Environmental Almanac: If you're chilly, think about critters
By the time you're looking at this column, Punxsutawney Phil will have already seen his shadow or not, so we'll have that projection for how much more winter we must endure or get to enjoy, depending on your perspective.
But what about Phil? According to Joe Merritt, a mammal ecologist who is based at the Illinois Natural History Survey, a division of the University of Illinois Prairie Research Institute, he should have another month to himself, since wild groundhogs at our latitude generally do not come out of hibernation until early March.
And Merritt should know. He's a Pennsylvania native, and over the course of a 40-year career, he has specialized in studying 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.
I contacted Merritt with a simple question about Phil. But through a long and wide-ranging conversation, I learned much, much more about the fascinating strategies mammals use to cope with cold, many of which scientists are still working to understand.
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 the upper 50 F range. So Merritt et al. would say bears undergo a period of "winter lethargy."
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 of a hibernating Arctic ground squirrel (a relative of the 13-lined ground squirrel common in Illinois) was less than 27 F!
Hibernation is also a more complex phenomenon than you might expect. Using radio telemetry, scientists have been able to monitor fluctuations in the body temperature of hibernating animals. In doing so, they've found hibernating animals experience regular episodes of arousal, during which their body temperature rises from its cool baseline, usually somewhere just above freezing, 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, although nobody fully understands yet 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 from 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.
Environmental Almanac is a service of the UI School of Earth, Society and Environment, where Rob Kanter is communications coordinator. Environmental Almanac can be heard on WILL-AM 580 at 4:45 and 6:45 p.m. on Thursdays.