Cooperation during parental care in birds: Climate, sexual selection, and social environment

Background/Question/Methods

Interests of the two sexes when caring for offspring overlap only partially. Resolution of this evolutionarily significant conflict ranges from uniparental care (full conflict) to equal biparental care (full cooperation). Previous comparative studies focused on transitions between uniparental vs. biparental care, whereas outstanding variability in the share of parental duties between the sexes when both sexes contribute to care remains virtually unexplored. Several factors were hypothesized to affect the degree of parental cooperation during care, including environmental harshness and variability, sexual selection, and social environment. Here we estimated parental cooperation in 659 species of birds by constructing a quantitative index based on eight parental activities spanning the whole breeding cycle. We tested alternative hypotheses for the evolution of parental cooperation in birds using comparative analyses controlling for phylogenetic uncertainty. We related parental cooperation to climatic conditions in the species’ range during the breeding season (mean and within-year and among-year variability in temperature and rainfall), sexual size dimorphism, extra-pair paternity, and adult sex ratio.

Results/Conclusions

We found that parents cooperated more in environments with low rainfall and in environments with more variable rainfall during the breeding season. However, these effects were not particularly strong and depended on the statistical approach used. Sexual selection and social envoronment were much stronger predictors of parental cooperation in our sample of species. Parents cooperated more in species with smaller sexual size dimorphism (i.e. social mating system), with lower frequency of extra-pair paternity (i.e. genetic mating system), and with more equal adult sex ratio (i.e. social environment). We conclude that several parental cooperation strategies might be adaptive in a given set of environmental conditions. Overall, parental cooperation strategies seem to be more tightly linked to the dynamics of sex roles, including mating systems and population sex ratios, than to the prevailing climatic means and variability.