Sperm quantity and quality effects on fertilization success in a highly promiscuous passerine, the tree swallow Tachycineta bicolor
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Sperm competition is widespread among animal taxa and considered a major force in sperm evolution. Recent comparative studies have indicated that sperm competition selects for high sperm production capacity and long and fast-swimming spermatozoa across species. Here, we examine the role of sperm quantity and quality for fertilization success of individual males in a Canadian population of tree swallows Tachycineta bicolor, a socially monogamous, but highly promiscuous passerine. Male fertilization success (the sum of withinpair and extrapair young) was significantly associated with the size of the cloacal protuberance (a proxy for sperm quantity), but not with sperm size or in vitro sperm swimming speed. In a multivariate analysis, both cloacal protuberance volume and relative sperm midpiece size (i.e. high mitochondrial loading) had significant effects on male fertilization success. However, relative sperm midpiece size was not associated with fertilization success in a simple regression. Further, both cloacal protuberance volume and relative midpiece size had significant effects on sperm velocity, both in simple regressions and in a multivariate analysis. The finding that males with large relative midpiece size had both higher fertilization success and faster swimming sperm, suggests an indirect link between sperm morphology and male fertility mediated through sperm velocity. In conclusion, both quantitative and qualitative sperm traits seem to affect male fertilization success in tree swallows.
KeywordsCloacal protuberance Sperm competition Sperm length Sperm motility Tachycineta bicolor Tree swallow
We are grateful to Susan A. Crowe, Kira E. Delmore and Hannah Munro for assistance with field work, and the staff at the Queen's University Biological Station for logistical support. Two anonymous referees are thanked for valuable comments on an earlier draft of the manuscript. This study was funded by grants from the Natural Sciences and Engineering Research Council of Canada (grant no. RGPIN/6691-2006) to R.J.R. and the Research Council of Norway (grant no. 170853/V40) to J.T.L. Sampling for this study was conducted under Queen’s University Animal Care permit 2005-021-R1, Canadian Wildlife Service (CWS) banding permit 10302, and CWS scientific capture permit CA0156.
Conflict of interest
The authors declare that they have no conflict of interest.
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