A Field Study of Growth in Microtus pennsylvanicus

The genus Microtus is of great and widespread importance to agriculturalists and students of animal ecology because of its periodic fluctuations in numbers. That the often cited, extremely high reproductive potential indicated by Bailey (1924) for M. pennsylvanicus is seldom attained in nature and that deviations from the potential vary in different stages of population development was indicated by Hamilton (1937a). A detailed knowledge of the factors influencing reproductive rate, therefore, is essential to an understanding of population dynamics. One of these factors is growth rate. Growth rate not only affects the ontogeny of the individual, but its effects are transmitted directly to the population. Laboratory studies of growth and development have been conducted by Ranson (1934) and others for M. agrestis, by Selle (1928) and Hatfield (1935) for M. californicus, by Bodenheimer (1941) for M. guentheri, by Morrison, et al. (1954) for M. oeconomus, by Cowan and Arsenault (1954) for M. oregoni, and by Hamilton (1937b, 1941) for M. pennsylvanicus. In general, these studies have indicated a rapid early growth averaging between 0.6 grams per day for the small M. oregoni to 1.0 grams per day for the large M. californicus. Female Microtus may begin to breed at an age of 3 to 4 weeks, and the male a week or two later. A period of relative sterility following puberty is recognized by Bodenheimer and by Cowan and Arsenault and is suggested for the male by Leslie and Ranson (1940). The young of M. oregoni born after late July do not attain puberty during the same season, and mice born late in the season grow slower than earlier captive litters.