Dc. Dearborn et al., Sexual dimorphism, extrapair fertilizations, and operational sex ratio in great frigatebirds (Fregata minor), BEH ECOLOGY, 12(6), 2001, pp. 746-752
Across taxa, the presence of sexual ornaments in one sex is usually correla
ted with disproportionately great parental effort by the other. Frigatebird
s (Fregatidae) are sexually dimorphic, with males exhibiting morphological
and behavioral ornaments, but males and females share in all aspects of par
ental effort. All other taxa in a clade of 237 species exhibit biparental c
are, but only frigatebirds exhibit pronounced sexual dimorphism. We tested
for the presence of two factors that could contribute to the evolution of m
ale ornaments in great frigatebirds: a high frequency of extrapair fertiliz
ations 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 be
havior was strongly male biased, with typically five or six males available
per female. Our results suggest that extrapair fertilizations are not resp
onsible for the exaggeration of sexual ornaments in male frigatebirds, and
that operational sex ratio may be related to sexual dimorphism in this spec
ies. Further work is needed to determine whether the male-biased operationa
l sex ratio creates the variance in male reproductive success that would be
needed to drive the evolution of male ornaments.