come too close. The snake then strikes out and grabs the passing lizard. At night the splitjaw tries a different method. It hunts down the lizards using its senses of smell and sight. While holding most of its body close to the ground, the snake raises up its head a few inches (6 centimeters or so) and flicks its tongue. The tongue picks up scent chemicals in the air. It then slowly sneaks up on the lizard by slithering forward almost in a straight line, and when it gets near enough, strikes out to grab the animal.


How do snakes, such as the keel-scaled splitjaw, get to islands? Although most people do not consider snakes to be swimmers, many of them can swim quite well for at least short distances. This explains how they reach islands close to shore, but sometimes snakes are found on islands far out in the ocean. In this case, some of them may have floated by climbing onto a large branch that was broken off a coastline tree and fell into the surf, or possibly they may have stowed away on a boat or a plane and slithered on shore after landing on the island. Another possibility is that a bird snatched up a snake on the mainland and held it in its claws to kill and eat later, only to accidentally drop it when it was flying over an island. Even though snakes can reach islands in many ways, some islands still have few, if any, of these animals. For example, only one species of land-living snake occurs on Hawaii. The snake, called the Brahminy blind snake, came to Hawaii from Asia probably in a shipping carton.


The keel-scaled splitjaw snake is mainly active at night, although it does do some hunting during the day. It usually stays on or under the ground, probably spending a good deal of its time in small moist tunnels, or burrows, which provide a safe hiding place. Splitjaw snakes will also climb up shrubs and tree limbs, sometimes reaching heights of 8 feet (2.5 meters). Scientists knew very little about the reproduction of this species until the Jersey Wildlife Preservation Trust, a group in the United Kingdom that tries to save endangered animals by breeding them in captivity, were able to get two captive snakes to mate successfully in 1982. The female laid eggs. Since then, other female keel-scaled splitjaws have laid eggs, too. No one has observed the snakes mating in the wild, but in captivity, they seem to mate most successfully from March to July and lay eggs from May to October. A female typically lays three to eleven soft-shelled eggs at a time, possibly laying them in a hidden spot, such as within a pile of leaves or inside a hollow tree trunk. Females may stay with the eggs for a while. When they hatch in about three months, the young are bright orange.


People rarely see this snake in the wild.


The World Conservation Union (IUCN) and the U.S. Fish and Wildlife Service consider the Round Island casarea splitjaw to be Endangered, or facing a very high risk of extinction in the wild throughout all or a significant portion of its range. It once lived on the nearby and much larger Mauritus Island, but habitat loss, combined with the presence of non-native species, wiped out the splitjaws. On Round Island, the snakes had to survive the loss of the rainforest. In the 1970s, people became aware of the problems faced by the snakes and other animals on Round Island and set out to remove the non-native goats and rabbits that were eating the native plants, and therefore destroying the soil conditions needed by the snake. Now, to protect the rainforest further, only scientists and conservationists are allowed to visit Round Island. Plans are under way to remove non-native animals from a few other nearby small islands where the splitjaws once lived and possibly release some captive-bred splitjaws there. The hope is that the snakes will survive to breed and produce a wild population.

The U.S. Fish and Wildlife Service lists the smooth-scaled splitjaw as Endangered, but the World Conservation Union (IUCN) lists it as Extinct. No one has seen that species, also known as the Round Island bolyeria boa, since 1975.



Burnie, David, and Don Wilson, eds. The Definitive Visual Guide to the World's Wildlife. New York: DK Publishing, 2001. Page 379.

Day, David. The Doomsday Book of Animals. London: London Editions Limited, 1981.

Greene, Harry W. Snakes: The Evolution of Mystery in Nature. Berkeley: University of California Press, 1997.

Web sites

"Round Island Keel-Scaled Boa (Casarea dussumieri)." http://www.arkive.org/species/GES/reptiles/Casarea_dussumieri/more_info.html (accessed on September 8, 2004).