Accuracy and precision in estimation of age of Norwegian Arctic polar bears (Ursus maritimus) using dental cementum layers from known-age individuals

Validation of age estimation from tooth cementum growth layers was conducted for 32 polar bears (Ursus maritimus) of known age, by two readers. Both readers correctly estimated age for 24% of the bears, and 50–53% were within the year of correct age. The age of young animals (age 1–8) was overestimated, while ages for bears over 8 years were underestimated. Comparison between the readings of the two readers indicated that the precision was low. Further, one of the readers reread tooth slides earlier prepared and read by another age estimation laboratory. There was a large discrepancy between these readings indicating a bias in the ages estimated. We conclude that age estimation of polar bears can be difficult, particularly in populations where individuals may forage throughout the year. As tooth growth layers may deposit differently for bears from different areas, and as different laboratories may read the same slides according to different criteria, an evaluation of the methods should be conducted for all populations, based on a significant number of tooth slides, with a broad age range, from animals of known age.