Variation in mandible shape and body size of house mice Mus musculus in five separate New Zealand forest habitats

This study investigates variation in house mouse Mus musculus body size and mandible shape across New Zealand, using geometric morphometrics and biomechanical advantage analyses. The Mus phylogroups currently known in New Zealand include Mus musculus domesticus, M.m.musculus and M.m.castaneus. We examined samples of house mice inhabiting five different podocarp and beech forest environments across the North and South Islands (Pureora Forest, Zealandia Wildlife Sanctuary, Craigieburn Forest Park, Eglinton Valley and Hollyford Valley). Significant variation in mandible shape and body size was found between all five forest populations. South Island mice had larger bodies and greater mechanical advantage in the temporalis muscle compared with their North Island counterparts. Zealandia Sanctuary mouse mandibles were broader and shorter than South Island mouse mandibles, and had greater masseter muscle advantage. Centroid size and body weight, but not head-body length, varied significantly with two distinct genetic haplotypes. Finally, annual rainfall was the most significant covariate with mandible shape. Higher rainfall locations were generally associated with soft-food related mandible shapes, while lower rainfall correlated with hard-food mandible shapes. This preliminary investigation provides the framework for further research into mandible shape and body size variation in New Zealand house mice.