Sexual selection in northern water snakes, Nerodia sipedon sipedon: Examination of the mating system and correlates of male reproductive success using microsatellite DNA markers

Abstract

<p>Determining how sexual selection operates on males and females can lead to a better understanding of the evolution of specific traits that influence fitness. Of the few studies that have investigated sexual selection in reptiles, most have relied on estimates of mating success from field observations, without knowing whether these behavioural data reliably reflect true reproductive success. However, recently developed molecular tools allow researchers to more accurately assign parentage and thus, measure reproductive success. In this study, I isolated ten northern water snake (Nerodia sipedon ) microsatellite loci which were then used to conduct genetic studies on wild populations of this species. My major findings include that: (1) There is fine-scale (<2 km) genetic structure among three water snake populations. (2) Based on paternity assignments involving approximately 200 males and 800 offspring, more than half of the litters were multiply sired, which is lower than previous estimates for this species, and the incidence of multiple paternity was greater where females were more clumped and male competition for mates more intense. (3) Estimates of mating success based on field observations do not reliably predict genetically-determined measures of reproductive success. (4) Securing additional mates resulted in a marginal increase in reproductive success for females, but a large increase for males: the opportunity for sexual selection was more than 70 times greater for males than females. (5) Small male body size was not correlated with reproductive success and therefore, the evolution of female-biased sexual size dimorphism cannot be explained by a reproductive advantage of small males. However, males who were more "active" (i.e. more field observations) had higher reproductive success. The results of this study support the view that, to effectively study sexual selection in many reptile species, it is necessary to measure reproductive success using genetically-based techniques.</p>