The Great Python vs Boa Debate

     One of the most common cases of mistaken identity in the snake world involves the members of the python and boa families. Now, this identity crisis is not entirely unfounded, because these animals are very similar, but that does not negate the fact that they are their own distinct families with their own evolutionary histories. 

     Before we can talk about how pythons and boas are different, we must highlight their similarities which are remnants of their shared ancestor. Both families of snakes are considered “primitive snakes”, because they still retain two lungs and the vestigial remnants of leg and pelvic bones. These whispers of legs from the past can be observed near the cloaca where there are often two bony projections known as spurs sticking out from between the wide belly scales of the snake in question. Boas and pythons are also constrictors that subdue their prey by squeezing them with extremely muscular bodies as opposed to injecting venom, like a viper or cobra. These families of snakes can also be found in a wide variety of habitats ranging from arid deserts and scrubland to tropical rainforests that have shaped the diversity of these animals.

     The 59 species that fall in the Boidae family can be found all over the globe (with the exception of Australia) from the waterways to the treetops and are the animals that we know as boas. The main thing that is unique to the Boidae family is that these species (with three exceptions) are viviparous, meaning that they give live birth after a period of embryonic development within the body of the female. Besides live birth, infrared-sensitive receptors can also be found along the upper lip of many boa species that may or may not be situated within specialized depressions or pits, these sensors allow the animal to “see” the heat radiating off of their warm-bodied prey. (This particular sense is often highlighted when discussing pit vipers.) Beyond these traits that link the family together, the boas are a diverse family with examples that range from the massive anaconda (Eunectes murinus) and the dainty emerald tree boa (Corallus caninus).

     The 40 species that make up the Pythonidae used to be grouped in with the Boidae family until recent DNA analysis determined that the pythons diverged onto their own evolutionary path much earlier than originally thought. Unlike boas, pythons are generally terrestrial, or ground-dwelling, with a few exceptions that have arboreal or semi-aquatic lifestyles, and are only found in what is known as the Old World (Africa and Asia) and Australia. Pythons are also oviparous, or egg-laying with no exceptions, however, the rearing of those eggs does vary from species to species. Traditionally, it has been thought that once a snake lays their eggs, they leave them to incubate and hatch on their own, but species like the rock python (Python sebae) have been recorded coiling around the eggs and aiding incubation by generating heat with muscle contractions. Like their boa counterparts, many pythons also have infrared-sensitive receptors, but in the Pythonidae family, these sensors are always contained within pits along the upper lip of the snake.

     Despite these differences, there are species of boas and pythons that look very similar, like the emerald tree boa (Corallus caninus) and the green tree python (Morelia viridis). These similarities have come about because of a phenomenon known as convergent evolution where two species have independently evolved similar traits as a result of similar selective pressures. What this means is that these snakes look alike because they live in similar environments, not because they are closely related.  Convergent evolution is also seen when you look at the body shapes of dolphins and sharks. Despite their similarities, pythons and boas are their own distinct families that fulfill similar roles and sometimes look alike thanks to convergent evolution. 

References:

Herpetology fourth edition - ISBN: 978-1-60535-233-6

https://animals.sandiegozoo.org/animals/boa

https://www.livescience.com/53785-python-facts.html

https://www.iflscience.com/plants-and-animals/pythons-and-boas-evolved-in-parallel/all/

• The research linked in this article will be cited as well, but may not be available if you don’t have access through a university or other type of organization. 

Esquerre, Damien, and Keogh, J Scott. “Parallel selective pressures drive convergent

     diversification of phenotypes in pythons and boas.” Ecology Letters, 2016.