Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [40]
By late fall after a good sugar maple seed year, or after finding a well-stocked bird feeder, chipmunks take trip after trip fully loaded, and all trips lead into the hibernation burrow system that has special granary chambers. These food stores are especially needed in March when the snow is generally still deep. It is then the mating season and the male chipmunks burrow to the surface. There is no new food yet above-snow, but traveling on the crust is easy and those little ground squirrels with the most stockpiled food from the fall can then be the most single-minded in their pursuits.
Normally I don’t see a single chipmunk all winter long. They stay underground, entering into periods of torpor. But torpor is an option, not a necessity or a rule, as was apparent in the winter of 2000–2001 when throughout Maine and Vermont we had an exceptionally large mast crop in the fall. The sugar maples, red oaks, and beech all simultaneously produced bumper seed crops, whereas in many years they produced no seed at all. It was also a winter of exceptionally deep snow. Yet, despite the frequent storms that winter, the chipmunks came to our feeder all winter long.
A chipmunk’s availability of stored food affects whether it remains fully active or enters full torpor (Panuska 1959). But entry into torpor also requires a cold stimulus. Chipmunks are light sleepers; handled torpid chipmunks invariably become roused (Newman 1967). When that happens, their metabolic rates increase as much as fiftyfold within an hour. In contrast to the torpor induced by starvation, as in a nonhibernator close to death, the hibernating chipmunks’ low body temperature is not passive. At an air temperature of 0°C they regulate their body temperature near 6°C, rather than at 37°C when they are active. At air temperatures above 15°C, however, body temperature of hibernating chipmunks is no longer regulated, passively increasing with increasing air temperature (Newman 1967).
Like chipmunks, northern flying squirrels as already mentioned also do not fatten up for winter, nor do they put on a thick insulating fur as red squirrels do. Nor do they lay up stores of food. Instead, they solve the energy problem by huddling in groups in snug nests. Even at -5°C outside the nest, the temperature within the nest is not yet low enough for them to have to shiver to keep warm.
Unlike the eastern chipmunk, some ground squirrels from the western American mountains and deserts enter into hibernation torpor not in response to cold. They begin to hibernate in the hottest, driest part of the year and then continue to stay torpid through the winter (Cade 1963). These squirrels enter hibernation regardless of temperature, and also regardless of the absence or presence of food and water. One of these species, the golden-mantled ground squirrels (Citellus lateralis) has gained fame for the revelations that have emerged for the timing of its hibernation, through the experimental work of Kenneth C. Fisher and his student Eric T. Pengelley. Their subject animal, unlike eastern chipmunks, does not store food but instead fattens up prior to hibernation. So much to eat, so little time. How do they know when to begin? They consult an internal calendar.
Calendar-type timing was suspected when Fisher and Pengelley noted that their squirrels kept under constant light and temperature conditions in the lab at the University of Toronto stopped eating and drinking and went into hibernation in October, at the same time that those outside exposed to the natural environment did. For four consecutive years in one experiment, the squirrels’ cycles of feeding, fattening, and hibernation torpor all continued in the absence of all external cues, in somewhat shortened annual cycle of 324 to 329 days rather than 365 days. The cycling of behavior and physiology occurred in squirrels