Like many other species of both reptiles and mammals, some snakes do hibernate for a variety of purposes. Hibernation is when an animal's breathing, body temperature and heart rate are all greatly reduced in an effort to expend as little energy as possible. Many species hibernate through the winter, when resources are scarce, but some desert animals undergo a similar process, called estivation, during the dry summer months.
Many snakes in temperate regions of the planet, or regions that are far enough from the equator to have noticeably cool winters, will hibernate for the winter months. This is the same reason most other species hibernate, and the primary purpose is to conserve energy in a time of the year when food may be scarce and large amounts of energy will be required to keep warm. Different species of snakes will hibernate for different amounts of time. Garter snakes, for instance, hibernate less than bull snakes, who hibernate less than rattle snakes.
A second reason that a snake may choose to enter a hibernation like state is if it is pregnant. Females of a given snake species are, obviously, the only sex that engages in this form of hibernation. A pregnant female snake will not hibernate like most snakes do in winter, but will instead significantly decrease body functions. They will spend the summer months of their pregnancy basking in the sun, close to their dens, and may go in to full hibernation soon after giving birth.
Snakes that hibernate during the winter are not choosy about who they hibernate with. It is common, for instance, for snakes to hibernate with other members of the same species. It is also not uncommon to find snakes of different species hibernating in the same den. In South Dakota, for instance, hibernation dens may be filled with bull snakes, rattlesnakes, garter snakes and racers during cold winter months.
A physiological process that is similar to hibernating is fasting. Pregnant Northern Pacific female rattlesnakes, for instance, do not eat during pregnancy, which is one feature of their semi-hibernation state. A female that gives birth may then go directly in to hibernation, which will mean that the female rattlesnake has not eaten for an entire year. Snakes of both sexes also often fast before they shed.