An Analysis of Age and Growth Models Examining Size-At-Age Data for the Alameda Whipsnakes (Masticophis lateralis euryxanthus)

Effective management of rare wildlife species requires a clear understanding of population dynamics based on robust estimates of demographic parameters, yet these data are lacking for many taxa, including the federally and State-of-California threatened Alameda Whipsnakes (Masticophis lateralis euryxanthus, AWS). Alameda Whipsnakes are colubrids found over a limited geographical area at the margins of the densely populated San Francisco Bay Area region. Despite their threatened status, little has been published about the population dynamics of this species. Our main objective was to develop age and growth relationships and growth rates for the AWS. We completed a five-year capture-recapture study targeting a population in the Los Vaqueros Reservoir watershed in Contra Costa County, California, USA, and applied and compared the fit of three classic growth models (von Bertalanffy, logistic, and Gompertz) to length-at-age data. We also assessed growth rates of AWS across ages, and estimated female size and age at reproductive maturity. We found no evidence of sexual dimorphism in our sample. The logistic growth model produced biased results and was ignored. In a leave-one-out information criterion (LOOIC) model comparison of the von Bertalanffy and Gompertz models, the Gompertz model had 100% of the weight. Predicted age for the largest observed individual under this model was 8.3 years. As expected, predicted annual growth rates declined with age, specifically, from 0.61 mm/day at small sizes to 0.01 mm/day at large sizes. The model-predicted age of the smallest gravid female was 2.2 years. The new information about the AWS reported here is an important starting point for future efforts to understand the life history and quantify the population dynamics of this rare snake.