Boa constrictor

The boa constrictor is a large snake, although it is only modestly sized in comparison to other large snakes, such as the reticulated python, Burmese python, or the occasionally sympatric green anaconda, and can reach lengths from 3–13 ft (0.91–3.96 m) depending on the locality and the availability of suitable prey.[13] Clear sexual dimorphism is seen in the species, with females generally being larger in both length and girth than males. The usual size of mature female boas is between 7 and 10 ft (2.1 and 3.0 m) whereas males are 6 and 8 ft (1.8 and 2.4 m).[14] Females commonly exceed 10 ft (3.0 m), particularly in captivity, where lengths up to 12 ft (3.7 m) or even 14 ft (4.3 m) can be seen.[15] The largest documented non-stretched dry skin is deposited at Zoologische Staatssammlung München (ZSM 4961/2012) and measures 14.6 ft (4.45 m) without head.[16] A report of a boa constrictor growing up to 18.5 ft (5.6 m) was later found to be a misidentified green anaconda.[17]

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).[18] Some specimens of this species can reach or possibly exceed 45 kg (100 lb), although this is not usual.[19]

The size and weight of a boa constrictor depends on subspecies, locale, and the availability of suitable prey. B. c. constrictor reaches, and occasionally tops, the averages given above, as it is one of the relatively large subspecies of Boa constrictor.[14]

Other examples of sexual dimorphism in the species include males generally having longer tails to contain the hemipenes and also longer pelvic spurs, which are used to grip and stimulate the female during copulation.[20] Pelvic spurs are the only external sign of the rudimentary hind legs and pelvis and are seen in all boas and pythons.

Head shape of B. imperator

The coloring of boa constrictors can vary greatly depending on the locality. However, they are generally a brown, gray, or cream base color, patterned with brown or reddish-brown "saddles" that become more pronounced towards the tail. This coloring gives B. constrictor species the common name of "red-tailed boas." The coloring works as a very effective camouflage in the jungles and forests of its natural range.

Some individuals 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 with 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.[14]

A juvenile South American boa constrictor

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 family Boidae.[21] Boa constrictors also have two lungs, a smaller (non-functional) left and an enlarged (functional) right lung to better fit their elongated shape, unlike many colubrid snakes, which have completely lost the left lung.

Boa imperator has 50–95 dorsal scales, 223–252 ventral scales, 43–69 subcaudal scales, 19–25 supralabial scales and 1–3 anal scales.[22]

Their prey includes a wide variety of small to medium-sized mammals and birds.[25] The bulk of their diet consists of rodents, but larger lizards and mammals as big as ocelots are also reported to have been consumed.[24] Young boa constrictors eat small mice, birds, bats, lizards, and amphibians. The size of the prey item increases as they get older and larger.

Boa constrictors are ambush predators, so they often lie in wait for an appropriate prey to come along, then they attack. However, they have also been known to actively hunt, particularly in regions with a low concentration of suitable prey, and this behavior generally occurs at night. The boa first strikes at the prey, grabbing it with its teeth; it then proceeds to constrict the prey until death before consuming it whole. Unconsciousness and death likely result from shutting off vital blood flow to the heart and brain, rather than suffocation as was previously believed; constriction can interfere with blood flow and overwhelm the prey's usual blood pressure and circulation.[26] This would lead to unconsciousness and death very quickly.[26] Their teeth also help force the animal down the throat while muscles then move it toward the stomach. It takes the snake about 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 a week to several months, due to its slow metabolism.[27]

The effects of central fusion and terminal fusion on heterozygosity

Boa constrictors are viviparous, giving birth to live young.[28] They generally breed in the dry season—between April and August—and are polygynous; thus, males may mate with multiple females.[29] Half of all females breed in a given year, and a larger percentage of males actively attempt to locate a mate.[29] Due to their polygynous nature, many of these males will be unsuccessful. As such, female boas in inadequate physical condition are unlikely to attempt to mate, or to produce viable young if they do mate.[29] Reproduction in boas is almost exclusively sexual. In 2010, a boa constrictor was shown to have reproduced asexually via parthenogenesis.[30] The Colombian rainbow boa (Epicrates maurus) was found to reproduce by facultative parthenogenesis resulting in production of WW female progeny.[31] The WW females were likely produced by terminal automixis (see Figure), a type of parthenogenesis in which two terminal haploid products of meiosis fuse to form a zygote, which then develops into a daughter progeny. This is only the third genetically confirmed case of consecutive virgin births of viable offspring from a single female within any vertebrate lineage. In 2017, boa constrictors, along with Boa imperators and Burmese pythons, were found to contain a new set of sex determining chromosomes.[32] Males were discovered to contain a pair of XY sex determining chromosomes, while females have a XX pair.[32] This is the first time snakes were thought to contain male heterogamety, and since then has been found in ball pythons (Python regius) as well.[33]

During the breeding season, the female boa emits pheromones from her cloaca to attract males, which may then wrestle to select one to breed with her.[21] During breeding, the male curls his tail around the female's and the hemipenes (or male reproductive organs) are inserted. Copulation can last from a few minutes to several hours and may occur several times over a few week period.[34] After this period, ovulation may not occur immediately, but the female can hold the sperm inside her for up to one year.[34] When the female ovulates, a midbody swell can be noticed that appears similar to the snake having eaten a large meal.[34] The female then sheds two to three weeks after ovulation, in what is known as a post-ovulation shed which lasts another 2–3 weeks, which is longer than a normal shed.[34] The gestation period, which is counted from the postovulation shed, is around 100–120 days.[34] The female then gives birth to young that average 15–20 in (38–51 cm) in length.[25] 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 stillborn or unfertilized eggs known as "slugs". The young are independent at birth and grow rapidly for the first few years, shedding regularly (once every one to two months). At 3–4 years, boa constrictors become sexually mature and reach the adult size of 6–10 feet (1.8–3.0 m), although they continue to grow at a slow rate for the rest of their lives.[7] At this point, they shed less frequently, about every 2–4 months.[35]