Foraging plasticity in a highly specialized carnivore, the endangered black-footed ferret

The extirpation of black-footed ferrets (Mustela nigripes) from the wild resulted from the rangewide decline of prairie dogs (Cynomys spp.) brought about by poisoning campaigns, the arrival of an exotic disease, and habitat loss. It is widely accepted that ferrets are an obligate, near monophagous, dietary specialist of prairie dogs and that high-density prairie dog colonies are necessary for effective recovery. To test the extent to which ferrets are dietary specialists, we measured the stable isotopic values of 321 ferrets of known age and sex as well as of their potential prey (e.g., prairie dogs, mice, ground squirrels, and rabbits). Our results confirmed that prairie dogs are the most common diet item for ferrets, although ferrets possessed greater foraging plasticity than previously reported, consuming substantial quantities of other species. The degree to which ferrets were specialized on prairie dogs differed between age-sex groups. Adult male and juvenile ferrets had equivalent diets, with prairie dogs constituting nearly 75% of their assimilated diet. In contrast, adult females obtained over one third of their diet from other species, notably mice. However, female ferrets appeared to have provisioned prairie dogs to their dependent offspring. Conservation of ferrets, one of North America's most endangered mammals, will require prairie dogs, not just as prey, but also for the prey-rich habitat that their colonies provide.