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Elephas maximus and Stegodon orientalis were two keystone proboscideans in southern Asia that coexisted mainly after the Middle Pleistocene in many regions. The long-term paleoecology and possible foraging competition of these two species have not been intensively investigated yet. Here, we applied stable isotope (C, O) analysis to the tooth enamel of coexisting Elephas maximus, Stegodon orientalis, and other associated mammalian species in Quzai Cave, southern China, dated to the early Late Pleistocene, to explore their paleoenvironmental context and foraging ecology. The delta C-13 values of Elephas maximus were widely distributed between -17.9 parts per thousand and -11.9 parts per thousand (n = 10), while Stegodon orientalis delta C-13 values ranged from -16.7 parts per thousand to -14.7 parts per thousand (n = 7). These results suggest that Elephas maximus was possibly a mixed feeder with a broader range of dietary resources than Stegodon orientalis, which probably browsed on a narrower range of plant resources in more densely forested landscape. A chronological comparison (from 8 Ma to recent) of published delta C-13 data for these two species from Asia showed that none of them were dietary specialists. However, Elephas had a more flexible foraging ecology and a stronger ability to exploit abrasive grasses than Stegodon. The niche partitioning and perceived different foraging behaviors of Stegodon and Elephas might have reduced the level of interspecific competition and allowed them to coexist during the Pleistocene. Moreover, the high-level of ecological flexibility of Elephas might have helped them to survive until the present day, while Stegodon eventually went extinct by the terminal Pleistocene similar to 12 ka. An extensive comparison and evaluation of the delta C-13 data from fossil mammals in mainland southeast Asia during the Early to Late Pleistocene suggests that southern China was dominated by C-3 vegetation throughout the Pleistocene, in contrast with the evidence of C-4 biomes in neighboring Laos, Cambodia, and Thailand. Southern China experienced relatively stable environments during the Pleistocene, which can be attributed to the wide range of mountainous regions that acted as ecological refugia from human interference and climatic fluctuations, and allowed the preservation of high biodiversity. The isotopic data we present here provides new evidence about the ecological complexity of mainland southeast Asia and elucidates the need for more systematic research to investigate extinction models and ecological conservation in this region. (C) 2019 Elsevier Ltd. All rights reserved.