What Kills a Wolverine?

Spend enough time guiding in Alaska’s backcountry and you are bound to find something you can’t explain. This happened to me recently as I was leading a canoe trip down the upper Noatak River in Gates of the Arctic National Park.

My group, a family of four, my co-guide,and I were camped on the shore of the lake where we’d been dropped off earlier in the day. It was one of those sublime Brooks Range days with scattered clouds, and lots of blue sky. Pingo Lake, as its known, is a popular drop off for paddlers, and one of the few places along the upper Noatak where you can find impacts from people. A group or two a week pass thought during the peak months of June and August, hardly a crowd, but enough to have some impacts on the tundra, and probably, on how wildlife use the lake.

Late in the afternoon, as I was starting to think about dinner, two of my clients came up to say they’d found a dead bear cub back down the portage trail that connects the lake to the river. Curious, I walked the short distance to their find.

Sure enough, not far off the trail, and partially obscured by some willows, was a patch of dark brown fur. As I approached, something looked off about the dead animal. The hind legs, were pointing up, and while the claws were indeed long and aggressive, the foot was not shaped like a bear’s. I walked around to the front of the animal and realized the distinctive face was not that of a bear, but a wolverine.

People don’t often see wolverines. On the dozens of trips I’ve led to Alaska’s arctic, I’ve seen a wolverine on only three occasions. Most often, we see only the hind end bobbing away over the tundra in the distinctive gait of a mustelid. Rarely do we get anything more than a distant glimpse. So a dead wolverine, laying on the tundra next to a frequently used lake, was downright weird.

The wolverine appeared to be an adult, and had probably been there a few days to a week. There was a notable odor, though not yet overwhelming, and insect larvae were well established beneath the corpse (read: maggots). Using a stick I lifted the upper lip and got a look at the clean, only slightly worn teeth. It was not obviously emaciated and was intact.

The one strange thing was it’s posture. As I mentioned earlier, the legs and belly were up, and the chest, head, and forepaws were down. Now wolverines are flexible critters, but not that flexible. A little leverage with my stick, and I found that the back flopped loosely, the spine apparently broken about half way down.

Something had killed this wolverine. I was confused. This was an apparently healthy animal, dead and broken, but un-eaten. I certainly wouldn’t tangle with a wolverine, and I didn’t know any animal that would.

Wolverines are predators and scavengers. Fierce, agile and capable, they hunt everything from ground squirrels to caribou calves. There are even a few scattered records of wolverines (which rarely weigh more than 40lbs), taking down full grown caribou. They scavenge carcasses left by wolves and bears, and will occasionally, steal from other predators.

This brief video I recorded in 2013 on a tundra ridge above the Kokolik River in Northwest Alaska. To date, this is the best sighting of a wolverine I’ve ever had. Pay attention to the loping gait and sudden, observant stops, which bely the fact that, despite their size, wolverines are really just big weasels.

There aren’t a lot animals in the Noatak capable of killing a wolverine. Grizzly bears, wolves, or an angry moose seemed the only possibilities. The mystery unsolved, I returned to camp, still wondering.

Today, with the knowledge of the internet at my fingertips, I did a little digging. There isn’t a lot known about wolverines. These are widely scattered, rare, and little observed animals. Many studies rely on radio-collars for their information, and that’s where I found some data on wolverine mortality.

Most of the wolverines in the studies I read died from some human cause, either trapping, roadkill, or on railroad tracks. Predation, however, was a notable cause of mortality in every study I read.

Wolves, Mountain Lions, and other wolverines have all been implicated in deaths of wild wolverines. One of these, the mountain lion, does not occur in Alaska, so I could easily eliminate that one as a possible cause of death for my Pingo Lake animal. The broken back, and otherwise intact carcass made me believe that a large animal, rather than something of equivalent size, must have been responsible. I suspect that were another wolverine the killer, there would be damage to the body, scratches or cuts. Frankly, I can’t imagine a wolverine going down without a fight.

If not another wolverine, and not a mountain lion, then a wolf or pack of wolves seemed the most likely.

But why would a wolf do this? One paper I read (Boles 1977), hypothesized, “Predation as a cause of mortality may be exacerbated when wolverines scavenge kills in the presence of other carnivores. The role of more efficient carnivores as producers of carrion may be essential to survival in some areas, but the beneficiary may risk serious injury or death.”

In short, when a wolverine tries to steal a kill from a wolf (or other large predator) then the wolves are likely to get mad, and the wolverine may pay a high price for that theft. It’s a good theory, and would fit our dead beast were it not for one glaring thing: there was no nearby kill.

Then I stumbled on a reference to Adolph Murie’s classic 1963 book “A Naturalist in Alaska”, where Murie recorded three incidents of wolves pursuing wolverines without provocation. All three of these wolverines survived, climbing trees to escape to the jaws of the wolves. In the Noatak, there are no trees, and thus no arboreal escape routes.

In a 1962 issue of the Journal of Mammalogy, I found the story of one wolverine who did not escape. Through tracks in the snow, the author deduced that a pack of wolves stumbled on a wolverine foraging on a frozen hunter-killed caribou carcass. With no tree to escape, the wolves trapped the wolverine, and killed it. Notably, however, they did not eat it.

I found other reports of wolves killing wolverines in a 1977 issue of The Canadian Field Naturalist. These two incidents were also the result of unprovoked attacks.

Wolves may see wolverines as competitors for food and harassing, driving them away, or killing them may leave more food for the pack. Or perhaps, wolves simply don’t like these strange, weasly creatures skulking through the tundra.

In the various papers and reports I perused, I did not find any examples of bears killing wolverines. Perhaps, the wolverines are too small and dexterous to be worth the effort, or perhaps bears simply don’t see wolverines as a competitor worthy of their attention.

The poor dead creature in the Noatak Valley was likely killed by wolves, though I can’t say that for certain. Later in the trip we heard wolves howling, their barks echoing over the tundra. Too we saw another wolverine, this one running full-tilt up a hill away from our passing canoes. I think I understand the wolverines shyness better now. They are not the fearless, even reckless predators, that I’d romanticized. No, like so many other animals of the arctic, they are vulnerable to larger predators. It’s no wonder they run. I think, in their paws, I too would make myself scarce.

Their rareness is all the more reason to treasure the rare opportunities I’ve had to see this species. If I stay patient, perhaps I’ll have more chances of viewing this enigmatic predator, but I’ll be perfectly happy if I never encounter a dead one again.

Information in this post comes from the following papers and books:

Boles, B.K. 1977. Predation by wolves on wolverines. Can. Field-Nat. 91:68-69.

Burkholder, B.L. 1962. Observations concerning wolverine. J. Mammal. 43:263-264.

Krebs, J. et al. 2004. Synthesis of Survival Rates and Causes of Mortality in North American Wolverines. The Journal of Wildlife Management, Vol. 68(3): 493-502.

Murie, A. 1963. A Naturalist in Alaska.