8 Things You Didn't Know About Boa Constrictors

Boa constrictors are some of the most famous snakes, partly because they're popular as pets, but also because of their vast size — sometimes 13 feet (3.9 meters) long and weighing more than 100 pounds (45 kilograms), the Smithsonian’s National Zoo & Conservation Biology Institute says.

There are more than 40 species of boas and they live in deserts, tropical forests, and savannas from Mexico to Argentina. Cousins of the green anaconda, these weighty serpents captivate researchers for many reasons (for instance, did you know they still have some remnants of legs?). Discover what makes the great boa constrictor so fascinating.

1. All Boas Are Constrictors, but There's Only One Boa Constrictor

"Boa" is a common name for more than 40 constricting snake species, all nonvenomous members of the family Boidae. It's also the name of a genus within that family, though, and the genus Boa contains only one recognized species, the boa constrictor. This is one of the fairly rare cases when a species' common and scientific names are the same (other examples: Aloe vera and Tyrannosaurus rex).

Boa constrictors are New World snakes, native to habitats from northern Mexico through Central and South America. There are several distinct subspecies, including the red-tailed boa (from the northern Amazon basin), boa constrictor amarali (from the southern Amazon basin), boa constrictor occidentalis (from Paraguay and Argentina), and boa constrictor nebulosa (from Dominica).

2. Boa Constrictors Give Birth to Live Babies

Boa constrictors are ovoviviparous, meaning the eggs stay inside the mother's body until they're ready to hatch, after which she gives birth to live young. Those baby boas hit the ground slithering, and are independent within minutes of their birth. Most clutches contain about 30 neonates, according to the Oakland Zoo. They average 6 to 24 inches (15 to 61 centimeters) in length at birth, but grow up to 3 feet (0.9 meters) within several months. They reach sexual maturity at 3 or 4 years old, at which point some individuals may stretch more than 6 feet (1.8 meters) long.

3. They Don't Kill Their Prey by Suffocation

Boa constrictors are ambush hunters, often hanging from trees until they can grab a passing animal with their jaws. Once that happens, they create two or more loops with their bodies to fully encircle their prey. By wrapping around the rib cage, they can compress their victim's vital organs and also monitor its heartbeat, letting them know when the deed is done.

Scientists long believed that boas kill with suffocation, but a 2015 study found they actually use an even quicker method: They cut off their victims' blood supply. Many animals can survive a relatively long time without air (as demonstrated when humans are revived after nearly drowning), and because boa constrictors' natural prey can be dangerous—often equipped with sharp claws, teeth, hooves, or beaks—the snakes work as quickly as possible. After a large meal, a boa constrictor may not need to eat again for weeks.

4. They Still Use What's Left of Their Legs

Like all snakes, boa constrictors evolved from four-legged ancestors. They are considered primitive snakes, though, because they still have some ancient features that have since faded away in most snake species. That includes two functioning lungs (others use only one lung, an adaptation to their elongated body shape) and remnants of legs called "pelvic spurs." Boas no longer need legs for locomotion, but they continue to use their vestigial limbs, which resemble claws protruding from their underbellies. Males use them for mating, for example, and they're said to come in handy during combat, too.

5. Living With Boas Is a Long Commitment

Boa constrictors are naturally solitary snakes that can adapt relatively well to captivity. That said, no human should enter such a relationship lightly. Wild boa constrictors can live for 20 to 30 years, and in captivity, they've been known to surpass 40. That's a long time to feed and care for any pet, but especially one that requires so much habitat maintenance to prevent problems like scale rot.

Zoos are often unable to take orphaned boa constrictors because of how much space they require, and they should never be released into the wild because they can cause major ecological problems. Boa constrictors rarely pose a direct threat to people, but large snakes should always be handled with extreme care and fed with more than one person present.

6. In the Wild, They Help Control Rodents

While wild snakes can seem frightening to some people, boa constrictors—like most snakes, and predators in general—provide valuable ecosystem services in their natural habitats. Snakes, of course, eat mice, rats, and other rodents that sometimes raid human food supplies and cause harm to humans beyond giving them a scare.

7. They Have Strong Senses, Despite Not Having Ears

Snakes don't have external ears, but they make up for it with their acute vibration sensitivity. They can detect sound vibrations and even the smallest underground movements in their jaw bones, and their eyes can see in the ultraviolet spectrum. Like all snakes, boa constrictors have split tongues that pick up odor molecules and detect where, exactly, the smell is coming from.

8. They Are in Danger

Boa constrictors have not been assessed by the International Union for Conservation of Nature and Natural Resources (IUCN), but most subspecies are on the CITES Appendix II list ("not necessarily now threatened with extinction but that may become so") and the boa constrictor occidentalis is on the Appendix I list ("the most endangered"). Wild populations have declined due to habitat loss, road mortality, and overcollection for the pet trade, especially on offshore islands.