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

The diet of stoat, Mustela erminea, in the Italian Alps was studied from Ma y to October 1996 and 1997 through faecal analysis. Seats were collected al ong twenty transects and near dens within a 10-km(2) study area. In the sam e area, seasonal variation in the available biomass of different prey types was estimated using capture-mark-recapture (small rodents), pitfall trap g rids (insects), and fruit counts. About 60 % of seats (n = 734) contained s mall rodents, indicating that they were the main prey for stoat. However, t he frequency of occurrence of fruits in the diet increased significantly in August, after rodent biomass had dropped by more than 50 % in July, but in creased again to previous August levels. Fruit consumption decreased in Sep tember 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 pre y. We conclude that primary prey, rodents, is always harvested, suggesting that the costs of harvesting rodents, abundant throughout spring-autumn, ar e never high. The consumption of alternative prey is independent of its ava ilability, and increases only when relative benefits of harvesting alternat ive prey (fruits) increases, which is consistent with optimal foraging theo ry. (C) 2001 Editions scientifiques et medicales Elsevier SAS.