Precision and Accuracy of Estimating Age of Maine Black Bears by Cementum Annuli

We investigated the precision and accuracy of age estimation by cementum annuli counts for Maine black bears (Ursus americanus). Precision of age estimation was assessed by: 1) a repeated measures analysis of variance design to evaluate effects of reader, reader experience, bear sex, bear age class, and trial; and 2) pair-wise comparisons of estimated years of birth (YOB) from series of premolars removed from bears over periods ranging from 2 days to 12 years. Age estimation accuracy was assessed through pair-wise comparisons of cementum-assigned YOB to known YOB for known-age bears. Experienced readers assigned significantly (P = 0.0001 ) lower age estimates than inexperienced readers; greater differences in age estimates occurred in old bears. Least variation in age estimates occurred in experienced readers' estimates for young bears (SD = 1.08 yr). Experienced readers estimated age more precisely over 3 trials than inexperienced readers (P = 0.0051). YOB estimates from multiple teeth removed from individual bears showed decreasing agreement with increasing time between tooth removal (P = 0.002), and decreasing agreement with increasing age of bears (P < 0.001). Teeth removed later in life yielded later YOB estimates than teeth removed earlier. Cementum age estimates are accurate for bears <6 years of age, but may underestimate age in older bears. Managers using cementum age estimation should recognize the technique's limitations in precision and accuracy, and minimize changes in personnel and methodology to reduce variation in estimates over time. Int. Conf. Bear Res. and Manage. 8:415-419 Cementum annuli are considered the most accurate indicators of age for black bears (Marks and Erickson 1966, Stoneburg and Jonkel 1966, Hildebrandt 1976, Rogers 1978, Matson's Lab 1988). However, published reports on the reliability of this age estimation technique are supported by limited data. We found no published tests of the precision of cementum age estimates for bears, and tests of accuracy were based on small sample sizes containing few old animals (Marks and Erickson 1966, Sauer et al. 1966, Stoneburg and Jonkel 1966, Matson's Lab 1988). This paper presents results of our investigations of precision and accuracy of cementum age estimates for Maine black bears. We thank C.D. Dyke and D.J. Sebastian-Laskowski for assistance in assigning cementum age estimates. D.M. Kane constructed the figures, and commented on the manuscript. This work is part of Maine black bear research, funded under Federal Aid in Fish and Wildlife Restoration Projects W-67-R and W-82-R. METHODS Premolars (PM1,2, PM ,2) were extracted from hunterkilled bears from 1 September 30 November 1984, and from live bears captured throughout the year during 1975-1988. Premolars were stored in collection envelopes <6 months before being decalcified in a 20% formic acid solution for 44 hours, then immersed in buffered water for a minimum of 18 hours. Buffered water was prepared by mixing 3,500 ml sodium phosphate dibasic solution (9.46 g NaHPO4/L distilled H20) with 389 ml sodium phosphate monobasic solution (9.20 g NaH2PO4*H20/L distilled H20). Four central sagittal sections (8 gLm) were removed from each premolar, using a freezing cryostat/ microtome. Sections were mounted on microscopic slides, air-dried for >20 min, and immersed in a solution of 550 ml buffered water and 200 ml Giemsa stain for 20 min. Sections were viewed through binocular microscopes using 100X magnification and transmitted light. Age estimation was based on techniques reported by Willey (1974). Each age estimate was obtained as the sum of all complete annuli identified on the cross-section. Effects of "double" annuli (Hildebrandt 1976) were not considered in assigning estimates; incomplete annuli or accessory lines (Sauer et al. 1966, Rogers 1978) were discounted. Precision of Age Estimation Variation Within and Among Readers. -A 100-slide sample was chosen from 1,309 slides prepared from premolars collected from hunter-killed bears. Slides were chosen on the basis of age estimates that had been previously assigned by our most experienced reader (> 1,600 readings). They were randomly selected without replacement until each of the following categories contained 25 slides: young males (0-4 yr), old males (>8 yr), young females (0-4 yr), and old females (>8 yr). Sex-age categories were selected to ensure sufficient separation for investigating the effects of sex and age, but the distribution of bear ages within each category was not controlled. Sex of bears was obtained from hunters, 416 BEARS-THEIR BIOLOGY AND MANAGEMENT through mandatory registration reports. All slides in the sample were readable, and represented the normal range of processing variation experienced in our laboratory. Six readers (3 experienced, 3 inexperienced) each assigned an age estimate to each slide on 3 separate occasions, yielding 1,800 age estimates for analysis. Experienced readers had read >600 slides of black bear premolar sections, including about 12 slides from knownage bears, prior to this study. Inexperienced readers were instructed in the age estimation process, but had viewed <24 slides prior to reading the sample. Slides were reordered and renumbered after each reading of the sample to reduce reader familiarity with individual slides. Individual readers viewed the entire sample in a session; 1-3 days elapsed between sessions. Betweenand within-tooth comparisons of age estimates were made using repeated measures analysis of variance. Precision was further quantified using confidence interval analysis. Variation Within Bears. A sample of 115 bears, each with 2-5 premolars removed over intervals of 2 days to 12 years, provided 185 within-bear comparisons of YOB estimates. Premolars were removed from bears estimated at 1-18 years of age. Collection date for each premolar was referenced during age assignment. Although teeth were processed by different laboratory personnel during the collection period, all within-bear comparisons were based on age estimates assigned by one experienced reader. The initial premolar removed from each bear provided a YOB used as a base for pairwise comparisons with YOB estimates from subsequent premolars. Agreement of YOB estimates, and trends in differences between YOB estimates, were examined relative to bear sex, time between removal of teeth, and bear age at the time of tooth extraction. Accuracy of Age Estimation Fifty-six premolars were collected from 41 knownage bears <6 years of age, and assigned age estimates by one experienced reader. Premolar collection dates were referenced during age assignments. Accuracy of the resulting cementum age estimates was assessed through comparisons with actual ages.