Age Determination of Black Bears by Cementum Layers

Abstract

Microscopic examination of decalcified, sectioned, and stained teeth of three wild known-age black bears (Ursus americanus) shows that the layers present in the cementum may be used for age determination. The data also reveal that the primary factor governing seasonal changes in cementum production, which in turn result in the formation of annuli, may not be denning. Recently much interest has been directed toward studying the population dynamics of the large North American carnivores. This work, however, has been seriously hindered by the lack of an accurate technique for age determination. The use of differential tooth growth as a technique for age determination was first attempted by the marine mammalogists Scheffer (1950) and Laws (1952, 1953). Since then it has been used successfully on many marine mammals (Nishiwaki and Yagi 1953, Nishiwaki et al. 1958, Mansfield and Fisher 1960, and Kleinenberg and Klevezal 1962) and for aging many North American ungulates (Sergeant and Pimlott 1959, Low and Cowan 1963, and McEwan 1963). Rausch (1961) observed external ringlike markings on the roots of canine teeth from Alaskan black bears. He found these markings represented the boundaries between successive increments of dentin and could be used for determining the age of individual bears through the sixth summer of life. Mundy and Fuller (1964) used the cementum of M3 as an indicator of age for the grizzly bear ( Ursus spp. ). Although they lacked known-age specimens, they obtained a positive correlation between the number of annuli represented in the cementum and the zygomatic breadth of the skull.3 The present study is concerned primarily with testing the merits of using the cementum as an age indicator for black bear by using teeth from the three known-age wild animals. Our thanks are extended to Dr. H. D. Fisher, under whom the laboratory work was conducted, for his advice and criticism of the manuscript. We would also like to thank Miss Hilary Short, Dr. G. E. Rouse, Ian McAskie, and Miss Marilyn Bennett for their technical assistance. MATERIALS AND METHODS Canine teeth of ten unknown-age skulls obtained from the museum of the University of British Columbia were used for experimenting and refining the technique. Three s ctioning methods were attempted: thin grinding, acetate peel, and histological sectioning. The histological sectioning proved the most successful, and was used for the known-age teeth (Fig. 1) from the three tagged wild bears captured in northwestern Montana. The teeth were cut in half along the gum line and the bottom halves were placed in 30 percent formic acid (or 15 percent nitric acid) to decalcify. The decalcified teeth were placed in a satu1 This study was jointly supported by the University of British Columbia Research Fund and Montana Federal Aid Project W-98-R. 2 Present address: Department of Zoology, University of Montana, Missoula. 3See also Marks and Erickson (1966), this issue of The Journal of Wildlife Management.