Boas and pythons are both members of the same suborder, Serpentes. Boas belong to the family Boidae and pythons to the family Pythonidae. The species are so similar that Pythonidae is sometimes referred to and classified as as a sub-family of Boidae, and snake keepers refer to both boas and pythons as "boids" for short.
Both groups of snake are common in the pet trade, are non-venomous constrictors and some of the species tend to be similar in appearance, with longer, thick bodies. Boas and pythons are closely related to each other and have some similarities but bigger differences.
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The largest difference between boas and pythons is how they give birth. Pythons lay eggs, and boas give birth to live babies. However, boas do not give live birth in the sense that mammals do, but are ovoviviparous, meaning they develop in a yolk sac and clear membrane and emerge from this within the mother's body. If these baby boas do not develop properly in the womb they will look like shriveled yellow eggs when born, commonly called "slugs". There are a few exceptions, as one species of sand boa lays eggs.
Most boas live in the western hemisphere, although there are a few species found in Africa and island nations such as Madagascar and Fiji. Pythons are only native to the eastern hemisphere, and are found in Asia, Australia and Africa. An established Burmese python population is now found in Florida, but these pythons are an introduced species.
Boas and pythons both have tiny vestigial limbs remaining from when their ancestors had legs. These can be seen today as tiny "spurs" near the base of their tail. Both groups also have two lungs, whereas most species of snake have only one lung.
The primary difference in the snakes' anatomy is that pythons have a postfrontal bone in their skulls as well as extra rows of teeth that boas do not possess.
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Boas and pythons use constriction to kill their prey and are able to swallow prey items much larger than the snakes themselves because of their ability to disarticulate their jaws. Even the largest of both families do not pose a large danger with regards to eating adult humans, however fatalities have been observed when pet owners were suffocated by boas and pythons due to this constriction.
Both groups of snake actually have the honor of being the largest snake in the world. The anaconda, a species of boa, grows to be the heaviest snake in the world while the reticulated python grows to be the longest.
Most large boa constrictors range in size from around 4 feet to 12 feet depending on the subspecies and gender, with the anaconda being the only "giant". Pythons, on the other hand, have several species of "giant" pythons that can reach over 20 feet in extreme cases. These include the Burmese python, the African rock python and the reticulated python.