Boa constrictors are non-venomous snakes famous for their method of subduing prey: squeezing, or constricting, it to death. Though they are not as long as their relatives, anacondas and reticulated pythons, boa constrictors rank among the longest snakes in the world.
Size and appearance
The longest reported boa constrictor was 13 feet (4 meters) long, according to the University of Michigan’s Animal Diversity Web (ADW). However, boa constrictors are generally between 6.5 and 9.8 feet (2 and 3 m) long. They can weigh more than 100 lbs. (45 kilograms), according to National Geographic (opens in new tab). Females are often larger than males, according to a 2003 article published in the journal Amphibia-Reptilia.
By comparison, green anacondas reach up to nearly 20 feet (6 m) long, and reticulated pythons are commonly 16 to 25 feet (4.8 to 7.6 m) long, according to the ADW.
Boa constrictors have saddle-like markings running the length of their bodies. The markings are dark on a light tan or gray background, according to Bill Heyborne, a herpetologist and professor of biology at Southern Utah University. The saddle markings become more prominent near the tail.
Boa constrictors also usually have marks on their heads, though the configuration can vary from species to species, said Heyborne. Typically, a stripe runs from the snout to the back of the head, and a dark triangle between the snout and the eyes continues behind the eyes, where it slopes down toward the jaw. Boa constrictors sometimes have spots across their bodies.
Boa constrictors have small, hooked teeth that they use to grab and hold prey. If their teeth fall out or become damaged, they can regrow them. Boa constrictors do not have fangs, but their jaws can stretch incredibly wide, allowing them to swallow large prey, according to National Geographic (opens in new tab).
Boa constrictors, like all members of the Boidae family, have pelvic spurs. “Boas possess spurs on either side of the vent [cloaca], at the posterior of the body,” said Heyborne. “The spurs connect internally to rudimentary leg and pelvic bones, and are in fact, the remnants of hind legs.” The spurs are larger in males than females.
Range and habitat
Boa constrictors are New World snakes, meaning they live exclusively in the Western hemisphere, according to the ADW. They are found throughout the Americas, ranging from northern Mexico through Central America and into Peru on the west side of the Andes and south to Argentina on the east side. They are also found on islands in the Caribbean and off the Pacific coast.
Previously, scientists thought that all members of the family Boidae were New World snakes, but certain boa species have been discovered in remote areas such as Mauritius and New Guinea, according to the San Diego Zoo.
Though stories often depict boa constrictors living in jungles, their habitats are actually much more diverse. When they do reside in rainforests, they tend to stick to the edges or clearings. They can also be found in dry tropical deserts and semi-deserts, woodlands, scrub and agricultural areas. They are often found near streams or rivers, according to the ADW.
Boa constrictors are typically solitary, said Heyborne. They are mostly nocturnal, though they will sometimes come out during the day to sun themselves in cooler temperatures.
These snakes, especially young ones, are semi-arboreal, said Heyborne. Though they are good swimmers, they prefer to be on dry land or in trees. Because of their large size, they tend to move in a straight line instead of slithering back and forth like smaller snakes, according to the San Diego Zoo. Sometimes they hide in the burrows of medium-sized mammals.
“Like all snakes, they must shed their skin in order to grow,” said Heyborne. When they’re shedding, boa constrictors “tend to become more easily stressed and thus more aggressive,” though that tends to vary from snake to snake and across populations.
Also when shedding, boa constrictors’ eyes cloud over as a lubricating substance develops under the old skin layer to shield the eye. This affects their vision and as a result, they often stay still for a few days until their vision returns to normal, Heyborne said. When not clouded, boa constrictors have excellent vision. Along with their tongues, which gather sensory information as they do for most snakes, boa constrictors’ eyes are the primary tools through which they perceive the world, especially since boa constrictors do not have heat-sensory pits like most other members of the Boidae family do.
Hunting and diet
Boa constrictors tend to be ambush predators that sit and wait for desired prey to pass from their tree perch or burrow, said Heyborne. He noted that active foraging behaviors have been noted in places with less dense prey populations.
Boa constrictors’ diets are composed mostly of small mammals like rats and squirrels. According to the Smithsonian National Zoological Park, bats are a favorite food, which boa constrictors catch while hanging from trees or the mouths of caves, snatching their prey as it flies by.
Though most of their prey is not astoundingly large, boa constrictors will eat anything they can get their enormous, stretchable jaws around. This includes monkeys, pigs and deer. According to the ADW, it takes boa constrictors up to six days to digest a meal.
Constricting to kill
Probably the best-known characteristic of boa constrictors is their method of killing. Boas are not venomous; rather, they kill their prey by constriction, or squeezing, it to death. There are, however, some common misconceptions about how that constricting works, said Heyborne.
One is that boa constrictors crush or break the bones of their prey. Another is that they suffocate it, squeezing the prey’s lungs too tightly to work. Scientists held this belief until quite recently, when a paper revealed what happens to prey animals during constriction. “It turns out that the squeezing overwhelms the circulatory system,” explained Heyborne. “Blood cannot get to the brain, and the animal dies within seconds due to ischemia.”
Once the prey is dead, the boa constrictor swallows it whole.
“Boas tend to be solitary, until mating time,” said Heyborne. Boa constrictors typically mate during the dry season, which in their range tends to be from April until September, according to a study published in the Journal of Herpetology. The same study found evidence suggesting that females do not mate every year, while males do. The researchers hypothesized that females may only decide to mate when in top physical condition. Nevertheless, females can mate with more than one male per season. Females are widely scattered, and males must work to find them; to help with this task, females emit a scent from their cloacas when they wish to mate. (Cloacas are single openings found on the posterior of snakes used to access reproductive organs, urinary tracts and intestinal canals).
Boa constrictors are ovoviviparous, meaning they give birth to live young, said Heyborne. Inside the mother, individual clear membranes protect embryos by regulating temperatures. When they are born, the baby snakes have to push their way through the membranes. Their gestation period is about five to eight months, depending on the local temperature. Females give birth to litters that range from 10 to 64 young, with the average being around 25.
Once born, boa constrictors are fully independent. Their appearance doesn’t change much as they age, though their colors might fade a bit. Young boa constrictors tend to spend a lot of time in trees, said Heyborne. “As they grow larger and heavier, they will spend more time on the ground.” They reach sexual maturity around 2 or 3 years.
There are at least nine subspecies of boa constrictors, according to the ADW. The Integrated Taxonomic Information System (ITIS) lists 10. The number changes regularly as scientists conduct research. In fact, a comment on the Boa constrictor report on ITIS says:
As of late 2014, it appears that some subspecies and synonyms under Boa constrictor … may need to be split out and allocated to one or more separate species. For example, there are indications that Boa constrictor imperator … may represent a separate species, possibly to contain all the Central American representatives; however, until more complete sampling and analysis (and examination of type specimens) have been completed, it remains unclear which subspecies and synonyms would belong under the proposed species Boa imperator (or other species?), and which would remain under Boa constrictor.
As of November 2015, according to ITIS, the taxonomy of boa constrictors is:
Kingdom: Animalia Subkingdom: Bilateria Infrakingdom: Deuterostomia Phylum: Chordata Subphylum: Vertebrata Infraphylum: Gnathostomata Superclass: Tetrapoda Class: Reptilia Order: Squamata Suborder: Serpentes Infraorder: Alethinophidia Family: Boidae Genus & species: Boa constrictor Subspecies:
- Boa constrictor amarali — Amarai's boa constrictor
- Boa constrictor constrictor — red-tailed boa constrictor
- Boa constrictor imperator — common Northern boa constrictor
- Boa constrictor longicauda — Tumbes Peru boa constrictor
- Boa constrictor mexicana — Mexican boa constrictor
- Boa constrictor nebulosa — Dominican clouded boa constrictor
- Boa constrictor occidentalis — Argentine boa constrictor
- Boa constrictor orophias — St. Lucia boa constrictor
- Boa constrictor ortonii — Orton's boa constrictor
- Boa constrictor sabogae — Pearl Island boa constrictor
When most people hear the words “boa constrictor,” they usually think of the red-tailed boa, said Heyborne. They have dark saddle-like markings on a tannish background. “The saddles become more prominent and more brightly colored (reddish) toward the tail end, giving the snakes their common name,” he said.
They are very popular as pets. According to Reptiles Magazine, animals bred in captivity often have brighter coloration than those found in the wild. Pet owners should be aware that they grow quickly, reaching six feet in just a few years and growing up to 12 feet long.
Boa by another name
Several different snakes are called "boas" but are actually not members of the Boa genus. Many of them are popular as pets. For example, the Jamaican yellow boa, or simply the “yellow snake,” is a member of a different Boidae genus, Epicrates subflavus. It is found only on the island of Jamaica and is the island’s largest terrestrial predator, according to an article in Molecular Ecology. They can grow to be about 6 feet long.
Similarly, the emerald tree boa is a brilliant green snake, also in the Boidae family but not a subspecies of Boa constrictor. True to their name, emerald tree boas (Corallus caninus) are typically a stunning green color, though the shade can vary from bright as fresh grass to a duskier olive. Emerald tree boas live in South America and range from Venezuela to Peru and Bolivia to Brazil. They get bigger the farther they are into the Amazon Basin, according to the Smithsonian National Zoological Park.
Rosy boas are relatively small snakes, and also are not true boa constrictors. They are members of the Charinidae family named Charina trivirgata. They grow to just over 3 feet. They are native to the United States, being found in eastern California and Arizona as well as in Northern Mexico and Baja California, according to Arkive.org. Rosy boas live in deserts, scrublands and rocky hills. They are secretive snakes that live in burrows during the day.
Rubber boas are close cousins of the rosy boas and are also found in North America. They live in northern, cooler climates like the Pacific Northwest and British Columbia. Some also live in Southern California. These snakes get their name from their distinctive smooth skin and tendency to curl themselves up in what looks like a rubber ball when threatened, according to Seattle’s Burke Museum.
Boa constrictors are not listed on the International Union for Conservation of Nature's Red List of Threatened Species. However, they are on the Convention on International Trade in Endangered Species (CITES) Appendix II, which means they are not currently under threat of extinction but that may change if trade is not carefully controlled. The subspecies Boa constrictor oxidentalis is more endangered and on the CITES Appendix I list.
Some populations of boa constrictors have come under threat from hunters seeking their attractive skins and meat. They have also faced habitat loss from both urban and agricultural development as well as increased danger from road vehicles. Boa constrictors have also been overly collected for the pet trade, according to the ADW.
Boa constrictors rarely, if ever, attack people, except in self-defense, according to the ADW. People, even children are too big for a boa constrictor to swallow.
Morphs are varieties of snakes (and other animals) that have been selectively bred to produce markings, eye colors and sizes that are different than they are in the wild.