Morphometric characteristics of free-ranging Eurasian lynx Lynx lynx in Switzerland and their suitability for age estimation

Knowledge of the age of individual animals is crucial to mess population dynamics, disease epidemiology and to successfully implement conservation strategies. Morphometric data reflect complex interactions of factors such as age and sex, and may also depend on genetics, population density, food availability, pathogen load and climate. The aims of this study were to assess the suitability of morphometric characteristics as an ageing tool for lynx during their growth period and to provide baseline data for the Eurasian lynx populations in Switzerland. Seventeen body measurements of 180 free-ranging Eurasian lynx lynx lynx of known age, captured or found dead in Switzerland between 1981-2017 were compiled by sex and age class (juveniles, subadults, adults) and tested for significant differences between males and females, age classes, and populations (Jura Mountains, Alps). Classification tree analysis (CART) was performed to create an ageing tool based on physical characteristics. Generalised linear models revealed a significant effect of age and sex on measurements but no differences were found between populations. The growth pattern was characterised by a rapid increase of all parameters in the first year of life, followed by a slowdown in the subadult age class; the adult class corresponded to the post-growth period. Sex differences became apparent at the age of 9-11 months and were most pronounced in adults. The developed classification trees allowed us to correctly categorise 93% of the females and 92% of the males as juvenile, subadult or adult. In conclusion, classification trees based on body measurements can be used to place lynx into broad age categories and represent a standardised, non-invasive, fast, cost-free and very user-friendly tool. These trees can be successfully combined with tooth wear evaluation and deliver age information with an accuracy acceptable in the context of various epidemiological investigations and of the selection of individuals for translocation.