No evidence of inbreeding depression in sperm performance traits in wild song sparrows

Sylvain Losdat, Ryan R. Germain, Pirmin Nietlisbach, Peter Arcese, Jane M. Reid

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Inbreeding is widely hypothesized to shape mating systems and population persistence, but such effects will depend on which traits show inbreeding depression. Population and evolutionary consequences could be substantial if inbreeding decreases sperm performance and hence decreases male fertilisation success and female fertility. However, the magnitude of inbreeding depression in sperm performance traits has rarely been estimated in wild populations experiencing natural variation in inbreeding. Further, the hypothesis that inbreeding could increase within-ejaculate variation in sperm traits, and thereby further affect male fertilisation success has not been explicitly tested. We used a wild pedigreed song sparrow (Melospiza melodia) population, where frequent extra-pair copulations likely create strong post-copulatory competition for fertilisation success, to quantify effects of male coefficient of inbreeding (f) on key sperm performance traits. We found no evidence of inbreeding depression in sperm motility, longevity or velocity, and the within-ejaculate variance in sperm velocity did not increase with male f. Contrary to inferences from highly inbred captive and experimental populations, our results imply that moderate inbreeding will not necessarily constrain sperm performance in wild populations. Consequently, the widely observed individual-level and population-level inbreeding depression in male and female fitness may not stem from reduced sperm performance in inbred males.
Original languageEnglish
Pages (from-to)1842-1852
Number of pages11
JournalEcology and Evolution
Issue number3
Early online date12 Jan 2018
Publication statusPublished - Feb 2018

Bibliographical note

We thank the Tsawout and Tseycum first nations bands for allowing access to

Mandarte, everyone who contributed to long-term field data collection, and Greta Bocedi and Lukas Keller for constructive comments. This study was funded by Marie Curie Actions, UK Royal Society, Swiss National Science Foundation, the Natural Sciences, Engineering Research Council of Canada and the European Research Council.


  • genetic relatedness
  • inbreeding
  • paternity
  • reproductive strategies
  • sexual selection
  • sperm quality


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