Anatomical response to the vole population cycles in the Swedish red foxes (Vulpes vulpes)

The size of animals is often related to food abundance in the year an individual is born and thus the mean size of the animals often varies between birth cohorts. Neglecting these complications may result in false conclusions in analysis of morphological variation. In this study I examine how the lengths of the skulls and the long bones of red foxes Vulpes vulpes varied between birth cohorts and in relation to fluctuations in their main food (voles). Foxes, born in the northern half of Sweden when the voles are extremely common, have larger skulls and longer legs than foxes born when the voles are scarce. The relation between the abundance of voles and the size of the skulls and the long bones is less pronounced southwards and ceases in central Sweden. Neglecting these complications may result in false conclusions. When the voles are scarce the reduction of the size of the skull is larger counted as a percentage, than the shortening of the long bones. This indicates that in northern areas with much snow and long winters it is more important for the foxes to retain the length of the legs than to keep the length of the skulls.