Sexual size dimorphism in seabirds: sexual selection, fecundity selection and differential niche-utilisation

Seabirds exhibit a range of sexual size dimorphism (SSD) that includes both male-biased (males > females) and female-biased SSD (males < females). Here we use phylogenetic comparative methods to test the selective processes that may influence their SSD. Using phylogenetically independent contrasts we show that the sizes of males and females are scaled isometrically in seabirds. We also test three functional hypotheses of SSD: sexual selection, fecundity selection and differential niche-utilisation. First, we found support for the sexual selection hypothesis, even though seabirds are socially monogamous and, as a consequence one might expect sexual selection to be weak. We show that SSD is correlated with an aspect of sexual selection, the agility of male displays, since in species that exhibit aerial displays the males are smaller (relative to the female) than in species in which the males display on the ground. Second, our results are not consistent with the fecundity selection hypothesis, since contrary to the predicted trend, female seabirds lay larger eggs in male-biased species than in female-biased ones. Finally, our results are not consistent with a previous study of the differential niche-utilisation hypothesis, since we found no relationship between SSD and ocean primary productivity in the breeding areas. Taken together, we suggest that seabird SSD is most consistent with the sexual selection hypothesis via the agility of male displays. Nevertheless, further data and tests are required to establish whether different resource utilisation by males and females may also select for SSD.