In Canada, grizzly bears are moving north and are already in polar bear territory, Van Valkenburgh and Slater said.
The life scientists—whose co-authors include UCLA undergraduates Leeann Louis and Paul Yang and graduate student Borja Figueirido from Spain's Universidad de Malaga, Campus Universitario de Teatinos—studied two adult male skulls from museums, one of a polar bear from Canada, the other of a grizzly from Alaska. They built 3-D computer models of the skulls and then analyzed their biomechanics.
"We can apply muscle forces to the skull to simulate biting, and we can measure how hard the animal could bite. We can measure stress and strain in the skull as well," Slater said. "We found that while the stresses in the grizzly bear skull are relatively low, the same bites in the polar bear produce much more stress. Combined with other evidence from Blaire's laboratory, this tells us that the smaller teeth of polar bears are less suited to diets that consist of plants, grass, vegetation and berries."
"Polar bears would not be able to break up the food as well in their mouths and would not digest it as well," Van Valkenburgh said.
In the timeline of evolution, polar bears evolved from the brown bear very recently, and the two are very closely related, Van Valkenburgh and Slater said. Genetic studies indicate that the split between polar bears and brown bears occurred only 500,000 to 800,000 years ago—the most recent split between any of the eight bear species.
Despite the recentness of the split between these two species, their skulls and teeth are extremely different, probably as a result of where they live (arctic versus temperate regions) and the differences in their diets. Grizzly bears have very large molar teeth, while polar bears have teeth that are much smaller. Polar bears eat seal blubber, which is soft and does not require much chewing, while brown bears consume many plants.
The biologists investigated the rate at which skull shape has evolved in the bear family. They found that the rate of evolution in the branch of the bear family tree leading to the polar bear was twice as fast as the rates in other branches of the tree; it appears that skull shape evolved extremely rapidly in polar bears.
Polar bears probably evolved very rapidly in response to glacial climates during the ice ages, Slater said.
"You don't see many bears that look like polar bears, and the difference in skull shape evolved very rapidly," Slater said.
Follow U.S. News Science on Twitter.