Cervids, and especially the red deer Cervus elaphus, are among the most regularly and abundantly recorded ungulates in Pleistocene/Paleolithic bone assemblages. Numerous Pleistocene or Holocene subspecies have been described, reinforcing their status as essential proxies for environmental and chronological reconstructions. Despite this, at the beginning of the Late Pleistocene, their diversity seems to have decreased. In this study, we analysed teeth and some postcranial elements of Cervus and Capreolus from northeastern Iberia and southeastern France to clarify their morphological characteristics and ecological adaptations. We describe a transitional form in northeastern Iberia between the western European stock and the current form C. e. hispanicus. Such sub-speciation processes are connected to biogeographical factors, as there were limited exchanges between north-eastern Iberia and the northern Pyrenees, whereas the north-western part of the peninsula seems more connected to the northern Pyrenees. The anatomical plasticity (morpho-functional adaptation and body size) of red deer is connected to dietary flexibility (dental meso- and microwear). Conversely, Capreolus shows greater morphological and ecological homogeneity. Body size variations seem directly correlated with their ability to browse throughout the year. The marked differences between the eco-bio-geographical responses of the two taxa can be explained by their habitat selection.