Mammals in cold times
By KEITH JENSEN and ANDREW HEBDA*
ONCE WINTER sets in, it seems that mammals become more inconspicuous. We often assume that, being intelligent, they are all hibernating. This is not true. Even those that hibernate do not necessarily stay asleep in the fall and reawaken when spring beckons. Mammals have one of three strategies for making it through the cold weather-migrate, hibernate or tolerate.
Migrating is truly for the birds; a common tern, for instance, has recently been recaptured in South Africa, over 26,000 km from where it started in Finland. Some insects also fly off to warmer climates. It is, however, much more difficult to cover great distances on four feet. White-tailed deer move into winter "yards" where there is less snow and more deer traffic, making it easier for them to get around; but they still have to endure the cold. Little brown bats remain here, but migrate to hibernation caves and mines where they can congregate in large numbers. The largest flying mammal in North America, the hoary bat, flies from the Maritimes to the warmer climes of the southern US.
There's more to hibernating than just sleeping. Body temperature, breathing rate and heart rate drop; the activity of the body-its metabolism-slows down. The need for energy (fat and food) is reduced. Hibernating mammals survive on stores of fat for nutrition and insulation. A special, high-energy fat, called brown fat, is used to quickly heat the body. Little brown bats and long-eared bats, once they reach their caves, use their brown fat reserves for periodic wakenings so they can move about, and even mate (see shunpiking, November, 1996). Prior to Groundhog Day, a woodchuck rolls up into a ball inside a den, usually under a tree. The body temperature can be as low as 3_C and the heart beats only five times per minute. Though not considered true hibernators, bears do cool down to as low as 31_C from 38_C. Amazingly even though they are unconscious and do not eat, drink or void wastes, the females give birth to, and suckle, their cubs in mid-winter.
If hibernation or escape are not possible, the only thing left to do is bear the cold weather. Skunks, porcupines, raccoons, flying squirrels and chipmunks do not hibernate, but enter a deep sleep called torpor. During fair weather, they will awaken from their slumber and move about in their sheltered trees, logs and dens. Red squirrels are more active, and rely on caches of food, or middens, to tide them until spring. Even moles and shrews continue to be active. Small mammals are not able to store up a lot of fat or have six-inch long hairs for insulation, so they nest or den up under the fallen leaf layer, under trees, and in the spaces within the rubble found near rock faces. Once the frost sets into the ground their burrowing activity is more restricted. Voles dig runways under the leaf litter and snow, and scurry about to their caches, sheltered from the much colder air above. Snowmobiles can crush their tunnels, and cross country ski tracks can turn a cozy passage into a long, deadly-cold superhighway. Years of low snowfall, like this one, means reduced insulation from the cold, and less protection from predators. Otter, red fox, lynx and others are very active, presenting further challenges to muskrats, voles, snowshoe hared, and other hardy species.
Mammals use different strategies to survive the harsh conditions of winter-they do not all disappear into slumber or to Florida. In fact, winter is a fantastic time to look for their signs such tunnels, tracks and chewed branches.
*Keith Jensen is Senior Naturalist and Andrew Hebda is Curator of Zoology, at the Nova Scotia Museum of Natural History, Halifax.
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