Polar bears can be assigned to age classes based on examination of the skull and dentition. As with other mammals, progressive closure of skull sutures is adequate to separate young and adult polar bears (Manning 1964). Hensel and Sorenson (1980) assigned living polar bears to approximate age classes based on reproductive status, physical measurements, and tooth replacement and wear. Ages of cub-of-the year and yearling polar bears can be assigned without error by observation of dental eruption despite huge variation in size and weight in these age classes. In the autumn and early winter of their first year, permanent canines of young polar bears appear as small cones barely longer than the incisors. Early in the second year of life, canines have grown, but still appear conical in overall cross section. By autumn and early winter, as the second birthday approaches, canines have taken on the shape of mature teeth with a distinctive base and crown. More precise assignment of polar bear ages can be made, as in other ursids, by counting cementum annuli in microscopic cross sections of tooth roots (Stoneberg and Jonkel 1966; Hensel and Sorenson 1980; Calvert and Ramsay 1998).