When cold winds blow and hard frosts come to Maine, a silent change takes place in the animal kingdom. Many creatures we enjoy watching through the warm months begin to disappear from view. Having built up reserves of body fat in late summer and early autumn, the animals take shelter in places that allow them to survive until the following spring. We refer to this phenomenon as “hibernation.”

In my research, I found that scientists are unsettled on a firm definition of hibernation. It is broadly referred to as a condition of suppressed metabolic activity over an extended period of deep sleep, or torpor. This is not limited to winter – species such as desert lizards go into a deep sleep in periods of extremely hot or dry weather. This is referred to as “estivation.” Other species can sleep for months at any time of year if food supplies are limited, thus conserving energy until lean times are over. Scientists are actively studying the triggers for these dormant states, hoping to gain more knowledge of the phenomenon. There have even been attempts to put human volunteers into a state of hibernation, with an eye toward possible health benefits.

Winter is a lean, cold time for many species, so hibernation is a key survival technique for them. Lizards, frogs and turtles are able to dig into the mud in ponds and rivers. Since they are cold blooded creatures, their body temperatures will fall dramatically, and their pulse and respiration rate will drop to less than 10% of normal. Because so little oxygen is available to them in these conditions, frogs have evolved to absorb oxygen through their skin.

Ground dwellers like woodchucks and badgers retreat underground to deep burrows, where the earth generally remains above 50 degrees F year round. Snakes often gather in underground dens, or stone basements, in nests that can number in the hundreds. Mice and other rodents make every effort to gain entry and build cozy nests in barns, garages or cabins like mine!

Many small mammals do not have enough body mass to survive for months of cold and fasting. While they can survive body temperatures that fall as low as 27 degrees F, they concentrate enough energy in their brains to stay alive. Still, they must rouse themselves occasionally to eat. The squirrels who hoarded acorns last autumn sleep near a cache so they can grab a high energy snack when needed. Raccoons and skunks will leave their hideouts to take field trips during warm spells to forage (I’ve seen raccoons pulling down standing corn during January thaws).

Bears have been studied extensively during hibernations that can last up to eight months. This is made easier because bears sometimes take shelter above ground under boulders or stumps, or in piles of logs. You have probably seen television films of researchers crawling into dens with black bears to monitor their health, or to change batteries in monitoring devices. Bears give birth in their winter dens so researchers can also check on the cubs, as long as mom doesn’t wake up to protect her brood. Despite not eating for many months, the mother produces rich milk to allow her cubs to gain weight until they emerge from the den. The mother lives off her stored fat content, but also absorbs protein and her body can recycle urine to avoid dehydration.

So if your favorite animals seem to have gone scarce these days, don’t worry too much for their safety. Over thousands of years they have adapted survival methods to get through periods of harsh weather or scarcity. Imagine how good it must feel for hibernators to emerge in those early warm days of spring to find bright sunlight, the grass greening and a new year ahead of them.

If you like Ed Robinson’s writing, check out his two Nature Notes books! Click here for more information.