Boa Constrictor

The **boa constrictor** (*Boa constrictor*), also called the **red-tailed boa** or the **common boa**, is a species of large, heavy-bodied snake that is frequently kept and bred in captivity. The boa constrictor is a member of the family Boidae, found in tropical North, Central, and South America, as well as some islands in the Caribbean. A staple of private collections and public displays, its color pattern is highly variable yet distinctive. Ten subspecies are currently recognized, although some of these are controversial. This article focuses on the species *Boa constrictor* as a whole, and on the nominate subspecies *B. c. constrictor*.

## Description

Boa constrictor has long been famous as one of the largest species of snake. In reality, boa constrictors are fairly modest-sized boids and are dwarfed by the other competitors for this title. The maximum length reported in B. constrictor was slightly over 4 meters. Individuals are generally between 2 and 3 meters in length, although island forms are commonly below 2 meters. Within populations, females are usually larger than males. However, the tails of males may be proportionally longer than those of females because of the space taken up by the hemipenes. Boa constrictor coloration and pattern are distinctive. Dorsally the background color is cream or brown that is marked with dark "saddle-shaped" bands. These saddles become more colorful and prominent towards the tail, often becoming reddish brown with either black or cream edging. Along the sides, there are rhomboid, dark marks. They may have smaller dark spots over the entire body. The head of a boa constrictor has 3 distinctive stripes. First is a line that runs dorsally from the snout to the back of the head. Second, there is a dark triangle between the snout and the eye. Third, this dark triangle is continued behind the eye, where it slants downward towards the jaw. However, there are many variations on appearance. At least 9 subspecies are currently recognized by some authorities, although many of these are poorly defined and future research will undoubtedly modify this taxonomy. Currently acknowledged subspecies include: B. c. constrictor, B. c. orophias, B. c. imperator, B. c. occidentalis, B. c. ortonii, B. c. sabogae, B. c. amarali, B. c. nebulosa (Dominican boa, recently elevated to full species), and B. c. longicauda. Most of these subspecies are distinguished largely by their range rather than appearance, but regional (subspecific) variation in form, size, and coloration does occur.

As in most members of the family Boidae, boa constrictors possesses pelvic spurs. These are hind leg remnants found on either side of the cloacal opening. They are used by males in courtship and are larger in males than in females. Males possess hemipenes, a double-penis, of which only one side is commonly used in mating. Although heat-sensing pits are common in Boidae, they are absent in B. constrictor. Thus, this species is presumed to have no specialized thermosensory abilities. The teeth of boa constrictors are aglyphous, meaning they do not possess any elongated fangs. Instead, they have rows of long, recurved teeth of about the same size. Teeth are continuously replaced; particular teeth being replaced at any one time alternate, so that a snake never loses the ability to bite in any part of its mouth. Boas are non-venomous. Boa constrictors have two functional lungs, a condition found in boas and pythons. Most snakes have a reduced left lung and an extended right lung, to better match their elongated body shape.

## Range and Habitat

Boa constrictor is an exclusively New World species which has the largest distribution of all neotropical boas. Boa constrictors range from northern Mexico south through Central and South America. In South America the range splits along the Andes mountains. To the east of the Andes, B. constrictor is found as far south as northern Argentina. On the west side of the mountains, the range extends into Peru. Boa constrictors are also found on numerous islands off the Pacific coast and in the Caribbean. Islands included in the boa constrictor range are: the Lesser Antilles, Trinidad, Tobago, Dominica, and St. Lucia. Some islands off the coast of Belize and Honduras are also inhabited by this species.

Boa constrictors occupy a variety of habitats. Primary habitat is rainforest clearings or edges. However, they are also found in woodlands, grasslands, dry tropical forest, thorn scrub, and semi-desert. Boa constrictors are also common near human settlements and often found in agricultural areas. Boa constrictors are commonly seen in or along streams and rivers in appropriate habitats. Boa constrictors are semi-arboreal, although juveniles tend to be more arboreal than adults. They also move well on the ground and can be found occupying the burrows of medium-sized mammals.

## Development

Fertilization is internal, with mating facilitated by the pelvic spurs of males. Boa constrictors are ovoviviparous; embryos develop within their mothers' bodies. Young are born live and are independent soon after birth. Newborn boa constrictors resemble their parents and do not undergo any metamorphosis. As in other snakes, boa constrictors shed their skins periodically as they age, allowing them to grow and preventing the scales from becoming worn. As a boa grows, and its skin is shed, its coloration may gradually change. Young snakes tend to have brighter colors and more contrast between colors, but most changes are subtle.

### **Reproduction**

Males are polygynous; each male can mate with multiple females. Females may also have more than one mate in a season. Females are usually widely scattered and courting males must invest energy into locating them. Most female boa constrictors do not appear to reproduce annually. Usually about half of the female population is reproductive each year. Furthermore, females likely become reproductive only when they are in good physical condition. While a higher percentage of males seems to reproduce each year, it is likely that the majority of males also do not reproduce annually. Boa constrictors generally breed during the dry season, usually from April to August, though the timing of the dry season varies across their range. Gestation lasts for 5 to 8 months depending on local temperatures. The average litter has 25 young but can be anywhere from 10 to 64 young.

Maternal investment in young is considerable and requires the mother to be in good physical condition. Since young boa constrictors develop within the mother's body, they are able to develop in a thermoregulated, protected environment and they are provided with nutrients. Boa constrictor young are born fully developed and are independent within minutes of birth. Male reproductive investment is largely spent in finding mates.

## Lifespan

Boa constrictors are potentially long-lived, perhaps averaging around 20 years old. Captive boas tend to live longer than wild ones, sometimes by as much as 10 to 15 years.

## Behavior

Boa constrictors are solitary, associating with conspecifics only to mate. However, Dominican populations which will occasionally den together. Boa constrictors are nocturnal or crepuscular, though they bask in the sun to warm themselves in cool weather. They periodically shed their skins (more frequently in juveniles than adults). A lubricating substance is produced under the old skin layer. When this occurs, the snake's eye can be seen to cloud up as this substance comes between its eye and the old eye-covering. The cloudiness affects their vision and boas will often become inactive for several days until the shedding has completed and their vision is restored. During shedding, the skin splits over the snout and eventually peels back from the rest of the body. Boa constrictors are most often observed in trees or on the ground near streams and rivers.

#### **Home Range**

Boa constrictors defend territories that change over time. Territories may be abandoned if resources or conditions decline.

**Communication and Perception**
Like most snakes, boa constrictors rely on strong vomeronasal senses. Their tongues flick continuously, bringing odor molecules into contact with the chemosensory (vomeronasal) organ in the top of their mouths. In this manner, they constantly sense chemical cues in their enviornment. Boa constrictors have good vision, even into the ultraviolet spectrum. In addition, they can detect both vibrations in the ground and sound vibrations through the air through their jaw bones. They do not have external ears. Unlike most boids, boa constrictors lack thermosensory pits.

## Food Habits

Boa constrictors are carnivorous generalists. The main bulk of their diet consists of small mammals, including bats, and birds. However, they will eat any animal they can capture and fit in their mouths. Boa constrictors capture prey through ambush hunting, although occasionally they actively hunt. They can rapidly strike at an animal that passes by a branch that they are suspended from, for example. They are non-venomous and prey is dispatched through constriction. Boa constrictors wrap their prey in the coils of their body and squeeze until the prey asphyxiates. This is especially effective against mammals and birds whose warm-blooded metabolism demands oxygen at a rapid rate. Once dead, the prey is swallowed whole. Interestingly, if captive boa constrictors are presented with dead prey, they still constrict the food item before consuming it. It takes boa constrictors 4 to 6 days to fully digest a meal.

## Predation

When threatened, boa constrictors will bite to defend themselves. Though there are few references to predation on boa constrictors in nature, they are certainly killed and consumed by numerous reptilian, avian, and mammalian predators. Young boas are especially vulnerable.

## Ecosystem Roles

Boa constrictors are predators on birds and small mammals, including bats. They are important predators of rodents and opossums, especially, which can become pests in some areas and carry human diseases.