Seasonal Changes in Body Mass, Composition, and Organs of Northern Red-Backed Voles in Interior Alaska

Abstract

Northern red-backed voles ( Clethrionomys rutilus ) undergo a pronounced annual cycle in body mass and are heaviest in summer and lightest in winter. We trapped voles throughout 1994 to determine how changes in body composition and organ size contributed to this cycle. Body mass peaked in summer for females and spring for males. Seasonal changes in body mass were primarily due to changes in lean mass. Body mass was 30–50% lower in winter than summer, and water content of lean mass was lowest in winter. Total body fat was low throughout the year but peaked (as with body mass) in spring (males) or early summer (females). Energy reserves in the form of fat depots are apparently most crucial during the breeding season. A low relative ash content in early summer was possibly due to a cation imbalance in the diet. Absolute and relative sizes of different body components contributed to the annual cycle in total body mass. All body components (except brown adipose tissue) declined in absolute mass, dry mass, and percent water during autumn, with skeletomuscular components contributing most to loss of total body mass. Most body components declined in proportion to declines in total body mass. However, liver, reproductive tract, and muscle mass of males declined proportionally more than total body mass; heart, brain, and bone declined proportionally less. Whole body analyses suggest that the annual cycle of body mass in C. rutilus is driven by seasonal changes in optimal body size. Component analyses are consistent with the hypothesis that the primary selective force driving seasonal changes in body components is the enhanced overwinter survival of C. rutilus with relatively small body size.