Boa

I

Introduction

Boa, common name for nonvenomous snakes that make up the boa family. All boas are constrictors—that is, they kill their prey by squeezing it to death and then they swallow it whole. Boas use the coils of their body to suffocate their prey, rather than crushing it, as is commonly believed. They can stretch their jaws wide apart to swallow animals larger than their heads. Depending on the size of its prey, a boa may take several weeks or longer to digest its food. Many of the exceptionally large snakes of the world, including the anaconda, are members of the boa family, but not all boas are large. Some are no more than 50 cm (20 in) long when fully grown. Boas vary greatly in color and pattern.

II

Evolution of Boas

Members of the boa family retain very reduced hipbones and hind legs, which can be seen externally as a pair of small, movable spurs sometimes used in mating. These primitive features show that snakes evolved from lizards that once had fully functioning legs. Boas are thought to be one of the earliest groups of snakes to evolve and are known from the fossil record as far back as the Cretaceous Period, which ended 65 million years ago. The modern distribution of boas around the world in North, Central, and South America; Africa; Europe; Asia; and on many islands, including the West Indies, Madagascar, New Guinea, and some Pacific Islands; reflects ancient periods when the land masses on Earth were connected or much closer together.

During the Paleocene Epoch about 60 million years ago a huge boa named Titanoboa lived in South America. Scientists found its fossil bones in a coal mine in Colombia and estimate it grew to more than 13 m (43 ft) long and weighed more than a ton, making it the largest known snake. Titanoboa lived in a very hot, tropical environment and probably spent most of its time in water, where it could prey on crocodiles.

Boas and pythons are closely related types of snakes that both kill by constriction, have two functioning lungs, and retain remnants of hind legs. The two groups also differ in several ways. Anatomical distinctions include certain bones in their skulls. Some boas and all pythons have special heat-sensing receptors around their mouths to detect warm-blooded prey—boas more often have heat-sensitive lip scales while pythons have organs sunk in pits. Most boas give birth to live young from eggs that are retained inside the mother’s body, while all pythons lay eggs that hatch externally. Boas are found worldwide, except in Australia, while pythons are found in Africa, Asia, Australia, and the Pacific islands. In some classifications, pythons are considered members of the boa family that belong in a separate subfamily of their own. Other classifications place boas and pythons in separate families.

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III

Types of Boas

Probably the best-known boa is the boa constrictor, found in the jungles of Central and South America. Boa constrictors are usually yellowish or grayish, with a pattern of dark brown dorsal saddles; the tail is often reddish-orange. Most adults do not exceed 2 to 3 m (6.6 to 10 ft) in length, although unusually large individuals may reach 4 m (13 ft). Stories of larger boas may have caused boa constrictors to be confused with anacondas (water boas) or with the pythons of Asia. Boa constrictors usually hunt at night and eat birds, small mammals, and sometimes lizards. They have special heat-sensitive lip scales.

Tree boas, such as the brightly colored emerald tree boa, live in trees and have strong, prehensile tails. With their triangular heads, elliptical eye pupils, and long, curved front teeth, they resemble the venomous vipers. Tree boas primarily prey on birds and have large heat-sensing pits around their mouths.

Some smaller boas, such as the rubber boa, range throughout much of the northwestern United States. Found as far north as British Columbia, Washington, and Montana, the rubber boa, whose shiny brown skin resembles rubber, is a burrowing animal. Because of its blunt head and tail, it is sometimes called a two-headed snake. Rubber boas mainly eat baby rodents.

IV

Boas and People

Although boas are generally docile, they can deliver a painful, but nonvenomous, bite with sharp, backward curving teeth. Large specimens are capable of strangling a human if they wrap themselves around a person’s neck. Boas are often displayed in zoos and have become popular as pets. The snakes are widely bred in captivity for the pet trade and for scientific research or for conservation purposes.

Only one species of boa is currently listed as endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) imposes some restrictions on the export of certain species of boas.

Scientific classification: Boas belong to the family Boidae. The boa constrictor is classified as Boa constrictor, the emerald tree boa as Corallus canina, and the rubber boa as Charina bottae.

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