Opposing effects of group size on reproduction and survival in African wild dogs

For cooperative breeders, we hypothesize that the effects of group size on reproduction and survival might run in opposition if the benefits of grouping cannot be shared without cost. We tested this hypothesis by examining relationships between group size, survival, and reproduction in African wild dogs (Lycaon pictus), cooperative hunters with highly cohesive packs within which reproduction is monopolized by the dominant male and female. The production and survival of pups are known to increase with increasing pack size, but the effect of pack size on adult survival has not been examined previously. Data from 366 individuals over a period of 6 years showed that the survival of adults decreased with increasing pack size, with a 25% difference between the largest and smallest packs after controlling for the effects of age, sex, social status, year of study, and pack identity. Several tests confirmed that undetected dispersal is unlikely to have produced this pattern. These results suggest that cooperative breeding in wild dogs cannot be fully explained by mutual direct benefit, thus reinforcing the prior inference that kin selection plays an important role in the evolution of their cooperation. The results also weaken support for the hypothesis that wild dogs are extinction prone due to group-level Allee effects. More broadly, the relationship of effects of group size on survival and reproduction might be predicted by considering whether cooperation yields benefits that accrue to all group members (e.g., through cooperative vigilance) or benefits that must be apportioned to individuals (e.g., through cooperative hunting).