Boa Constrictor (Boa constrictor): Facts, Size and Habitat

Appearance

The boa constrictor has a thick, muscular body and an arrow-shaped head that is clearly wider than the neck. Its background color is usually tan, brown, gray or cream, patterned with darker, saddle-shaped blotches along the back. Toward the tail the saddles often turn reddish-brown or brick red, which gives the species its popular name “red-tailed boa.” The pattern is excellent camouflage among dead leaves and tree roots.

The head carries distinctive stripes: one runs along the top from the snout to the back of the head, and others run from the snout through the eyes toward the jaw. Color varies greatly from place to place, and some populations are darker, paler or more contrasting than others.

Like all boas and pythons, boa constrictors have small pelvic spurs on either side of the vent. These claw-like spurs are the last visible remains of hind legs, and males usually have longer ones, which they use during courtship. Boas can detect heat from warm-blooded prey with sensitive cells in their lips, although unlike some relatives they lack obvious heat pits.

How big do boa constrictors get?

Boa constrictors are large snakes, but they are modest next to giants such as the green anaconda and the reticulated python. Size depends on the population and on food supply. Females are larger and heavier than males, a pattern called sexual dimorphism.

Very large claims should be treated with caution. A famous report of a 5.6 m (18.5 ft) boa constrictor turned out to be a misidentified green anaconda.

Habitat and range

The boa constrictor lives in a remarkable range of habitats, from humid tropical rainforest to dry forest, savanna, thorn scrub and semi-desert. It is a good swimmer and is often found near rivers and streams, and it commonly shelters in the burrows of medium-sized mammals, in hollow logs and among tree roots.

In the narrow, modern sense, Boa constrictor lives in South America, from Colombia, Venezuela and the Guianas south through the Amazon Basin, Brazil and Bolivia to Paraguay and northern Argentina, as well as on Trinidad and Tobago and other nearby islands. The closely related boas of Mexico and Central America are now often treated as a separate species, described below. Our article Where do boa constrictors live? covers the full range and the introduced populations in Florida, Puerto Rico and Aruba.

Taxonomy: one species or several?

For a long time, all mainland boas from northern Mexico to Argentina were treated as one species with many subspecies. Genetic studies have shown that the boas of Mexico, Central America and northwestern South America form a distinct lineage, and many herpetologists now recognize them as the Central American boa, Boa imperator. Boas on some islands have also been raised to species rank, including Boa nebulosa of Dominica, Boa orophias of Saint Lucia and Boa sigma of the Tres Marías Islands off western Mexico.

Not every authority accepts all of these splits, so books and websites still differ. In everyday English, “boa constrictor” is often used for all of these red-tailed boas.

Diet and hunting

The boa constrictor is a carnivore that eats a wide variety of animals. Rodents such as rats, mice, squirrels and agoutis make up much of its diet, along with opossums, bats, birds and lizards such as iguanas and tegus. Large boas sometimes take bigger prey, including monkeys, armadillos and young deer. Young boas eat small rodents, birds, lizards and frogs.

Boas are mainly ambush predators. A boa lies still in cover, often near a trail or a fruiting tree, and strikes when prey comes within reach, gripping it with rows of small, backward-curving teeth. It then throws coils around the animal and tightens them each time the prey breathes out. Research on boas has shown that constriction kills mainly by stopping blood circulation, not by suffocation or crushing bones, and that boas can feel the heartbeat of their prey and squeeze until it stops; read How do constrictors kill prey? for the details.

The prey is swallowed whole, usually head first. Digesting a meal takes about 4–6 days depending on its size and the temperature, and after a large meal a boa may not eat again for weeks or even months.

Behavior

Boa constrictors live alone and meet other boas mainly to mate. They are mostly nocturnal or active at dusk, but they may bask in the sun during the day, especially when nights are cool. Young boas often climb into trees and shrubs to hunt, while large adults spend more time on the ground.

When threatened, a boa may hiss loudly and strike. Boas from Central America are often described as quicker to hiss and bite than those from South America. A bite from a large boa can be painful, but it is rarely dangerous to people, and wild boas avoid humans whenever they can.

Reproduction, life cycle and lifespan

Boa constrictors give birth to live young instead of laying eggs. The eggs develop inside the mother’s body, a mode of reproduction often called ovoviviparity. Mating usually takes place in the dry season, and males may mate with several females. Females can store sperm for months before their eggs are fertilized.

Pregnancy lasts several months. The female then gives birth to a litter of about 10–65 young, usually around 25, each roughly 38–51 cm (15–20 in) long. The babies are independent at birth and receive no parental care. They grow fast in their first years and reach maturity at about 3–4 years, then keep growing slowly for the rest of their lives.

Boas have also surprised scientists. In 2010, researchers reported a female boa constrictor that produced all-female litters without any contact with a father, a process called parthenogenesis, or “virgin birth.” Genetic tests confirmed the young had no father. Such births appear to be rare.

Little is known about how long boa constrictors live in the wild. In zoos and private collections they commonly live 20–30 years. The oldest reliably recorded boa constrictor, named Popeye, lived just over 40 years and died at the Philadelphia Zoo in 1977.

Predators and threats

Young boas are eaten by birds of prey, wild cats, caimans and other predators, and some are taken by larger snakes. Large adults have few natural enemies apart from jaguars, large crocodilians and people.

People are the main threat. Boa constrictors are killed for their skins and meat and out of fear, collected for the pet trade and lose habitat to deforestation and farming. Between 1977 and 1983 alone, about 113,000 live boa constrictors were imported into the United States.

Conservation status

The IUCN Red List classes the boa constrictor as Least Concern because it has a very large range, tolerates many habitats and has no evidence of a rapid overall decline. International trade is controlled under CITES Appendix II, and the Argentine boa (Boa constrictor occidentalis) is listed on CITES Appendix I, the strictest level, because of past overcollection for the skin trade. Some local populations, especially on small islands, are more vulnerable.

Myths and facts

Myth: boa constrictors are venomous. Boas have no venom glands and no fangs. Their many small teeth are designed to hold prey, not to inject anything.

Myth: boas crush their prey’s bones. Constriction rarely breaks bones. The coils stop the prey’s blood circulation, which causes unconsciousness and death within minutes.

Myth: boas and pythons are the same thing. They look alike and both constrict, but they belong to different families. Boas (family Boidae) live mainly in the Americas, with smaller groups such as the sand boas of Africa, Asia and southeastern Europe and others on Madagascar and Pacific islands, and most give birth to live young. Pythons (family Pythonidae) live in Africa, Asia and Australia and lay eggs, which the female often guards by coiling around them.

Myth: a boa constrictor can swallow a person. Even the largest boa constrictors are far too small to eat an adult human. Only a handful of the very largest pythons have been reliably recorded doing so.

Myth: boas must eat every day. Snakes have a slow metabolism. A wild boa that has eaten a large meal may go weeks or even months before hunting again.

Fact: boas have two lungs. Most snakes have lost or greatly reduced the left lung to fit their long, thin shape. Boas keep a small left lung alongside the large, working right lung, one of several features that mark them as an older branch of the snake family tree.

Relationship with people

In many rural parts of Latin America, boas are valued for eating rats and other animals that damage crops and stored food, and in some areas they help limit opossum numbers. They are also frequently killed on sight, even though they are harmless to adults.

Outside their native range, boas released or escaped from captivity have established wild populations in places such as Puerto Rico and Aruba, where they prey on native birds and lizards. To see how boas differ from their Old World look-alikes, read our comparison of python vs boa.