Wolf meat?
Bison no target for predators
By MICHAEL MILSTEIN
Gazette Wyoming Bureau
YELLOWSTONE NATIONAL PARK - Take one look at a bison - big, shaggy animals built like bulldozers - and you'll no doubt understand why Yellowstone National Park's newest predators are keeping their distance.

"Wolves are avoiding bison," said Yellowstone wolf biologist Douglas Smith.

Although some had hoped that wolves would apply natural control to the bison herds that ignite debate when they overflow from the park in winter, so far it's not happening. Wolves reintroduced to Yellowstone in 1995 appear to be preying almost exclusively on elk, apparently the easiest marks among the park's varied assemblage of wildlife.

Since November 1995, biologists have watched about 600 wolf-elk "interactions," instances in which wolves approached a potential prey animal. In that same period, they have seen wolves approach bison just 41 times, suggesting that wolves rarely scrutinize bison as prey.

Although elk are more numerous in the park than bison - bison numbered about 2,200 at last count - Smith said "bison are slower moving, they feed more in the open and they're easier to locate, so you would think they'd make attractive prey." Some wolf packs have staked out territories that are home to large numbers of bison.

Biologists have documented about 600 elk killed by wolves in Yellowstone, although it's likely wolves have in fact killed many more, Smith said. They have similarly documented only five bison killed by wolves: Of those five bison, two were calves, two were killed in late winter when bison are weak and at their most vulnerable and the final bison had an injured leg.

"The pattern we're seeing is that wolves aren't killing bison unless they're calves or there's some problem with them," Smith said. "The older, sicker, weaker animals seem to be picked on the most."

Of 257 known and probable wolf kills documented in Yellowstone in 1997, about 90 percent were elk. Other prey included eight moose, six mule deer, two bison, one beaver and six unidentified animals.

It's not as if wolves cannot kill bison: Wolf packs in Canada's Wood Buffalo National Park feed almost exclusively on bison, the most abundant prey there, Smith said. Two of the bison killed in Yellowstone fell prey to a wolf pack roaming the Pelican Valley, where bison are virtually the only prey in the winter.

About 120 wolves now dwell in Yellowstone. When they have a choice of prey, observations show, wolves zero in on the easiest targets through a sly test that may offer the best explanation of why they kill so many more elk than bison.

According to observations by biologists, about three-quarters of elk confronted by wolves take off running and, when they do, wolves almost always take off after them. One out of every four elk ends up as a meal.

In contrast, almost nine of every ten bison approached by wolves show no sign of flight - flight may appear to the wolves as evidence of weakness.

"About 85 to 90 percent of the time, bison stand their ground and the wolves leave," Smith said. "What this tells us is the animals that stand up to wolves and don't run have a very low probability of being killed."

Smith stressed that he is not downplaying the effect of wolves on prey populations, but instead noting that wolves are not indiscriminate killers: They do not want to expend unnecessary energy to bring down prey any more than a person in search of food wants to bypass a nearby grocery store for one that's farther away.

"Bison are the most formidable prey for wolves in North America and moose are a close second," Smith said. "They can kill them (bison), but they have to work harder."

He predicted, though, that as wolf numbers grow and wolves become more familiar with Yellowstone and its wildlife, wolves will begin to kill more bison.

In Yellowstone's Madison and Firehole River drainages, Montana State University graduate student Rose Jaffe has begun a study of wolf-killed animals in hopes of determining what factors make them vulnerable to wolves. As of mid-January she had documented 17 wolf kills, including two bison calves, but she and her advisor Robert Garrott, an MSU professor, said it was too early to draw any conclusions about why the wolves had targeted those animals.

Jaffe approaches wolf-kills like a detective, examining the carcass to see if the animal was diseased or malnourished and inspecting the tracks of both the prey animals and the wolves that killed it to tell how far the wolves had chased their unlucky meal, how deep the snow was and what kind of terrain they had crossed.

"Sometimes it's a snap but most times it takes a lot of sleuthing to tell what happened," Jaffe said. "I feel like I'm reading a story in the snow. I can tell where a wolf sat down and wagged its tail or chewed ice balls out of its paws."