Understanding how genetic variations and environmental constraints influence morphological differences is crucial to interpreting phenotypic diversity. The mandible is a key structure widely used to make inferences about feeding ecology and habitat conditions in early ungulate growth. However, most studies focus on interspecific comparisons, and few address mandibular plasticity and variations at inter- and intrapopulation scales. In this study, we used a 2D geometric-morphometric approach to analyse how these factors shape the mandibles of two Cervini species characterised by different contexts, populations and belonging to distinct lineages: the red deer (Cervus elaphus) and the rusa deer (Rusa timorensis). For the European red deer, we found clear variations in mandibular morphology along a north-south transect from Norway to southern Spain. These variations are related to the need for different biomechanical forces to masticate vegetal resources with distinct physical properties, under contrasting climates. The mandibles of red deer in southern Europe are more robust, probably due to the higher proportion of highly fibrous, tough herbaceous monocotyledons in their diet. In contrast, northern populations have slender mandibles, reflecting the consumption of a softer diet. The American population of red deer from Argentina shows a different trend, both in size and shape. A recent study showed that New Caledonian rusa deer constituted a single genetic population. Here, we identified significant intrapopulation differences between contrasting local habitats. Gouaro-Déva rusa deer are significantly smaller, which we interpret as the consequence of greater stress, impacting the growth and ontogenic development of these animals. Deer from tropical forests have a more robust mandible than in grassy areas, aligning with the biomechanical needs of their respective diets. Morphological variations in deer jaws at intraspecific and interspecific scales provide insights into the ecology and environment of these animals, and offer potential for future (paleo)ecological research using geometric morphometric approaches.