In cooperative breeding systems, inclusive fitness theory predicts that helpers more closely related to the breeders should be more willing to provide costly alloparental care, and thus have more impact on breeder fitness. While philopatry of offspring from previous years is often the primary source of helpers, helpers may nevertheless vary in their relatedness to the breeders, some even being unrelated. When helpers of different relatedness are present in one breeding group, it can be difficult to determine whether their fitness impacts differ. Here, we used functional linear models to evaluate the per capita effects of helpers on eight breeder fitness components based on helper relatedness to the breeding pair in the red-cockaded woodpecker (Dryobates borealis).
Results/Conclusions
In support of inclusive fitness theory, helpers more related to the breeding pair made greater contributions to five fitness components: probability of attempting a nest, clutch size, nest success at both egg and nestling stages, and male breeder survival. Male helpers of all relatedness levels increased pre-fledging survival. In conflict with inclusive fitness theory, less related male helpers made higher contributions to female breeder survival. Less related female helpers made negative contributions to two fitness components: nest success at the egg stage and female breeder survival. These findings suggest that both inclusive fitness benefits and other benefits (e.g., increasing the probability of becoming a breeder) may underlie helping behaviors in the red-cockaded woodpecker. To date, most support for inclusive fitness theory has been predicated on positive associations between relatedness and behavior, rather than actual fitness consequences. Our results found that fitness benefits scale with relatedness in most cases, supporting the theory of inclusive fitness, while demonstrating the application of an underused statistical approach to disentangle high-dimensional ecological phenomena.
