ヒグマの歯の年輪形成時期および歯の種類による年輪数について The Number of Annuli and the Time of Annuli Formation in Different Kinds of <i>Ursus arctos yesoensis</i> Teeth
The age of <i>Ursus arctos yesoensis</i> was known to be determined by teeth annuli, but the number and clarity of annuli sometimes differed from bear to bear or between the different kinds of teeth obtained from the same individual. In order to examine the characteristics of teeth annuli, 2 bears of known age (No.5 and 7) and 6 bears of unknown age were employed. The age of the bears of unknown was estimated by examining J<sub>1</sub> and P<sub>4</sub> teeth, which exhibited clear annuli. The teeth of J<sub>1</sub>, J<sub>2</sub>, J<sub>3</sub>, C, P<sub>1</sub>, P<sub>4</sub>, M<sub>1</sub>, M<sub>2</sub> and M<sub>3</sub> of the lower jaw obtained from No.7 bear and some others were analyzed for detecting morphological and numerical differences of annuli. The age estimated from the annuli of C was sometimes younger than the actual age by one year due probably to the delayed development of the canine. The annuli numbers observed in the other kind of teeth coincided with one another (Fig. 9-17). There were 2 assertions that in the first one, the annuli formed in the period before the hibernation, and that in another, it begun to form in the fall and completed in April or May of the next year. In No.1 (11 years and 8 months) bear which was killed at the end of September, J<sub>1</sub> showed 11 annuli in the inner part of the cementum, and the marginal cement layer was unstained. However, a little stained short line was found in it, indicating a new annulus formation (Fig. 1). P<sub>4</sub> of No.2 (3y. & 9m.) bear which was killed in early October showed almost the same feature of new annulus formation as that of No.1 (Fig. 2). J<sub>1</sub> of No.3 (14y. & 10m.) bear which was killed at the end of November had 15 annuli, of which the outmost one was stained faintly (Fig. 3). J<sub>1</sub> of No.4 (2y. & 10m.) bear which was killed at the end of November showed about the same features as that of J<sub>1</sub> of No.3 (Fig. 4). P<sub>4</sub> of No.5 (5y. & 1m.) bear which was killed in the middle of Feburuary had 5 distinct annuli, and the outmost annulus was clear and perfect (Fig. 5). J<sub>1</sub> of No.6 (11y. & 2m.) bear which was killed at the end of March had 11 clear annuli, and an unstained cementum layer was developing on the outer side of the outmost annulus (Fig. 6). P<sub>4</sub> of No.7 (5y. & 3m.) bear which was killed in the middle of April showed 5 dark stained annuli, and an unstained cementum layer developed in some extent (Fig. 7). J<sub>1</sub> of No.8 (10y. & 4m.) bear which was killed at the beginning of May showed 10 annuli with a well developed marginal unstained cementum layer (Fig. 8). It was clear that the formation of new annulus began in the middle of September and continued up to the end of April. The No.5 bear was castrated male, so that it was suggested that the sex hormone had no relation to the annuli formation (Fig. 24). In this species the so-called accessory lines appeared, which closely resembled annuli and were also reported in grazzly bear. Therefore, in such a case, several kinds of teeth should be examined for an exact age determination. The mechanism of the accessory line formation and its significance are not known at present.
日本応用動物昆虫学会誌 18(3), 139-144, 1974