Behavioral Ecology Vol. 13 No. 6: 808-815
© 2002 International Society for Behavioral Ecology
Male reproductive success and sexual selection in northern water snakes determined by microsatellite DNA analysis
a Department of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada b Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1, Canada
Address correspondence to P.J. Weatherhead, who is now at the Program in Ecology and Evolutionary Biology, University of Illinois, Shelford Vivarium, 606 E. Healey, Champaign, IL 61820, USA. E-mail: pweather{at}uiuc.edu. H.L. Gibbs is now at the Department of Evolution, Ecology and Organismal Biology, Botany and Zoology Building, The Ohio State University, 1735 Neil Avenue, Columbus, OH 43210-1293, USA. G.P. Brown is now at the School of Biological Sciences, A08, University of Sydney, Sydney, NSW, Australia.
Male northern water snakes (Nerodia sipedon) have high variance in reproductive success relative to females. We used DNA-based paternity analyses from a 3-year study of two marsh populations of water snakes to investigate the factors that contribute to variation in male success. Male traits investigated included body size, condition, tail length, home range size, activity during the mating season, and genetic profile (genetic similarity to females, heterozygosity, and genetic variability [d2]). We successfully assigned > 80% of offspring to sires from a sample of 811 offspring from 45 litters. Male reproductive success did not vary significantly with body size, tail length, condition, home range size, or the number of microsatellite loci at which males were heterozygous, nor with other features of their genetic profiles. However, we found evidence of positive assortative mating by size in the marsh in which receptive females were not spatially clumped. Also, males that were most active during the mating season were more successful, particularly where females were not clumped. We failed to find evidence of selection acting on male size through variance in reproductive success, indicating that sexual selection does not have an important influence on sexual size dimorphism in this species (males are smaller than females). We propose that males are smaller than females because the lack of advantage to large size allows males to adopt a low-energy, low-growth strategy that reduces their risk of predation outside the mating season.
Key words: assortative mating, DNA loci, heterozygosity, microsatellites, Nerodia sipedon, northern water snake, paternity analysis, sexual selection, sexual size dimorphism.
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