Accuracy and Precision of Estimating Age of Gray Wolves by Tooth Wear

Both accuracy (i.e., the proximity of the age estimates to the true ages) and precision (i.e., the repeatability of age estimates) of our age estimates and those of the commercial aging service were evaluated. We followed the recommendations of Campana et al. (1995), who examined the value of statistical and graphical methods for determining the consistency of fish age estimates. First, age-bias graphs were developed for diagnosing systematic differences between age determinations based on tooth wear and by the commercial aging service that counted cementum annuli. The age-bias graphs show known ages of the wolves along the horizontal axis and estimates of age by each reader on the vertical axis. Age estimates that are parallel but separate from the 1:1 equivalent line, or that diverge as the lower or upper age limit is approached, indicate systematic bias.

Next, to compare the precision of the 4 biologists, we estimated the coefficient of variation for their estimates relative to the known ages. We calculated individual coefficients of variation of the age estimate for each wolf, then averaged across wolves to produce a mean coefficient of variation for the reader (Campana et al. 1995). An estimate of the coefficient of variation was expressed as the ratio of the standard deviation to the mean (Campana et al. 1995). Higher coefficient of variation values indicate lower precision.

We used linear regression of known ages and estimated ages to test for significant differences from a slope of 1 and an intercept of zero. A slope other than 1 would reflect inconsistency in the age estimate compared to known age. An intercept other than zero would indicate a systematic bias between the estimate of age by a reader and known age. We then used the 20 wolves of known age to describe wear on incisors, canines, and carnassials characteristic of each yearly age class from 1-14 years of age. We also developed charts illustrating typical tooth wear in 2-year increments on incisors, canines, and carnassials that can be compared to the teeth of live wolves or museum specimens to estimate their age (Figs. 1, 2).

Fig. 1. Progressive wear on wolf incisors and canines in 2-year increments from 1 to 12 years of age. Wear on incisors typically progresses beyond the lobes on the first 2 upper and lower incisors at 8 years of age, leaving approximately 5 mm of enamel. At 10 years of age, 2-4 mm of enamel remain on the first and second incisors. Length of canines is reduced 30-50% with 10-16 mm of enamel remaining. Beyond 12 years of age, incisors may be worn to the roots, with a few peg-like stumps projecting above the gum line, or the gums may cover the roots. Length of canines is reduced 50% with 10 mm of enamel remaining.

Fig. 2. Progressive wear on wolf carnassials (upper premolar 4 and lower molar 1) in 2-year increments from 6 to 12 years of age. Wear is visible on tips of major prominences at 5 years of age and profiles flatten slightly by 6 years. Deep wear on the posterior cusp of the lower carnassial after 10 years of age results from occlusion with the first upper molar, not the upper carnassial.