Boas birth live young; pythons still lay eggs
Boid snakes—boas—range the Americas and pockets of Africa, Eurasia, and Pacific islands. They share primitive jaw gear and pelvic spurs with pythons, yet lack postfrontal bones and premaxillary teeth, and they give birth instead of laying eggs. And despite the legend, they kill by stopping breath rather than by crushing.
Boidae are nonvenomous constrictors. The green anaconda of South America is the heaviest snake and second-longest known. Like pythons they have elongated, mobile upper-jaw bones that widen the gape. Most keep a fairly rigid lower jaw with a coronoid piece and a vestigial pelvis whose hind-limb remnants show as anal spurs beside the vent—larger and more used in males during courtship. Pythons were once filed inside Boidae as the subfamily Pythoninae, but they turned out not to be close relatives despite the surface resemblance.
Distinguishing traits versus pythons include absence of postfrontal bones and premaxillary teeth, live birth, and labial pits (when present) set between scales rather than on them. Ranges barely overlap: boines dominate the New World, but other boids inhabit Africa, southern Eurasia, Madagascar, New Guinea, and the Solomons. Greatest abundance of evolutionary lineages sits in the Americas and Caribbean, with outliers from southeastern Europe and Asia Minor through Arabia to Indonesian islands. That undoes the old rule of thumb that boas belong to the New World and pythons to the Old.
Kills come by constriction: coils tighten until the prey cannot inhale and asphyxiates; recent work also stresses circulatory arrest from extreme pressure. Large individuals take cat-sized mammals; green anacondas may swallow subadult tapirs. Meals go down whole and may digest for days or weeks. Despite fearsome size, most boas are not aggressive toward humans when unprovoked. Contrary to popular belief, even the giants do not crush their victims; prey is barely deformed before it is swallowed. Classification remains contested, and most non-boine lineages, such as the Erycidae and Sanziniidae, are often raised to full families of their own.
Source: Boidae
