Reproductive delays between mating and birth may provide a previously unconsidered avenue for post-copulatory sexual selection in mammals. In particular, delayed fertilization could provide an enhanced opportunity for sperm competition by extending the time for ejaculates to interact in the female reproductive tract. We tested the prediction that species with delayed fertilization exhibit greater degrees of sperm competition than those without delays by examining testis volume (a proxy for sperm competition) in 38 species of bats. Examination of fluid-preserved museum specimens of bat species with and without delays revealed that species with delays (in particular those with delayed fertilization) had significantly larger testes than species without them. Although it predicts the presence of delayed fertilization, hibernation did not predict relative testis size. We conclude that, once they evolve, reproductive delays may facilitate sperm competition.
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Acknowledgments The authors thank the museum personnel who helped make this project possible: LA County Museum; J. Dines, Museum of Vertebrate Zoology; E. Lacey, C. Corbin, The American Museum of Natural History; D. Lunde, N. Simmons, E. Westwig. We also thank P. Brennan, E. Charnov, T. Garland, Jr., C. Oufiero, P. Heidman, and members of the Hammond and Zuk labs who provided useful comments. We would also like to thank Brock Fenton and an anonymous reviewer for helpful comments that greatly improved the quality of this paper. This work was supported by a UC Mexus Dissertation Research Grant to Orr. During the publication process, Orr was supported in part by the National Science Foundation Postdoctoral Research Fellowship in Biology under Grant No. (DBI-1202871).
- Delayed fertilization
- Sexual selection
- Sperm competition
- Testes size