Weasels are cute, natural-born killers

Weighing as much as a cup of walnuts and resembling a squeaky dog toy, the short-tailed weasel is easy to underestimate.

The spunky little creature that springs through snow drifts and woodpiles all over mainland Alaska is one of the fiercest predators around. The ermine — another descriptor for the short-tailed weasel — is one of two species of weasel in Alaska, along with the least weasel.

In the basement of the University of Alaska Museum of the North, curator of mammals Link Olson once held up the skull of a short-tailed weasel that came to the collection from a high snowy slope of Denali. 

Olson showed how the ermine’s jaw, unlike ours, is designed to move only up and down. The lower jaw locks into the skull to strengthen the force of its bite.

“They have fewer molars than us,” Olson said, noting that their all-meat diet of voles, shrews, pikas, birds, fish and insects does not require much grinding. “They’ve evolved to use their mouth to catch, kill, slice and swallow.”

Searching for pika once in Denali National Park, Olson saw a weasel hopping by with a vole in its mouth. Olson squeaked like a rodent to see if he could stop the weasel. It stopped and “appeared to be struggling to make up its mind — flee with the meal it already had or go nab another one?”

In Poland, where people call short-tailed weasels “stouts,” researchers have studied surplus killing, in which weasels sometimes kill more voles than they can eat and cache them in the very tunnels the voles had made.

The scientists concluded that weasels did indeed go on killing sprees. But they found the creatures engaged in that behavior more in cold temperatures. During cold snaps, they would hunt less and rely more on their caches of tiny carcasses. 

The researchers wrote that rather than a “behavioral aberration,” the weasel’s excess vole killing was a useful adaptation to life in the far north.

While holding the ermine skull between his index finger and thumb, Olson also noted how the weasel is designed to plunge into holes no larger than its head.

“They can follow voles and pikas into really tight confines,” he said. “But it’s a tradeoff.”

The weasel’s long body leaves it more vulnerable to heat loss than animals like pikas, which are almost spherical. Weasels — with fur that turns snow-white in the winter — don’t hibernate. Like their relative the wolverine, they are always on the move.

“Weasels can’t just hunker (to stay warm),” Olson said. “They have to be out hunting all the time.”

Since the late 1970s, the University of Alaska Fairbanks Geophysical Institute has provided this column free in cooperation with the UAF research community. Ned Rozell is a science writer for the Geophysical Institute. A version of this story appeared in 2015.