We're not alone coping with the cold
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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.
