Size of Internal Organs and Forage Quality of Herbivores: Are There Differences between Cycle Phases in Microtus Voles?

Abstract

We studied the possible role of food quality in cyclic vole populations by using the size of internal organs as an indicator of forage quality. We trapped field (Microtus agrestis) and sibling (M. rossiaemeridionalis) voles during two increase and two decline phases of the 3-yr population cycle in western Finland, recorded total body mass and weighed their pancreas and liver. There were no obvious differences in the relative size of organs related to phase of the cycle, which suggests voles from declining populations do not exhibit pancreatic hypertrophy. These results do not support the hypothesis that proteinase inhibitors in food plants that have adverse effects on metabolic processes of herbivores, and which are induced by grazing, cause regular cycles in small rodent populations.