From what we currently know about the evolution of bears, ecological radiation has been governed primarily by adaptation to specific food sources and habitat types. Diversification away from Miacid ancestors involved a decrease in obligate carnivory through increased consumption of insects and plants; bears became omnivores. Fruit may be a larger component in the diets of sun and spectacled bears than in diets of other bears (Walker 1964); but data are too limited for quantitative comparison. Sloth bears are the most specialized for consumption of termites and ants.
Grizzlies and secondarily brown bears apparently make the heaviest use of plant and animal foods specific to areas without climbable trees, although some Asian black bears, sloth bears, and spectacled bears also make considerable use of tall grasslands and scrub lands. Polar bears are specialized for killing seals on the sea ice. Arboreality among the black-colored species serves both to aid foraging and for refuge from enemies. The transition away from arboreality to terrestrial and sea ice habitats involved increasing capabilities (in grizzly bears) for digging out plant and animal foods, as well as capabilities (in grizzly and polar bears) for predation on large mammals and for combative defense against large enemies or groups of enemies (e.g.,wolves). Ancestral bears dwelt primarily in temperate habitats. Subsequent geographical and ecological radiation has included adaptations to tropical habitats (sun, sloth, and spectacled bears) and arctic habitats (black bears,grizzly/brown bears, and especially polar bears). Other evolutionary changes in bears seem secondary to these, as illustrated by variations in body size. Size variations during the evolution of a species and current differences among species are discussed here; concurrent geographical variation within species is discussed and analyzed in Chapter 9:I. Then in 9:II, factors governing all these kinds of size variations are discussed.
SUMMARY & CONCLUSIONS