Sperm design and variation in the New World blackbirds (Icteridae)
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Post-copulatory sexual selection (PCSS) is thought to be one of the evolutionary forces responsible for the rapid and divergent evolution of sperm design. However, whereas in some taxa particular sperm traits are positively associated with PCSS, in other taxa, these relationships are negative, and the causes of these different patterns across taxa are poorly understood. In a comparative study using New World blackbirds (Icteridae), we tested whether sperm design was influenced by the level of PCSS and found significant positive associations with the level of PCSS for all sperm components but head length. Additionally, whereas the absolute length of sperm components increased, their variation declined with the intensity of PCSS, indicating stabilising selection around an optimal sperm design. Given the diversity of, and strong selection on, sperm design, it seems likely that sperm phenotype may influence sperm velocity within species. However, in contrast to other recent studies of passerine birds, but consistent with several other studies, we found no significant link between sperm design and velocity, using four different species that vary both in sperm design and PCSS. Potential reasons for this discrepancy between studies are discussed.
KeywordsSperm competition Sperm morphology Sperm velocity Phylogenetic analysis Passerine birds
We thank J. Homan, P. Llambias, N. López, D. Ortiz, L. Reinhardt, J. Ross, C. and F. Ruiz, K. Soria and A. Trutsch for their help in the field. Additional sperm samples were provided by the Alaska Bird Observatory, Bird Collection of the Smithsonian National Museum of Natural History, Cornell Lab of Ornithology, Museo de La Plata Buenos Aires, Museu de Zoologia da Universidade de São Paulo, Museum of Wildlife and Fish Biology UC Davis, Proyecto Recuperación de la Mariquita, M. Avery, A. Azpiroz, M. Conway, K. Ellison, D. Elwonger, A. Fonseca, S. Immler, W. Janousek, R. Keith, D. Maddox, R. Meese, B. Ortiz, B. Peer, T. Pepps, N. Perlut, R. Rehmeier, J. Rivers, V. Rohwer, B. Sandercock, W. Ward, C. Willis and G. Young. We also thank the anonymous referees for their constructive comments on our manuscript. S. L. was supported by the Janggen-Poehn Foundation, the Swiss National Science Foundation, a Sheffield University Overseas Research Student Award, a Lauff Research Award and an NSF LTER Graduate Research Award; T. R. B. was funded by the Leverhulme Trust. All samples were collected under licence of the respective institutions or collaborators and, within the US, under an additional collective Federal Fish and Wildlife Permit (MB 131466).
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