Host to the largest concentration of big mammals in the conterminous United States Yellowstone National Park is one of the best places in North America to see large carnivores in predatory action.

On the afternoon of May 31, visitors to the park – only recently reopened on a limited basis after COVID-19-related closure – saw predatory action that’s especially rare to witness: a grizzly bear killing a bison. Michael Daus managed to capture cellphone footage of the encounter, which took place along the Firehole River at a trailhead in Yellowstone’s Midway Geyser Basin (Editor's note: Be prepared for distressing footage and equally distressing, cheesy music):

Initially charged by the young bison, the grizzly ends up grappling its victim from behind, tearing into the bovid’s back as it staggers along. The struggling pair totter across a bridge over the river, then end up in the flow itself. The grizzly ultimately dispatched its hefty quarry along the riverbank. Park staff later moved the carcass on account of its proximity to the parking lot and trail. 

According to Daus, the attack unfurled over about 17 minutes. It certainly made an impression. "To have it happen close by and relatively safe for the circumstances we were in," he told the Billings Gazette, 'that’s a treat."

Yellowstone’s grizzlies are among the most carnivorous of North America’s inland brown bears, which partly reflects this Rocky Mountain highland’s great abundance and diversity of large ungulates. Animal protein for the bears, however, also comes in the form of everything from pocket gophers and cutthroat trout to ants and army cutworm moths, which Greater Yellowstone grizzlies seek out on austere talus slopes above timberline in summer.

The two most important big beasts for Yellowstone grizzlies, calories-wise, are elk and bison. These are commonly eaten as scavenged carcasses, an especially critical protein source in spring for lean "silvertips" (as grizzlies are sometimes called) freshly out of their winter dens. The 1990s reintroduction of grey wolves to Yellowstone has translated into a smorgasbord for grizzlies, too, given the infusion of wolf-killed carrion that resulted; grizzlies, especially big males, usually dominate wolves and enthusiastically run them off their kills.

But Yellowstone grizzlies can be capable ungulate predators in their own right. They’re especially adept at hunting elk calves in late spring and early summer: methodically scouring the sagebrush for bedded-down calves, rushing elk herds ambush-style with explosive speed, and sometimes testing herds at a lope in search of lagging youngsters:

In the fall, grizzlies also occasionally take down bull elk that are weakened or wounded from all the exertions of the rut. 

The 4,000 to 5,000-odd bison of Yellowstone mainly feed grizzlies as carrion, but – as Daus’s video dramatically proves – they sometimes fall prey to the bears. This can happen, for example, in the spring, when bison weakened by the park’s gnarly winters are more vulnerable. As with elk, bull bison injured or tapped out from the fights and chases of the rut might be taken down by a large grizzly.

Speaking to the Billings Gazette, Yellowstone’s long-serving bear management biologist Kerry Gunther – who reckoned the griz in Daus’s footage was a subadult – said he’s seen a few grizzly attacks on bison, and has found buffalo carcasses with apparent bear bite marks on the spines. "We had a collared bear that would occasionally kill adult bison," he told the Gazette. "So some bears get somewhat proficient at it."

Bison, though, are no easy prey: not for a grizzly, not even for a pack of wolves. Huge, remarkably fleet, and well-armed, the bovids also range around in large herds, and may mount communal defence of vulnerable members such as young calves. Bison bulls commonly roam on their lonesome, but are also the biggest of their kind – they may weigh a ton – and can be mighty surly. 

Bison are no easy prey: not for a grizzly, not even for a pack of wolves. Image © Jack Dykinga

Adult male grizzlies – which in Yellowstone consume a higher percentage of meat on average than sows – are likely the most effective hunters of bison. But one of the observed predation incidents in Yellowstone involved a female (sow) griz attacking a young bull bison. Although that badly wounded bison ended up being put down by park rangers on account of its proximity to visitors, a report on the event suggested the grizzly – which had wandered off temporarily after a prolonged, stop-and-start attack on the bison – would likely have finished the job.

Another sow with cubs was seen running down and killing a bison calf in Yellowstone’s Hayden Valley in 2000, successfully holding her ground against the mother bison’s charges. A paper in Ursus documenting that predation noted that it’s fairly unusual to see a bison cow and a calf of that age – estimated at seven to eight weeks old – away from large mixed herds, which may have made the pair that much more vulnerable to opportunistic predators.

Among the more interesting recorded observations of grizzly predation on bison in Yellowstone was logged in March 2000. Observers watching wolves harrying bands of bison in the Pelican Valley noticed grizzly bears keying into the hunts. At one point, a griz rushed in on a calf wolves had previously attacked, only to be rebuffed when nine bull bison – which had been proactive in the calf’s defence – encircled it. Later, when the wolves managed to seize the ill-fated little buffalo, a grizzly ran up and – despite an initial attempt by two wolves to drive it off – commandeered it from the pack: a case of kleptoparasitism, when one critter robs food off another. (Feel like going down a kleptoparasitism spiral? Kick things off with wild dogs and cheetahs, then move on to bald eagles and sea lions.)

Yellowstone was the last refuge for free-roaming plains bison when the animal was nearly eliminated in the late 1800s. Though bison herds have since been restored in many small pockets of their former range, and though grizzlies roam elsewhere, the Greater Yellowstone Ecosystem remains the primary place where the two creatures actually overlap.

After almost being wiped out completely, bison herds have since been restored in many small pockets of their former range, like Lamar Valley in Yellowstone National Park. Image © NPS/Neal Herbert

It’s thus one of the few remaining stages for the ancient relationship between the grizzly and the bison: a relationship that once encompassed a vast swath of North America. Bison in historical times ranged across much of the continent; prior to widespread persecution in the 19th and early 20th centuries, meanwhile, grizzlies thumped along from the fringe of the Arctic down to Mexico, and from the Pacific coast to the eastern frontier of the Great Plains. 

Surely in no part of their generous shared real estate were bison so important to grizzlies than on the mixed- and short-grass prairies of the Plains. This enormous steppe famously supported a plethora of large mammals comparable to African grasslands, including pronghorn, elk, and – most notably – plains bison in the tens of millions. 

Today grizzlies are mostly gone from the Great Plains – though they’ve made increasing forays back onto their prairie haunts in Montana and Alberta – but the great bears were once widespread there. The gallery forests and thickets of prairie rivers provided plenty of herbivorous forage, but it’s speculated the annual bonanza of buffalo carcasses constituted a major and critical part of a Great Plains grizzly’s diet. These would be bison killed, for example, by winter weather, disease, wolf packs (the grey wolves of the Great Plains were known as "buffalo wolves"), and Native Americans, who, especially before acquiring the horses that made bison-hunting more selective, often killed large numbers of bison en masse via "buffalo jumps" and other skilful corralling methods. 

On its 1804-1806 expedition exploring the western United States, Meriwether Lewis and William Clark’s Corps of Discovery observed grizzlies attracted to the Great Falls of the Missouri River on the Great Plains by plentiful carcasses of drowned buffalo.

And surely, as the aforementioned Yellowstone incidents attest, Great Plains grizzlies occasionally killed bison themselves: calves separated from the herd, winter-weakened animals, bulls gored during the rut and limping around alone. (For much more on the deep-time dimensions of the grizzly/bison story, check out this CounterPunch piece by David Mattson, who conducted years of fieldwork on Yellowstone grizzlies.)

The ancient relationship between the grizzly and the bison once played out across a vast swath of North America. Image Janko Ferlic

The recent reintroduction of plains bison to Alberta’s Banff National Park, the oldest national park in Canada, restores another small piece of the mighty grazer’s shared dominion with the grizzly bear. In 2017, grizzly tracks were observed during the spring calving season around the enclosure in which bison were staged in the park’s Panther Valley before being released. 

Now that the Banff bison are free-roaming, there’s certainly the opportunity for grizzlies to scavenge their remains and perhaps prey on them – the first time local bears have had the chance since the mid-19th century. In an article in the Rocky Mountain Outlook earlier this month, though, Banff bison-reintroduction project manager Karsten Heuer noted, "From everything we’ve been seeing so far, grizzly bears and wolves haven’t quite figured out how to take advantage of this new potential prey source."

Among the other relatively few places where grizzlies and bison still cross paths are, conceivably, Montana’s National Bison Range in the foothills of the Mission Mountains (though that refuge is mainly the haunt of black bears) and some scattered locales in northwestern Canada and interior Alaska where wood bison– the boreal form of the American bison, a tad bigger and lankier than its plains cousin – roam.

If one of the continent’s most ambitious conservation and "rewilding" schemes comes to full fruition, there’s the chance grizzlies and plains bison will once again rub shoulders someday out on the wide-open prairie.

The American Prairie Reserve aims to establish the biggest wildlife sanctuary in the Lower 48 states on the northern Great Plains of north-central Montana, restoring bison, elk, swift foxes, and other native creatures to more than three million acres of remote grassland and breaks. (It’s a complicated project that’s sparked heated controversy among local ranchers.) If grizzlies continue edging eastward onto the Montana plains from the Rocky Mountain Front as they have in recent years, they may eventually – as some hope – resume their place in the American Prairie Reserve’s ecological fabric: once again to scavenge bison meat, draw the ire of grumpy bison bulls, and maybe, once in awhile anyway, drag down a lone, lame buffalo in some windswept shortgrass draw.

Top header image: Knight725/Flickr