Estimating Ages of Mule Deer: An Evaluation of Technique Accuracy

Ages estimated from (1) molar tooth ratios (Robinette et al. 1957), (2) visual evaluation of molariform tooth wear at a check station, and (3) regression of eye lens weight (Y) on age (X), were compared to assumed known ages obtained from counts of cementum annuli in the first permanent incisor of mule deer (Odocoileus hemionus). Ages of 62 of 100 mule deer about 26 months and older were not correct when estimated to the nearest year from the molar tooth ratios. Ages of 43 of 116 (37 percent) were not correct when estimated from visual evaluations of permanent molariform tooth wear at a check station and assigned to one of 5 age groups (2, 3+4, 5+6, 7+8, and 9+ years). A regression of eye lens weight (Y) on age (X) for 177 mule deer of both sexes was fitted with the equation loge Y = 0.3585710.94080 1/X with a correlation coefficient of -0.98. A deer whose lens weight exceeded about 0.9 g, corresponding to an estimated age of 16 months from birth, had a confidence interval greater than 12 months in width and could not be assigned to a single-year age-class with 95 percent confidence. None of the three methods examined was found satisfactory for accurate individual age estimation. Reliable interpretations of population dynamics of deer require that age structures be estimated with minimal error. Various techniques have been developed for age estimation of Odocoileus deer. These include: (1) tooth replacement and wear criteria (based on known-age deer) given in qualitative form (Severinghaus 1949, Ryel et al. 1961), and quantitative form (Severinghaus 1949, Ryel et al. 1961, Robinette et al. 1957, Brown 1961); (2) eye lens weight (Lord 1962, Lueth 1963, Longhurst 1964, Larsen 1964, Hoffman and Robinson 1966, Connolly et al. 1969); and (3) cementum annuli counts for Odocoileus and other cervids (Sergeant and Pimlott 1959, Mitchell 1963, Low and Cowan 1963, McEwan 1963, Gilbert 1966, Ransom 1966, 1A contribution from Colorado Federal Aid Project W-105-R, and based, in part on J. A. Erickson's M. S. Thesis, Colorado State University, Fort Collins. 2Present address: U. S. Forest Service, Intermountain Forest and Range Experiment Station, Boise, Idaho. Reimers and Nordby 1968, Erickson and Seliger 1969). Little has been published on the magnitudes of error involved in either qualitative or quantitative methods of estimating mule deer age from wear of permanent molariform teeth. Among qualitative estimates made to the nearest year, Brown's (1961: 22) data show a 60 percent error in a sample of 47 known-age deer (0. h. columbianus) 2.5-7.5 years of age, and Ryel et al. (1961:312) show a 43 percent error in a sample of 39 known-age deer (O. virginianus) 2.5-5.5 years of age. An indication of the error involved in quantitative estimates is found in Robinette et al. (1957) and Brown (1961) who used wearreflecting measurements of molariform teeth of known-age deer. Both works show considerable overlap in the 95-percent confidence intervals about the mean measurement ratios for successive year classes. Estimation of deer age from eye-lens weight has yielded conflicting results. Lueth (1963) judged the technique inac-