Body size-dependent refuges in voles: an alternative explanation of the Chitty effect

In cyclic vole populations the body size of voles tends to vary with population density, voles being smaller in the decline and low density phases than in the peak phase of the cycle. We have studied this ‘Chitty effect ̓ using fi eld measurements of vole (Microtus agrestis, M. rossiaemeridionalis) body size and predator densities, and laboratory experiments on the minimum passable hole size for voles and their major predator, weasels (Mustela nivalis nivalis). We found that many voles can pass through smaller holes than the smallest weasels, which implies that these small voles can have a refuge from weasel predation. Consequently, predation pressure is expected to be greater on larger voles. In the fi eld we found a negative association between the mean body size of voles and weasel abundance. Our laboratory and fi eld results suggest that size selective predation might be an important cause of the observed smaller size of voles in the decline phase of cycles.