Hibernators Large and Small

Winter strategies for species without central heating span migrating, toughing it out and hibernating. That last option - damping down the metabolism - has been taken up by species ranging in size from large to small.

Black Bears: In the late summer and early fall these New England neighbors double to triple their food intake in order to add an inches-thick layer of body fat. Come fall the bears find a confined space for a den such as under the trunk of a fallen tree. The den offers some physical protection from the elements, but is not warmer than the outside temperature. Body temperature drops to about 90F degrees and heart rate slows to ten beats per minute (bpm). This semi-hibernating state last for months, but if disturbed a black bear can achieve a fully roused state within minutes. One large puzzle is how bears manage the winter-long rest without needing to eat, drink, urinate or defecate, nor suffer from the bone loss and muscle atrophy humans would undergo from the equivalent months of bed rest.  

Woodchucks: Also known as groundhogs, these mid-sized mammals are true hibernators. They gain thirty percent in body weight, almost entirely as fat, before entering a den in late October to begin a months-long state of torpor: body temperature dropped to 40F degrees, heart rate dropped to about five bpm and breathing rate decreased to less than one per minute. Roughly every two weeks the hibernating animals rouse to full awareness, go to the bathroom and undergo a day or two of normal sleep in order to catch up on their dreaming (confirmed by rapid-eye-motion type sleep). If this coincides with February 2nd then it is Groundhog Day. One puzzle: what late summer signal triggers the beginnings of over-eating to gain all that weight?

Chipmunks: These ground-living relatives of squirrels add some body fat in the fall and store caches of food – primarily seeds – in their underground burrows. The burrows extend below the frost line. Chipmunks enter an intermittent hibernating state. Body temperatures approach 40F degrees. Heart rate slows from 200-300 bpm to under 10. Every few days the chipmunk warms back up to close to 100 degrees and becomes active. Separate areas of the den serve as food larder, bathroom and bedroom. Come late winter the males will begin to surface-wander in search of dens of females. One consequence - spring litters tend to have only one father, whereas summer litters are likely to have multiple fathers.  

Yellow Jacket Wasps: In the fall the virgin queens leave their mother's nest to mate. Like honey bees, yellow jacket queens will mate with 3-10 males. The males die. The fertilized females hibernate under tree bark on in another dry place. Insects have a few hibernating strategies to choose from. Some synthesize glycerol (antifreeze) in order to lower their freezing temperature. Some select a dry place to stay and supercool without ice crystal formation. Others allow themselves to freeze, and in fact synthesize freeze-promoting proteins, with the idea that freezing s-l-o-w-l-y and then remaining frozen all winter reduces risk of large ice crystal formation which would poke holes in cells. Yellow jacket queens employ glycerol and supercooling. Coming out from under hibernation is not triggered by an internal signal. Rather, the yellow jacket queens depend on the warming temperatures of spring to bring them back to life.

Warmer winters in Alabama and other south eastern states have led to an interesting phenomenon. Rather than the nest dying from the cold there is enough food and warmth to get through the winter as an active colony. Come spring, the colony re-expands, and related daughter queens set up housekeeping next door. So instead of scattered annual colonies there is a multi-year, condominium-like complex making up a super-colony of tens of thousands of related but not genetically identical yellow jacket wasps.