Boa constrictor

Identification

The boa constrictor is a large, heavy-bodied snake with small scales on the snout, a vertical pupil and no heat-sensing pits on its lips². It is light brown with a chain of dark brown, saddle-like markings along the back, which turn red and black on the tail^2,3. Three dark stripes run along the head: one down the middle to the neck and one through each eye^2,3.

Adults are generally 2–3 m long, and boas over 3.5 m are rare^3,9,10. One of the largest known skins, from Brazil, measures 4.45 m, and the snake was probably about 4.55 m long with its head¹⁰. Females grow larger than males, and males have larger spurs, the tiny remains of hind legs, on each side of the vent^2,3.

Distribution

The Reptile Database lists this species from Colombia, Venezuela, Guyana, Suriname, French Guiana, Peru, Bolivia, Brazil, Paraguay and Argentina¹. The Reptiles of Ecuador project also records it in Ecuador’s Amazon lowlands and in Trinidad and Tobago². Boas from Mexico and Central America were once included in this species, but they are now treated as separate species, Boa imperator and Boa sigma, so Boa constrictor in the strict sense lives only in South America¹.

The Reptile Database lists several subspecies, including the Argentine boa (B. c. occidentalis)¹. Introduced boas also live outside the native range, for example in Florida in the United States¹.

Habitat

The boa constrictor lives in many kinds of habitat, including rainforest edges and clearings, woodland, grassland, dry tropical forest and thorn scrub³. It is often found along streams and rivers, and also near farms and human settlements³. It mostly stays on the ground, but it can climb to perches up to 13 m high, and young boas spend more time in trees than adults².

Behaviour

The boa constrictor is mainly active at night². By day it usually rests coiled on the ground, in a tree or in a tree hole². It lives alone except when breeding⁹. When threatened it pulls its neck into an S-shaped coil and may give a long, loud hiss². Boas are sometimes eaten by other snakes, including green anacondas².

Diet

Boas catch prey both by waiting in ambush and by actively searching, then wrap it in their coils and constrict it until it suffocates². They eat lizards, birds and mammals such as rodents, armadillos, marsupials, bats and small monkeys, and they frequently take domestic dogs, cats and rabbits². In north-eastern Brazil, the stomachs of road-killed boas held opossums, tegu lizards, rodents and a ground-dove⁴.

Reproduction

Breeding takes place in the local dry season, and as many as 10 males have been found tangled around a single female². Males use their spurs during courtship². After 5–8 months, the female gives birth to 10–64 live young; the eggs hatch inside her body^2,3. Females probably do not breed every year, and often only when they are in good condition³. There is also evidence that females can occasionally produce young without mating (parthenogenesis)¹.

Venom and medical significance

The boa constrictor has no venom; boas have no venom glands and no venom-injecting fangs⁵. It has no fangs, but it does have rows of long, backward-curving teeth³. It kills its prey by constriction, not by venom². Anyone bitten by a large snake should have the bite checked by a doctor.

Conservation

The boa constrictor is listed as Least Concern on the IUCN Red List (published 2021)⁶. It is widespread and occurs in many protected areas, but some populations may be declining². Across the Amazon, wild boas are caught for the pet trade and for their skin and meat, and several hundred live boas and skins are shipped each year from Iquitos, Peru².

International trade is controlled under CITES Appendix II, and the Argentine boa (B. c. occidentalis) is listed in Appendix I, the strictest category⁷.

Similar species

The green anaconda (Eunectes murinus) is another large, harmless constrictor of tropical South America^5,8. Even experts have mixed them up: a famous 5.64 m “boa constrictor” killed in Trinidad during the Second World War was later found to be a green anaconda¹⁰. The anaconda is dark olive with round black spots and has its eyes and nostrils on top of its head⁸. The boa is light brown with dark saddle-like markings, three dark head stripes and a red-and-black tail². These differences are not a safe basis for handling any snake. Keep your distance from every snake you cannot identify.

In captivity

The boa constrictor is the most common pet snake in Ecuador, and wild-caught boas are shipped to pet dealers around the world². Owners need to plan for an animal that grows to at least 2–3 m and lives 20–30 years in captivity⁹, and some boas have lived up to 40 years². The Royal Veterinary College recommends an enclosure of at least 6 × 3 × 2 feet (about 1.8 × 0.9 × 0.6 m) for an adult, with a warm-to-cool temperature gradient and branches to climb⁹.