American bison are built for snow, but deep winter is still a challenging seasonal gauntlet—not least in the high interior of Yellowstone National Park, a legendary stronghold for these shaggy, humped bovids in the heart of the U.S. Rocky Mountains.
The season’s harsh weather and deep snow-cover pose the greatest threat: Winterkill is a big-time mortality factor for bison (often informally called “buffalo”). But it’s also one of the times of year when they’re most vulnerable to an ancient antagonist: the grey wolf.
On January 27th, David Swindler of Action Photo Tours caught rare footage of a successful bison hunt by what’s been one of the largest wolf packs in Yellowstone in recent years: the Wapiti Lake Pack. The event took place in the Hayden Valley, an expansive meadowed basin on the Yellowstone Plateau that’s both a linchpin of the Wapiti Lake Pack’s territory and part of the core range of Yellowstone’s central bison herd. (The Hayden Valley was also where, a few years ago, we reported on the Wapiti Lake wolves ganging up on a grizzly bear.)
Swindler told me by email that he and his fellow observers counted approximately 19 Wapiti Lake wolves participating in the buffalo hunt. “The bison circled the younger calves when they noticed the wolves approaching,” he said. “Some of the larger members of the herd would occasionally face off with the wolves in an effort to drive them away. But the Wapitis have learned to be very patient and persistent.”
According to Swindler, the wolves harried the bison defending the herd’s rear guard, wearing them down, ever watchful for an opportunity to strike. “When the herd started to run,” he said, “the defenders followed. This allowed the wolves to attack the bison from behind, eventually singling out a female. It didn’t take long for the wolves to take her down once she had split off from the herd.”
American bison—which come in two forms, the rangier wood bison of boreal Canada and Alaska and the slightly smaller but stockier plains bison of more southerly grasslands—are the biggest terrestrial mammals in North America, with bulls capable of weighing a ton or more. Their size, strength, endurance, and propensity for communal defence make them challenging prey for wolves, to say the least.
Calves and older bison compromised by injury, disease, or other conditions appear to be most targeted, but even a healthy adult buffalo can become vulnerable to a wolf pack when separated from a herd and (as perhaps this doomed Yellowstone cow was) exhausted from wheeling and running in deep snow. Indeed, on the Boreal Plains of northeastern Alberta, wolves most heavily prey on wood bison during late winter, and particularly when the snowpack exceeds 30 centimetres deep.
Grey wolves and bison have been tussling in North America since the Pleistocene, but drastic reductions in the geographic ranges of both due to hunting and other anthropogenic effects mean they overlap today in only a small fraction of their historical shared habitat. The trickiness bison pose as potential wolf chow (as well as the more general concept of a predator encountering a “novel” prey species) appears to be reflected in ecological trends observed when one or the other species has been reintroduced to former haunts.
In multiple regions where bison or wolf reintroductions have occurred, researchers have documented significant lag-times before wolves appeared to start attempting to take down buffalo. A 2023 study on wolf scavenging and predation on several reintroduced wood-bison herds in northwestern Canada suggested it took between a decade and a half-century for bison to show up on lupine menus. Increasing signs that wolves are consuming buffalo in these areas “provide unequivocal evidence that these reintroduced bison populations are becoming integrated into local food webs and beginning to be under selective pressure by their main predator,” the study authors wrote, “both of which are hallmarks of ecological restoration.”
By comparison, in Yellowstone, where wolves were reintroduced in 1995 and 1996, it actually didn’t take very long for some to start hunting bison: More than two dozen wolf-killed bison were confirmed between the spring of 1995 and the spring of 1999. However, elk—an abundant prey species significantly less dangerous to hunt than buffalo—were and remain the primary quarry of Yellowstone wolves. In the early days of “re-wolfed” Yellowstone, the Mollie’s Pack became particularly known for tackling bison: a special skill apparently born of the fact that elk didn’t winter in the Pelican Valley, centre of the Mollie’s wolves’ domain, leaving bison the only significant source of ungulate meat during that long, snowy season.
A 2014 study focused on wolf/bison interactions in Yellowstone since the wolf reintroduction suggested that a larger pack size—on the order of a dozen or more wolves—may be correlated with greater success preying on bison as compared with taking down elk.
Yellowstone National Park reports that bison have increasingly figured into the diets of the park’s wolves over the decades, though most of that feeding comes in the form of scavenging buffalo carcasses, and elk remain the number-one prey species. As Swindler’s footage shows, Yellowstone wolf packs may be more accustomed nowadays to giving bison-hunting a go, but that doesn’t mean it’s an easy undertaking.
Predator vs Prey
Predator vs Prey
Animal Behaviour
