Snow has covered the ground across most of Pennsylvania for several weeks, and a lot of winter lies ahead. Wildlife, perhaps mammals most of all, are ready for the challenge.
What is now Pennsylvania was home to, or at least visited by, 71 mammal species when William Penn ascended the Delaware estuary in 1682. The larger, more wilderness-dependent mammals such as the wolf, eastern elk and mountain lion disappeared from the state by the mid 1800s, yet 63 mammal species are known to remain today. To live at roughly 40 degrees north latitude, all the survivors have developed some adaptation to winter's cold and snow. Some mammalian strategies for winter survival entail complex internal changes and some are evident in external appearance, while others are simple modifications of daily activity in response to harsh conditions.
On Jan. 3, Powdermill Nature Reserve in Rector, Westmoreland County, will hold a lecture titled "How Animals Survive Freezing Temperatures." Hosted by avian ecologist Andrew Vitz, it starts at 1 p.m. Powdermill is a biological research station of the Carnegie Museum of Natural History (www.powdermill.org).
Familiar winter adaptations of mammals include those shown by snowshoe hares and ermine, which molt their brown fur in the fall, in response to shorter day length, and don a coat to match the snow. As evidence that Pennsylvania lies at the southern fringe of the need for this trait, some hares retain brown patches on the legs and head.
Close relatives of the ermine, least and long-tailed weasels in northern Pennsylvania turn white. In southern counties the pelt remains unchanged or acquires a paler brown hue. The hare's large feet and heavily furred toes help it travel over deep snow without sinking.
Though sleeping through the winter is considered a common mammalian response to cold and snow, only a few Pennsylvania mammals are true metabolic hibernators. These include the woodchuck, the two jumping mouse species and all 11 bats common to the state. In true hibernation, respiration and heartbeat slow dramatically, and body temperature falls to near freezing. Woodchucks feed heavily to accumulate fat through the autumn, increasing their weight by a third, before descending into an underground den, which they plug with an earthen "door." Temperature within the burrow remains at about 56 degrees throughout the winter.
Some bats hibernate in particular communal patterns on the roofs of caves or mines. The endangered Indiana bat forms dense radiating clusters that may contain (or did so before the species declined) nearly 3,000 bats per square meter. Though true hibernators, bats may awake periodically throughout the winter to seek warmer roosts within the cave.
Many familiar mammals, like the chipmunk, do not hibernate but enter a state of deep "torpor" or inactivity for a span of several days, particularly during harsh weather. Chipmunks pass such days inside a system of tunnels, burrows and chambers used specially for resting and storage of food.
Other small mammals, especially meadow and red-backed voles, have adapted to snow cover in the same way. They remain active in winter, travelling throughout a network of tunnels under the snow, enjoying the insulating quality of the snowpack itself. In Pennsylvania, however, snow is no certainty and voles live out the same niche in snowless winters by using voids in rock crevices and tunneling under leaf litter to reach the seeds and grasses they seek as food.
Flying squirrels huddle together for warmth in leafy nests high in trees and colonies of beavers curl up in lodges insulated by mud and surrounded by ice. Because they don't hibernate and need food all winter, beavers cache supplies of tender branches and twigs near the lodge where the food is accessible even under the ice.
Sharing habitat with the beaver, muskrats have an interesting way of maintaining breathing holes in frozen lakes and ponds. From beneath the ice, muskrats dig a hole to the surface and push submerged vegetation up through the aperture, creating a small opening that resists further freezing.
In a complex and poorly understood adaptation, otters and other members of the weasel family employ delayed implantation of the fertilized embryo as a way of adapting to winter.
Black bears have developed a similar adaptation. Bears breed in early summer but the egg does not implant on the uterus until the beginning of winter, after the female has had an opportunity to build sufficient fat reserves to sustain her through the period of inactivity and to nurse her cubs, born inside the winter den.
Even deer have acquired adaptations to winter in this region. Hollow fur provides effective insulation. In severe conditions deer will reduce their movement to conserve calories and "yard up" in groups under dense hemlock stands or in protected hollows to escape the wind.
One mammal still in the process of adapting to the cold is the opossum. Once absent from Pennsylvania's northern tier, this marsupial now lives statewide, perhaps in response to gradually milder winters. Its naked frostbitten ears and often ice-abbreviated tail are testament that it is not quite prepared for winter's "cold snaps."
First Published January 3, 2010 5:00 AM