Extreme sexual size dimorphism, sexual selection, and the foraging ecology of Montezuma Oropendolas

Sexual differences in body size usually are considered to be a product of either sexual selection or ecological divergence between the sexes. I tested the predictions of these hypotheses to explain extreme sexual dimorphism in size in a Neotropical blackbird, the Montezuma Oropendola (Psarocolius montezuma). Sexual dimorphism was more pronounced in wing length and relative mass, traits that appear to be important in courtship and competition for mates, than in bill length or tarsus length. Male dominance rank, which is an indicator of mating success, was positively associated with body size (i.e. high-ranking males were larger than low-ranking males). These results suggest that sexual selection acts to increase male body size, and hence contributes to sexual size dimorphism in this species. Differences in foraging behavior and diet also existed between the sexes and were related to sexual size differences. The small size of females allowed them to forage on food resources that were difficult for the larger males to reach (e.g. insects in rolled leaves at the ends of branches). Females, though, did not appear to be restricted from those resources exploited by males (primarily epiphytal bromeliads). Although it was not possible to reject the ecological-divergence hypothesis, my results suggest that sexual foraging differences are a consequence rather than a cause of sexual dimorphism in size in this species.