Eye Lens Weight as a Criterion for Age Classification of Wild Woodchucks

The study of animal populations requires the knowledge of the age of individuals to better understand population structure, turnover rates, and reproductive characteristics. Many methods have been used to evaluate the age of animals from wild populations. The eye lens weight technique, refined by Lord (1959) and revised by Friend (1967a,b), has been used for small game animals (Keith and Cary 1979) as well as big game (Keller and Landry 1976). The age of woodchucks (Marmota monax L.) has been estimated primarily from morphological criteria such as body length, weight, molt and coloration, and incisor width (Snyder et al. 1961, Davis 1964). Davis (1964), and S. W. Ruckel and P. S. Scanlon (pers. commun., 1978) were the first to use the eye lens weight method on woodchucks. Davis (1964) noted that the growth of the eye lens of woodchucks >12 months was linear and that the inclusion of juveniles rendered the regression line nonlinear. During a study of woodchucks in an agricultural region, animals of known age were harvested and the regression line of eye lens weight as a function of age was determined. This paper presents the regression line determined for this population, which can be used to classify age of wild woodchucks from harvest samples. We also examine the influence of eye lens pairs with 1 l% weight differences, primarily in juveniles, on regression equations to facilitate the use of the technique on small samples.