COMPARATIVE EVIDENCE FOR THE EVOLUTION OF SPERM SWIMMING SPEED BY SPERM COMPETITION AND FEMALE SPERM STORAGE DURATION IN PASSERINE BIRDS |
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Authors: | Oddmund Kleven Frode Fossøy Terje Laskemoen Raleigh J Robertson Geir Rudolfsen Jan T Lifjeld |
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Institution: | National Centre for Biosystematics, Natural History Museum, University of Oslo, P.O. Box 1172 Blindern, NO-0318 Oslo, Norway;E-mail:;Department of Biology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway;E-mail:;E-mail:;Department of Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada;E-mail:;Institute for Biology, University of Tromsø, N-9037 Tromsø, Norway;E-mail:;E-mail: |
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Abstract: | Sperm swimming speed is an important determinant of male fertility and sperm competitiveness. Despite its fundamental biological importance, the underlying evolutionary processes affecting this male reproductive trait are poorly understood. Using a comparative approach in a phylogenetic framework, we tested the predictions that sperm swim faster with (1) increased risk of sperm competition, (2) shorter duration of female sperm storage, and (3) increased sperm length. We recorded sperm swimming speed in 42 North American and European free-living passerine bird species, representing 35 genera and 16 families. We found that sperm swimming speed was positively related to the frequency of extrapair paternity (a proxy for the risk of sperm competition) and negatively associated with clutch size (a proxy for the duration of female sperm storage). Sperm swimming speed was unrelated to sperm length, although sperm length also increased with the frequency of extrapair paternity. These results suggest that sperm swimming speed and sperm length are not closely associated traits and evolve independently in response to sperm competition in passerine birds. Our findings emphasize the significance of both sperm competition and female sperm storage duration as evolutionary forces driving sperm swimming speed. |
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Keywords: | Clutch size extrapair paternity postcopulatory sexual selection sperm length |
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