Raven scavenging favours group foraging in wolves

Wolves, Canis lupus, routinely live in large packs that include unrelated individuals and mature offspring. Studies. show that individual wolves that live in large packs suffer reduced foraging returns. Therefore, group hunting and group living (sociality) in wolves is generally thought to be favoured by indirect fitness gains accrued through kin-directed altruism. However, we show that kin-directed altruism cannot account for groups that include mature offspring or unrelated individuals. We also present an analysis that incorporates a previously ignored feature of wolf foraging ecology, namely the loss of food to scavenging ravens, Corvus corax. By accounting for this process, we show that individuals in large packs do indeed accrue foraging advantages. In the hypothetical absence of this scavenging pressure, an individual would maximize its rate of prey acquisition, and minimize its risk of energetic shortfall, by foraging with just one other individual. However, incorporating the effect of scavenging by ravens leads to a dramatic increase in the predicted group size. Our analysis indicates that per capita gains are highest in the largest observed packs. The greater food-sharing costs in a larger pack are more than offset by smaller losses to scavengers and increased rates of prey acquisition. Thus, in contrast with previous interpretations, the selfish benefits of social foraging appear to contribute to the maintenance of sociality in wolves after all. We explore whether such benefits favour group living in various social carnivores that hunt large prey and are thus vulnerable to scavenging. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.