Bristol Snakes 3: Madagascar Tree Boa

In common with most zoos, the majority of Bristol’s snake collection consists of boas, pythons, and various North American colubrids. These snakes are about the easiest to maintain in captivity, as just about all of them either feed on rodents and other mammals in the wild or can be trained to east them in captivity. Maintaining the various specialist invertebrate, fish, or amphibian in captivity is much more time consuming, so people whose ideas of the potential variety of snake species and ecological specialisations are derived from zoo or pet snakes are often surprised at what snakes can do.


Nonetheless, boas can be quite variable in themselves. While many people think of them as purely New World species, just as all pythons are native to the Old World, in fact boas have a worldwide distribution, though only in South America do they reach the size of giant snakes capable of feeding on larger mammals. The largest of the Old World boas is the one addressed in this post, the Madagascar Tree Boa Sanzinia madagascariensis. Reaching a length of 2m and a body width as much as a mans’ forearm, it is one of the top predators of Madagascar.

It is an adaptable species, living everywhere from dry forest to rainforest, and although it can climb well and hunt in trees it also spends a lot of time on the ground, unlike more specialist rainforest species such as the Garden Tree Boa from the Amazon rainforest. As with most boas and pythons, it is nocturnal, feeding on rodents, birds, smaller lemurs and anything else it can catch. As with many of its relatives, it has heat sensitive pits on its upper lip which enables it to detect warm blooded prey even in pitch darkness, which it kills by constriction.

Sanzinia head - note labial pits

Like other boas, Sanzinia are ovoviviparous, which is the term used to describe many live bearing reptiles – essentially they retain the eggs within the mother’s body and only release them on the point of hatching. This means that all the nourishment for the developing embryo is supplied at the start of the pregnancy, unlike with mammals where a placenta provides nutrition as the embryo develops. Numbers of young depend on the size of the female but are usually less than 12. Neonates are around 25cm long and brick red, unlike the greenish adults, and can feed on small mice from birth. Reports from captive bred animals suggest the babies are more likely to climb than the adults, and in the wild probably take various geckos and other arboreal lizards as well as mammals.

Green phase Sanzinia

They seem to have quite a low metabolism, and once adult seem to only feed once a month or so, especially the males. This is probably an adaptation to the extremely variable environments in much of their range, where food in some years is extremely scarce. It is likely that a female takes at least one year, probably two, after reaching maturity to build up enough reserves to produce a litter, and another year or two subsequently between litters to recharge them. Give the fairly low litter size, this means that the total lifetime reproductive output of a female, even given a long lifespan (similar size boas often reach 20 years or more in captivity) is quite low. Despite this, at present it is listed by the IUCN as a Least Concern species, although deforestation in Madagascar and some collection for the illegal pet trade is a potential threat. Our Sanzinia were actually confiscations from the illegal trade in this species. One potential complication is that it occurs as two subspecies, S.madagascariensis madagascariensis in the east of the island and S.madagascariensis volontany in the western side, and these may in fact be separate species and require separate conservation measures. Even so, both of the forms are widespread and can make use of human-modified habitats, so they are probably in no immediate danger.

(images from Wkipedia and top image taken by myself at Bristol)