The wolf, the central modern bison predator
Wolves are the only currently extant non-human predator that consistently takes healthy adult bison. The two species evolved together across the Great Plains and the boreal forest for millions of years and the predator-prey relationship is therefore ancient. Modern wolf-bison ecology is best documented in Yellowstone National Park, where wolves were reintroduced in 1995 and 1996 and have since formed packs that vary in their specialisation on different prey species.
Some Yellowstone packs specialise on elk; others have developed bison-hunting habits. The bison-specialist packs include the Mollies pack on the park's northern range and several packs in the Hayden and Lamar Valleys. These packs take bison year-round but with much greater frequency and success in late winter, when deep snow restricts bison mobility and the cumulative effects of the winter reduce bison body condition. A late-winter bison hunt by a pack of eight to twelve wolves can take down even a healthy adult cow; the technique involves chase to exhaustion across deep snow, harassment of the herd to isolate a target, and a coordinated attack from multiple angles once an individual is separated.
Bison defence against wolves centres on the herd cluster. A bison herd that detects approaching wolves will close ranks with calves and weaker animals in the centre and with adult bulls and cows facing outward. Wolves cannot break a tight cluster of adults and must work to isolate an individual. Yellowstone observations show wolf attempts succeeding in roughly one in ten encounters; the great majority of approaches end with the wolves withdrawing and the herd intact.
Wood bison in the Mackenzie River drainage and in Wood Buffalo National Park live with sustained wolf pressure as well. The wood bison herd at Slave River Lowlands and the Pink Mountain herd in British Columbia have substantial wolf populations sharing the range. The predator-prey ecology is similar to Yellowstone's: pack hunting of primarily winter-weakened animals, with bison herd defence usually successful but with ongoing mortality at a sustainable rate.
Grizzly bears
Grizzly bears take bison opportunistically. The most common pattern is calf predation: a grizzly that locates a calf separated from the cow or a calf in a distracted herd may make a single charge and take the calf before the herd can respond. Yellowstone records calf losses to grizzlies each year, although the total is a small fraction of the calf cohort.
Direct grizzly predation on healthy adult bison is rare but documented. A large male grizzly can in principle take a bison cow or a young bull, but the risk to the grizzly from a counter-charge or from herd-mate intervention is substantial. The more common grizzly-bison interaction is scavenging: a grizzly that locates a wolf-killed bison carcass will routinely displace the wolves and take over the carcass. Yellowstone bison carcasses from winter wolf kills produce substantial spring food for the bear population, and the spatial overlap between wolf-bison kill sites and known grizzly travel corridors is a well-documented ecological pattern.
Black bears occasionally take bison calves but the interaction is rare. Adult bison are well beyond the predator capacity of a black bear.
Mountain lions
Mountain lions (cougars) are abundant across much of the modern bison range but they do not significantly prey on bison. The species specialises on deer and elk and is too small to take bison without unacceptable risk of injury to itself. There are occasional documented incidents of mountain lions taking very young calves or sick adult bison, but the predation rate is too low to register in any of the major bison-herd ecology studies. Mountain lions, wolverines, and the few remaining lynx in bison range are essentially non-predators of bison.
The historic predator landscape
Before European arrival, the bison shared the Great Plains and the boreal forest with a richer predator community. Wolves were everywhere. Grizzly bears extended down onto the Plains as well as into the mountains; the so-called Plains grizzly was a grassland-adapted bear that historically followed bison herds, taking calves and weakened adults. Black bears occupied wooded riparian areas across the eastern Plains. Cougar populations extended through much of the bison range.
More dramatically, the bison evolved with a deeper Pleistocene predator community that no longer exists. The American lion (Panthera atrox), the short-faced bear (Arctodus simus), the dire wolf (Aenocyon dirus), and the Smilodon and Homotherium saber-toothed cats all preyed on the steppe bison and on the larger Pleistocene bison species before the end-Pleistocene megafauna extinction approximately 11,700 years ago. Modern bison anatomy (the massive head, the heavy shoulder hump, the dense forequarter musculature) is partly a legacy of evolution against this larger predator community. The modern bison is over-built relative to the threat it actually faces in protected park settings.
Indigenous Plains peoples as bison hunters
Plains Indigenous peoples were the most important bison predator across the historical range for at least 12,000 years before European arrival. Lakota, Cheyenne, Crow, Blackfoot, Comanche, Kiowa, and many other Plains nations developed sophisticated bison-hunting techniques: communal jumps where herds were driven over cliffs (the Head-Smashed-In Buffalo Jump site in southern Alberta is the most famous, with documented use for nearly 6,000 years), pound traps where herds were funnelled into enclosures and dispatched, ambush hunting at water crossings, and individual stalking with bow and lance.
The acquisition of the horse in the 17th and 18th centuries transformed the efficiency of Plains bison hunting. Mounted hunters using the bow or, later, the firearm could take bison from individual animals up to substantial harvest volumes. The mounted bison hunt became the central economic activity of the Horse Plains cultures and remained so until the commercial European-American hide hunt collapsed the bison population in the 1870s and 1880s. The cultural and economic devastation of the bison collapse on Plains nations is a major theme in 19th and 20th century North American history and is the central context for the modern InterTribal Buffalo Council's bison restoration programmes.
The 19th century commercial hide hunt
The 19th century commercial hide hunt was an extinction-level predation event with no ecological precedent. Between roughly 1865 and 1885, professional hunters operating with high-powered rifles (the .50 Sharps was the standard) killed bison primarily for hides, which were shipped east to be processed into industrial belting and into commercial leather goods. Annual kill rates in the peak years are estimated at one to two million animals across the entire range.
The hide hunt was effectively unregulated and the federal government either permitted or actively encouraged the kill as a means of forcing Plains Indigenous peoples onto reservations by removing their primary food source. By 1889 the wild bison population was estimated at fewer than 1,000 animals in total, and most of the survivors were in a few protected pockets (Yellowstone's small herd, a remnant in northern Alberta, the Pablo-Allard private herd in Montana). The commercial hide hunt was the largest single predation event in the species' entire evolutionary history.
Modern human harvest
Humans remain the largest source of bison mortality today by a substantial margin. Commercial bison ranching slaughters tens of thousands of bison per year for meat, the primary product driving the modern bison economy. Conservation herd management harvests, particularly the seasonal shipment of Yellowstone bison to slaughter when the herd exceeds the park's target population, account for several hundred to several thousand additional bison per year. Indigenous treaty-rights hunting on lands adjacent to Yellowstone and elsewhere accounts for additional harvest. Hunting on Indigenous land trusts and on Custer State Park surplus animals adds more.
The total annual human harvest of bison is on the order of 70,000 to 80,000 animals per year across North America. Compared with this, natural predation by wolves and bears amounts to perhaps a few thousand calves and a few hundred adults across the entire continent annually. The bison's ecological future depends primarily on land and habitat decisions rather than on management of natural predation.
Frequently asked questions
Why don't wolves attack bison more often?
Because the success rate against a defended bison herd is low and the risk of injury to the wolves is high. Wolves are opportunistic predators and focus on prey species where the energetic return per attempt is favourable. Elk and deer offer a higher return on effort than bison. Wolf packs that specialise on bison do so when other prey are scarce or when local conditions (deep snow, weakened bison) make bison an easier target.
Did wolves originally control bison populations on the Plains?
No, not as the primary regulator. The historical Plains bison population was so large (30 to 60 million animals) that the wolf population could not significantly affect it. Drought, fire, disease, and Indigenous hunting were larger regulators of bison numbers than wolf predation. Wolves were part of the ecosystem and took a steady background fraction of calves and weakened adults, but the bison population was not wolf-limited.
Are there any bison-specific predators today?
No. No predator species specialises exclusively on bison. The wolf is the only species that significantly takes adult bison and wolves take many other prey species as well. Grizzlies, mountain lions, and the smaller carnivores all have broader diets.
Do bison ever attack predators?
Yes. A defending cow with a calf will charge a wolf, a bear, or any other perceived threat and is fully capable of injuring or killing the attacker. Yellowstone has recorded multiple cases of wolves killed or injured by defending bison cows. The counter-charge is part of the calculus that limits predator attempts on healthy herd animals.