Sociobiology of the myrmecophagous sloth bear in Nepal

Abstract

Ursids have adapted to environments ranging from the tropics to the arctic, and although the family is noted for its omnivory, some species have specialized food habits. The sloth bear (Melursus ursinus) has specialized on insect prey, particularly termites and ants, and exhibits some characteristics and behaviors that are common among myrmecophagous mammals. We examined whether myrmecophagy has affected its sociobiology. During 1990-1994 we studied a high-density population of sloth bears in Royal Chitwan National Park, Nepal. We found extensive seasonal overlap among home ranges of adults of the same sex (>50%) and between subadults and adults of both sexes (>70%). Moreover, overlap zones between adjacent ranges were used in proportion to their area. This, and observations of unrelated bears feeding or traveling in proximity to one another (not at concentrated food sources), suggested a high degree of mutual tolerance in this population. However, subadults and females with young may have temporally avoided other bears by limiting their activity to daylight hours. Predators (which were chiefly nocturnal) may also have affected the activity patterns of these (the most vulnerable) bears, and were probably responsible for the females' habit of giving birth in an underground den, fasting for several weeks so as not to leave cubs unattended in the den, and carrying the cubs on their back for 6-7 months after leaving the den. The young left their mother at 1.5 or 2.5 years old (this varied by family) and remained together and (or) later rejoined a sibling or another subadult, possibly to form a coalition against either predators or older bears. We documented few mortalities and no permanent juvenile dispersal in this study, but we also found few subadults in our study area, which indicates undetected mortality or dispersal. We cannot discount the possibility that some aspects of the sociobiology of sloth bears (e.g., cub-carrying, mutual tolerance) are related to myrmecophagy, but the social system and life-history traits of this species seem to fit well within the range observed among other ursids.