With little fat or fur to keep it warm, the short-tailed weasel must hunt down prey daily to survive winter
Deep in a burrow beneath a blanket of snow, a short-tailed weasel curls into a ball, its systems working double-time to generate heat. All is well--for now. But if the weasel furnace is not stoked with a mouse or vole before tomorrow, the predator's fate could abruptly change. "In winter, weasels continually teeter on the brink of starvation," explains longtime weasel observer Mikael Sandell, professor of wildlife ecology at the University of Agricultural Sciences in Umeå, Sweden. "If they don't eat every 24 hours or so, they may die."
Nature seems to play a cruel trick on this tiny hunter, turning some of its most valuable attributes against it when winter rolls in. Protected by only a thin coat of fur and a modicum of insulating fat, its slim, go-anywhere body becomes a liability in cold weather. Its frenetic temperament burns enormous amounts of energy. If that weren't enough, just when easy prey becomes scarce, the animal's need for food skyrockets. For the short-tailed weasel, winter is the deadliest of foes.
Over time, however, the little predator has learned to cope with its inanimate enemy. Predatory prowess usually keeps the weasel larder well stocked. A winter disguise provides camouflage. Sleight-of-tail trickery foils enemies. And, thanks to a rare reproductive adaptation, pregnant weasels can postpone delivery during the harsh winter months, then give birth when conditions are more favorable.
The short-tailed weasel, or ermine, belongs to the same family as mink, ferrets, badgers, martens and skunks. Its closest relatives are the long-tailed weasel and the least weasel, look-alike cousins that differ from ermine only slightly in size, coloration and prey preference. All are quick, stealthy predators. The three species occupy virtually all of North America (except the desert Southwest), from sea level to an altitude of 10,000 feet. At home in forest, farmland, prairie and tundra, short-tailed weasels also roam the boreal latitudes of Europe, Asia and parts of New Zealand, where they have been introduced by people.
Brown on top and yellowish below during the summer months, the ermine's coat turns solid white in winter--except, that is, for the tail tip, which remains coal black year-round. Males measure a foot or so in length, including 3 or 4 inches of tail, and weigh about 3 ounces. Females are a few inches shorter and usually weigh about an ounce less.
Capable of climbing trees, swimming rivers and hunting day or night, the versatile weasel eats everything from earthworms to waterfowl, though it usually dines on mice. Rising on its hind legs to search for prey, the weasel-with flattened head and pointed nose--suggests a cobra with fur, a bundle of concentrated predatory energy. Says Sandell, "The weasel is one of the most efficient predators of all."
A keen sense of smell guides the weasel to its prey, which it typically assaults in a bounding flourish. When a rodent retreats below ground, the hunter simply follows. A quick bite to the back of the neck or base of the skull, and the prey is dead.
In 1983, however, Sandell recorded an unusual situation in which a weasel chose not to kill. He fed the caged animal live mice, which it quickly dispatched and ate. Except one. "Curiously," he says, "the weasel and one mouse became like friends."
For several weeks, the odd couple lived together, the weasel killing and consuming two mice per day, sometimes climbing over its roommate to get to them. Eventually, Sandell released the weasel into the wild, leaving the healthy mouse behind. Why was the mouse spared? Sandell speculates that the young weasel may have left its family just as it was learning to recognize siblings. "Perhaps the mouse became a substitute brother or sister," he says.
Were it not for winter, life might be a breeze for the weasel. But the very attributes that make the animal an accomplished hunter cause problems in the cold. The weasel's slim body, ideally suited for negotiating narrow rodent tunnels, has a high surface-to-volume ratio, causing the weasel to lose heat fast. Also, its short, fine fur--an advantage when squeezing into tight places--provides little warmth.
With a heart that beats several hundred times a minute, the hyperactive weasel burns a lot of fuel. "Weasel metabolism is about twice that of similarly sized animals," says Sandell. "This makes them quick, effective hunters, but it also makes them hungry all the time."
The perpetual need for energy prevents a weasel from gorging itself then fasting a few days, as many other predators do. Each day, a weasel must consume prey--two or three mice or voles, for example--equal to half its own weight. (If humans ate as much, they would devour 50 to 75 pounds of groceries per day.) When blizzards make hunting difficult, the animals may perish.
In winter, many prey species stop reproducing, which deprives weasels of inexperienced young rodents just when their energy needs are greatest. It doesn't help that local rodent populations periodically crash, forcing weasels into new territory. "At first, you see lots of weasel tracks in the snow as they search everywhere for prey," says Sandell. "Then the tracks disappear, as they quickly starve." Indeed, starvation is the leading cause of death for short-tailed weasels.
Even so, the animals are hardly defenseless against winter perils. In a burrow appropriated from its prey, a weasel might spend 20 hours of every day at energy-saving rest. When food is abundant, it may continue to hunt long after it is sated, stockpiling surpluses (sometimes 30 or 40 animals) against leaner times to come.
A few years ago, one of Sandell's colleagues in Sweden attached radio collars to 20 water voles she was studying. Then a female weasel with young moved into the area. In two days, all the voles were dead, the tiny radio collars left in a heap. "The weasels ate the voles and spit out the radios," says Sandell. "In 48 hours she killed 20 animals nearly as large as herself."
Amid the bounty of summer, male weasels leave their home territories to locate females and mate. On such a schedule, it seems the females would enter winter with extra mouths to feed. But a remarkable clock-stopping adaptation puts weasel parenthood on hold. For ten months, the fertilized eggs lie dormant within the female. Come spring, with prey abundant once more, the eggs begin growing again, culminating with the birth of four to nine young in April or May.
With a life expectancy of only 18 months, weasels don't waste time when it comes to reproducing. Adult females breed while still nursing their young. Perhaps more astonishing, females become fertile when about six weeks old; adult males enter the nest and breed with them, sometimes even before their eyes are open. By fall, the young have become independent adults and most females are pregnant.
In winter, the entire wild world gets hungry, and many weasels fall prey to hawks, owls and other predators. For protection, northern ermine change color from chocolate or reddish-brown to white. Triggered by autumn's declining daylight hours, this molt makes weasels all but invisible against snowy backgrounds.
"Turning white in winter likely saves a lot of weasel lives," says Roger Powell, associate professor of zoology and forestry at North Carolina State University. "In the snow, a brown weasel would be extremely easy to see."
But even in the dead of winter, the end of the animal's tail remains black, an inconsistency that had long puzzled biologists. In 1978, Powell set up an experiment to test his theory that the ebony tip served a purpose: to distract predators.
At the Brookfield Zoo near Chicago, he painted a flat rooftop white to simulate snow, then tethered red-tailed hawks (trained to attack artificial weasel models) at one end. A special jess and restraining line allowed the birds to fly the length of the roof. Using a pulley apparatus, Powell repeatedly sent a fake weasel "scurrying" across the roof. The hawks attacked repeatedly--not the weasel's body, but the black tail tip. When Powell substituted totally white weasel dummies for those with black tips, the hawks' successful hits increased. Apparently, the black spot did confuse the birds. "When they missed, the hawks either grabbed for the black tip or slammed on the brakes at the last second, as though they had just realized their mistake," says Powell. "In the wild, this would give the weasel an extra chance to stay alive."
The animal's talent for survival is not always greeted with enthusiasm among people, who often speak with disdain of "weasel words" or "weaseling out" of a chore. Over time, the very name has become synonymous with "liar," "sneak" and "cheat." Ermine infamy probably stems from the animal's rare killing sprees that litter barnyards with dead fowl and outrage farmers. As a result, weasels have long been persecuted for their supposed bloodthirsty and gluttonous ways.
It's a bum rap, says Powell: "The weasel's negative reputation is largely unjustified." We shouldn't apply human values to animals, he says, nor should we blame weasels for crimes they didn't commit. One study in North Carolina found that most of the chicken deaths attributed to weasels were actually the work of rats.
But what of the occasional barnyard massacres actually committed by weasels? "Surplus killing," as biologists call this behavior, is normal for a hungry predator uncertain about its next meal. "The weasel is stocking up on food, just as people do at the grocery store," says Powell. "The notion that weasels are evil is a myth."
Montana writer Gary Turbak, a frequent contributor to this magazine, has never weaseled out of a story assignment. Dwight Kuhn photographed the short-tailed weasels pictured here in captivity near his home in Maine.