Morphological and functional divergence of the lower jaw between native and invasive red foxes

The introduction of European red foxes in Australia in the late mid-nineteenth century has resulted in the spread of this invasive species across the continent. The morphological and functional divergence of this relatively recently introduced population has not been explored to date, yet it may provide unique insights into adaptability of this widespread carnivore to very different environments. Here we used three-dimensional geometric morphometric approaches and dissections to explore differences in mandible form and function between two populations: one from France and the other from Western Australia. Bite force was predicted for Australian foxes using partial least squares (PLS) regression models based on the observed covariation between estimated bite force (from muscle dissections) and mandible form in French foxes. Muscle contributions were estimated based on Euclidean distances between landmarks that provide insights into muscle lever arms. Despite the greater sample size, Australian foxes show reduced variability in mandible shape compared with French foxes. The mandibles of adult French foxes tend to be slightly smaller and they also strongly differ in shape from the Australian foxes in functionally important areas of the mandible such as muscle insertion areas. This is accompanied by significant differences in the predicted bite force, even relative to size, and muscle contribution: the bite of Australian foxes is weaker and they show greater use of their temporalis muscle compared to French foxes. The reduced variability suggests a founder effect or stabilizing selection on a specific morphology, which was supported by statistical tests. The corresponding anatomical traits suggest different functional demands likely due to differences in diet or competition. Future studies investigating the drivers of variation in mandible shape in native and invasive populations, including data from the original source of the Australian introductions, are needed to better understand the observed differences