Boas are a group of nonvenomous, constricting snakes (family Boidae), most of which are found in tropical America and in Madagascar. Boas bear live young, and in this way they differ from the Old World
pythons, which lay eggs. Boas are of ancient derivation, retaining some of the features of their lizard-like ancestors, such as paired lungs (modern snakes have only one), tiny remnants of hind limbs (often called spurs), and a characteristic bone in their lower jaw, the coronoid, which is not found in advanced snakes. Boas have no poison fangs, and they kill their prey by squeezing, though their prey is often bitten first.
Boas are classified in the family Boidae, whose members have small nostrils and small eyes with elliptical, vertical pupils. Boas are among the most ancient groups of living snakes, having been present in the Cretaceous period, 200 million years ago, when dinosaurs still walked the earth. Although boas made up a major part of the snake fauna in the past, today only about 40 species of boas are known. Two quite distinct subfamilies are recognized, the true boas, which are mainly tropical rainforest animals from South America and Madagascar, and the sandboas from the deserts and other arid regions of the Northern Hemisphere. The green anaconda (Eunectes murinus ) of South America is the largest boid snake and probably the heaviest snake species in the world. An average individual is 10–15 ft (3–4.6 m) long. A number of wild green anacondas weighing more than 300 lb (136 kg) have been recorded.
All primitive snakes (other than the burrowing blindsnakes) have eyes with vertically elliptic pupils (cat-eyes) that can open widely in the dark. Boas have a stout body, short tail, and a green, brown, or yellow body with either blotches or diamond patterns. They tend to be most active at night, whether they inhabit deserts or rainforests. Beyond this general similarity, however, the South American boas are highly varied in their habits as well as in their overall appearance.
The boa constrictor, (Boa constrictor ) and the West Indian boas (Epicrates ) are primarily ground-dwellers, although they may also climb trees, but they show no specializations for a particular lifestyle. As in other boas, the young feed on small animals such as lizards, whereas the large adults tend to feed on larger mammals and birds.
The West Indian boas are found on many Caribbean islands. In general, each island has a single, unique species. The exception is the island of Hispaniola (Haiti and the Dominican Republic) which has three species—Epicrates fordi and E. gracilis, each only about 3 ft (1 m) long, and E. striatus, a much larger snake that reaches a length of 8 ft (2.5 m) or more. The latter species is also found on a number of islands in the Bahamas. On several Caribbean islands boas gather at cave entrances at night, snatching bats out of the air as they exit or enter the cave.
Probably the best known representative of the true boas is the boa constrictor of tropical America, from Mexico to Argentina. Although often depicted as a giant, man-eating snake in lurid stories or movies, this boa seldom grows to a length of more than 10 ft (3 m); the record is 16 ft (5 m). Boa constrictors are gentle and easy to care for, and have become one of the favorite pet snakes in recent years.
The green anaconda (Eunectes murinus ) of South America may be the largest snake in the world. It has fairly reliably been reported to grow to 35 ft (11 m) long, although 25 ft (7.6 m) is the maximum seen in recent times. These dark green snakes with black marking are river-dwellers, and are highly adapted for an aquatic existence. The anaconda’s eyes and nostrils are positioned on top of its head to allow it to see and breathe while the rest of its body is completely submerged. Anacondas lie submerged in the water at night waiting for peccaries (pigs) to come down to drink. Besides feeding on mammals, anacondas are also known to eat birds, crocodilians (caimans), and turtles. Like other boas, anacondas give birth to 40 or more live young.
The tree boas (Corallus and relatives) are highly modified for a life in the trees. Tree boas have slender bodies and their prehensile tails make them excellent climbers. They also have large eyes for nocturnal foraging, and long teeth for catching sleeping birds and other tree-dwellers, such as lizards, rodents, and opossums. Tree boas, like all boas, have heat-sensitive grooves between the labial scales under their nostrils that locate warm-blooded prey, even in total darkness. The emerald tree boa (Corallus caninus ) and the green tree boa (Boa canina ) are especially well-adapted for an arboreal life, with green, white-marked coloration making them almost invisible in the trees.
The West Indian boas (Epicrates ) are common on those islands lacking the snake-eating Indian mongoose, which was introduced on several islands in the 1800s to control the rats in the sugarcane fields. However, boas are almost extinct on most islands where the mongoose occurs. As is the case with so many attempts to introduce exotic animals for a particular purpose, the mongoose did not do the job that was expected. Instead of eating rats, the mongoose preferred chickens to rats and also ate ground-nesting birds and terrestrial lizards and snakes, some of which have been driven to extinction.
The distribution of the true boas is rather odd. Although most of them inhabit tropical America, there is one group of three species, the Pacific boas (Candoia ), that are found on the other side of the Pacific Ocean, in the Fiji islands and on other islands north and east of New Guinea. Although they appear to be closely related to the American species, these Pacific boas live in an area more than 4,000 mi (6,440 km) away. It is not easy to explain this disjunct distribution, but a parallel case is found in the Fiji iguanas (Brachylophus ), whose closest relatives are also in tropical America.
Most sandboas are relatively small snakes, less than 3 ft (1 m) in length, currently found in southern Asia and northern Africa, and in western North America. In most ways the sandboas are very much like the true boas of South America, but because most of them live in relatively treeless areas, they are more adapted to burrowing in the sand than to climbing.
Constricting snake— One that kills its prey by wrapping its body around it to stop its breathing movements.
Like the South American boas, they feed on small animals, such as lizards and rodents, which they kill by constriction. Because of their subterranean habits, however, the sandboas tends to have small, compact heads that can be pushed through the soil, and short, stubby tails that can act as “pushers.” Their tail verte-brae are specialized and can be recognized in the fossil members of this family. Such sandboa fossils are known from many localities in western Europe and eastern North America that are very distant from the areas sandboas currently inhabit. The two American sandboas are the rubber boa of the dry pine forests of the western states of Washington and Oregon, and the rosy boa of the southwestern desert regions. The latter is a handsome snake that is a favorite of pet owners.
As in the true boas, there is a strange situation in sandboa distribution and relationship. On the island of Madagascar, off the southeast coast of Africa, there are two boas that resemble those of South America. One of these is specialized as a tree boa, the other is so similar in appearance to the South American boa constrictor that some experts have placed it in the same genus. Recent research, however, suggests that despite their superficial similarities the Madagascan boas are related to the sandboas, and probably represent an ancient division of that group.
Cogger, Harold G., David Kirshner, and Richard Zweifel. Encyclopedia of Reptiles and Amphibians. 2nd ed. San Diego, CA: Academic Press, 1998.
de Vosjoli, Philippe, Roger Klingenberg, and Jeff Ronne. The Boa Constrictor Manual. Rev ed. Irvine, CA: BowTie Press, 2005.
Ross, R.A., and G. Marzec. The Reproductive Husbandry of Pythons and Boas. Stanford, CA: Institute for Herpetological Research, 1990.
Tolson, P.J., and R.W. Henderson. The Natural History of West Indian Boas. Excelsior, MN: R & A Publ., 1994.
Herndon G. Dowling