Abstract
To resolve conflicting field observations regarding the action of sexual selection, we used breeding experiments and paternity analysis of the 927 resulting offspring to assess how male size, condition, tail length, genetic similarity to the female, and variation in operational sex ratio (OSR) affected male reproductive success and the incidence of polyandry in northern watersnakes (Nerodia sipedon). Only size affected male mating success. Large males were more successful, but only when male size varied substantially and competition among males was intense (i.e., male-biased OSR). The conditional nature of the size advantage may explain why studies of free-living watersnakes have produced inconsistent results regarding the relationship between male size and mating success. Size differences between males did not affect the proportion of offspring each male sired within multiply sired litters. We found positive size-assortative mating, but only when the OSR was female biased, suggesting that smaller males had improved access to females when competition among males was reduced, but that competition with larger males still restricted mating opportunities of small males to less preferred, smaller females. Most litters (58%) were multiply sired and larger females were more likely to produce multiply sired litters, similar to free-living watersnakes. There was no association between the incidence of multiple paternity and OSR, however, suggesting that polyandry is not simply a function of opportunity, with females passively waiting for males to court them.
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Acknowledgements
We thank G. Blouin-Demers, G. Brown, E. O'Grady, M. Prosser, C. Verrault, F. Phelan, F. Connor, R. Green, S. Sommerer, J. Gilchrist, D. Bender, K. Metz, J. Svec, H. McCracken, T. Volk, A. Moenting, C. Yourth, P. Begin, S. Doucet and J. Jones for help constructing the enclosures and/or help in the field. We thank L. DeSousa, B. Woolfenden, M. Kasumovic and L. Blanchard for help in the laboratory and G. Brown and D. Bender for providing statistical advice. Financial support was provided by the Natural Sciences and Engineering Research Council of Canada, the province of Ontario, Carleton University, McMaster University and the University of Illinois
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Kissner, K.J., Weatherhead, P.J. & Gibbs, H.L. Experimental assessment of ecological and phenotypic factors affecting male mating success and polyandry in northern watersnakes, Nerodia sipedon . Behav Ecol Sociobiol 59, 207–214 (2005). https://doi.org/10.1007/s00265-005-0026-7
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DOI: https://doi.org/10.1007/s00265-005-0026-7