Accuracy of Moose Age Determinations from Incisor Cementum Layers

Abstract

Incisors from 68 known-age moose (Alces alces) were sectioned and the ages of the animals estimated from cementum layers. Mean age determined in this manner overestimated true age by 0.5 yr with errors ranging between minus 1 and plus 3 yr. Cross sections were preferred to longitudinal sections. The major error resulted from difficulties in distinguishing discrete cementum layers. Other sources of error are discussed and the annual sequence of cementum deposition is described. J. WILDL. MANAGE. 42(3):558-563 Age determination of ungulates from cementum layers in teeth has become an accepted technique during the past 2 decades. There are inherent errors with the technique, however, and when relating age to growth, reproduction, or population dynamics, the age estimate should be relatively accurate and the frequency and magnitude of errors known. The errors appear small for some species. Complete agreement of age estimates with known ages was found for 20 black-tailed deer (Odocoileus hemionus columbianus) (Low and Cowan 1963), 46 whitetailed deer (0. virginianus) (Lockard 1972), 16 mule deer (0. hemionus) (Erickson and Seliger 1969), and 18 elk (Cervis canadensis) (Keiss 1969). Nevertheless, Sauer (1973) reported that 16 percent of the age estimates from cementum layers of 69 known-age teeth of white-tailed deer were in error. Similarly, Ranson (1966) did not find total agreement between observable cementum layers and known age in white-tailed deer. Previously published reports on moose age determination using cementum layers (Sergeant and Pimlott 1959, Markgren 1969, Wolfe 1969) suggested a range of accuracy of +1 or 2 yr. None of these works were based on large numbers of known-age teeth. This report describes the accuracy and precision of age estimates from the cementum layers of incisors from 68 known-age moose, compares results from longitudinal and cross sections of cementum, and describes the annual sequence of cementum deposition. We are grateful to E. Kootuk and the members of the Alaska Department of Fish and Game who were responsible for securing and sectioning the known-age teeth and to W. Heimer and D. McKnight for valuable criticism of an early draft of the manuscript. This work was supported by Federal Aid in Wildlife Restoration Project W-17-9. MATERIALS AND METHODS Incisors (1,) were collected from 68 moose tagged as calves in the Matanuska and Tanana Valleys, Alaska and later recovered from hunter kills, live recaptures, train and auto collisions, and natural mortality. If both I, were available, then longitudinal and cross sections of the teeth were prepared. Longitudinal sections were prepared by grinding lateral surfaces of the tooth to produce a thin, medial, dorsal-ventral section of the root. Alternate sides were ground until the section became translucent and cemen558 J. Wildl. Manage. 42(3):1978 This content downloaded from 207.46.13.124 on Wed, 22 Jun 2016 05:47:15 UTC All use subject to http://about.jstor.org/terms ACCURACY OF CEMENTUM MOOSE AGES. Gasaway et al. 559 tum layers could be distinguished readily using a binocular microscope at 10 to 30x magnification. The dorsal cementum has the most distinctive layers and was the portion used for estimating age. A water-bath grinder with a fine abrasive wheel was preferred since it reduced odor, prevented burning of the tooth, and was self-cleaning. Tooth sections were stored in a solution of 70 percent ethanol, 29 percent water, and 1 percent glycerine. Serial cross sections of the 11 root were cut with a lapidary saw fitted with a fine tooth blade (modified from Gilling and Buonocore 1959). Oil was dripped over the tooth to prevent burning. Five 0.3 mm sections were prepared from the proximal half of the root and stored in the above solution. The time required for an experienced individual to cut cross sections is approximately the same as grinding longitudinal sections; however, the initial cost of equipment to cut cross sections is greater. Cementum layers were counted and age was interpreted generally as described by Sergeant and Pimlott (1959). In this study age was reported in whole years for convenience of analysis, although the actual age of each moose exceeded its reported age by the number of months it lived past its last birthday. Annual cementum deposition consists of translucent (light) and opaque (dark) layers when viewed with transmitted light (Fig. 1). Opaque layers are usually deposited during mid to late winter and appear to be mostly complete by April or 2 months prior to the animal's birthday. On a moose's birthday the number of opaque layers theoretically equals its age in years. An age estimate to the nearest month can be made when the month the animal died is known. Most births occur during late May in Alaska; therefore, if P~B~pna~Jla~ 3 + ~PF~.~ r~u~ :?~i~L,