An Evaluation of Whole Body Potassium-40 Content for Estimating Lean and Fat Mass in Pigeons

A severe limitation in studies of avian ecological energetics is the lack of an accurate, non-invasive technique for determining whole body fat storage in living birds. We explored a technique of assaying total body potassium as a predictor of lean mass (LM) and then derived fat mass (FM) by subtracting LM from total body mass. Body potassium (K), present in lean tissue but not in fat, was estimated noninvasively from naturally-occurring radioactive40K which occurs as a fixed ratio to total body K. We assayed 29 pigeon (Columba livia) carcasses for40K and then measured LM from body composition analyses in which fat mass was extracted using petroleum ether. The40K results were regressed against LM using five different combinations of independent variables. Regression equations were tested by comparing predicted LM (and FM predictions by subtraction from body mass) to measured LM values obtained from a separate group of pigeons. Whole body assay of40K was not a useful predictor of LM in pigeons (r2= 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 r2to 0.97, but body mass alone explained 96% of the variability in LM.