No evidence that sperm morphology predicts paternity success in wild house wrens
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Postcopulatory sexual selection (PCSS) in internally fertilizing vertebrates is a topic of great interest, yet relatively little is known about the characteristics of sperm and ejaculates that confer an advantage in PCSS. In this study, we investigated several measures of sperm morphology that potentially contribute to fertilization success under PCSS. We tested whether sperm morphology related to success in PCSS (via extra-pair paternity) in house wrens (Troglodytes aedon). We found no evidence that sperm morphology differed between extra-pair sires and the within-pair males they cuckolded, nor that sperm morphology correlated with the proportion of within-pair offspring sired, the number of extra-pair offspring sired, or the total annual reproductive success. Male behavioral strategies may affect the probability that their sperm compete with other males’ sperm and that their sperm succeed under competition. Effects of these behavioral strategies, as well as differences between males in sperm number, could mask the effects of sperm morphology on the outcome of PCSS. Despite moderate levels of extra-pair paternity, selection on sperm may be relatively weak in house wrens. Further work is needed to understand general patterns in how sperm morphology relates to fertilization success within species.
KeywordsPasserine Extra-pair paternity Postcopulatory sexual selection Sperm competition Sperm morphology Troglodytes aedon
We thank Paula Cohen and Bob Doran for access to their microscope; Dan Fergus, Kelly Zamudio, Sigal Balshine, Kern Reeve, Wes Hochachka, Jon Lambert, Emma Greig, Melissah Rowe, Arild Johnsen, the NBB Behavior reading group, and the Webster and Lovette labs for thought-provoking discussions; and two anonymous reviewers for comments that improved the manuscript. Paulo Llambías and Taza Schaming initiated the study population. This research was supported by a National Science Foundation Graduate Research Fellowship, the Nordic Research Opportunity Fellowship, and grants from the American Ornithologists’ Union, Animal Behavior Society, Cornell University Department of Neurobiology and Behavior, and Cornell Lab of Ornithology to ERAC, and an Einhorn Discovery Grant to KL. This manuscript was part of ERAC’s PhD dissertation, completed at Cornell University.
This study complied with the current laws of the countries in which it was performed. Animal use was approved under Cornell Institutional Animal Care and Use Committee (Protocol 2007–0123), and the appropriate state and federal bird banding permits were obtained (numbers 1231 and 20954, respectively).
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