Boas: Boidae

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BOAS: Boidae



Boas come in many sizes, from small to very large. The adults of some species grow to less than 1 foot (about 0.3 meters) in length, but some are immense. The boa constrictor (kun-STRIK-tuhr), for example, reaches nearly 14 feet (4.3 meters) in length, and the green anaconda can grow to 25 feet (7.7 meters) in length and 300 pounds (136 kilograms). Among all the boa species, females are usually larger than males.

The boas are split into two subfamilies. One includes the sand boa, rubber boa, rosy boa, and eleven other species, none of which grows to much more than 4 feet (1.2 meters) in length. They all have small eyes, narrow heads on thick necks, large scales on the end of their snouts, and short tails. The tail in a snake is the part of the body behind the vent, a crosswise opening on the belly side of the snake and toward the rear of the animal. The other subfamily includes the anacondas, boa constrictors, and other mostly larger snakes. The smallest is the Abaco boa, which reaches just 31.5 inches (81 centimeters) in length, and the largest is the green anaconda, which can be about ten times as long. Members of this subfamily have large heads on smaller necks, large eyes, and long tails. The anacondas are different in that they have distinctively soft and loose skin.

Depending on the species, boas may be red, orange, yellow, green, brown, or gray and may or may not have patterns of blotches or spots on their backs. Some have scales that shine in different colors when the light strikes them in certain ways, and, in a few, the color of the skin changes completely from dark in the daytime to light at night. For example, the Fiji Island boa can switch from black to pale pink within six hours. Two features that all boas share are the presence of heat sensors on the front of the face and two little bits of bone, known as spurs, that look like small claws. One spur lies on each side of the vent. The spurs are always noticeable in males but are sometimes small and not easily seen in females.


Boas live in many places around the world, including South America, Central America, North America, Europe, Africa, and Asia. They are also present in New Guinea and on many islands throughout their range.


From fields to forests and marshes to deserts, boas live in many different habitats. Some of the sand boas make their homes in deserts, while others, like the viper boas, live in wet forests. Some species prefer warm climates, but others are able to exist in very cold areas, like southern Mongolia in Asia. The boa constrictor is unusual in that it can survive well in a wide variety of habitats, from deserts to rainforests in warmer climates and also grasslands in cooler areas.


While boas may spend some time slinking through their habitats looking for animals to eat, most of them are ambush hunters, which means that they find a good spot, wait motionless for a prey animal to wander by, and then strike out to grab it. The heat sensors on their faces help them "see" the heat coming from the prey, which helps them to hunt at night. The sand boas ambush prey by burying themselves in the sand and waiting for lizards or small mammals. Amazon tree boas coil around tree branches to ambush birds, and Puerto Rican boas sit still in the entrances to caves and watch for bats. Green anacondas, which are also called water boas, often lurk underwater until a passing fish or other animal comes within striking distance. Members of the boa family are constrictors, which means that the snake will kill its prey by looping its body around the animal and squeezing, cutting off the animal's air until it is dead. While most boas eat small mammals, birds, or reptiles, the green anaconda and a few of the giant species eat quite large animals, including deer and crocodilelike caimans (KAY-muhns). Some reports, although extremely rare, indicate that green anacondas have killed and eaten humans.


Boas frequently come out during the day to sunbathe, or bask, which warms their bodies. They are most active, however, at night. Some of their most interesting behaviors are seen in the ways they defend themselves. When threatened, many sand boa species roll the body into a ball with the head buried in the middle, and some of the short-tailed species poke out the tail to trick the attacker into thinking it is actually the head. The snake can survive a bite to the tail much better than a bite to the head. The Fiji Island boa flattens its head and neck much like a cobra, which makes the snake look bigger and may frighten off an attacker. Some of the larger boas hiss, strike, and bite when they feel threatened. They may also ooze a bad-smelling material from the vent area.

During breeding season, the males of some species wrestle over females, sometimes biting one another. In most species, the females give birth to baby snakes. A few, like the Calabar ground boa (sometimes mistakenly called a ground python, which confuses it with the python family), lay eggs.


Many of the smaller species have little contact with humans. Some people hunt the larger boas for their skins and/or meat or to make medicines. Several species are popular in the pet trade.


People are often surprised that a snake that looks so small can even get its mouth around what look to be impossibly large animals that make up its diet. A green anaconda, for example, can eat an entire deer. Snakes are able to do it, in part, because their lower jaws are different from those in a human. Unlike a person's lower jaw, a snake's jaw is split into left and right sides that are connected by stretchy muscle and tissue, called ligament (LIH-guh-ment). As the snake's teeth grasp the prey animal and draw it into the mouth, the lower jaw—one side at a time—moves forward and pulls the animal farther inside. The snake's head and then its neck stretch like elastic to become much wider than normal, so the prey can fit inside its body.


According to the World Conservation Union (IUCN), one species is Endangered, facing a very high risk of extinction in the wild in the near future. In addition, four are Vulnerable, which means that they face a high risk of extinction in the wild in the near future, and two are Near Threatened and are at risk of becoming threatened with extinction in the wild in the future. The U.S. Fish and Wildlife Service lists three species as Endangered. The low numbers of these snakes result from loss of their habitat and, in the case of the endangered Mona boa, from cats and rats that have been introduced to the area and prey on the snakes. In addition to these listed snakes, one species may be extinct, that is, no longer alive. Scientists have only one record of this species, called Cropan's boa or Corallus cropanii, which dates back to the mid-1900s.


Physical characteristics: Boa constrictors are usually brown with darker brown and somewhat triangular markings running down the back. The markings may become reddish on the tail, which explains their other common name: redtail boa. The snakes have heads that are wider than their necks and long tails that they use to cling to tree branches. Boas can become quite large, with the longest reaching 13.8 feet (4.2 meters).

Geographic range: They are found in Mexico, throughout Central America, in much of South America, and on various nearby islands along the coasts of these areas.

Habitat: The boa constrictor lives in many habitats, including evergreen and deciduous jungles, rainforests, near-desert areas, grasslands, and farm fields. Boas are good climbers and are often found in trees.

Diet: Boa constrictors usually dine on small mammals, like rats and squirrels, but also on birds, iguanas (ee-GWA-nuhs), and other large lizards. Large boas, which do most of their hunting on the ground, sometimes eat bigger animals, such as porcupines. Young boas are much more likely than adults to hunt for prey in trees.

Behavior and reproduction: Boas hunt for food mostly at night, spending the day inside cracks in tree trunks, in burrows made by tunneling animals, or in some other hiding place. Scientists know little about their mating behavior in the wild. Females, which give birth to baby snakes rather than eggs, may have twenty-one to sixty-one young at a time. The babies are about 19.5 inches (49.5 centimeters) long at birth. The young can have their own young once they are two to four years old.

Boa constrictors and people: Boa constrictors are rather common in the pet trade. They are often seen in farm fields, where the snakes find, kill, and eat many pest animals.

Conservation status: This snake is not endangered or threatened. ∎


Physical characteristics: The emerald tree boa has a bright green back with white, diamond-shaped markings. The snakes have large, almost heart-shaped heads and long tails. They are not venomous (VEH-nuh-mus), that is, not poisonous, but have long front teeth— sometimes up to 1.5 inches (3.8 centimeters). Adults can grow to about 7.3 feet (2.2 meters) in length.

Geographic range: The emerald tree boa lives in the northern half of South America, near the Amazon River.

Habitat: This tropical species spends most of its life in trees, often in those with branches that hang over rivers.

Diet: An ambush hunter, the emerald tree boa waits patiently in trees for birds or small mammals, including monkeys, to approach. It then strikes out, grasps the animal with its long front teeth, and wraps its prey with its strong body. It then squeezes the animal to death before eating it.

Behavior and reproduction: This snake spends most of its time coiled around or looped over branches in trees. From this perch, it watches for a passing bird or other animal for its next meal. This is a live-bearing species, which means that the females give birth to baby snakes rather than laying eggs. The babies are often red or orange, but sometimes green. All change to green as they get older.

Emerald tree boas and people: Emerald tree boas are sought in the pet trade, but laws are helping to protect them in many countries.

Conservation status: This species is not endangered or threatened. ∎


Physical characteristics: A long and large-bodied snake, the green anaconda can reach a length of 25 feet (7.6 meters) and 300 pounds (136 kilograms). An average adult is about 10 to 15 feet (3 to 4.6 meters). It is a dark green snake with round, black spots down the back and a black stripe behind each eye.

Geographic range: This snake lives in the northern half of South America and on the West Indies island of Trinidad.

Habitat: Also known as the water boa, the green anaconda is often found in freshwater marshes, swamps, ponds, and slow-moving streams or along their shores. The young often climb onto low branches along the water's edge.

Diet: Prey include birds, fish, turtles, crocodilelike caimans, and mammals, such as deer and monkeys. The snake kills the animals by coiling its body around them and squeezing.

Behavior and reproduction: Green anacondas are ambush hunters, waiting in the water near the shoreline for prey animals to approach. They sometimes wander onto land to sunbathe, or bask. The breeding season is in the dry season, when several males will approach a female for a chance to mate with her. The females give birth to twenty to forty-five baby snakes. Some of the young can be quite large, ranging from about 2 to 3 feet (61 to 91.4 centimeters) in length.

Green anacondas and people: Green anacondas and people have little contact. Their large size and bad temper make them poor pets. Although green anacondas can and do eat humans on extremely rare occasions, most stories of such activity are untrue.

Conservation status: This species is not endangered or threatened. ∎



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