Foraging theory predicts that animals should forage so as to maximize their net rate of energy gain or to minimize their risk of starvation. In situations where prey numbers fluctuate dramatically, theory predicts further that foragers will eat 'optimal' prey when it is abundant but expand their diet to include other prey types when the optimal prey is scarce; this is the alternative prey hypothesis. Here, we test this prediction by analyzing the diet of a mammalian predator, the feral house cat Felis catus, during periods of scarcity and abundance of the long-haired rat Rattus villosissimus. We also investigate whether the body condition of feral cats differs during different stages of the prey population cycle. Feral cats were shot during culling operations in semi-arid grassland habitats in central Queensland, Australia, and the stomach contents later identified. We found that the body condition of feral cats did not differ between phases of the prey population cycle, but the diets of cats culled when long-haired rats were scarce were significantly more diverse than when this rodent was abundant. Rats comprised about 80 % of cats' diet by volume and frequency of occurrence when they were present, whereas birds, reptiles and invertebrates comprised the bulk of the diet when rats were not available. We conclude that, whilst feral cats are often thought to be specialist predators, they may be better considered as facultative specialists that will shift their diet in predictable ways in response to changes in the abundance of primary prey.

No abstract

Seeds are commonly viewed as the mainstay of the diet of desert rodents. We describe the diet of a common Australian desert rodent, the sandy inland mouse Pseudomys hermannsburgensis, using direct observations of free-living animals and analysis of the stomach contents of preserved specimens. Direct observations showed that animals forage mostly on the ground surface and eat seeds from a wide range of plant species, as well as invertebrates and occasional green plant material. Stomach content analysis revealed no differences in the presence or absence of these three major food groups between seasons or the sexes. However, invertebrates were more prominent in the diet of mice during prolonged, dry, population ‘bust’ periods compared with post-rain population ‘boom’ periods, with this dietary shift probably reflecting a scarcity of seeds during the busts. The results confirm that seed is an important component of the diet of P. hermannsburgensis, with 92% of stomachs containing seed. The results also support the classification of the species as omnivorous rather than granivorous, with 70% of stomachs containing invertebrates and over half the specimens analysed containing both seeds and invertebrates. We suggest that dietary flexibility is important for rodent persistence in Australia’s climatically unpredictable arid regions.