Colorado’s Ermine

Exploring the Abundant Lives of High Country Mustelids

Imagine this … You are riding up the chairlift on a snowy winter morning. The crisp mountain air fills your lungs as you sit back to appreciate the beautiful backdrop of the Colorado Rockies. And, suddenly, out of the corner of your eye, you see a flash of movement in the snow below. In this split second, the creature may have appeared to be a white blur — a sprinting snowball.

And yet, it is more likely you caught a glimpse of the ermine, a term specifically referring to the white winter coat phase of the short-tailed weasel.

“In this part of Colorado, most people who use the term ermine are referring to either long-tailed weasels or short-tailed weasels, the smallest Mustelids in the state,” explains Hannah Rumble, community programs director at Walking Mountains Science Center. The similarly shaped long-tailed weasel and short-tailed weasel both belong to the Mustelidae family, along with other carnivorous mammals such as badgers, martins, otters and other members of the weasel family.

“Each of the two weasels turns white in the winter in order to camouflage with snow, except for the black tips of their tails. It is thought that the black tip distracts predators from the rest of their bodies if they’re being chased in the snow,” explains Rumble. In the longer summer months, the coat of both weasel species becomes a light brown color, with the long-tailed weasel having a more yellow belly compared to the white belly of the short-tailed weasel.

The key to this transformation can be found in the amount of daylight and its change over time. As days approach winter, the decreasing amount of daylight triggers hormone reactions, which, in turn, cause a decrease in the production of natural melanin-containing pigments. As new fur grows and daylight continues to decrease, the weasels’ brown summer coat will shift to a bright white instead. Likewise, once days become longer and temperatures rise, dark pigment levels will increase, and brown fur will reappear. In addition to aided camouflage and stealth, the winter white color change also serves to improve the insulative properties of the fur. “The air
pocket left by the lack of melanin in each hair also provides more insulation in the winter,” explains Rumble.

Such a trait is vital for survival in a snowy environment such as the Vail Valley and helps to explain why these animals can be so difficult to spot. In fact, many small mammals, including the ermine, may not be above the snow much at all in the winter months and prefer to spend their time under the surface in what is known as the subnivean. “Essentially, subnivean means below the snow,” explains Cal Orlowski, wildlife biologist and mountain sports ranger with White River National Forest Service. “There’s kind of a whole other world of activity that’s going on below the snowpack,” he continues. “For example, all the small rodents — mice, rats, voles — are still active; they don’t necessarily hibernate in the winter. And, they are able to survive by creating tunnels and channels through the snowpack under the upper layers of the snowpack.”

And, the ermine will use these subnivean tunnels, too, following the trails of their prey.

In addition to providing a source of travel and hunting, the network of snow tunnels protects the ermine from the harsh winter weather. “The snow actually provides some insulation, so that the temperatures below the snowpack are quite a bit warmer than the temperatures
on the surface of the snowpack where you have wind and big temperature fluctuations,” explains Orlowski. Due to their elongated body plan, these weasel species must find methods to retain body heat, including using such tunnels, as well as supporting an incredibly high metabolic rate.

“Their long body shapes help them navigate subnivean tunnels more easily, but this shape also causes a lot of heat loss in the winter, so they must eat a lot each day to stay warm,” explains Rumble. “As a result, weasels are known to kill in ‘excess’ and sometimes pile up carcasses for lean times,” she adds. Their hunting ability is unique in that both short-tailed and long-tailed weasels frequently kill animals larger than themselves, a trait rarely observed in the animal kingdom. “They can kill animals up to three times their size!” Rumble exclaims.

As far as interacting with these ferocious furballs, observing from afar and sharing the area is the best way to go. “Being respectful and keeping safe distances is a good rule of thumb for observing any wildlife,” Orlowski shares, especially for species that may not be used to human interaction. “And, [make] an attempt to avoid unnecessary snow compaction,” he adds. As adaptive and intriguing as underground snow tunnels are, they are still susceptible to collapse and extremely important to keep in mind.