Why Wolves Hunt in Packs: Satiety in Numbers
From Earthwatch Institute Monday, May 17, 2004

Earthwatch-supported research shows that group foraging by wolves may have evolved in response to hordes of scavenging ravens

For decades, scientists have puzzled over why wolves live and hunt in packs of six or more, rather than leading solitary lives like most carnivores. A paper co-authored by Earthwatch-supported ecologist Dr. Rolf Peterson shows that larger wolf packs are favored by an ecological force overlooked until now: the loss of food to scavenging ravens, sometimes more than 50 at any given kill.

"Raven scavenging may be the 'ultimate' force explaining why wolves live in packs," said Peterson, professor of Wildlife Ecology at Michigan Technical University and principal investigator of the Earthwatch-supported Moose and Wolves project in Isle Royale National Park, Michigan. The number of ravens at kills, and their potential impact on wolf ecology, has only been documented by other studies in the last decade.

"Other scavengers may be locally important, but only the raven coexists with wolves throughout its worldwide range, and only the raven is ever-present in winter, when wolves travel in packs," Peterson added.

The paper in press in the journal Animal Behaviour is based on 27 years of data on wolf pack size and predation on moose on Isle Royale, an island in Lake Superior. Lead author John Vucetich (Michigan Technical University), Peterson, and co-author Thomas Waite (Ohio State University) apply these data to show that by consuming moose more quickly, a large pack loses less food to scavenging ravens.

Previous studies of wolves have struggled to explain why wolves foraged in packs, because the cost of sharing food among pack mates leads to each individual getting less food. Peterson and his colleagues use their data to confirm these costs, which favor a group size of only two, but find that the cost of sharing food in larger packs is more than offset by decreased losses to ravens.

The findings of Peterson and his colleagues may also help explain group foraging in other social carnivores, such as African lions, spotted hyenas, and African hunting dogs. At the very least, they are a significant contribution to the conservation and management of northern ecosystems where wolves are still found.

"As wolf populations recover in parts of their range, a major source of year-round food is returning to help support scavengers," said Peterson, highlighting the role of the wolf as a "keystone" predator. "This study indicates how significant the scavengers may be, in turn, to the evolutionary history of wolves. It's a two-way street."

Although most of the data for the study were collected in the winter, Earthwatch teams assisting Peterson in the summer also relocated some moose kills and collected data on the moose's sex and age. These data are part of a long-term, comprehensive study by Peterson and colleagues, supported by Earthwatch volunteers, on the predator-prey ecology of wolves and moose.

"Since 1988, Earthwatch volunteers have provided essential physical and financial assistance to wolf-moose each summer at Isle Royale," said Peterson. "Summer fieldwork provides additional information about the genetic make-up of the wolves and tells us the condition of the moose population on which the wolves rely."

Earthwatch Institute is an international nonprofit organization that supports scientific field research by offering members of the public unique opportunities to work alongside leading field scientists and researchers. The Institute's mission is to engage people worldwide in scientific field research and education to promote the understanding and action necessary for a sustainable environment.

For more information, see "Raven scavenging favours group foraging in wolves." John A. Vucetich, Rolf O. Peterson, and Thomas A Waite. Animal Behaviour, 2004, in press. doi:10.1016/j.anbehav.2003.06.018