Diet of stoats (Mustela erminea) in an Alpine habitat:The importance of fruit consumption in summer

Abstract

The diet of stoat, Mustela erminea , in the Italian Alps was studied from May to October 1996 and 1997 through faecal analysis. Scats were collected along twenty transects and near dens within a 10-km 2 study area. In the same area, seasonal variation in the available biomass of different prey types was estimated using capture-mark-recapture (small rodents), pitfall trap grids (insects), and fruit counts. About 60 % of scats ( n = 734) contained small rodents, indicating that they were the main prey for stoat. However, the frequency of occurrence of fruits in the diet increased significantly in August, after rodent biomass had dropped by more than 50 % in July, but increased again to previous August levels. Fruit consumption decreased in September and October, although available biomass of fruits remained constant. Thus stoat included a large amount of alternative food in their diet when fruits were mature and their availability, and probably their profitability, increased. We were, however, unable to measure absolute consumption of prey. We conclude that primary prey, rodents, is always harvested, suggesting that the costs of harvesting rodents, abundant throughout spring–autumn, are never high. The consumption of alternative prey is independent of its availability, and increases only when relative benefits of harvesting alternative prey (fruits) increases, which is consistent with optimal foraging theory.