Résumé
Understanding the evolution of complex social behaviours, such as cooperative breeding, is a fundamental problem in evolutionary biology, which has attracted much theoretical and empirical interest. Variation within and between species in the frequency of helping behaviour has been typically associated with variation in direct costs and benefits due to ecological constraints, or with indirect fitness payoffs (i.e. kin selection). Here, we provide the first evidence that individual variation in cooperative behaviour within a natural population also has a heritable component. Using a seven-generation pedigree in a wild population of western bluebirds (Sialia mexicana), we show significant heritable variation for the propensity to help rather than breed, as well as for the probability of having a helper at the nest. We also document a strong positive relationship between a bird's lifespan and its prospect of receiving help when breeding, in accordance with earlier comparative studies across species. These findings provide useful insights into the possible mechanisms which have led to the evolution of cooperative breeding and other social systems.