Sometimes the simple way is best: an evaluation of total body electrical conductivity (TOBEC) and a morphological index as predictors of total body water in the northern brown bandicoot, Isoodon macrourus

Total body electrical conductivity (TOBEC) can be used as a non-invasive measure of body water and/or lean mass, but there are no validations of this technique in small marsupials. The ability of TOBEC to describe body water content in the northern brown bandicoot, Isoodon macrourus, was compared with that of a morphologically based index. The TOBEC Index, E, was significantly related to total body water, calculated using tritium dilution, by a cubic polynomial. Using this relationship TOBEC was able to predict total body water to within 3.9 +/- 16.8 g (s.e., n = 13) and percentage total body water to within 0.4 +/- 1.8% (s.e., n = 13) of the values calculated by tritium dilution. The ratio of mass to hindleg length gave the best predictive power of the morphological indices tested for total body water and was significantly related to body water by a quadratic polynomial. Total body water predicted from the mass to hindleg-length ratio averaged 2.9 +/- 14.0 g (s.e., n = 13) lower, and the percentage total body water averaged 0.2 +/- 1.6% (s.e., n = 13) lower, than that derived from tritium dilution. Both techniques were able to make very strong predictions of body water, well within the ranges of variability that might be expected to be ecologically significant in this species. Contrary to expectations, the morphological index was as good a predictor of body water as TOBEC in this calibration set of animals. This was the first validation of the use of electrical conductivity to predict body water in a peramelid marsupial, and only the second in any marsupial. The power of TOBEC to describe body water in this study of a peramelid, coupled with the previous validation in a diprotodontid, suggests that the technique may be more generally applicable to marsupials, but the equal power and convenience of the morphological index suggests that the latter may be more appropriate for this species.