Variation in sperm morphometry and sperm competition among barn swallow (Hirundo rustica) populations
Spermatozoa vary greatly in size and shape among species across the animal kingdom. Postcopulatory sexual selection is thought to be the major evolutionary force driving this diversity. In contrast, less is known about how sperm size varies among populations of the same species. Here, we investigate geographic variation in sperm size in barn swallows Hirundo rustica, a socially monogamous passerine with a wide Holarctic breeding distribution. We included samples from seven populations and three subspecies: five populations of ssp. rustica in Europe (Czech, Italy, Norway, Spain, and Ukraine), one population of ssp. transitiva in Israel, and one population of ssp. erythrogaster in Canada. All sperm traits (head length, midpiece length, tail length, and total length) varied significantly among populations. The variation among the European rustica populations was much lower than the differences among subspecies, indicating that sperm traits reflect phylogenetic distance. We also performed a test of the relationship between the coefficient of between-male variation in total sperm length and extrapair paternity levels across different populations within a species. Recent studies have found a strong negative relationship between sperm size variation and extrapair paternity among species. Here, we show a similar negative relationship among six barn swallow populations, which suggests that the variance in male sperm length in a population is shaped by the strength of stabilizing postcopulatory sexual selection.
KeywordsBarn swallow Extrapair paternity Hirundo rustica Sperm competition Sperm size
We are grateful to all people that assisted with field work, especially Frode Fossøy in Canada, Bjørn Aksel Bjerke in Norway, and Luz Garcia-Longoria in Spain. A special thanks to Gustav Thorsø Mohr for allowing us to trap inside the barn at Thorsø Herregård. We thank two anonymous reviewers for helpful comments on an earlier draft of the manuscript. This study was supported by funding from the Czech Science Foundation (to TA, project no. P506/12/2472), a Fondazione Cariplo grant (to NS, grant no. 2009–3496), the Ministry of Culture of the Czech Republic (to JC, grant no. DKRVO 00023272), the Natural Sciences and Engineering Research Council of Canada (to RJR), the Norwegian Research Council (to JTL, OK, LEJ and TL), the Samuel Freeman Charitable Trust (to TAM), and the Spanish Ministry of Economy and Competitivness (to IGH, FdL and AM, grant no. CGL 2012–36665).
All authors declare that the present study complies with the current laws and ethical standards of animal research in Canada, Czech Republic, Israel, Italy, Norway, Spain, and Ukraine.
Conflict of interest
The authors declare that they have no conflict of interest
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