Asiatic black bears (Ursus thibetanus) and sun bears (Helarctos malayanus) are ecologically similar and coexist extensively across Southeast Asia. We used foraging signs identified to bear species to examine their food selection and dietary overlap relative to food abundance, nutrition, and phenology in 3 habitats in Thailand. We posited, based on ecological theory, that coexistence of these 2 species would be explained through resource partitioning; our data, however, did not support this hypothesis. We conducted 71 sign transects and recorded 730 bear signs, mainly claw marks on trees that bears climbed for food. Both species fed predominantly on fruit; we documented 93 plant species from 42 families that bears consumed. Insects were of secondary importance. Bears of the 2 species selected fruit trees of the same families and genera in each habitat, especially lipid-rich Lauraceae and Fagaceae, tracking fruiting phenology through time. Diet overlap was high, even during periods of diminished fruit availability. We propose a number of mechanisms that may have promoted coexistence of these 2 species. For example, sun bears consumed proportionately more insects than did black bears; insectivory may help sustain the smaller-sized sun bears in the face of competition over fruits with black bears. Also, competition over fruits was reduced by both species cropping a lower proportion of common fruit trees than rarer fruit trees, thereby leaving a potential surplus for the other species. Furthermore, food resources were generally abundant and available year-round: about half the trees in the forest were potential food trees for bears. Bear populations likely were depressed below carrying capacity by previous hunting; as they recover, more competition for resources and greater niche divergence could ensue.
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Vol. 94 • No. 1