Many studies assume that it is beneficial for individuals of a species to be heavier, or have a higher body condition index (BCI), without accounting for the physiological relevance of variation in the composition of different body tissues. We hypothesized that the relationship between BCI and masses of physiologically important tissues (fat and lean) would be conditional on annual patterns of energy acquisition and expenditure. We studied three species with contrasting ecologies in their respective natural ranges: an obligate hibernator (Columbian ground squirrel, Urocitellus columbianus), a facultative hibernator (black-tailed prairie dog, Cynomys ludovicianus), and a food-caching non-hibernator (North American red squirrel, Tamiasciurus hudsonicus). We measured fat and lean mass in adults of both sexes using quantitative magnetic resonance (QMR). We measured body mass and two measures of skeletal structure (zygomatic width and right hind foot length) to develop sex- and species-specific BCIs, and tested the utility of BCI to predict body composition in each species. Body condition indices were more consistently, and more strongly correlated, with lean mass than fat mass. The indices were most positively correlated with fat when fat was expected to be very high (pre-hibernation prairie dogs). In all cases, however, BCI was never better than body mass alone in predicting fat or lean mass. While the accuracy of BCI in estimating fat varied across the natural histories and annual energetic patterns of the species considered, measuring body mass alone was as effective, or superior in capturing sufficient variation in fat and lean in most cases.

许多研究认为，一个物种的个体较重或具有较高的身体状况指数（BCI）是有益的，但没有考虑不同身体组织组成变化的生理相关性。我们假设 BCI 与生理上重要的组织（脂肪和瘦肉）质量之间的关系取决于年度能量获取和消耗模式。我们研究了在各自自然范围内具有对比生态的三个物种：专性冬眠者（哥伦比亚地松鼠，Urocitellus columbianus），兼性冬眠者（黑尾草原土拨鼠，Cynomys ludovicianus）和储存食物的非冬眠者（北美红松鼠（Tamiasciurus hudsonicus）。我们使用定量磁共振 (QMR) 测量了男女成年人的脂肪和瘦肉质量。我们测量了体重和两项骨骼结构测量（颧骨宽度和右后足长度），以开发性别和物种特异性 BCI，并测试 BCI 在预测每个物种身体成分方面的实用性。身体状况指数与去脂体重的相关性比与脂肪质量的相关性更一致且更强。当脂肪预计非常高时（冬眠前的草原土拨鼠），这些指数与脂肪呈最正相关。然而，在所有情况下，BCI 在预测脂肪或去脂体重方面从来都不比单独的体重更好。虽然 BCI 在估计脂肪方面的准确性因所考虑物种的自然历史和年度能量模式而异，但在大多数情况下，单独测量体重在捕获脂肪和瘦肉的足够变化方面同样有效或优越。