Spanning an area of nearly 9,000km2 - roughly the size of Cyprus - Yellowstone National Park is one of the largest national parks in North America. It’s also widely considered to be the first of its kind in the world; established by the 42nd US Congress and signed into law on March 1st, 1872.
Yellowstone is as famous for its wildlife as it is for its iconic geysers. The park itself is home to nearly 300 species of birds, 16 species of fish, five species of amphibians, six species of reptiles, and 67 species of mammals, making it one of the most diverse ecosystems in Earth’s northern temperate zone.
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The majority of these mammals are quite small, but a handful fall into the category of megafauna - animals with a body mass exceeding 45kg. During the last Ice Age, North America was inhabited by a wide range of megafauna, such as woolly mammoths, sabre-toothed tigers, and giant ground sloths.
Most of these cold-adapted animals have since disappeared, leaving today’s megafaunal diversity a mere shadow of its former richness. Yet in Yellowstone, traces remain of what life on the continent was like before rapid climate change and human activity drove many large animals to extinction.
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Like their ancestors from prehistory, many of Yellowstone’s megafauna are deadly - not only to other animals, but also visiting tourists. Here are 10 of the deadliest animals found in the world’s oldest national park…
10 deadliest animals in Yellowstone National Park
Bison
They may look docile, but these unpredictable giants are responsible for injuring more tourists in Yellowstone National Park than any other animal, including bears and wolves.
According to a 2019 report published in the journal Human-Wildlife Interactions, 25 people were injured by bison in Yellowstone between 2000 and 2015. Although concerning, this represented a significant decline from the 56 injuries recorded between 1978 and 1992 - a period that also saw two people killed by bison.
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What makes bison so deadly is their size; males stand about 1.8m tall at the shoulder, reach lengths of up to 3.5m, and weigh in at nearly a ton. While females only grow half as large, they can still be deadly, especially if tourists get too close - park guidelines recommend staying 25 yards away from bison at all times. They’re surprisingly agile and fast too, capable of reaching speeds of up to 35mph.
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Yellowstone is the only place in the US where bison have lived continuously since they ranged across North America thousands of years ago. In 2024, conservationists estimated there were as many as 5,400 bison living in the park, split between two primary breeding herds: northern and central.
Grizzly Bears
There are few animals more feared than ‘grizzlies’, and for good reason - these giant apex predators are the size of a small car, as fast as a galloping horse, and have jaws strong enough to crush bones.
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Since Yellowstone National Park was established in 1872, seven people have been killed by grizzly bears. Only one of these attacks was classified as predatory; the others involved surprise encounters and/or bears who’d caught a whiff of human food.
The last few decades has seen Yellowstone’s grizzly bear population increase, from 136 in 1975 to a peak of 1,030 in 2024. This ‘bear boom’ hasn’t led to an increase in attacks. In fact, the number of bear-infliced injuries has dropped dramatically from 45 per year in the 1960s to just one injury per year in the 2000s.
A 2022 study, also published in the journal Human-Wildlife Interactions, found the per capita risk of being killed by a grizzly bear in Yellowstone was one fatality for every 26.2 million park visits. In other words, you’re more likely to die driving to Yellowstone than from a grizzly once you get there.
Black Bears
While half the size and not quite as aggressive as grizzly bears, black bears still pose a significant threat to tourists visiting Yellowstone.
These bears have killed one person since 1872 and between 1979 and 2022 injured five, according to statistics compiled by Kerry Gunther, a member of the North American Bears Expert Team. The majority of these attacks took place at campsites and seemed to occur as a result of the black bears in question seeking out human food.
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In 2025, a 29-year-old hiker was injured by a bear on the Turbid Lake Trail northeast of Yellowstone Lake, marking the park’s first bear attack since a grizzly injured a hiker in the Mammoth Hot Springs area in 2021. Despite the hiker originally identifying the bear as a black bear, experts have since suggested it was more likely a grizzly, based on its location, size, and reported behaviour.
Not much is known about the population of black bears in Yellowstone, and whether their numbers have been affected by the increase in the population of grizzly bears since the 1970s. However, they are said to be ‘common’ throughout the park and tourists are advised to remain vigilant and follow guidelines wherever they’re hiking.
Wolves
If you’ve ever heard of the concept of rewilding, you’ve likely heard of the wolves of Yellowstone - a classic ecological success story. By the early 20th century, wolves had been virtually wiped out of Yellowstone as a result of government-implemented predator control programs. However, in 1995 they were reintroduced as part of the Yellowstone Wolf Project.
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This program triggered a beneficial trophic cascade that curbed excessive elk populations and allowed plants like aspen and willow to recover. These plants brought beavers back to the park, which stabilised riverbanks and increased biodiversity. Today, the park is home to nine beaver colonies - just one of the many positive consequences of the reintroduction of wolves.
Now, you may think the reintroduction of a dangerous apex predator may have led to an increase in the number of attacks on tourists… but that isn’t the case. Since modern records began, there are no reported instances of wolves attacking humans in Yellowstone. That said, tourists are still warned about the dangers of habituating them by giving them food, which may encourage them to approach groups rather than run away.
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Such behaviour has been documented in other parts of North America, say the International Wolf Center. In 2005, a 22-year-old was killed near a remote mining camp in Canada where wolves had grown accustomed to raiding a nearby landfill site for food.
Elk
Of the large mammals found in Yellowstone, elk are by far the most abundant. At their peak numbers in summer, an estimated 10,000 to 20,000 individuals from six to seven herds inhabit the park, with thousands gathering around Mammoth Hot Springs - one of the most popular destinations for visiting tourists.
Having so many large, aggressive 300kg ruminants in a relatively small area crowded with people obviously creates significant safety risks.
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To combat these risks, the National Park Service are forced to bring in extra staff during busy weeks of the year whose primary job is to prevent people getting too close to bulls hopped up on pheromones. These bulls have been documented smashing into cars, chasing people, and even goring them with their metre-long antlers.
It’s not just the males that are aggressive. In 2019, an employee working at the park was hospitalised after being attacked by a female elk during calving season. Just a year earlier, two other people also suffered injuries after being attacked by a female elk in the same area.
Moose
Standing 2m tall at the shoulder and weighing nearly 700kg, moose are the largest members of the deer family. While largely solitary and skittish, these giants aren’t exactly ‘gentle’ - when threatened, they can become incredibly aggressive and have been known to ram vehicles and stomp on tourists.
In Alaska, more people are injured by moose (5-10 people) than by bears (3-4 people) each year, according to the state’s Department of Fish and Game. The majority of moose-related injuries take place on the roads, where reported collisions involving moose can reach as many as 500 cases annually.
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And it’s moose that come off worse from these collisions - roughly 800 die across Alaska every year as a result of collisions with cars, many of which go unreported.
There may be several documented cases of moose attacking humans, but none of these have taken place in Yellowstone. The National Park Service says this is largely down to the fact that tourists don’t encounter them very often. Fewer than 200 individuals currently live in Yellowstone, and the population has been declining over the last 40 years as a result of habitat loss and hunting outside the park.
Cougars
There are only an estimated 45 cougars living in Yellowstone, yet they’re considered one of the area’s top predators, preying primarily on elk but also deer, pronghorn, bighorn sheep, and even moose.
These elusive cats, also known as mountain lions, were largely eradicated from Yellowstone in the early 1900s as a result of the same predator control programs that virtually wiped out the park’s wolves. However, they naturally re-established themselves in the early 1980s, possibly making their way from habitats in central Idaho.
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The northern range of Yellowstone is prime habitat for cougars, but they’re known to live throughout the park in summer. They’re seldom seen though, and according to the National Park Service very few cougar-human confrontations have actually taken place inside the park.
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Most cougar attacks have been documented in the western United States and Canada, with Vancouver Island, California, and Colorado having the highest concentrations of incidents. In the last 100 years, approximately 130 attacks on humans have been reported, 29 of which resulted in fatalities.
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Prairie Rattlesnakes
While Yellowstone’s large mammals may command most visitors’ attention, it’s important they also keep an eye out for smaller animals underfoot, such as snakes.
The only venomous snake found in Yellowstone is the prairie rattlesnake - a metre-long, highly camouflaged species whose range extends over much of North America’s Great Plains, from southwestern Canada to northern Mexico.
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Like other rattlesnakes, the prairie rattlesnake is highly venomous, possessing potent venom that has neurotoxic properties and tissue-destructive effects. A single bite can cause severe pain, swelling, and - in some cases - long-term tissue damage. However, bites are rarely fatal if treated immediately with antivenom.
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According to the National Park Service, only two snake bites are known to have taken place during the history of the park.
Bald Eagles
The US’ most symbolic bird might not be dangerous to humans, but it’s deadly to Yellowstone's large diversity of fish, birds, and small mammals, as well as any pets that tourists may bring to the park.
In 2023, four occupied bald eagle nests were documented on Yellowstone Lake alone, plus a further nine in other areas of the park. Their numbers have been steadily increasing since they were listed as an endangered species in 1967, prompting a delisting in 2007.
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Like others that live outside the park, Yellowstone’s bald eagles primarily feed on fish, though during winter they’ve been documented eating more waterfowl and scavenging carrion. None have been documented targeting pets, though as opportunistic feeders it is recommended tourists keep small pets close near areas where bald eagles have been sighted.
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Despite their name, bald eagles aren’t actually bald. Instead, their name is derived from the old English word balde, meaning ‘white-headed’.
Humans
It could be argued the deadliest animals in Yellowstone are, in fact, humans. The park may be home to powerful predators such as bears and wolves, but it’s human activity that has historically caused the most profound and long-lasting harm to wildlife - for example, the eradication of wolves and cougars in the early 20th century.
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A lot of incidents involving animals have also been caused by tourists failing to adhere to the National Park Services’ guidelines for observing wildlife. In these cases, it’s often human actions - such as getting too close, feeding animals, or provoking them - that escalates encounters and turns them deadly.
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There’s growing concern that overhunting is adversely affecting wildlife populations in Yellowstone too. In 2022, more than 500 wolves were killed in Montana, Idaho, and Wyoming over the course of just a few months. This included nearly 20% of the wolves that sometimes range outside of Yellowstone, which threatens to undermine the decades-old Yellowstone Wolf Project.
The hunting of wolves may be banned in Yellowstone, but it’s legal in surrounding states and even encouraged in some. In Montana, for example, legislators set a goal in 2021 to reduce the state’s wolf population to “at least 15 breeding pairs.”
