The Boa constrictor is a large snake, although it is only modestly sized in comparison to other large snakes such as the reticulated python and Burmese Python, and can reach lengths of anywhere from 3–13 feet (0.91–3.96 m) depending on the locality and the availability of suitable prey. There is clear sexual dimorphism seen in the species, with females generally being larger in both length and girth than males. As such, the average size of a mature female boa is between 7–10 feet (2.1–3.0 m), whilst it is 6–8 feet (1.8–2.4 m) for the males. It is common for female individuals to exceed 10 feet (3.0 m), particularly in captivity, where lengths of up to 12 feet (3.7 m) or even 14 feet (4.3 m) can be seen. A report of a Boa constrictor growing up to 18.5 feet (5.6 m) was later found to be a misidentified green anaconda.
The Boa constrictor is a heavy-bodied snake, and large specimens can weigh up to 27 kg (60 lb). Females, the larger sex, more commonly weigh 10 to 15 kg (22 to 33 lb). Some specimens of this species can reach or possibly exceed 45 kg (100 lb), although this is not usual.
The size and weight of a Boa constrictor depends on subspecies, locale, and the availability of suitable prey. Several populations of Boa constrictors are known as “dwarf boas”, such as the population of B. c. imperator on Hog Island. These smaller subspecies are generally insular populations. B. c. constrictor itself reaches, and occasionally tops, the averages given above, as it is one of the relatively large subspecies of Boa constrictor.
Other examples of sexual dimorphism in the species include the fact that males generally have longer tails to contain the hemipenes and also longer pelvic spurs, which are used to grip and stimulate the female during copulation. Pelvic spurs are the only external sign of the rudimentary hind legs and pelvis, seen in all boas and pythons.
The coloring of Boa constrictors can vary greatly depending on the locality. However, they are generally a brown, grey or cream base color, patterned with brown or reddish brown “saddles” that become more pronounced towards the tail. It is this coloring that gives Boa constrictor constrictor the common name of “red-tailed boa”, as it typically has more red saddles than other Boa constrictor subspecies. The coloring works as very effective camouflage in the jungles and forests of its natural range.
There are also individuals that exhibit pigmentary disorders such as albinism. Although these individuals are rare in the wild, they are common in captivity where they are often selectively bred to make a variety of different color “morphs”. Boa constrictors have an arrow-shaped head that has very distinctive stripes on it. One runs dorsally from the snout to the back of the head. The others run from the snout to the eyes and then from the eyes to the jaw.
Boa constrictors can sense heat via cells in their lips, though they lack the labial pits surrounding these receptors seen in many members of the boidae family. Boa constrictors also have two lungs, a smaller (non-functional) left and enlarged (functional) right lung to better fit their elongated shape, unlike many colubrid snakes which have completely lost the left lung.
Dependent on subspecies Boa constrictor can be found from northern Mexico through Central America(Belize, Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica and Panama) to South Americanorth of 35°S (Colombia, Ecuador, Peru, Venezuela, Trinidad and Tobago, Guyana, Belize, Suriname,French Guiana, Brazil, Bolivia, Uruguay and Argentina). Also in the Lesser Antilles (Dominica and St. Lucia), on San Andrés, Providencia and many other islands along the coasts of Mexico and Central and South America. The type locality given is “Indiis” – a mistake, according to Peters and Orejas-Miranda (1970).
Boa constrictor flourishes in a wide variety of environmental conditions, from tropical rainforests to arid semi-desert country. However, it prefers to live in rainforest due to the humidity and temperature, natural cover from predators and vast amount of potential prey. It is commonly found in or along rivers and streams, as it is a very capable swimmer. Boa constrictor will also occupy the burrows of medium-sized mammals, where it can hide from potential predators.