title: Maintenance of sperm variation in a highly promiscuous wild bird.
abstract: Postcopulatory sexual selection is an important force in the evolution of reproductive traits, including sperm morphology. In birds, sperm morphology is known to be highly heritable and largely condition-independent. Theory predicts, and recent comparative work corroborates, that strong selection in such traits reduces intraspecific phenotypic variation. Here we show that some variation can be maintained despite extreme promiscuity, as a result of opposing, copulation-role-specific selection forces. After controlling for known correlates of siring success in the superb fairy-wren (Malurus cyaneus), we found that (a) lifetime extra-pair paternity success was associated with sperm with a shorter flagellum and relatively large head, and (b) males whose sperm had a longer flagellum and a relatively smaller head achieved higher within-pair paternity. In this species extrapair copulations occur in the same morning, but preceding, pair copulations during a female's fertile period, suggesting that shorter and relatively larger-headed sperm are most successful in securing storage (defense), whereas the opposite phenotype might be better at outcompeting stored sperm (offense). Furthermore, since cuckolding ability is a major contributor to differential male reproductive output, stronger selection on defense sperm competition traits might explain the short sperm of malurids relative to other promiscuous passerines.
authors.affiliation: Section of Ecology, Department of Biology, University of Turku, Turku, Finland. s.calhim@gmail.com
authors.forename: Sara
authors.initials: S
authors.lastname: Calhim
authors.forename: Michael C
authors.initials: MC
authors.lastname: Double
authors.forename: Nicolas
authors.initials: N
authors.lastname: Margraf
authors.forename: Tim R
authors.initials: TR
authors.lastname: Birkhead
authors.forename: Andrew
authors.initials: A
authors.lastname: Cockburn
mesh_terms: D000818:Animals; D000835:Animals, Wild / Q000502:physiology*; D005260:Female; D008297:Male; D012098:Reproduction; D012726:Sexual Behavior, Animal / Q000502:physiology*; D020308:Songbirds / Q000502:physiology*; D013094:Spermatozoa / Q000166:cytology*