Weasel in the Woodpile

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The once common belief that weasels, especially the short-tail (ermine), don seasonal camouflage by changing color from dark to white at the time of the first significant snowfall is based, not on myth or legend, but on relatively recent scientific research. Until the late 1800s, biologists believed that weasels shed their brownish summer pelage and replaced it with a thicker white winter coat in response to the lowered temperatures of autumn.

That theory was refuted when researchers noticed that captive weasels kept in heated buildings still molted. The researchers also noted that their mustelid prisoners began molting within 48 hours of the first real snowfall—and thus was born the theory that the seasonal onset of snowfall controls the timing of the weasel's annual change from brown to white.

We now know that the weasel's biannual color change stems not from snowfall but from a decreasing photoperiod: When days become shorter in late fall, the decreasing daylight triggers the weasel's pituitary gland to molt the summer coat and simultaneously to inhibit the production of hormones that produce the pigments coloring the weasel's fur. Come spring with its lengthening days, this phenomenon reverses itself, replacing the shed white fur with a dark summer coat.

The myth that the weasel conceives through its mouth and gives birth through an ear originated in the fertile imaginations of ancient Greek storytellers. While the truth isn't quite that strange, it's highly unusual in a couple of ways.

In a process known as induced ovulation, the female weasel releases her egg, not on a regular timetable like most mammals, but only when it's needed—at the instant of copulation. There's good reason for this: Adult weasels are generally loners, so, rather than chancing a mating meeting while the female's egg is not viable, nature invented on-demand ovulation to make certain that an egg will be ready and waiting whenever a male might happen along to fertilize it.

The weasel's second unusual reproductive trait is called delayed implantation, and applies to the long-tailed and short-tailed species, but not to the least weasel, which can mate and give birth at any time of year. This biological anomaly allows mating to take place during summer, when weasels are out and about and the most likely to meet—but the resulting fertilized egg goes on hold, not implanting in the uterus until late winter. This months-long delay assures that the young will be born during spring, when food is plentiful and living is at its easiest.

Weasel kits are born with a strong hunt ing instinct, but must learn strategy and tactics from their mothers (with some help from the fathers in the long-tailed species). The kits mature rapidly and, by the winter following their birth, are fully grown and fending for themselves.

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