Biologist presents findings on wolverines

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A wolverine in Glacier National Park.

The devil bear. The little wolf. The skunk bear.

Despite being a member of the weasel family topping out at about 40 pounds, the wolverine’s abundance of nicknames reflects its larger-than-life personality.

Perhaps most telling, its scientific name, Gulo gulo, is Latin for “glutton.”

Rick Yates, a U.S. Forest Service wildlife biologist, spent from 2002 to 2007 studying the elusive carnivore’s behavior, trapping and tracking wolverines over hundreds of square miles in Glacier National Park. Many of his observations underscore the mustelid’s legendary appetite, and are the subject of his next scientific paper.

“It’s not on a regular basis, but when [a moose] is stuck in deep snow, they’ve been known to take them down. They can jump on their back and crush their spine with their jaws,” he said during a recent presentation in Kalispell.

Yates said Glacier National Park offered a unique chance to study wolverine behavior, given the park’s relative cornucopia of potential prey.

Along with Eastern Mennonite University Professor Jeffrey Copeland, Yates set out more than a decade ago to document wolverine behavior in the park, particularly nesting habits. But Yates soon realized the animal droppings they found along the way could unlock a bevy of additional information.

“I just felt it was something we should take advantage of. Over the five years, I think we had 189 scats, and that’s a pretty good sample itself,” Yates said, adding that they worked with the University of Wisconsin, which analyzed the composition of the scat samples. “I looked at it as a treasure trove of what these animals are eating.”

As it turns out, they’re eating just about everything: from deer, sheep, squirrels and marmots to fish, pine needles, soil and even bones. Yates said wolverines are one of the only animals that can live off bones, with traces found in 90 percent of the samples and composing up to 92 percent of a single scat.

“It’s a heck of a strategy for survival, but you can see why they’d be named ‘the glutton,’” he said.

In 80 percent of the samples, Yates said, the wolverines had eaten a significant amount of conifer needles, although the jury is still out on the reason for that behavior. He noted that wolverine urine has a particularly high level of turpines, of which the pine needles are presumably the source.

One of the biggest surprises from the scat analysis was the presence of the northern bog lemming, rarely seen in spite of its transcontinental range.

“I don’t know the last time one of these was recorded, but obviously the wolverines know about them,” Yates said.

Reflecting their opportunistic nature, many of the wolverines’ droppings also contained traces of cattle and fish, which Yates said were likely scavenged rather than actively hunted.

He added that this was the first time researchers had identified the seasonal differences in wolverine diets. Through the winter, they subsist mainly on big game animals but switch to rodents during the summer, when they emerge from hibernation and present comparatively easier, more plentiful meals.

However, the scientists turned up a lot more than scat, with most of the researched focused on trapping individual animals to learn more about their distribution and behavior in the park.

Trapping had to be undertaken in the winter when bears are generally inactive. Resembling tiny, windowless log cabins, each of the 12 traps was armed with a transmitter to allow the team to respond and collar the animal within a few hours of capture.

Yates, Copeland and their team caught and tracked 28 wolverines.

“We had trappers saying, ‘You can’t catch them multiple times. They’re too smart,’” Yates recounted. “So I said, ‘Well, you know there’s a trick to it. ... We don’t kill them. It’s much easier to get them the second time around that way.’”

Each wolverine was outfitted with a radio collar that also contained a motion detector to record activity levels, as measured by the frequency of the animals’ movements. Averaged over time, the data showed 12-hour activity cycles, in which the animals’ movements would spike twice each day, with corresponding rest periods in between.

Individual males’ territories were exclusive to one another, with smaller ranges for individual females located within the males’ larger territory.

“Each male associates with two, maybe three females, and that’s his ‘clan,’” Yates said. “So he breeds with them and also helps to raise the young with them.”

Yates added that male wolverines are capable of recognizing their offspring, even after they begin to mature. In one case, he said they tracked a male who came across a set of fresh set of tracks left by his issue, catching up to the youngster and then leading it to a cache of food buried in the snow. Yates also observed males babysitting the kits while the mother was out foraging for food.

Globally, wolverines are found across the northern hemisphere’s arctic and subarctic regions, most abundant in Russia and Mongolia, and distributed throughout Alaska, Canada and several northern states, particularly in the Northwest.

“In Glacier, their [individual] home range is about 100 square miles. ... But farther up north, their home ranges are much larger, up to 300 square miles. These are some of the smallest home ranges anyone has documented, which has to do with how productive Glacier Park is.”

He added that Logan Pass is perhaps the easiest place in the world for the casual wildlife gawker to spot a wolverine. That said, wolverine sightings are far from common. And that’s one hole in existing data that Yates and Copeland hoped to fill.

As it turned out, mortality for juvenile wolverines was very high, with about 70 percent of those tracked dying before reaching age one. Avalanches and legal trapping were partially responsible, with predation by a rival wolverine responsible for one early death, plus there were two that Yates believed had been killed by golden eagles. In another case, a young wolverine appeared to have fallen from a cliff. The ones that survive their first year, however, have a much higher survival rate.

In addition to the radio collars, Yates said the team used a fixed-wing aircraft to fly over the park, picking out wolverine tracks in the snow below and following them to dens. Using GPS to save the location, they would return after the seasonal thaw to gather more detailed information.

“We were able to document probably around 15 den sites at Glacier,” he said. “We about doubled the number of den sites that have been found in the lower 48. They’re very difficult to find and are often in some very inaccessible areas.”

Generally, wolverines site their dens under fallen trees or boulders underneath eight to ten feet of snow. They sometimes change dens after giving birth in a “natal den,” moving afterward to a “maternal den” to wean the kits for about 10 weeks.

Beyond helping to track dens, the collar’s radio tracking provided some astounding examples of wolverines’ ability to quickly traverse rugged terrain, which Yates termed “mountaineering with impunity.”

In one case, a wolverine raced 4,900 feet up Mount Cleveland, Glacier’s highest peak — stopping just short of the summit — in 90 minutes.

Why? Yates said the wolverine appeared to be taking a “shortcut” to a dead animal on the other side of the peak.

Yates said wolverines would cross through the park’s rugged landscape so quickly that they had to reprogram the collars to provide updates every five minutes to get a more detailed sense of their paths. Yates noted the animals’ dependence on deep snows creates concern for their longevity in the area since a warming climate already seems to be shortening the snowy season.

He wouldn’t comment on the fight over whether to list wolverines as a threatened species under the federal Endangered Species Act, but noted that very little substantial data has been collected on the animals to inform that decision.

“Certainly the more information the better, but not much has been published on their den sites and habitats, or the nature of those habitats,” Yates said. “We just want to add to the knowledge base of what wolverines need to survive.”

That said, the Glacier Park study indicated a potentially fragile population. Yates said based on their studies of reproductive and survival rates for wolverines, the addition or subtraction of just one female in the park would have made the difference between a stable population and a declining one.

Reporter Samuel Wilson may be reached at 758-4407 or by email at

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