We’re not alone coping with the cold

If you’re like most Midwesterners, you’ve had about enough of winter by now. And it probably does nothing to brighten your day for me to point out that Punxsutawney Phil saw his shadow on Sunday. So let’s leave that prognosticating groundhog behind, and contemplate some of the fascinating wintering strategies of other furry creatures.

I learned about these through a recent conversation with Joe Merritt, who is a mammal ecologist with the Illinois Natural History Survey (at the U of I Prairie Research Institute). Over the course of a forty-year career, Merritt has specialized in exploring how mammals cope with winter. Among other projects, he has studied shrews under the snow in Siberia, pikas in the mountains of Tibet and the variety of small mammals that inhabit the Rocky Mountains of Colorado.

Take hibernation, for example. If asked to name an animal that hibernates, many people would say bears. As it turns out, though, bears do not hibernate—at least according to the definition scientists use. Yes, they retreat to a den for months on end, and during that time don’t eat, drink, urinate or defecate. In addition, their metabolism is suppressed and their heart rate slows. But—and this is a big “but” for mammalogists—the body temperature of bears in winter remains somewhere in upper 50 degree range. So Merritt et al. would say bears undergo a period of “winter lethargy.”

[An Arctic ground squirrel takes a peek from its burrow. Photo by Øivind Tøien.]

In contrast, the body temperature of most “true hibernators” drops all the way into the 30s. And at least one, Merritt pointed out, can survive with its body temperature below freezing. The coldest core temperature recorded for a hibernating arctic ground squirrel—a relative of the thirteen-lined ground squirrel common in Illinois—was less than 27 degrees!

Hibernation is also a more complex phenomenon than you might expect. Using using radio telemetry, scientists have been able to monitor fluctuations in the body temperature of hibernating animals. In doing so, they’ve found these critters experience regular episodes of arousal, during which their body temperature rises from its cool baseline all the way up to normal.

These spikes are very costly to the animals—that is, they use up a great deal of energy—which suggests there must be good reasons for them. But nobody yet fully understands exactly what those reasons are. One reason seems to be that warming up enables animals to experience REM sleep, which is necessary to maintain brain function over time.

While hibernation is fascinating, it’s actually a fairly uncommon way for mammals to cope with winter. Of the 60 mammal species native to Illinois, only 16 hibernate, and 12 of those are bats.

How do the rest get by? Body mass is important, since it enables bigger animals to store energy--and “bigger” here starts with tree squirrels, raccoons and opossums. Insulation helps, too—a nice layer of subcutaneous fat and a fur coat. Shivering generates heat when circumstances demand it. And hanging out with friends.

In a recent study of least shrews, which range form Central America to Wisconsin, Merritt found that the cold related energy needs of individuals were reduced by nearly half when they packed into communal nests.
Maybe the question there is how they get along well enough to stay in such tight quarters. I know some mammals who are getting cranky from being cooped up this winter.