Known as delayed implantation, this reproductive technique is rather common among northern mammals, and it offers a number of advantages. As noted, courtship can occur during the easy time of the year, when movement isn't hampered by snow and cold; this way, too, a female gets a jump on the spring, bearing her young early so they are growing up in a season when the world is awash in young — and unwary — prey animals. This is especially important with predators, like mustelids, whose young have a long development period. Baby fishers do not even open their eyes until they are nearly seven weeks old, and are not fully weaned until they reach four months of age.

Getting a Good Night's Sleep
One northern animal takes delayed implantation to perhaps its greatest extreme. The black bear mates in midsummer, not long after the female has booted her previous litter of cubs (now a year and a half old) out on their own. Here again the embryos stop developing after a few cell divisions and become quiescent. But unlike weasels, which remain active all winter, female bears den up in early winter — perhaps choosing a rock den, a hollow tree, or a blowdown. Bears are not true hibernators like bats or woodchucks, but their metabolic rate does slow by about half, and they become sleepy and lethargic.

In early winter, the embryos attach themselves to the uterine wall and begin to develop. About six or eight weeks later — usually sometime in late January or February, depending on latitude — the somnolent female gives birth, scarcely aware of the act. The newborn cubs, covered with a thin sheen of black fur and looking like tailless rats, weigh less than twelve ounces and are, in proportion to the size of their mother, among the smallest mammalian babies. They find their way to the female's nipples despite being blind, deaf, and unable to smell. Research has shown that they react only to warmth — and the nipples are the warmest part of the mother's body.

Through the remainder of the winter, the cubs nurse on high-fat milk and grow with only the periodic, drowsy attentions of their mother. When she finally rouses and the family emerges from the den two months later, the cubs weigh about five pounds — bright-eyed, active furballs that bear almost no resemblance to the tiny infants they were just eight weeks before, with a head start on the season of growth that lies ahead of them.

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Seasonal Guide to the Natural Year: New England & New York is available from the Adventurous Traveler Bookstore. Click here to order!