Abstract
<title>Abstract</title> <p> Mandible morphology plays a critical role in carnivore feeding ecology, yet how these structures vary within a species across distinct ecological contexts remains underexplored. Arctic foxes ( <italic>Vulpes lagopus</italic> ) in Iceland are known to differ in feeding behaviour between coastal and inland habitats. To evaluate whether foxes from these different habitats differ in mandible morphology, and whether such differentiation increases with age and population density, this study examined mandible shape variation in 348 foxes across three regions, comparing juveniles and adults, over a period of forty years (1979–2018). Analysis based on elliptical Fourier coefficients revealed significant regional differences in mandible morphology. Coastal Westfjords differed from inland Eastern and Southern populations, with the difference primarily concentrated in the mandibular ramus, specifically in the angle between the ramus and the mandible body, indicating differences in biomechanical demands of feeding, consistent with the dietary divergence between ecotypes. Population size at birth was associated with mandible shape in adults but not juveniles, suggesting that early-life demographic conditions contribute to morphological differentiation that becomes apparent with age. The effect was strongest in the Westfjords and South populations. These results demonstrate phenotypic divergence in a functional trait within a single species and indicate that mandible morphology is shaped by both ecotypic affiliations and demographic conditions experienced during early development. These findings provide insights into the adaptive capacity of Arctic foxes and have implications for understanding how predators respond to changing ecological conditions. </p>