Boa Constrictor
The boa constrictor (Boa constrictor), also called the red-tailed boa, is one of the largest snakes in the Americas and one of the few animals known by its full scientific name. It lives in South America east of the Andes, from the Guianas and Trinidad to the dry Chaco of Argentina, in rainforests, savannas, dry woodlands and the edges of farmland. Most adults measure two to three meters, with females larger than males, and the longest reliable records pass four meters. It is not venomous: it kills by constriction, squeezing until the prey's blood flow stops. Juveniles eat lizards and small birds, while adults take opossums, rodents, bats, birds and sometimes small deer. It gives birth to live young, usually about 25 per litter. The species is Least Concern, but the Argentine boa, its southern subspecies, is protected by CITES Appendix I.
Where do boa constrictors live?
The boa constrictor lives in South America east of the Andes, one of the broadest ranges of any large snake. It is found in the Amazon of Brazil, Peru, Ecuador, Colombia and Bolivia, in Venezuela, the Guianas and on Trinidad and Tobago, and south through the Cerrado, the Pantanal and the Atlantic Forest to the Chaco of Paraguay, Bolivia and Argentina, where its southern subspecies, the Argentine boa, reaches Córdoba, San Luis and Mendoza. Few snakes use so many habitats: rainforest, gallery forest, savanna, dry thorn scrub, mangroves and the edges of farmland and towns. It is mostly a lowland snake, common below 1,000 meters. It shelters in hollow logs, burrows of other animals and tree holes, and young boas spend much of their time in trees. The boas of Mexico, Central America and the Pacific side of Colombia and Ecuador belong to a close relative, the Central American boa (Boa imperator), which many scientists now treat as a separate species.
How long do boa constrictors get?
Most adult boa constrictors measure between two and three meters, and females are clearly larger and heavier than males, which rarely pass two and a half meters. A typical large female weighs 10 to 15 kilograms, although exceptional animals have passed 45 kilograms. The longest reliable records are slightly over four meters, and the largest known skin, kept at the Bavarian State Zoological Collection in Munich, measures 4.45 meters without the head. Stories of boas of five or six meters are usually confusions with anacondas or pythons. Size also changes from place to place: the boas of some islands and dry regions stay smaller, while those of the Amazon tend to be the biggest. Newborns measure about 35 to 50 centimeters. Even a large boa is much lighter than a green anaconda of the same length, since its body is slimmer.
Red-tailed boa, lampalagua and other names
The boa constrictor is one of the very few animals whose common name is the same as its scientific name: Carl Linnaeus called it Boa constrictor in 1758, and the name stuck. In English it is also known as the red-tailed boa, for the reddish brown saddles near the tail that stand out against its tan or gray body. Several subspecies are recognized. The best known is the Argentine boa (Boa constrictor occidentalis), called lampalagua in Argentina, a darker, almost black and silver snake of the dry Chaco and the Monte. It is the only form of the boa listed in CITES Appendix I, after decades of hunting for its skin. In Brazil the boa is called jiboia. The northern boas, from Mexico to the Pacific coast of Ecuador, were long treated as the subspecies imperator and today are usually considered a separate species, the Central American boa.
How a boa constrictor kills its prey
The boa constrictor's most defining feature is its muscular body. For a long time it was believed that constrictors suffocate their prey, but a 2015 study led by Scott Boback at Dickinson College showed that a boa's squeeze stops the blood flow: within seconds the prey's blood pressure collapses and the heart and brain stop receiving oxygen, so death comes much faster than by suffocation. The boa also feels the heartbeat of its prey and keeps squeezing until it stops. It does not crush bones. Unlike pythons and tree boas, it has no heat sensing pits on its lips; it finds prey mainly by smell, flicking its tongue, and by sight. Its flexible jaws, joined by stretchy ligaments, let it swallow animals much wider than its head, always head first. Its pattern of brown saddles on a tan or gray background is excellent camouflage on the forest floor.
What does a boa constrictor eat?
The boa constrictor is an opportunistic carnivore whose menu changes as it grows. Young boas, which spend more time in trees, eat mostly lizards, frogs, small birds and mice. Adults move to larger prey: rats, opossums, agoutis, armadillos, bats, which they sometimes catch at the entrance of caves, birds such as parrots and chickens, iguanas and other reptiles, and now and then monkeys, ocelots or small deer. It is an ambush hunter that waits motionless on a branch, at the edge of a trail or near a burrow until an animal passes within reach, and then strikes, grips with its teeth and coils around it. After a large meal it can go weeks or even months without eating, because its slow metabolism gets a lot of energy from a single prey. Near towns it is often found hunting rats and chickens, which is why some farmers value it and others kill it.
Behavior and reproduction
Boa constrictors are solitary and mostly active at dusk and at night, spending the day in hollow logs, burrows, rock crevices or dense vegetation. They are ambush predators that rely on patience and camouflage rather than chasing their prey. Juveniles climb a lot, while large adults spend more time on the ground, though they can still climb well and swim with ease. When threatened a boa hisses loudly, a sound that can be heard several meters away, and may strike to warn. Boas breed in the dry season. Several males may follow the scent of a female, and after mating she keeps the young inside her body for five to eight months before giving birth to live babies, usually about 25 and up to 65, that are independent from birth. Females can store sperm, and in 2010 researchers in the United States documented a boa that gave birth without any male, a rare case of virgin birth.
Can a boa constrictor kill a human?
A wild boa constrictor is no danger to an adult. It is not venomous, it is far too small to swallow a person, and in the wild it avoids people and hides or flees when found. Its bite, given when it is grabbed or cornered, can leave many small puncture wounds that bleed and should be cleaned, but it is not life threatening. The rare deaths caused by large constrictors almost always involve much bigger species, such as reticulated pythons and Burmese pythons kept as pets, not boas. Still, a large boa is strong, and handling one alone or wearing it around the neck is a bad idea. Where boas live near people they are usually helpful neighbors, since they eat rats and mice. The real danger runs the other way: many boas are killed on sight by people who confuse them with venomous snakes.
Is the boa constrictor endangered?
No. The IUCN classifies the boa constrictor as Least Concern, reflecting its wide distribution and its ability to live in disturbed habitats. That does not mean it is safe everywhere. Deforestation driven by agriculture and cattle keeps reducing its habitat, especially in the Chaco, the Cerrado and the Atlantic Forest. For decades hundreds of thousands of boas were taken from the wild for their skins and for the international pet trade, which is why the species is in Appendix II of CITES, which regulates that trade. The Argentine boa, or lampalagua, suffered so much that it was moved to Appendix I, which bans its international trade; its habitat in the dry Chaco has shrunk by about a third. Many boas also die on roads or are killed out of fear. Island populations and heavily hunted areas are the most vulnerable.
