北海道におけるヒグマの冬篭り穴について

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Hitherto reports concerning the hibernation den of the Yeso brown bear have been very few, though in Hokkaido the bear occurs more commonly than elsewhere as the game statistics show. Recently (since 1930) an average of 500 bears have been killed annually. Nevertheless, in Hokkaido relatively few dens have been described, two by INUKAI (1932) and one by PEELLE (1931) . In the USA, by comparison, CRAIGHEAD et al. (1972) reported 17 dens of the grizzly bear from Yellowstone Park. Adding to the above, LENTFER et al. (1912), REYNOLDS et al. (1976) and others reported on the denning habits of American brown bears. One of the chief reasons why observations in Hokkaido have been so scarce is that the habitat of Ursus arctos yesoensis is somewhat different from that of the other bear countries. In Hokkaido forests inhabited by bears mostly comprise mixed stands of coniferous and broad-leaved trees with very thick and tall undergrowth including sasa bamboo, bushes and weeds. Therefore, to explore or even penetrate the trackless forest is very difficult indeed. In winter in Hokkaido the bear habitats are covered with deep snow, and as the animals hibernate under the snow cover it is hard to discover of their dens. Recently, however, the development of Hokkaido has been much advanced, permitting safe access to deep forest for investigations of the bionomics of Ursus yesoensis. Thus up to the present we have now been able to study 20 hibernation dens from various parts of Hokkaido, extending from the most northerly to the most southerly localities. The necessary conditions determining the site of the hibernation den in accordence with our observations, are as follows: a mountainside slope of steep inclination (23°to 40°), a vegetation cover comprising tree stands with thick undergrowth, an assurance of deep snow cover in winter, an abundance of rich natural bear food, remoteness from human disturbance etc. It is noted that neither natural caves nor holes in trunks of trees are used for the hibernation den by the bear in Hokkaido. Neither have dens been found on flat places nor on slopes lacking winter snow cover. Some people believe that hibernation dens are found on sunny southern mountain slopes. This is not true, there being no definite orientation of the slope on which the den is made. The location of the den is always several tens of meters above the bottom of the slope, so as to avoid avalanche danger. Generally the hibernation lair of the Yeso brown bear is a horizontal ellipsoid hole consisting of two parts, namely the narrow entrence and the inner wide bed chamber. The entrence of most of the dens opens on the surface of the slope, although sometimes it opens in a shallow vertical hole dug by the bear. The latter kind of den is generally found in gradual slopes. Usually, the entrence leads directly into the bed chamber. However, there are some dens in which the entrence is connected with the bed chamber by a 0.3 to 0.6 m long tunnel. The dimensions of the bed chamber are wide enough for sitting or lying. The longitudinal length of the chamber is 0.9 to 2.5m, the width is 0.76 to 1.8m, and the height is 0.65 to 1.1m. The bottom area is 0.51 to 3.6m2. From the ceiling of the chamber tiny plant roots of various lengths hang in dense masses. These roots prevent the falling of soil from the ceiling, at the same time strengthening the whole structure of the chamber. From the above we can understand why these bears select forested or bushy slopes for their hibernation dens. Generally the floor of the bed chamber is covered with bedding materials about 5 to 20 cm thick. However, a few dens lack such bedding altogether. The materials utilized consist of dried fallen leaves, and dried leaves and broken stems of sasa bamboo. The chamber is redolent of the pleasant odour of dried fresh plant materials. The bedding is collected by the bear from around the den.
日本哺乳類学会の論文

北海道におけるヒグマの冬篭り穴について

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