Food Fight: Wolves Pack Up to Out-eat Ravens

New research on the wolves of Isle Royale may shed light on a mystery that has long puzzled biologists: Why do some predators band together to hunt?

“Most species of predators live solitary lives,” says John Vucetich, a research assistant professor of wildlife ecology at Michigan Technological University. “Biologists have always wanted to know why the few exceptions live in groups.”

In his observations of wolves and ravens, Vucetich may have found the answer: Predators that hunt in groups lose far less meat to scavangers. Earlier, scientists had guessed that wolves hunting in a large pack would bring down more food per wolf, so each individual would get more to eat. However, studies showed that wasn’t the case. “According to their calculations, wolves in big packs got less food,” Vucetich said. “It didn’t make sense.”

So Vucetich examined the methodology and discovered a problem. Scientists had calculated the amount of food available based on the weight of the prey killed, not on the amount that the wolves actually ate. “Then I thought, ’OK, what you kill isn’t relevant; it’s what you consume,’” he said. “What happens after the kill? Suddenly, scavengers are really important.”

For wolves, ravens are the really important scavengers. “You never see wolves without ravens nearby,” Vucetich said. “They are ubiquitous at kills.” On Isle Royale National Park, located in Lake Superior, five to 15 ravens are found on the carcasses of moose killed by wolves. And on the mainland, the numbers can be far higher: About 100 ravens were once counted around the carcasses of a few wolf-killed deer.

“So we asked the questions: How much can a raven take per day? It can eat and stash about two pounds. How much can a wolf eat? Up to 18 pounds in a few hours,” Vucetich said. “We put the pieces together, and we found that in bigger packs, both the pack and the individuals actually get more food, not less.”

Ravens are intelligent, fast and agile. It’s useless for wolves to waste energy chasing them away from a carcass because the ravens come right back, so they don’t. Instead, wolves simply out-eat the ravens, and thus the advantage of a large pack becomes clear.

Based on his discovery, Vucetich thinks any predator that hunts large prey has to have some strategy to deal with scavengers. They can make their kills in dense cover, like tigers, or haul the carcass off to a secluded spot, like leopards. Lions usually hunt in groups, which helps when a gang of hyenas shows up.

Solitary hunters can find themselves at a loss when confronted with unexpected scavengers. “A colleague of mine saw a mountain lion kill an elk in the open,” Vucetich said. Mountain lions usually hide their large prey after a kill, and their behavior hasn’t adapted to deal with scavanging ravens. “Ravens found the kill, and the mountain lion went nuts trying to chase them away.” “The moral of the story is, if you’re eating something big, you have to have a way to deal with scavengers.”

Vucetich’s work is supported by the National Science Foundation, Isle Royale National Park and Earthwatch. His article, “Raven Scavenging Favours Group Foraging in Wolves,” has been published in the June 2004 edition of the journal Animal Behaviour and is posted online at http://dx.doi.org/10.1016/j.anbehav.2003.06.018 The co-authors are Rolf Peterson, of Michigan Technological University, and Thomas Waite, of Ohio State University.