Yellowstone wolves lose the thrill of the hunt when bears are around

JACKSON HOLE, Wyo. — If you are a wolf living in Yellowstone National Park, bears mess with you. They show up uninvited and steal kills from your pack. And when scavenging bears drive you away from tasty carcasses, you and your fellow wolves will—strangely enough—kill less often.

The reasons for this unexpected finding are explored in a new study by researchers at the University of Montana, Yellowstone National Park, Norwegian Institute for Nature Research and others. The work was published in the scientific journal Ecological Monographs.

“In both Yellowstone and Scandinavia, previous research had shown how the presence of bears led to wolf kill rates that were lower,” said Dr. Matthew Metz, a research associate with the Yellowstone Wolf Project, who earned his doctorate in wildlife biology from UM in December. “This was exciting because it showed that wolf foraging behavior doesn’t occur in a vacuum—it is affected by other apex predators on the landscape.”

This latest study delved into the competitive mechanisms that lead to decreased kill rates by wolves, and examined whether they were the same between continents.

Research has shown that larger cat species kill more often when sharing hunting grounds with bears. Bears drive them away from kill sites, and the cats are forced back to hunting, driving up their predator kill rates. Metz said this dynamic differs for wolves.

“What we did was break down the wolf foraging sequence,” he said. “We studied their searching time and their handling time—the amount of time they spend eating and digesting their kills.”

The researchers found that wolf handling times sometimes increased when bears were around, including for the wolf-bear interactions during summer in Yellowstone National Park. Aimee Tallian, a scientist with the Norwegian Institute for Nature Research and the lead author of the international study, said they suspect wolves stick around more to defend their kills, or they move back and forth from kills sites more often to avoid confrontations with bears.

Metz said wolf behavior also is greatly affected by the seasons. In winter, wolves experience less competition when bears hibernate. In the summer, wolves are denning and raising pups, which affects their foraging behavior since they must care for their young.

The primary ungulate prey of wolves—elk in Yellowstone and moose in Scandinavia—are born in large pulses in the early summer, and the presence of newborn prey drives up kill rates. However, adult prey become hardier as their nutrition improves during warmer months, and bears have emerged from hibernation to complicate matters.

“Relatively little had been known about how bears affected the foraging dynamics of wolves,” Metz said. “Our work starts to fill in the gap by demonstrating that the dynamics do differ and provides another reminder of how changes in ecosystem complexity—in this case the presence of bears—affects the behavior of other species.”

What’s it like doing research in one of America’s most-treasured national parks?

“Northern Yellowstone is an amazing place to work,” he said. “There are mountain peaks with amazing views that let you see all the way to the Tetons on a clear day. However, we also have picked our way through miles of super-dense forest ‘regen’ resulting from the 1988 fires to search for wolf GPS locations in the absolute middle of nowhere. But the sum total of all those days over many years led to an amazing data set.”