Variation in the size of foxes in Scotland

Foxes become larger from south to north in Scotland, independently of the climate, the prey taken or the productivity of the areas in which each population lives. In areas where fluctuating vole populations are important as food, foxes born in high vole years are no larger than those born in poor years. However foxes became smaller after a severe winter in north-east Scotland, followed by a gradual increase in the size of each year class through a series of years with mild winters and expanding rabbit populations. It is suggested that food availability is determining the average size of foxes through selection, and the north/south cline in size is the result of increased hunting hours at higher latitudes during winter. The selective advantage of different size animals under conditions of different food availability is discussed.