Hunters and freeloaders

Natural History, Sept, 2004 by Nick W. Atkinson

Puzzling over the whys and wherefores of wolf packs, evolutionary biologists have noted that they are often larger than they "should" be. Most carnivore species hunt alone, and so each individual gets to eat what it kills. Group hunting, by contrast, makes it possible to take more prey, but the bounty must then be shared. And if a pack exceeds a certain number of animals (a number based on such variables as available food, the risks of nutritional shortfall, and the energy requirements of territorial defense), competition for food becomes so intense that its members should, theoretically, fare better if they hunt alone. Yet wolf packs frequently exceed the expected optimum number.

To resolve the paradox, say John A. Vucetich, an ecologist at Michigan Technological University in Houghton, and his colleagues, the focus needs to shift from the wolves to their hangers-on: ravens. More mobile than wolves, ravens often manage to steal a large proportion of a wolf pack's kill. One raven can consume or hoard four pounds a day from a large carcass. The key, say the investigators, two of whom have studied the wolves of Isle Royale National Park in northern Michigan for decades, is that wolves in large packs lose less food to ravens, simply because the carcass gets consumed fasten That single benefit of belonging to a large group outweighs the costs of having to hunt more often and share the kill among more packmates. ("Raven scavenging favours group foraging in wolves," Animal Behaviour 67:1117-26, June 2004)