Thieving birds may drive canines to form big packs.

Wolf packs often turn out to be bigger than predicted by the theories of animal behaviorists, and a new analysis points to a previously underappreciated factor: the scrounging genius of ravens.

Observations of wolves for 27 winters on an island in Lake Superior permitted a detailed analysis of factors affecting the amount of food a wolf gets in winter, says Thomas Waite of Ohio State University in Columbus. The analysis repeatedly suggested that the economics of feeding would work out best if wolves hunted in pairs, but in real life the wolves typically form packs of six or more.

With the new consideration of the food that wolves lose to ravens scavenging on the pack's kill, it makes sense for wolves to hunt in larger packs, the team reports in an upcoming Animal Behaviour.

One of the coauthors of the new study, Rolf Peterson of Michigan Technological University in Houghton, has led an unusually long study of wolves in winter on Isle Royale. The data dash several ideas about pack size, says Waite. For example, a pack is not necessary to bring down big prey, he says. In 11 instances, Peterson's team saw a wolf by itself kill a moose.

Waite, Peterson, and John Vucetich, also of Michigan Tech, used the Isle Royale data. When they adjusted for the energy that a wolf expends on hunting, the results again suggested that wolves would do best hunting in pairs. Kinship didn't explain the large packs, nor did estimates of the probability that a wolf might go a longtime without food.

A moose is such large prey, however, that one or two wolves can't eat it all at once. When researchers calculated that a wolf hunting with one partner loses more moose meat to ravens than a wolf hunting in a big group does, the big packs made sense. A pair of wolves typically loses about 37 percent of a carcass to ravens, whereas a pack of six loses only 17 percent.

According to raven specialist Bernd Heinrich of the University of Vermont in Burlington, ravens arrive within a minute of a kill. "It's not what they eat on the spot, it's what they haul off," he explains. He says that he finds it plausible that these birds could have a "major effect" on wolves' diet.

The wolf pack is unusual, explains Waite. About 85 percent of carnivore species prowl as loners.

"Scavenging may be a common selective factor for carnivore sociality," says Waite. "We really do need a test of this across other species."