Diet and bite force in red foxes: Ontogenetic and sex differences in an invasive carnivore

Abstract

Bite force is often used as a predictive indicator of an animal's feeding ecology, although the premise that there is a direct link between diet and cranial morphology can be difficult to test empirically. Studies that have examined this question tend to rely on generalizations of a species' diet, and age and sex differences are rarely considered. Red foxes (Vulpes vulpes) are introduced predators in Australia, allowing large sample size collection through culling for comparison of skull morphology (size, morphometry, weight), demographics (age, sex) and diet (stomach contents). Over half (57%) of the 540 animals we sampled were juveniles (<1 year old; dispersing from their natal sites) and most variation in skull morphometry was driven by age; adults had significantly more robust skulls than juveniles, with greater estimated bite force. Sexual dimorphism (body mass and body length) was reflected in longer, heavier skulls of males. We also recorded significant sex and age differences in diet. Sheep carrion comprised 47-65% of diet volume; however, adult females ate less sheep but had more mice and invertebrates in their diet than males or juveniles of both sexes. This dietary separation for adult females does not appear to be directly due to estimated bite force constraints, but probably rather prey availability, which may reflect feeding behaviour and space use patterns. Juveniles (both sexes) showed as much consumption of sheep carrion as adult males, despite their lower estimated bite force than adults. This is the first study that directly compares ontogenetic and sex differences in the diet of a carnivore together with their cranial morphology and estimated bite force, and highlights limitations of inferring diet partitioning from skull morphology alone.