Growth and sexual size dimorphism in northern water snakes (Nerodia sipedon)

Understanding the evolutionary basis for sexual size dimorphism (SSD) in a species first requires determining the extent to which observed SSD is a result of sex differences in growth or in size at maturity, as opposed to being a consequence of factors such as differential survival or catchability of the sexes. We used data from a nine-year study to assess how these factors contribute to female-biased SSD in Nerodia sipedon in eastern Ontario. Females grew substantially faster and larger than males, although both sexes approached their asymptotic size at the same rate. Smaller individuals of both sexes grew faster, and snakes grew faster in years with higher mean air temperatures during the activity season. Variation in growth rate was unrelated to relative fat content of individuals in the spring. We found no evidence that either the sex or size of individuals affected their probability of capture. However, comparisons of estimated ages of males and females, and a null growth-based analysis, both indicated that large, old males were overrepresented in capture samples, probably as a result of differential survival of males and females. The over-representation of large males caused us to slightly underestimate SSD caused by sex differences in growth. A comparison of sex-specific growth patterns in our population of N. sipedon with those in two other N. sipedon populations suggests that intraspecific variation in SSD in this species may be primarily a consequence of differences between populations in selection pressures on males.