The effect of predation risk on body weight in the field vole, Microtus agrestis

The effect of predation risk on body weight in subadult Microtus agrestis w as studied in the laboratory and in the field. Exposing voles in the labora tory to faecal odours from domestic cat, captive fox and captive mink led t o a larger decrease in individual body weight compared to control voles whe n the distance between cover and food bowl was short, despite equal intake of food in both groups of voles. Increasing the distance between cover and food bowl to 50 cm resulted in very low food intake in voles exposed to pre dator odour compared to control voles. In the field, voles from grids where mammalian and avian predators were excluded generally lost less or gained more weight in autumn and winter than voles from corresponding control grid s. Despite some variation in individual weight changes during autumn and wi nter voles from exclosures gained more weight than control voles in early s pring immediately before onset of reproduction. Mean weights throughout aut umn, winter and spring were found to be affected not only by individual wei ght changes but also by immigration. When immigration was low, mean weights on predator exclusion grids were higher than on predated grids. Accordingl y, the result of reduced predation risk in the laboratory as well as in the field seems to be higher body weights in the individual and often also in the population. In the field this was especially evident at the onset of br eeding in spring, potentially resulting in a higher reproductive potential. The study is the first of its kind to our knowledge to show the same trend in weight development in the laboratory as well as in the field.