The association between the emergence of cooperative breeding and clutch size

Association of social group with both life-history traits and brain size in cooperatively breeding birds

Social group is associated with life-history traits and can predict brain size variation in cooperative primates and some other mammal groups, but such explicit relationships remain enigmatic in cooperatively breeding birds. Indeed, some compositions of social group in cooperative species (e.g., helper number and group size) would affect the fitness of breeders by providing alloparental care. Here, we conducted comparative tests of the relationship between the social group and both life-history traits and brain size across 197 species of cooperatively breeding birds using phylogenetically controlled comparative analyses. We did not find any correlations between helper numbers and both life-history traits and brain size. However, we found that maximum group size was positively associated with clutch size. Moreover, average group size has positive associations with body mass and relative brain size. Our findings suggest that helper numbers cannot promote variation in relative brain size, while larger groups may predict bigger brains in cooperatively breeding birds.

Evolution of delayed dispersal and subsequent emergence of helping, with implications for cooperative breeding

Cooperative breeding occurs when individuals help raise the offspring of others. It is widely accepted that help displayed by cooperative breeders emerged only after individuals' tendency to delay dispersal had become established. We use this idea as a basis for two inclusive-fitness models: one for the evolution of delayed dispersal, and a second for the subsequent emergence of helpful behavior exhibited by non-breeding individuals. We focus on a territorial species in a saturated environment, and allow territories to be inherited by non-breeding individuals who have delayed dispersal. Our first model predicts that increased survivorship and increased fecundity both provide an incentive to non-breeding individuals to delay dispersal, and stay near their natal territory for some period of time. Predictions from the first model can be well understood by ignoring complications arising from competition among relatives. Our second model shows that effects on relatives play a primary role in the advantage of helping. In addition, the second model predicts that increased survivorship and fecundity promote the emergence of help. Together, our models lead us to conclude that the emergence of cooperative-breeding systems is made easier by life-history features associated with high survivorship and fecundity. We discuss the implications of our conclusions for life-history-based hypotheses of cooperative breeding and social evolution.