There’s a battle ‘a-bruin (sorry for the nerdy science pun). Both polar bears (Ursus maritimus) and grizzly bears (Ursus arctos) have reputations as being fierce predators. Both are at the top of their respective food chains and lots of research has shown how aggressive they can be to their own kinds[1,2].
But what would happen if you pitted these two top predators together in a battle. Who would win the battle of a polar bear vs grizzly bear? The answer might surprise you. It certainly surprised me!
Researchers have found that, at least under certain conditions, grizzly bears are able to dominate polar bears at a shared feeding site.
In the post below, I’ll go into the details of how polar bears respond to grizzly bears, why we think grizzly bears win in a battle of polar bear vs grizzly bear, and how common interactions between the two species are.
Polar Bear vs Grizzly Bear: Who Wins?
Polar bears are drawn annually to bowhead whale (Balaena mysticetus) remains along the coast in northeastern Alaska. Large numbers of polar bears can occur here as it’s the only reliable food source for polar bears in the area who are stuck on shore until the sea ice returns. Up to 80 individual bears can occur here any given day in autumn.
These whale remains provide polar bears an important source of energy during a period of the year when they are generally not eating much and relying on stored body fat.
Grizzly bears are drawn to the whale remains too, for the exact same reason. They want to pack on the pounds before they go into hibernation in the coming months.
During a different study looking at polar bear behavior at the site of the remains, researchers noticed that polar bears would suddenly flee the remains when a grizzly bear came by. I’ve seen this with my own eyes too.
A grizzly bear caused a dozen or more BIG polar bears to run away, even though the grizzly bear did not exhibit any aggression and was half their size! It was an incredible sight!
And this is exactly what the research found. In most circumstances, a grizzly bear would win in fights at the whale remain site and force the polar bear to leave.
I never would have guessed that would happen. But the researchers have a few different hypotheses for why in a battle of a polar bear vs grizzly bear, the grizzly bear usually wins, which I’ll discuss below.
How Can Grizzly Bears Beat Polar Bears?
In the biological world, a species that is larger than another is often able to outcompete the smaller species. Even within bears, grizzly bears will sometimes kill black bears (Ursus americanus) if they encounter them.
In northern Alaska, where grizzly bear and polar bear ranges overlap, polar bears are significantly larger than grizzly bears. Male grizzly bears in northern Alaska have an average mass of 460 lbs (210 kg), whereas polar bears have an average mass of 800 lbs (400 kg).
So, on the basis of size alone, polar bears should kick a grizzly bear’s butt. So what’s going on here? There a couple of hypotheses to explain these unexpected results.
Hypothesis: Grizzly Bears are More Aggressive
One hypothesis for why grizzly bears win in a battle against grizzly bears is simply that they are meaner and more willing to fight than a polar bear.
Grizzly bears have a reputation for being “vicious” predators, and are known to be more aggressive than other bear species. Throughout their evolution, large Pleistocene predators such as wolves, large cats, and other bears resulted in natural selection for aggression in grizzly bears.
Grizzly bears are also omnivorous (i.e., they eat both plant and animals) and may be more likely to defend an energetically-rich food source compared to the lower quality food items that generally make up their diet.
Conversely, polar bears have access to energy rich prey most of the year and don’t usually have to worry about competing with other polar bears for it. During the late summer when they’re at the site of the whale remains, they generally are not aggressive to each other.
Further, because they primarily hunt on the sea ice, polar bears did not have to compete with the Pleistocene predators that grizzly bears evolved with.
This is not to say they are never aggressive towards one another, but probably less frequently than grizzly bears. Some of the scars I’ve seen on polar bears show that they were involved in some big fights. But it’s usually males competing with each other for access to mates.
Hypothesis: Grizzly Bears are Packing on the Pounds
Another hypothesis, and one that I think has a lot of support, is that grizzly bears are more willing to take risks fighting polar bears in autumn because they are trying to gain sufficient energy for their upcoming hibernation.
Polar bears don’t hibernate, and during late summer (when they most often interact with grizzly bears) they attempt to minimize their energy expenditures. Polar bears typically pack on their pounds in the spring when ringed seals (their primary prey; Phoca hispida) are most available.
It is likely more important for polar bears to avoid conflict and burning energy fighting grizzly bears because they know they have to wait another month or two for the sea ice to return. A grizzly bear really doesn’t have too much to lose. They need as much stored energy as possible to survive a long winter hibernation.
Hypothesis: Polar Bears are Risk Averse
A final hypothesis for why a battle of polar bear vs grizzly bear is typically won by grizzly bears is that polar bears may be more “scared” of a fight than grizzly bears.
As I mentioned earlier, grizzly bears are considered omnivores (i.e., they eat both plants and animals) whereas polar bears are strictly carnivores (i.e., they only eat animals).
Polar bears are highly adapted for hunting seals on the sea ice. From the color of their fur, their teeth, and claws, these features help polar bears to successfully hunt seals.
Grizzly bears are adapted for exploiting almost any resource. They are commonly seen eating berries, grubs, squirrels, and larger prey like moose and caribou.
Because grizzly bears can exploit so many different types of food, if they become injured they would likely be able to still find and consume food, even if it meant they couldn’t hunt larger prey any more.
If a polar bear was significantly injured, however, they could lose their entire ability to successfully hunt seals. Then they would be in a lot of trouble because there are not many resources to scavenge on the sea ice.
Thus, polar bears may be less willing to engage in a fight with grizzly bears over the whale remains because the cost of a potential injury outweigh the potential short-term energetic gains. It would just make sense to wait to resume feeding until after the grizzly bear leaves.
Polar bears and grizzly bears don’t typically interact with each other except during the summer when polar bears spend time on land. Most of the year polar bears are on the sea ice and grizzly bears are running rampant on land.
But when they do come together, the research indicates that in a battle of polar bear vs grizzly bear the grizzly bear appears to win. There were certainly observations of some polar bears beating grizzly bears in the study, but on average, the grizzly bear was the winner.