Seasonal variation of resting metabolic rate and body mass in free-living weasels Mustela nivalis.

Metabolic rates and body mass of mammals vary seasonally along with ambient temperatures and food availability. At the population level, seasonal changes in metabolic rate and mass can be due to selective mortality or emigration of individuals whose metabolic rate or mass differs from the average for the population. Alternatively, the metabolic rates of individuals can change seasonally, such that the population average increases or decreases due to shifts in the physiology of the overall population. The latter implies that individuals respond in a similar manner to changing seasonal conditions. We studied seasonal changes in body mass (BM) and resting metabolic rate (RMR) in free-ranging male weasels (Mustela nivalis) to test the consistency of these traits in individuals caught in different seasons of the year. At the population level, BM was remarkably stable across the seasons (F(3, 124)=0.25, P=0.9). In contrast, BM- corrected RMR varied significantly between seasons and was the lowest in winter (F(3, 135)=9.13, P<0.0001). We demonstrated that individual weasels were consistent in how their BM and RMR deviated from the seasonal means for the population (intraclass correlation, τ=0.78 and 0.33, respectively). This variation among individuals explained ~76% and 27% of the total variation of BM and basal metabolic rate, respectively. Hence, the relatively constant BM at the population level across seasons is due to a relative constancy of BM in individuals. Our study is one of relatively few research projects that demonstrate that seasonal changes in RMR observed in the wild population are in part due to a consistency in individual responses to changing environmental conditions.