Sexual dimorphism, extrapair fertilizations, and operational sex ratio in great frigatebirds (Fregata minor)

Across taxa, the presence of sexual ornaments in one sex is usually correlated with disproportionately great parental effort by the other. Frigatebirds (Fregatidae) are sexually dimorphic, with males exhibiting morphological and behavioral ornaments, but males and females share in all aspects of parental effort. All other taxa in a clade of 237 species exhibit biparental care, but only frigatebirds exhibit pronounced sexual dimorphism. We tested for the presence of two factors that could contribute to the evolution of male ornaments in great frigatebirds: a high frequency of extrapair fertilizations and a male-biased operational sex ratio. In 92 families sampled over two breeding seasons, there was only one extrapair fertilization. However, in both seasons, there were more males than females available for mating, and the sex ratio among individuals actively engaged in mate-acquisition behavior was strongly male biased, with typically five or six males available per female. Our results suggest that extrapair fertilizations are not responsible for the exaggeration of sexual ornaments in male frigatebirds, and that operational sex ratio may be related to sexual dimorphism in this species. Further work is needed to determine whether the male-biased operational sex ratio creates the variance in male reproductive success that would be needed to drive the evolution of male ornaments.