How do hibernating animals know when to wake up?

Not all hibernators wake at the same time. The length of hibernation varies by species and habitat. But bats in Africa, marsupials in Australia, tenrecs in Madagascar and other classic hibernators all wake at predictable times. What rings their alarm?

An animal in a dark burrow can’t feel warmer temperatures, or sense longer days. The signal to wake up comes from inside. Hibernators have an internal clock, a series of chemical reactions controlled by the hypothalamus of the animal’s brain.

External signals set the clock. Take a woodchuck, for example. The shorter days and cooling temperatures of autumn set its clock to time zero. The animal goes into hibernation, then wakes up about 180 days later. When its central nervous system sounds the alarm, a hibernator starts to shiver. This uses energy and generates heat.

When animals hibernate, their metabolic rate slows way down to conserve energy. A hibernating woodchuck, whose body temperature drops from 37 degrees Celsius to around 0 degrees Celsius, uses so little energy it can live for six months off the body fat that would last a little more than a week in its normeothermic, or waking, state.

Hibernators have a kind of fat that isn’t burned for energy during hibernation. This brown fat is used to generate heat when the animal gets cold, or after a big meal. Hibernators aren’t the only mammals who use their brown fat in this way. When you feel hot after eating a big meal, it’s partly from the generation
of heat by your brown fat cells.

All known deep hibernators arouse periodically during their hibernation to get rid of the metabolic waste that has built up. For the ground squirrel, it’s about every week. It re-warms for about three hours, then re-enters hibernation.

June 1, 2008

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