Seasonal Variations in the Body Composition and Nutrition of the Vole Microtus agrestis

The short-tailed vole, Microtus agrestis (L.), is found primarily in grassland, where it feeds chiefly on the green leaves and stems of various grasses (Godfrey 1953; Hansson 1971). Its population dynamics have been studied in detail in Britain by Chitty and his co-workers (Chitty 1952; Godfrey 1955; Chitty & Chitty 1962a; Chitty & Phipps 1966) and quantitative estimates have been made of its food supply (Chitty, Pimentel & Krebs 1968; W. N. Charles unpublished). Hitherto, however, little attention has been paid to the qualitative changes which occur during each year in the various grasses eaten by Microtus. This paper presents measurements of the pepsin-soluble content of several grasses at different times of year, in parallel with changes in weight, body composition and breeding condition of voles from isolated populations in two types of habitat-ungrazed limestone grassland and a young plantation. It will be shown that grasses have maximum pepsin solubility for only a short period in the year, and that most growth and reproduction of M. agrestis occurs outside this period; that the pepsin solubility of the grasses has little direct effect on the timing of changes in weight of different body components (except that the spring weight increase in males coincides with the spring flush of grass), and that in the area studied Microtus carries no appreciable energy reserve at any time of year.