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

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USGS Northern Prairie Research Center

Methods

We used skulls and teeth of 27 wolves from Alaska of known-minimum age, and for which estimates of actual age were available (Ballard et al. 1987, 1995), to develop criteria for assigning wolves to yearly age classes based on tooth wear. The known-minimum age of Alaska wolves differed by 0-7 years from the estimated age. We evaluated the accuracy and precision of tooth wear by testing the ages of 20 wolves of known age from Minnesota and Ontario. Ages were also estimated by Matson’s Laboratory (Milltown, Montana, USA) by sectioning canine and/or premolar teeth and counting cementum annuli.

The Alaska skulls were arranged in 1-year age classes from youngest to oldest, <1-13 years old (Table 1). Progressive wear on canines, incisors, and carnassials was described for wolves of each age class. Three of the authors (Ballard, Gipson, and Nowak) then independently estimated the ages of 20 known-age wolves from Minnesota and Ontario by comparing wear on their incisors, canines, and carnassials to the Alaska collection. Next, we collectively compared tooth wear on the 20 wolf skulls and arrived at a consensus age estimate of each wolf. We were not aware of the actual ages or origins of the wolves which included 12 wild wolves and 8 captives from Minnesota and Ontario.

Table 1.  Skulls of wolves from Alaska were used to define year age classes.
Assigned age classSkull numberThe basis for assigned ageKnown-minimum age in yr
 <1122027Killed as pup<1
 122044Killed as pup<1
 122065Killed as a pup, cementum annuli<1
 122127Killed as a pup, cementum annuli<1
 122151Killed as pup, cementum annuli<1
 122421Killed as pup<1
 122456Killed as pup<1
 122559Killed as pup<1
1-2122073Cementum annulin.a.a
 122152Tagged as pup, cementum annuli1.8
 122252Cementum annulin.a.
 122440Tooth wearn.a.
2-3122148Cementum annuli, tooth wearn.a.
 122170Cementum annuli, tooth wearn.a.
3-4122135Tagged as adult, cementum annuli1.8
 122143Cementum annuli, tooth wearn.a.
 122368Tagged as an adult, tooth wear3.0
4-561977Tooth wearn.a.
5-6122009Tagged as adult, cementum annuli3.8
 122018Tooth wearn.a.
 122038Tagged as adult, cementum annuli2.5
 122081Cementum annulin.a.
 122251Tagged as adult, cementum annuli1.8
6-7122255Tooth wearn.a.
7-8122136Tagged as adult, cementum annuli3.5
9-8122174Cementum annuli, tooth wearn.a.
13-14122094Tagged as an adult, tooth wear8.0
a n.a. = Not available.

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.

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