Evol Ecol Res 18: 225-252 (2017)     Full PDF if your library subscribes.

Exploring hypotheses for sexual size dimorphism in frigatebirds

Sarah A. Trefry1 and Antony W. Diamond2

1Department of Biology, University of New Brunswick, Fredericton, New Brunswick, Canada and  2Atlantic Laboratory for Avian Research (ALAR), University of New Brunswick, Fredericton, New Brunswick, Canada

Correspondence: S.A. Hudson (née Trefry), 5421 Robertson Road, Delta, B.C., V4K 3N2, Canada. email: sarah.trefry@gmail.com; sarah.hudson@canada.ca


Question: What selection pressures have led female frigatebirds to be larger than males?

Hypotheses: Intersexual competition hypothesis: Size differences are a result of selection for reduced foraging competition between the sexes. Aerial agility hypothesis: Size differences are a result of selection for increased aerial agility in males.

Species: Magnificent frigatebird (Fregata magnificens).

Location: Barbuda, Lesser Antilles.

Methods: Intersexual competition hypothesis: We compared regurgitated prey, stable isotopes of blood, claw, and feathers, and foraging parameters of males and females. Aerial agility hypothesis: We explored the strength and direction of selection on wing traits that influence flight performance, using fledging success as a measure of fitness.

Results: Intersexual competition hypothesis: We found no differences between the sexes in breeding season diets or in foraging parameters. Aerial agility hypothesis: Females experienced strongest selection for larger bills and longer wings, while selection on males favoured a shorter wing measurement. We also found a negative relationship between male wing loading and nest size.

Conclusions: Manoeuvrability may help males win aerial contests over nest material. Selection for male speed or agility may be an important driver in small male size.

Keywords: aerial agility hypothesis, Fregata magnificens, frigatebird, intersexual competition hypothesis, resource division, selection gradients, sexual size dimorphism.

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