Two cryptic species of mice coexist on Cyprus: the introduced house mouse and the endemic Cypriot mouse, that remained unnoticed until the beginning of the 21 st century. Their trophic position was investigated using isotopic ecology. The shape and biomechanics of the mandible provided a complementary insight into their respective diets. The Cypriot mouse exhibits generalist habits relying on various natural food resources, including invertebrates, while the house mouse exploits a broad spectrum of anthropic food resources. The Cypriot mouse has a large mandible optimized for chewing at the molars that facilitates consumption of large and hard food items presumably abundant in the natural vegetation of Cyprus. The small mandible size of the house mouse is compensated by a large masseter area and an optimization for incisor biting, making it an all-rounder tool for foraging on diverse non-natural items. This fine-tuning of generalist feeding behavior ensures efficient niche partitioning between the two species on Cyprus.