THE BENEFITS-OF-PHILOPATRY HYPOTHESIS FOR THE EVOLUTION OF COOPERATIVE BREEDING - VARIATION IN TERRITORY QUALITY AND GROUP-SIZE EFFECTS

Cooperative breeding systems are characterized by the presence of individuals that remain as nonbreeders in their natal groups past the age of reproductive maturity. Currently, the most widely accepted explanation of this nondispersal is that immature members of cooperative species are faced with ecological constraints on independent breeding, such as habitat saturation. However, current evidence indicates that reproductive opportunities in many noncooperative breeders (where immature individuals always disperse) are just as limited as those in cooperative species. Thus ecological constraints alone cannot explain the differences in the two dispersal patterns. As an alternative, we describe a "benefits of philopatry" hypothesis, which proposes that nonbreeding helpers remain at home only when there is a net fitness benefit to doing so. This is most likely to occur either when territories in a local area vary greatly in quality or when group cooperation leads to high variance in fitness of individuals in different-sized groups. The predictions of this model thus differ from those of the habitat-saturation model, which explicitly proposes that variance in fitness measures related to territory quality will be lower in cooperatively breeding species than in noncooperative forms. We test these ideas by comparing variance in reproductive success among territories in two cooperative and a social, but noncooperative, species. As predicted by the benefits-of-philopatry model, but not the habitat-saturation model, the among-territory coefficients of variation in the two cooperative species are similar to each other and significantly greater than those in the noncooperative species. These results suggest that local patterns of variation in quality among individual territories may play an important role in the evolution of different social systems.