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The Boa constrictor is a large, heavy-bodied species of snake. It is a member of the family Boidae found in 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, but also specifically on the nominate subspecies Boa constrictor constrictor.
Though all boids are constrictors, only this species is properly referred to as "Boa constrictor"; a rare instance of an animal having the same common and scientific binomial name. (The distinction is shared with Tyrannosaurus rex.)
All subspecies are referred to as "Boa constrictors", while the nominate subspecies, B. c. constrictor, is often referred to specifically as the "red-tailed boa". Within the exotic pet trade it is also known as a "BCC", an abbreviation of its scientific name, to distinguish it from other Boa constrictor subspecies such as the Boa constrictor imperator which is also regularly, and erroneously, referred to as a "red-tailed boa" or "common boa".
The Boa constrictor is a large snake, although 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 (0.91–4.0 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 (99 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 America north of 35°S (Colombia, Ecuador, Peru, Venezuela, Trinidad and Tobago, Guyana, 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.
Boa constrictors are solitary animals, and will only associate with conspecifics to mate. They are nocturnal; however, they may bask during the day when night-time temperatures are too low. As semi-arboreal snakes, young boa constrictor individuals may climb into trees and shrubs to forage; however, they become mostly terrestrial as they become older and heavier. Boa constrictors will strike when threatened, and will bite in defense. This bite can be painful, especially from large snakes, but is rarely dangerous. However, care must be taken to ensure that infection does not result from the injury. Specimens from Central America are more irascible, hissing loudly and striking repeatedly when disturbed, while those from South America tame down more readily. Like all snakes, Boa constrictors that are in a shed cycle will be more unpredictable. This is because the substance that lubricates between the old skin and the new will make the eyes appear "milky", blue, or "opaque", and as such the snake cannot see very well. This will cause it to be more defensive than it may be otherwise.
Prey includes a wide variety of mammals and birds. The bulk of their diet consists of rodents, but larger lizards and mammals as big as ocelots are also reported to have been consumed. Young Boa constrictors will eat small mice, birds, bats, lizards and amphibians. The size of the prey item will increase as they get older and larger.
Boa constrictors are ambush predators and as such will often lie in wait for an appropriate prey to come along at which point they will attack. However, they have also been known to actively hunt, particularly in regions with a low concentration of suitable prey, and this behaviour generally occurs at night. The boa will first strike at the prey, grabbing it with its teeth, it then proceeds to constrict the prey until death before consuming it whole. Their teeth also help force the animal down the throat whilst muscles then move it towards the stomach. It will take the snake approximately 4–6 days to fully digest the food, depending on the size of the prey and the local temperature. After this the snake may not eat for anywhere from a week to several months, due to its slow metabolism.
Boa constrictors are ovoviviparous, giving birth to live young. They will generally breed in the dry season—between April and August—and are polygynous, thus males may mate with multiple females. A half of all females will breed in a given year, and a larger percentage of males will actively attempt to locate a mate. However due to the polygynous nature of Boa constrictor many of these males will be unsuccessful. The reasoning being the fact that they are ovovivparous (i.e. eggs hatching inside the body). As such female boas without a good enough physical condition will be unlikely to attempt to mate, nor produce viable young if they do mate. In 2010, a boa constrictor was shown to have reproduced asexually via parthenogenesis.
During breeding season the female boa will emit a scent from her cloaca to attract males, who may then wrestle for the right to breed with her. During breeding the male will curl his tail around the female's and the hemipenes (or, male reproductive organs) will be inserted. Copulation can last from a few minutes to several hours, and may occur several times over a few week period. After this period ovulation may not occur immediately, however the female can hold the sperm inside her for up to one year. When the female ovulates, a mid-body swell can be noticed that appears similar to after the snake has eaten a large meal. The female will then shed two to three weeks after ovulation, with what is known as a post ovulation shed which will last another 2–3 weeks, which is longer than a normal shed. The gestation period, which is counted from the post ovulation shed, is approximately 100–120 days. The female will then give birth to young that average 15–20 inches (38–51 cm) in length. The litter size varies between females, but can be between 10 and 65 young with an average of 25, although some of the young may be still-borns or non-fertilised eggs known as "slugs". The young are independent at birth and will grow rapidly for the first few years, shedding regularly (once every one to two months). At between 3–4 years Boa constrictors become sexually mature and will have reached the adult size of between 6–10 ft, although they will continue to grow at a slow rate for the rest of their lives. At this point they will shed less frequently, approximately every 2–4 months.
This species does well in captivity, usually becoming quite tame. It is a common sight in both zoos and private reptile collections. Though still exported from their native South America in significant numbers, it is widely bred in captivity. When kept in captivity, they are fed mice, rats, rabbits, chickens and chicks depending on the size and age of the individual. Captive life expectancy is 20 to 30 years, with rare accounts of over 40 years, making them a long-term commitment as a pet. Proper animal husbandry is the most significant factor in captive lifespan, this includes providing adequate space, correct temperatures and humidity and suitable food items.
Young Boa constrictors should be started off in a relatively small enclosure, generally a secure plastic box or terrarium. This enclosure is then increased as the boa grows, as large open spaces are stressful for young snakes. Adults are often housed in 180 cm × 90 cm × 60 cm vivariums, however large females may need even larger enclosures. The general rule is that the minimum length of the enclosure should be two thirds of the snake's length. Snakes are generally kept in separate enclosures, but people have successfully kept females together. Male boas should not be kept together as they may fight. Glass, aquarium-style enclosures are seldom advised by snake keepers as they do not efficiently maintain temperature and humidity.
A thermal gradient should be provided, with a cool end and a warm end where the heat source should be located. The cool end should be maintained at 75–85°F (27–29°C), and the warm end at 86–92°F (30–33°C). Temperatures should not be allowed to rise above 95°F (35°C) or drop below 75°F (24°C). Cages that are too cold can cause many health problems, ranging from non-digestion of food to pneumonia. The necessary temperature can be provided by a heat mat, ceramic or specific light bulb or other alternative heating systems. All heat sources should be guarded, to prevent burns to the snake, and used in conjunction with a thermostat to prevent overheating. Humidity should be kept at 50%, and raised to 70% when the boa is in shed. However, high humidity should not be maintained for longer than a week, as this raises the risk of infections such as scale rot. Humidity levels can be maintained with a water bowl, and raised by adding more water bowls, moving the current water bowl closer to the heat source or misting the enclosure with a water sprayer. Boa constrictors do not need any special lighting, but should have approximately 12 hours of light and 12 hours of darkness every day to simulate a natural environment. Either a bulb or natural light can be used to provide such conditions.
Inside the enclosure a substrate, generally of newspaper or aspen shavings, must be provided. Untreated cedar or pine shavings are to be avoided as they contain oils that are toxic to snakes. A water bowl, large enough to provide adequate humidity and that the boa can coil within, must be provided. Another basic essential is adequate hides, at least two (one in the cool end and one in the warm end). Hides can be anything from empty plastic or wooden boxes to specially made hides from a reptile equipment retailer. The hides ensure that the snake feels secure, as stress can result in snakes refusing to eat. Shelves or secure branches are often provided so that boas can climb, but this is not an essential. Fake plants and other natural looking decorations are also commonly provided, but again they are not essential.
Young boas can be started on small to medium mice and then on to increasing size of rats. Most boas will never need a prey item larger than a large rat, however some big females (8 ft+) may require rabbits, guinea pigs and chickens. The general rule for feeding snakes is that a suitable prey item is the girth of the snake at its widest point, as with most snakes (including Boa constrictor) the left and right sides of the lower jaw are joined only by a flexible ligament at the anterior tips, allowing them to separate widely, while the posterior end of the lower jaw bones articulate with a quadrate bone, allowing further mobility, to allow the consuming of large prey. Young snakes can be fed once a week, to promote healthy growth. Adults only need to be fed once every month. Overfeeding (or powerfeeding) the snake can lead to a host of health problems later and can shorten its lifespan. It is natural for snakes to lose their appetite when going into shed as this is a stressful time for them, as such food should not be offered at this time. Water should be changed daily or every other day.
Boas should be handled regularly to maintain their docility. However, large boas should be handled by two people, as these snakes are incredibly powerful. Snakes should never be handled within 48 hours of feeding, due to a risk of regurgitation, or when in a shed cycle.
Boa constrictors are very popular within the exotic pet trade, and have been both captured in the wild and also bred in captivity. Today most captive Boa constrictors are captive bred, however between 1977 and 1983 113,000 live Boa constrictors were imported into the United States. These huge numbers of wild caught snakes have put considerable pressure on some wild populations. Boa constrictors have also been harvested for their meat and skins, and are a common sight at markets within their geographic range. After the reticulated python Boa constrictors are the snake most commonly killed for snake skin products, such as shoes, bags and other items of clothing. In some areas they have an important role in regulating the opossum populations, preventing the potential transmission of leishmaniasis to humans. In other areas they are often let loose within the communities to control the rodent populations.
In some regions Boa constrictor numbers have been severely hit by predation from humans and other animals, and over collection for the exotic and snake skin trades. However most populations are not under threat of immediate extinction, thus why they are within Appendix II rather than Appendix I.
There are 10 current subspecies of Boa constrictor, however much of these are poorly differentiated and it is thought that further research will redefine many of these subspecies. Some appear to be based more on location than biological differences, such as B. c. orophias (the St. Lucia Boa).
|Subspecies||Taxon author||Common name||Geographic range|
|B. c. amarali||Stull, 1932||Amaral's boa||Brazil, Bolivia and Paraguay|
|B. c. constrictor||Linnaeus, 1758||Red-tailed boa||South America|
|B. c. imperator||Daudin, 1803||Common northern boa||Central America and northern South America|
|B. c. longicauda||Price & Russo, 1991||Tumbes Peru boa||Northern Peru|
|B. c. melanogaster||Langhammer, 1983||Ecuadorian boa||Ecuador|
|B. c. nebulosa||(Lazell, 1964)||Dominican clouded boa||Dominica|
|B. c. occidentalis||Philippi, 1873||Argentine boa||Argentina and Paraguay|
|B. c. orophias||Linnaeus, 1758||St. Lucia boa||St. Lucia|
|B. c. ortonii||Cope, 1878||Orton's boa||South America|
|B. c. sabogae||(Barbour, 1906)||Pearl Island boa||"Pearl Islands" off the coast of Panama|
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