Estimating body composition of lizards from total body electrical conductivity and total body water

Nondestructive methods of estimating body composition are crucial for measuring energy budgets of free-ranging animals. However, most methods have proved to be either difficult or inaccurate for estimating lipid mass, particularly in small animals. I validated the use of total body electrical conductivity (TOBEC) and total body water (TBW) to estimate lean mass and nonpolar lipid mass in the lizard Sceloporus undulatus. Regression models based on either TOBEC or TBW were able to predict dry lean mass and wet lean mass within 5% of actual values. Estimates of nonpolar lipid mass, derived by subtracting predicted wet lean mass from total body mass, were highly correlated with actual nonpolar lipid mass. When total nonpolar lipid mass was greater than 0.1 g, the average percent error in predicted nonpolar lipid was 30% and 15% for the TOBEC and TBW methods, respectively. A sensitivity analysis suggests that, in most cases, TBW can be used to estimate nonpolar lipid mass within 15% of actual lipid mass. Both TOBEC and TBW measurements are relatively easy methods of tracking qualitative changes in body composition within individuals, but TBW should be used when quantitative estimates of nonpolar lipid mass are desired.