Persistent size variation in the anthophorine bee Centris pallida (Apidae) despite a large male mating advantage

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Abstract
Despite apparent directional sexual selection in favour of large body size, males of the anthophorine bee Centris pallida remain highly variable in body size. One possible cause of persistent size variation among males is geographic variation in the extent of the large male mating advantage. However, a study of a population in an area not previously investigated revealed that the large male mating advantage was as strong here as it has been elsewhere in other years. Although the reproductive benefits of being large were consistent in populations separated spatially and temporally, the intensity of bird predation on mate‐searching males varied greatly between locations. The bee‐killing birds focused exclusively on bees which were digging down to meet emerging females or fighting on the ground, never on flying males. Males which were collected on the ground by hand (to simulate avian predation) were significantly larger on average than flying males collected by sweep netting. Therefore, in some location in some years, sexual selection in favour of large body size may be opposed by natural selection exerted by predators, perhaps contributing to the maintenance of size variation in this bee.