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

ABSTRACT

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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