Determining the age of garter snakes (Thamnophis spp.) by means of skeletochronology

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Abstract
Individual and population-specific patterns of growth, and variations in these patterns, can be determined if the age of individuals is known; this allows the dynamics of populations to be more accurately modelled and projected into the future and provides a better understanding of the life history. Besides contributing to our understanding of the fundamental ecological issue of limitation of distribution and abundance, such knowledge is critical to management or conservation plans for many species. Skeletochronology, the reading of growth layers in bony structures, has been used extensively to determine the age of individual fish and, to a lesser extent, of reptiles and amphibians. This study evaluates the use of skeletochronology to determine the age of garter snakes (Thamnophis spp.), with emphasis on the development of techniques that allow the sampling of bone structures from live animals. Layers were observed in the caudal vertebrae of two species of garter snakes, using standard histological techniques, and were consistent in number within individual snakes, with some variability. The validity of growth layers as an indicator of age was established using snakes raised in the laboratory under differing hibernation regimes. Skeletochronology has the potential to be an important and useful technique for the study of age in snakes, but is very labour-intensive and is best used as part of a large, long-term project.