Body mass as an estimate of female body condition in a hibernating small mammal

In hibernating squirrels, the amount of energy stored as fat may influence several important demographic traits, but is difficult to quantify in living animals. Thus, several non-destructive indices of body condition are used, including simple indices that use body mass and scaled indices that correct body mass for structural size. However, the accuracy of these indices for hibernating squirrels is poorly known. We used measurements of total body electrical conductivity (TOBEC) from adult female Golden-mantled Ground Squirrels (Callospermophilus lateralis) to characterize body composition (lean mass versus fat mass) and condition (fat stores) at multiple stages in the circannual cycle. Body mass explained a high proportion of the variation in fat mass during the emergence and pre-hibernation stages, but less during the reproduction stage. Contrary to expectation, correcting for structural size did not markedly improve the condition index. Our results suggest that body mass is a good estimate of body condition during the periods of emergence and pre-hibernation fattening, and therefore may be useful to predict important components of fitness such as reproductive success and overwinter survival.