Red deer (Cervus elaphus) and reindeer (Rangifer tarandus) constituted major components of the European Middle Paleolithic faunas, and hence a key resource for hominid populations. In paleoenvironmental reconstructions, red deer and reindeer occurrences are typically considered as a tree-cover indicator and an open landscape marker, respectively. However, insights into the ecology of extant deer uncover a wide variety of feeding behaviors and occupied habitats. Exploring the feeding behavior of extinct eurytopic cervids constitutes a key to better apprehend paleoenvironments and their variations through time. By reflecting what has eaten an animal during the last few days or weeks of its life, dental microwear textures of herbivores constitute a bridge between a population and its environment. Here we analyzed, via Dental Microwear Texture Analysis (DMTA-SSFA), the diet of 202 R. tarandus and 116 C. elaphus preyed by the Neanderthal populations that occupied Combe Grenal. This site is one of the most important Mousterian archaeo-sequences in southwestern France (spanning MIS 6 to 3), characterized by an abundance of faunal remains (>12,000 remains) and ample variation in the lithic industries. Results illustrate the diversity of the food categories the two taxa were able to consume around Combe Grenal. The fact that both deer are eurytopic allows us to consider their feeding behavior as a good indicator of paleoenvironmental variations through time. DMTA-SSFA provide a more complex picture of Combe Grenal local paleoenvironmental variations than previously thought. These variations are discussed and contrasted with changes already documented in the sequence in hunted prey and Neanderthal productions (lithic industries and pigment use). Identifying and understanding ecological adaptations in fossil populations in response to environmental and climatic changes is a relevant approach to better understand the current adaptations of species to the climatic changes they are currently facing.