The One Thing Wolverines Can't Take On

by Douglas H. Chadwick
Spring 2010

Back in the Ice Ages, the ancestors of wolverines competed for prey and carcasses with saber-toothed cats, dire wolves, mega-bears and giant northern hyenas. Natural selection in this crowd did not favor the slow, shy or even slightly polite. Which helps explain why the 25- to 45-pound wolverines we know today singlehandedly bring down grown caribou and an occasional moose, and will fight a grizzly over the spoils.

But when I say these might be the toughest animals in the world, I’m also including the way wolverines relentlessly roam vast territories here along the top of the Rockies, taking on cliffs, icefalls and summits through some of the nastiest weather modern winters can throw at a mammal. Climbers and extreme skiers come back from such expeditions and tell riveting tales of survival. Wolverines just growl and keep going 24/7. Fierce and strong beyond all proportion to their modest size, they have nevertheless finally met a force they may not be able to overcome: climate change.

I volunteered on a study radio-tracking this species along the continental divide in Montana’s Glacier National Park, where winter starts in October and lingers into June. During the cold months, we would trap wolverines in stout log boxes and tag them with radios. Once the sedative we used wore off and we lifted the box’s lid to free them, some would sprint directly away. But others, like M1, a big gnarly guy with anger-management issues, would bound a few dozen feet and stop, as if they’d remembered the trap still held food and why the hell shouldn’t they go back to take it with them. M1 would circle us as if trying to make up his mind, and scentmark as he went, staking his claim. Once, he didn’t leap from the opened box at all but just perched on its lip glaring around at the two-leggeds, maybe figuring his chances of shredding what he saw as the competition.

When he did depart, he would sometimes lope several miles toward a headwall marking the east edge of the Great Divide, scale a nearly vertical 1,500-foot chute in about 20 minutes, and cross through Iceberg Notch to the Pacific side of North America in a plume of gust-driven flakes. The ridgeline would block his radio signal. And just like that, the wildest of mountaineers was gone, off to patrol the rest of his turf, which encompassed almost 200 square miles along the continent’s crown.

I still don’t really understand what makes wolverines tick. But I learned that Gulo gulo comes with a large heart. And big lungs, a huge stomach, a hefty thyroid gland and a higher metabolic rate than other animals its size. We’re talking about a powerful natural motor with a souped-up carburetor. To hold in the heat of this internal engine, wolverines, like many northern mammals, wear a double coat – a dense inner layer of air-trapping wool beneath a cover of long, stout guard hairs, which add extra insulation. Textured to resist absorbing moisture, the surface of wolverine guard hairs also excels at shedding frost.

A gulo’s crampon-clawed feet are enormous relative to its body, spreading its weight like snowshoes – a major advantage over most competitors and prey during the cold months. Long, harsh winters drain the energy reserves of hoofed animals post-holing through the snow, leaving some dead to be scavenged and others weaker by the day, more easily brought down for dinner. In steep terrain like Glacier, heavy snowfalls also mean more avalanches, which claim their own share of mountainside grazers. If buried deeply, the carrion keeps like meat in an ice chest until it melts out for gulos to gorge on through spring and early summer. Avalanches also
replace forests with vertical stripes and fans that start life over as meadows filled with wolverine summer snacks such as ground squirrels, mice and voles. Meanwhile, wolverines cache food in boulderfields with icy water running underneath and in snowbanks. Supplies in such larders may last months or even from year to year.

The list of adaptations that make winter a wolverine’s ally is impressive. Yet until scientists started to focus on climate change, no one gave much thought to how Ice Age-built creatures with a supercozy fur coat, smoldering metabolism and food cached in nature’s refrigerators are supposed to handle swimsuit weather in our ever-toastier Age of Industrial Exhaust.

In February, pregnant females go into snow dens and prepare to give birth. Fewer than two dozen dens have ever been discovered in the Lower 48, about half of them during the Glacier study. They were all at high altitudes and dug eight to 10 feet down into the snowpack. White as polar bears when born, baby wolverines weigh only a few ounces. They need all that snow overhead for insulation, especially when mom, their furry furnace, is away hunting. They also need to be too far under the surface for passing predators to find.

The kits won’t venture out until sometime in May. Wolverine biologist Jeff Copeland and ecologist Kevin McKelvey created a continental map showing where snow lasts through the first half of that month. Then they charted the range of Gulo gulo. The two patterns were nearly identical. When Copeland took a closer look at the species’ exact whereabouts, he discovered that the animals rarely occurred where the average maximum daily temperature in August exceeds 70 degrees F (22° C).

As it turns out, wolverines’ ties to a deep, persistent snowpack and places where summers don’t get too hot are what ecologists call obligate. Like the betterknown polar bear, they simply can’t get by without chilly conditions. Only about 300 remain in the Lower 48 today. The very least we two-leggeds can do is safeguard habitat corridors – especially north/south-running ones – to keep the surviving groups connected and give wolverines a better chance of adjusting to changing conditions. While we get dead serious about turning down the planet’s thermostat for all our sakes.

About the Author

Montanan Douglas H. Chadwick is a wildlife biologist and founding board member of Vital Ground, a nonprofit land trust that has helped safeguard more than 600,000 acres of key wildlife habitat and movement corridors. He has written hundreds of articles on natural history and conservation around the globe. His 11th book, The Wolverine Way, based on years of volunteer work with a wolverine study in Glacier National Park, will be published by Patagonia this spring.