Habitats include deserts, plains, forests and tropical rain forests. Heavy-bodied snakes are predominantly terrestrial. Boas are normally ovoviviparous, while pythons lay eggs, and boas have one less bone in the skull. Other snake families have no trace of pelvis or hind limbs and have lost the left lung. Like all snakes, they have no eyelids and the eyes are protected by a window-clear scale called the brille.
All scales tend to be smooth and shiny.
The eyes tend to be small in a somewhat triangular head that is only slightly wider than the neck. Boas and pythons are powerful constrictors, but contrary to belief, do not intentionally break a victim's bones and squeeze it to a pulp. Rather, they strike with open jaws and at the same moment loop several coils around the prey, squeezing until breathing and heart action stop. Giant boas are among the slowest of snakes. They can be quite fast in the water.