Some species of squirrels hibernate, while others do not.
More Info: The most common ground squirrel found in the Midwestern and Northeastern regions of the United States, the Eastern Gray squirrel, does not hibernate. It is less active in the winter months to conserve energy.
Squirrels that hibernate go into places called dens to sleep during the cold months. The generally hibernate for five to six months. Like most hibernating animals, their body temperature drops until it almost reaches that of the outside, which is below water's freezing point! A hibernating squirrel's heart will only beat a few times per minute, and its breathing rate also drops. Squirrels will wake up every few weeks for 12 to 20 hours and then go back into hibernation. Squirrels hibernate as an evolutionary adaptation that protects them against harsh conditions and poor food supply. When the hibernation period is over, squirrels' body temperatures rise quickly once again.
Some species of squirrels hibernate in the summer, which is called estivation. These squirrels live in hot climates, and it is the heat that poses the threat to their survival. In the Mojave Desert, located in California, a type of ground squirrel called the Mohave burrows into the ground to protect against hot temperatures. This type of hibernation begins between June and September and ends in February. Older squirrels go into estivation first because they need less stored food to survive. Mohave squirrels also become less active in cooler temperatures.
Like older squirrels, males go into summer hibernation first because they need less stored food. Female squirrels need enough food to feed their offspring. Ground squirrels will often have a litter of babies when they come out of estivation, which stay in the burrow for around six weeks.