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Physical characteristics, diet and life-cycle of Ursus thibetanus (japonicus)

Asiatic black bears have black fur (turning dark-brown in some individuals as they age) usually with a distinctive white patch on the chest that is often crescent-shaped. The marking is reminiscent of a

lunar crescent (see Figure 6), and its Japanese name tsukinowaguma means ‘crescent moon

bear’.6 The average weight of the Japanese black bear ranges between 60 and 120 kg in the adult male and 40 to 100 kg in the adult female. The average body

length is 110–140 cm.7 It is smaller than its ‘cousin’, the American black bear (Ursus americanus), but it is otherwise similar in appearance and shares some behavioural characteristics.8

The roaming range of the bear in Japan varies between 10 and 250 km² for a male bear and 4 and 100 km² for a female. There is significant variation from year to year, depending on the availability of food.9 When food is scarce, bears will expand their range in order to find adequate food, particularly in the months leading up to hibernation.10 This is particularly the case when mast trees have a poor season. Some bears will then extend their range into human-inhabited areas, which brings them into conflict with humans.

The bear is omnivorous, but vegetable matter constitutes the predominant part of its diet. Grasses, sedges, herbs and buds are preferred foods in spring, and berries and nuts in summer and autumn, when it needs to eat high-energy foods in order to build up body fat for the hibernation period.11 What

Figure 6: The Asiatic black bear (source: Harriet Corbett, Rox Graphics )

animal matter the bear does eat is derived predominantly from small animals such as frogs, crabs, ants and other insects.12 The bear will eat carrion, such as that of wild rabbits and serow (Capricornis crispus, or kamoshika in Japanese), but it is rare for the bear to hunt animals actively.13 Like the panda, the bear has evolved from a carnivore and its digestive system has not completely adapted to its largely vegetarian diet. It is therefore not efficient in its absorption of vegetable matter, and to sustain itself the bear needs to eat large volumes of food, particularly leading up to the hibernation period.14 One notable and yet unexplained characteristic of the bear’s dietary behaviour is its habit of tearing off the bark of trees (usually conifers) and gnawing at the exposed sapwood (see Figure 10). Worldwide, this behaviour has only been observed in the Asiatic black bear and the American black bear. 15

Bears usually hibernate over winter, leaving the den between April and May. Female bears with cubs born over the winter are generally the last to leave their dens. Their eventual emergence tends to coincide with the budding of beech trees and other plants in the area in which the den is located.16 It is thought that this timing allows the cub(s) sufficient time to learn to walk, and also coincides with more dense spring foliage which may serve to protect the cub(s) from potential predators.17 In the spring, the (adult) bear eats the new leaves of trees, shrubs and grasses; acorns and other nuts on the ground from the previous autumn; angelica and other plants from the dropwort family; the flowers of plants such as kobushi (Magnolia kobus) and tamushiba (Magnolia salicifolia), and, in areas of high snow fall, beech buds (both flower and leaf buds). When the budding period has finished, the bear also eats bamboo shoots. During spring it is thought that the bear’s level of activity and area of movement is fairly limited. As spring growth of flora advances, the bear climbs to increasingly high altitudes to feed on new growth.18

In the summer, the bear’s level of activity and the area of movement increases. During this period, the bear eats insects (which become more active during the summer); buds from the Rosaceae family of plants; and grasses. Summer is both the bear’s mating season, and the time that cubs born in previous years separate from their mothers. Little is known about the process and period of nurturance and separation, but it is thought that cub(s) hibernate with the sow through the

winter following the year in which they were born, and separate from the sow in the following summer.19

In the autumn months, the bear becomes intent on eating a large amount of food in order to build up fat reserves for the hibernation period. This period of hyperphagia coincides with the period when mast trees bear nuts, which are an important source of carbohydrates for the bear.20 These nut-bearing

Figure 7: Beech nuts, an important part of the bear’s diet during the autumn months (Photo: H. Suzuki)

trees include Fagus (beech, or buna in Japanese) (see Figure 7) and various species of Quercus (oak, or kashi in Japanese). During these autumn months, the bear’s area of movement varies according to the availability of food.21 As winter approaches, bears become increasingly single-minded about eating and may even become oblivious to danger—i.e., the presence of humans. Hunters report that bears are easier to hunt during this period due to their distractedness.22

Beechnuts do not mature until October, but recent research has shown that in years of poor masts, bears begin to appear outside their normal range (i.e. the usual range in a year of average or good masts) as early as July or August in their search for food. Researchers speculate that this is due to the fact that beech flowers are indicative of mast rates in the autumn. Thus, the number of flowers in the summer becomes a signal to the bear of the availability of its most important food source in the coming months, and triggers it to take compensatory measures by expanding its range.23

Whereas the Asiatic black bear mates in summer, like other bear species, there is a delayed implantation of the embryo in the uterus. This means that after copulation, the fertilised egg floats freely in the uterus until late autumn or early winter, when the female bear finds a den for the winter. Once the implantation occurs, the growth of the foetus takes about 60 days. This delayed implantation allows the birth of the cub(s) to coincide with the bear’s hibernation period, which means the birth can occur in relative safety. It is also thought that the delay allows for the gestation to abort if the pregnant sow does not build up enough body fat to assure both her own survival and that of her cub(s). This is corroborated by recent research which suggests that birth rates are higher in years which follow a good harvest of mast trees.24

Bears usually begin their hibernation between mid-November and mid-December. It is assumed that variations in timing are due to factors such as weather and nutritional levels, but there has not been any scientific verification of this.25 They den in hollow trees, under large rocks, in holes in the ground (often created by a fallen tree), small caves, and cavities between the raised roots of large trees.26 In areas with little snow, bears tend to prefer steep ravines, possibly to avoid contact with humans, particularly hunters and their dogs.27 Bears generally do not eat or even drink during the hibernation period.28

Cubs are born in the den during the hibernation period. Little is known about the birth and raising of cubs—in fact even the timing of the birth is not known precisely, though it is thought that it is generally mid- to late February.29 At birth, the cub is only about 200–400 grams, a mere one per cent of its mother’s weight, and its eyes are closed for the first week. 30 It is thought that bears usually have two cubs at one time, one male and one female, but it is also known for a bear to have only one cub, or to have triplets.31

One characteristic behaviour of this bear species is its habit of making ‘seats’, resembling large birds nests, in the branches of trees in which it feeds. These are most commonly referred to as

kumadana by upland Japanese, meaning literally, ‘bear shelves’ (see Figure 8). The bear makes these seats predominantly in the autumn, in mast trees such as konara (Quercus serrata) and

mizunara (Quercus crispula), (both types of oak),

kuri (Castanea crenata, chestnut), mizuki (Swida controversa, or dogwood) and beech trees. It

appears that these ‘seats’ are made when the bear feeds on the nuts in these trees: it breaks off branches one at a time, carefully removes and eats the nuts, and then places the finished branch under its body, to gradually build up a platform.32 There is common consensus that bears use these ‘seats’ for eating, but it has also been claimed that the seats are built to avoid insects and for resting, as noted in Chapter Three.33 However, this claim has not been verified by research, and as Watanabe (1984) points out, if this were the case, the ‘nests’ would be expected to be found in all types of large trees, rather than just mast trees.34