The Sloth Bear
The sloth bear, Ursus ursinus, arose from the oldest lineage of the true bears.  They diverged from the main lineage of bears about 6 million years ago.  Like the giant panda, it is likely that climatic fluctuations were important in isolating ancestral sloth bears in the south of Asia long enough to diverge as a separate species.  It is possible that a group of bears managed to colonize the Indian subcontinent either during a warm, interglacial period in the late Miocene era, or during one of the cold periods.  Cold periods occurred at that time about every 100,000 years. 

In one scenario, we could imagine that after the ancestral sloth bears had occupied India, the climate cooled again and a wall of glaciers descended from the Himalayas that may have blocked any movement of bears in or out of India.  Another possibility is that their ancestors moved south  across eastern Asia during a cold period, perhaps as far as the edge of the Himalayas.  Then, as the climate warmed, one group moved into the mountain foothills and became isolated there.  Such a group could have evolved around the margins of the Himalayas and perhaps expanded into lowland areas at a later period; maybe when Asiatic black bears arrived to compete with them.  Whatever happened way back then, the sloth bears were left alone to evolve in isolation, somewhere in southern Asia, and to acquire the unique characteristics that enabled them to survive.  During historic times they once occupied all of India and southern Nepal except for the very mountainous regions in the north.

Sloth bears are the insectivores of the bear world.  They have a number of interesting adaptations that enable them to exploit ants and termites as primary food sources.  Like the other bears in southern Asia, the sloth bear evolved from omnivorous, tree-climbing ancestors that avoided predation from the large cats and other carnivores.  Early sloth bears could not compete with more efficient carnivores in capturing prey, and were probably not large enough to drive the large predators from kills.  Carrion is occasionally used, but carrion decomposes too rapidly in the tropics to supply a reliable source of food.  As the sloth bear evolved it adapted to feeding on fruits and insects which were abundant.  Ants and termites in particular are widespread and represent a large and stable food source.  As early sloth bears utilized this resource, natural selection favored the development of large mobile lips.  The bear’s snout is also much more mobile than other species and the hair is greatly reduced.  In addition, the first pair of upper incisor teeth have disappeared, providing an opening for effectively sucking insects into the mouth.

Sloth bears are attracted to human developments in search of food such as honey or fruit, but their reliance on insects helps to keep them in more remote areas.  In two years of following bears with radio collars in Chitwan National Park in Nepal, none traveled outside the park.  If the population grows, however, subadults may be forced to disperse into human-occupied areas.  In other parts of their range, as habitat is reduced, sloth bears also are forced into marginal areas also.  They can be aggressive and dangerous, and bear parts are valuable commodities in the trade for Asian medicines.  Incentives for killing bears are therefore high.  Protection of all remaining habitat is a critical priority for conservation, and public education combined with strict enforcement of anti-poaching laws is also necessary.  To preserve the species in the long term, larger protected areas than those that currently exist will be needed, and it may be possible to reconnect some of the fragmented habitat.

Picture courtesy of Harriet Corbett, Rox Graphics, 866 Rd. 7RP, Powell, WY 82435, 307 645 3202, crowhart@wtp.net