Environmental uncertainty and the global biogeography of cooperative breeding in birds.

Understanding why organisms as different as amoebas, ants, and birds cooperate remains an important question in evolutionary biology. Although ecology can influence cooperation and conflict within animal societies and has been implicated in species differences in sociality, the environmental predictors of sociality across broad geographic and taxonomic scales remain poorly understood. In particular, the importance of temporal variation in selection pressure has been underestimated in most evolutionary studies. Environmental uncertainty resulting from climatic variation is likely to be an important driver of temporal variation in selection pressure and therefore is expected to impact the evolution of behavioral, morphological, and physiological traits, including cooperation. Using a data set of over 95% of the world's birds, we examine the global geography and environmental, biotic, and historical biogeographic predictors of avian social behavior. We find dramatic spatial variation in social behavior for which environmental and biotic factors--namely, among-year environmental variability in precipitation--are important predictors. Although the clear global biogeographic structure in avian social behavior carries a strong signal of evolutionary history, environmental uncertainty plays an additional key role in explaining the incidence and distribution of avian cooperative breeding behavior.