Changes in Patterns of Seasonal Growth of Microtus Pennsylvanicus

A field study of growth, in terms of both linear dimensions and weight, was made by successively trapping the small winter—active mammals found in a study area in a region with persistent winter snow cover. The animals trapped were primarily Microtus pennsylvanicus, with some data also on Blarina brevicauda, Clethrionomys gapperi, and Peromyscus sp. The reliability of the field measurements was determined by comparing successive measurements made on individual animals within short time spans. The results of these comparisons indicate that linear measurements can be made accurately on live, conscious animals in the field, and that linear measurements are much more reliable than weight measurements. Microtus pennsylvanicus young born in the spring and early summer reach adult size in 12 weeks or less. These animals undergo a fall loss in weight. Animals born in middle to late summer stop growing in the fall, maintain their weight and linear dimensions throughout the winter, and resume growth again in the spring. Blarina brevicauda individuals grow little in linear dimensions after entering the trappable population and do not appear to lose weight in the fall.