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I checked once again the next afternoon following another morning snowfall when the old tracks and the holes were obliterated. There were still no new tracks. Nor did new weasel tracks appear later.

This was apparently not the weasel’s den. It was probably the usurped den of a previous chipmunk victim that this weasel remembered. The most recent victim was likely caught outside its den; why else would the weasel have dragged it so far through the snow? That is, this chipmunk had probably not been in torpor and it got caught anyhow, because if it had been torpid it would have been inside its snug nest, which the weasel would have used while consuming its meal. Thus, staying warm to remain alert does not necessarily guarantee survival for the individual, at least not for this chipmunk. An individual’s odds are determined by its own specific circumstances, and small idiosyncrasies in the life circumstances of different species almost guarantee different strategies as well. Each animal’s existence is balanced on its own often conflicting mix of contingencies.

05

NESTS AND DENS

A large part of adapting to the winter world involves creating a suitable microclimate. Birds wear insulating feathers, mammals fur, and we wear adaptive clothing. But a number of animals, primarily beavers, bears, humans, birds, and some insects take a step beyond insulation, building nests or dens that supplement or take the place of body insulation.

In all animals, den construction is constrained by building materials, energy requirements for heating and cooling, defense, and accessibility. The Anasazi Indians of the southwestern United States built their homes high on inaccessible and defensible cliffs, choosing locations where an overhanging ledge offered shade at noontime in summer and exposure to direct sunshine in the winter when the sun is low. To the north and in Europe and Asia, when wood was not available, Ice Age hunters built huts framed with mammoth tusks and covered them with skins and sod; the early Eskimos did similarly, substituting whale bones for mammoth tusks. They of course invented the igloo, that marvel of simplicity and efficiency. Using hard-packed, fine-grained snow cut with a knife into blocks roughly twice as long as high, a man could build a house in less than an hour by spiraling the blocks upward, each slanted slightly inward.

In our own expansion into the winter world we at first relied on the retention of body heat, which is still the main source of heat in an igloo. Then we used fire as a supplement. The first use of fire for heating our hearth is attributed to European sites of about 500, 000 to 100, 000 years ago, when Ice Age hunters leaned poles against the cave entrance and covered them with hides. Not much changed in the way of heating (except for containing the fire in a fireplace) until Benjamin Franklin invented the Franklin stove, followed eventually by the invention of central heating. Instead of building nests, Ice Age people stayed warm and alive, much like a chickadee or a kinglet does in winter—by metabolically burning fat from the animals they killed.

Among possibly the first nests on earth, and still among the most impressively engineered ones, are those of insects who have been perfecting their building techniques for maybe 300 million years. Every autumn after the leaves fall in New England, I see the nests of one species of wasp, the white-faced hornet (Dolichovespula maculata), hanging conspicuously in the trees. Each nest is started in May by a single female that has hibernated through the winter. She uses her mandibles to scrape fiber from the dead wood of a branch, mixes it with her saliva for a papier-mâché slurry, and then makes tissue-thin strips of paper by adding one load at a time on the bottom edge of a slowly growing globe that will house her and eventually her entire brood of hundreds of eggs, larvae, pupae, and adult offspring. As her daughters come of age, they help their