Growth, Survival, and Reproduction in a Northern Illinois Population of the Plains Gartersnake, Thamnophis radix

Abstract

A six-year mark-recapture study, consisting of 567 captures of 216 individually marked snakes, and the birth of 889 young in 53 litters born to 39 wild-caught females were used to estimate demographic and life-history parameters of a Northern Illinois population of Thamnophis radix. Using the von Bertalanffy growth model, males were found to differ from females in asymptotic size but not in the rate at which they approached this size. Size of known age individuals together with growth rate estimates were used to assign age at first capture. Age and sex were then used as grouping variables to obtain age- and sex-specific survival using program MARK. The results suggest that males and females have approximately equal survival in the 0 and 1 age classes but that females have higher survival than males as adults (0.45 vs 0.35). Population estimates calculated using Schumacher-Eschmeyer and Jolly-Seber methods indicate an adult population size of 64 and 172 adults, respectively, corresponding to densities of 40 and 107 adults per hectare. Average female fertility increased from 6.4 in one-year-old females to 21 among six-year-old females. Detailed demographic studies such as this have utility in development of management strategies and theories concerning life-history evolution.