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

Results


Precision and Accuracy

Readers 2 and 3 as a team, most accurately estimated ages of the known age wolves from tooth wear (Figs. 3, 4, Table 2). Estimates made independently by readers 1 and 3 were 1-2 years higher than known ages (Table 2, Fig. 3). Age estimates by reader 2 were more precise, as indicated by a relatively small coefficient of variation of 19.5 compared to 26.7, 23.7, and 24.0 for readers 1 and 3, and the consensus of readers, respectively. All readers experienced difficulty in estimating the age of a 7-year old wolf with an underbite and an 11-year old wolf with broken and missing teeth (Figs. 3, 4). Estimates of age ranged from 3-5 years under actual age and 2-4 years over actual age for these 2 wolves, respectively.

Figure 3    Figure 4


Fig. 4.  Age bias graphs for 3 readers showing estimates of ages of 20 wolves from Minnesota and Ontario compared to known ages. On each graph age is shown in years for known ages on the X axis and estimated ages on the Y axis. The solid line is the 1:1 equivalence line; solid square below the 1:1 line represents the age estimate for a wolf with a severe underbite; solid circle on the upper right, above the 1:1 line, represents the age estimate for a wolf with broken and missing teeth from an old injury. Solid circles are age estimates for wild wolves; solid triangles are age estimates for captive wolves.

Fig. 3.  Age bias graphs for a consensus of the 3 tooth-wear readers (upper graph) and a commercial aging service (Matson's Laboratory, Milltown, MT 59851) using counts of cementum annuli to estimate ages of 20 wolves from Minnesota and Ontario compared to known ages. On each graph age is shown in years for known ages on the X axis and estimated ages on the Y axis. The solid line is the 1:1 equivalence line; solid square below the 1:1 line represents the estimate of age of a wolf with a severe underbite; solid circle on the upper right represents the estimate of age for a wolf with broken and missing teeth from an old injury. Solid circles are age estimates for wild wolves; solid triangles are age estimates for captive wolves.

Table 2.  Linear regression analysis relating estimated age to known age wolves from Minnesota and Ontario.  Values are comparison of known age to 3 individual readers, the consensus of readers, and counts of cementum annuli.
Independent
variable
Intercept (α) Slope (β) P-value
H0: α = 0 H0: β = 0 H0: β = 1
Reader 1 1.3 0.9 0.03 0.0001 0.31
Reader 2 0.9 1.0 0.12 0.0001 0.85
Reader 3 1.5 0.9 0.01 0.0001 0.40
Consensus 1.2 1.0 0.08 0.0001 0.66
Cementum annuli 0.4 0.8 0.12 0.0001 0.001

The commercial aging service accurately aged young wolves, but older wolves were underaged (Fig. 4, Table 2). The precision of the aging service was ±1 year except for a 9-year-old wolf that they aged at 6 years and a 14-year-old wolf that they estimated to be 11 years old. The relatively low coefficient of variation of 14.0 for the aging service reflected less variation than among the 3 readers. Age estimates by the aging service may have been influenced by poor intensity of annulus staining because some specimens had been in storage for ≥20 years and some were boiled prior to tooth extraction.

Progressive Tooth Wear

There were no detectable differences in tooth wear patterns between wild and captive wolves (Figs. 3, 4). Adult incisor, canine, and carnassial teeth are fully erupted by 26 weeks of age (Mech 1970). During the first year of a wolf's life they are bright white, and incisors appear molded with sharp ridges that project slightly beyond the teeth. These small projecting ridges occur along the lobes of incisors and along the posterior and anterior-medial edges of canines. Between 1 and 2 years of age, incisors and canines begin to show detectable wear (Fig.1, Table 3).

Table 3.  Wear on teeth of wolves associated with increasing age (yr).
Age Diagnostic wear on teeth
Incisors Canines Carnassials
 <1 Bright white, no visible wear, sharp edges project slightly beyond lobes. Bright white, no visible wear, small ridges project slightly on posterior edge of C1 and anterior-medial edges of C1. Bright white, no visible wear.
1-2 Slight wear on sharp edges of lobes. Slight wear on distal end of posterior small ridge of C1. No visible wear.
2-3 Central lobes of I1 and I2 slightly flattened, median lobes of I1-I3 flatten slightly. Tips slightly blunted, distal portion of C1 and C1 show wear on small ridges. No visible wear.
3-4 Flat tip on central lobes of I1-I3, flat surface on I1 extends into lateral lobe, and median lobe of I2 and I3 flatten. Tips show distinct, but rounded blunting. Visible wear on tip of most major prominences.
4-6 Flat surface of I1 progresses into median lobe, flat surfaces progress beyond lateral lobe on I1 and into lateral lobe on I2. Tips flattened. Wear on tip of all prominences.
6-8 Wear progresses beyond median lobe of I1 and I2, and lateral lobe of I2, reaches lateral lobe of I3. Tip clearly flattened, 1-2 mm of tip lost. Tip of all prominences flattened.
8-10 Wear progresses beyond all lobes of I1, I2, and I3, and reaches median lobe of I3. Visibly shortened profile with 3-5 mm of wear on tips. Profile of prominences almost flat.
10-12 Length of incisors reduced ≥50%, 2-4 mm of enamel remain, flat profile. Flat tip, length reduced 30-50%, 10-16 mm of enamel remain, distinct wear on anterior-posterior surfaces. Height reduced by ≥30%, posterior cusp of M1 worn almost to gum line.
≥13 Remaining incisors worn almost to gum line, some missing, and roots covered by gums. Tips blunt, length reduced ≥50% with ≤10 mm enamel remaining, anterior-posterior width reduced ≥30%. Prominences almost flat, posterior cusps of PM4 and M1 near gum line.

Among the 3 prominences on upper incisors, the central 1 projects well beyond those on each side. Lower incisors have only 2 prominences; a relatively large median one projects beyond the lateral prominence. Incisors do not correspond to an opposing tooth, except for the first lower incisor that is opposed by the wider first upper incisor. Each of the 2 lateral lower incisors occludes with 2 upper incisors. Incisors wear at a relatively consistent rate throughout life.

Canines are large spike-like teeth that curve slightly to the posterior, and when normally alined, the lower canine fits into a gap between the upper canine and the third upper incisor. Because canines do not oppose each other when the mouth is closed, there is little wear from occlusion. Wear on canines becomes apparent when wolves are 3-4 years old, possibly due to lack of tooth occlusion and continuous eruption of cementum deposits on their roots as the points are worn down (Allen 1974).

The carnassial teeth (lower molar 1 and upper premolar 4) provide a convenient cross-check for wear patterns on incisors and canines, especially where wolves have broken incisors and-or canines and the damaged teeth make the wolf appear older. Wear is visible on the pointed prominences of carnassials by 5 years of age, but the profile has not yet been distorted. After 5 years of age, the prominences and cutting surfaces become progressively flattened (Fig. 2, Table 3). Wear is due to chewing bone and other hard objects, and occlusion of lower molar 1 with upper premolar 4 and upper molar 1. Wear on the carnassials is easy to observe in immobilized wolves by pulling back the lips along the sides of the mouth. The profile of carnassials may be observed from the side while overall wear is best estimated by opening the wolf's mouth wide and looking from the front directly into the mouth.


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