Sexual selection, one of the central pillars of evolutionary theory, has powerful effects on organismal morphology, behavior, and population dynamics. However, current knowledge about geographical variation in this evolutionary mechanism and its underlying drivers remains highly incomplete, in part because standardized data on the strength of sexual selection is sparse even for well-studied organisms. Barber et al. use information on mating systems—including the incidence of polygamy and extra-pair paternity—to estimate the intensity of sexual selection in the world’s birds. Overall, their analyses suggest that climatic conditions leading to short, intense breeding seasons, or highly abundant and patchy food resources, increase the potential for polygamy in birds, driving latitudinal gradients in sexual selection. By presenting selection scores and associated uncertainty for 10,671 (>99.9%) bird species, Barber et al. provide a global template for future research on the causes and consequences of sexual selection, also highlighting gaps in knowledge. For example, further research is required to understand the mating systems of many tropical bird species – including the black-bellied sunbird (Cinnyris nectarinioides) pictured here.

Image Credit: Joseph Tobias