AN EVALUATION OF WHOLE-BODY K-40 CONTENT FOR ESTIMATING LEAN AND FAT MASS IN PIGEONS

A severe limitation in studies of avian ecological energetics is the l ack of an accurate, noninvasive technique for determining whole body f at storage in living birds. We explored a technique of assaying total body potassium as a predictor of lean mass (LM) and then derived fat m ass (FM) by subtracting LM from total body mass. Body potassium (K), p resent in lean tissue but not in fat, was estimated noninvasively from naturally-occurring radioactive K-40, which Occurs as a fixed ratio t o total body K. We assayed 29 pigeon (Columba livia) carcasses for K-4 0 and then measured LM from body composition analyses in which fat mas s was extracted using petroleum ether. The K-40 results were regressed against LM using five different combinations of independent variables . Regression equations were tested by comparing predicted LM (and FM p redictions by subtraction from body mass) to measured LM values obtain ed from a separate group of pigeons. Whole body assay of K-40 was not a useful predictor of LM in pigeons (r(2) = 0.51; mean absolute error was 14 +/- 7%). Absolute errors increased with FM predictions (96 +/- 50%). Adding body mass as an independent variable increased the r(2) t o 0.97, but body mass alone explained 96% of the variability in LM.