Now that you know how surviving bear species relate to one another and to more distant relatives, let’s take a quick glance at their ancestors as they have appeared and disappeared or migrated across the globe during the past several million years.
The traits by which bears cope with current environments were shaped by millennia of varying selection pressures. Information on past environmental conditions, and on how bears adapted to them, may help us understand and compensate for the limitations of their adaptation to modern conditions. We need to consider factors that once allowed extant bears to flourish while other species disappeared. Among these factors are habitat conditions and preferences, feeding habits, interspecific competition, and defense against enemies.
The phylogenetic record for ursidae, especially dating from the early Pleistocene, is one of the most complete for any mammal. Their durable bones have been preserved in caves where they denned and died (Kurten 1976). This record documents the origin and ecological radiation for most extant species from the Miocene and Pliocene, through Pleistocene glacial and interglacial periods, to the present. (Tables 1:1 & 1.2). Further evidence is provided by comparative study of the anatomy, karyotypes, and biochemistry of living species.
Deriving from carnivore stock, millions of years of selection pressures adapted most bears to increasingly herbivorous diets. This trend reached its zenith in the European and Florida cave bears, along with the giant panda, whose dentition and skull structure are specialized for chewing tough plant tissue. By contrast, giant short-faced bear and polar bear reverted to the ancestral predatory life style. Other ursids are omnivorous, eating mostly fruit, nuts, other low-fiber vegetation, colonial insects, and occasionally vertebrate prey. Some commonly dig for corms, bulbs, tubers, or rodent prey. Most bears eat invertebrates, especially colonial insects. But only the sloth bear has specialized for feeding on termites. It is also the only ursid to share the giant panda trait of being able to grasp food with one paw.
Bears inhabit biomes ranging from wet and dry tropical forests to the arctic tundra and sea ice, in Eurasia, North America, and South America. Like their ancestors, most bears are superb climbers, at least until they grow so large that size impairs climbing. Bears ascend trees to obtain food and for refuge from some enemies. Young of virtually all bear species may have climbed to refuge. But adults of the larger species rely for protection more on their combative ability. This strategy enables them to make fuller use of foods found in tree-poor habitats such as grasslands, shrublands, and polar sea ice.
Overall bears have been superbly adaptable, in part through morpho-physiological adjustments, but mostly through their intelligence and behavioral versatility.
PREFACE
