- Brown bears are found in northwestern North America, across northern Europe and Asia and in the Urals and Himalayas. They spend up to six months hibernating, usually in caves. After three months of hibernation, the females give birth in January and nurse their young until spring, losing up to 40 percent of their body weight. Autumn salmon runs in some areas help bears build up their body weight to prepare for hibernation.
- Squirrels are found all over the world, and there are many different species. Ground squirrels hibernate in shallow burrows for about six months. They go into a deep sleep and their body temperature may hover around the freezing point. Every two weeks or so, they will wake up and their body activity and temperature will return to normal briefly before they go back to sleep without leaving their burrow. Tree squirrels stay active all year, but stay in their nests to conserve body heat when it is cold.
- There are many species of mice, with different mice native to North America and Eurasia. Mice don't truly hibernate. Instead, they go into a state of hibernation-like torpor when food is scarce. Many mice live for about a year, so regular hibernation doesn't apply. Rather than trying to ensure that individual mice survive temperate-zone winters, the species' survival strategy is for any survivors to breed rapidly and replenish the population every spring.
- There are more than 1,000 species of bats in North America, Europe and Asia. Bats either hibernate or are migratory. Many species in the southwestern U.S. migrate south during the winter, while species inhabiting more northern areas tend to hibernate. Many bats hibernate in colonies in caves, packing themselves closely together to stay warm. Some mate in the autumn and give birth in the spring, after hibernation. In Europe, some species hibernate singly, in cellars, attics or barns and tend to have a high mortality rate during cold winters.
Brown Bears
Squirrels
Mice
Bats
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