The boa constrictor belongs to a specialized grouping of reptiles: the first class of vertebrates to evolve to become completely independent of water.

Boas still have vestiges of a pair of hind limbs, which are the most striking evidence of a link to their lizard origin. These pelvic remnants are clearly visible and are called anal spurs.

Boa constrictors often reach lengths of up to 4 m (13 ft.), and even-longer specimens have been recorded. They can live about 20 to 25 years. Boas are pinkish or tan in colour, with dark crossbands.

These snakes become sexually mature in three years. They give birth to live young rather than lay eggs; each snake is born encased in a sac-like membrane, which it must break open soon after. A boa constrictor will birth 15 to 40 young at a time.

The boa constrictor is found in the tropics of Central and South America, from Mexico to northern Argentina. It lives in deserts, wet tropical forests, open savannah and cultivated fields. Some boas live in underground holes, while others live in trees.

Boa constrictors are nocturnal. They eat mainly birds and small mammals such bats, opossums, mongooses, rats and agoutis. The snake's bite is not poisonous—they kill their prey by seizing it in their jaws, then entwining it in their powerful body coils and suffocating it. They are not a threat to humans.

The python and the anaconda belong to the same family as the boa.