Morphology and digestive efficiency of red foxes (Vulpes vulpes) and grey foxes (Urocyon cinereoargenteus) in relation to diet

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Abstract
Measurements of the skull and dentition, and digestive efficiency of red foxes (Vulpes vulpes) and grey foxes (Urocyon cinereoargenteus) were compared to determine whether differences in morphology and digestive ability could be associated with dietary preference. The longer jaws of the more carnivorus red fox increase the resistance moment arm when biting at the canines and carnassials, but greater size and mechanical leverage of the masticatory muscles in this species compensate for this, enabling the red fox to bite with forces that should be comparable to or exceed those of the grey fox. The relatively shorter masticatory moment arms of the more omnivorous-insectivorous grey fox should increase bite speed. Both fox species digested mice with equal efficiency, but the more omnivorous grey foxes had higher digestive efficiencies than red foxes when fed a diet of fruit. These morphological and physiological differences observed between sympatric red foxes and grey foxes may allow each species to exploit different foods more effectively.