Maintenance of sperm variation in a highly promiscuous wild bird

Sara Calhim*, Michael C. Double, Nicolas Margraf, Tim R. Birkhead, Andrew Cockburn

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    32 Citations (Scopus)

    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.

    Original languageEnglish
    Article numbere28809
    JournalPLoS ONE
    Volume6
    Issue number12
    DOIs
    Publication statusPublished - 15 Dec 2011

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