A tooth wear scoring scheme for age estimation of the Eurasian lynx (Lynx lynx) under field conditions

Abstract

Within the framework of conservation actions for the Eurasian lynx (Lynx lynx), ageing of individuals is required to assess suitability for translocation and to investigate population dynamics and disease epidemiology. We aimed to develop a standardised ageing tool for free-ranging Eurasian lynx, which would be non-invasive, time-and cost-effective, and applicable under field conditions. We used tooth pictures of 140 free-ranging lynx of known age from Switzerland. Tooth colour, calculus, number of incisor teeth and canine, premolar and molar tooth wear were recorded according to pre-defined criteria. Statistical comparisons among the categories of each criterion revealed significant differences for all criteria. Tooth colour and canine tooth morphology showed obvious and consistent alterations with age. Together with the molar tooth shape, premolar tooth tips, incisor teeth and amount of calculus, they pictured the age-related changes in lynx dentition. Crown fractures, enamel flaking and open pulp cavities were observed with an increasing prevalence over age but were also sporadically seen in juveniles. Based on the obtained results, we developed a classification scheme distinguishing six age classes: < 1 year, 1-2 years, 3-6 years, 7-9 years, 10-13 years, ≥ 14 years. The scheme was subsequently tested with the same lynx. Classification success rates of different readers ranged from 69 to 88% but errors did not exceed one age class. The homogenous tooth replacement pattern observed in lynx < 1 year allowed us to develop a separate ageing chart to age juveniles in months. The proposed scheme is a promising tool to objectively assign lynx to meaningful age categories.

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

“…Animals originated from the Swiss Alps (n = 101), from the Jura Mountains (n = 65) and from a recently reintroduced population nucleus in northeastern Switzerland (n = 14). Age was determined using the following methods (Marti and Ryser-Degiorgis 2018): age of lynx clearly recognised as juveniles (small body size and milk dentation) was calculated in months based on the known narrow birth period in May-early June (Schmidt 1998, Henriksen et al 2005, Breitenmoser-Würsten et al 2007 given that they were found either alive or as fresh carcasses; age determination for all other individuals was performed by counting tooth cementum annuli in canines or incisors (Matson's Laboratory, Manhattan, MT, USA), unless the exact age was known thanks to marking procedures of lynx at the kitten age. Animals known to be older than 2 or 3 years (for females and males, respectively) thanks to repeated captures or detections by phototrapping were categorised as adults (without data about their exact age in years).…”

“…Animals known to be older than 2 or 3 years (for females and males, respectively) thanks to repeated captures or detections by phototrapping were categorised as adults (without data about their exact age in years). Lynx age classes were defined as previously described (Marti and Ryser-Degiorgis 2018). Juveniles are lynx in their first year of life (i.e.…”

“…We differentiated between juveniles < 9 months and 9-11 months old based on their dentition (Marti and Ryser-Degiorgis 2018) to test for differences between 'old juveniles' and subadults. We differentiated between three arbitrarily chosen adult categories to assess potential Distance between the centre of the anus and the centre of the vulva/opening of preputium differences within the adult age class: young adults (up to 5 years; n = 10), intermediate (>5-10 years; n = 15) and old (>10 years; n = 7) adults.…”

“…We used a subset of 24 lynx, for which not only exact age and body measurements but also good quality tooth pictures were available, to compare the performance of the classification trees and of our tooth wear scheme (Marti and Ryser-Degiorgis 2018). The test set included eight juveniles between 9-11 months, eight subadults and eight adults, each group with four males and four females.…”

“…However, tooth evaluation is limited in animals with jaws severely damaged by traumatic events (e.g. fatal traffic accidents), and the discrimination of subadults from old juveniles and from young adults remains challenging (Marti and Ryser-Degiorgis 2018). Considering that lynx continue to grow after their first year of life (Andersen and Wiig 1984, Yom-Tov et al 2010, selected morphological measurements may contribute to a more accurate estimation of the age of young animals.…”