In the Austrian caves of Gamssulzen and Ramesch, two genetically distinct cave bears, Ursus ingressus and Ursus spelaeus eremus, apparently lived side by side for 15,000 years, together with brown bears Ursus arctos. The possible ecological partitioning of these three types of bears was investigated using multi-isotopic tracking of organic (delta C-13(coll), delta N-15(coll)) and inorganic (delta C-13(carb), delta O-18(carb), delta O-18(PO4)) fractions of bone. The cave bears from Ramesch, Ursus spelaeus eremus, were ecologically distinct from the cave bears from Gamssulzen, Ursus ingressus, both being ecologically distinct from brown bears from Ramesch, Ursus arctos. Both cave bear types were purely herbivorous but likely consumed different plant types and/or plants from different habitats, while brown bears included some animal proteins in their diet. Bone apatite delta O-18 values strongly suggest that both types of cave bears used isotopically distinct water sources, indicating that they may not have occupied the same landscape, either separated in space or in time due to climatic shifts. Therefore, the influence of environmental conditions strongly constrained the genetic structure of these bears. (C) 2010 Elsevier Ltd and INQUA. All rights reserved.