ESTIMATION OF THE METABOLIC-RATE OF THE DESERT IGUANA (DIPSOSAURUS-DORSALIS) BY A RADIONUCLIDE TECHNIQUE

We describe a method for measuring metabolic rates (MRs) of ectotherms using successive measurements of radionuclide body burdens which may provide an alternative means of measuring field metabolic rate (FMR). Although MRs can be measured in the laboratory, variability in food su pply and quality, temperature, activity, and other factors preclude th e direct application of such data to field conditions. Recently, the d oubly labeled water (DLW) technique has been successfully applied to e stimate FMR in a variety of animals, but this method is expensive, req uires special equipment, necessitates the sampling of blood or other b ody fluids, and may be unsuitable for certain species. We compared the rates of elimination of seven radionuclides with estimates of MRs (CO 2 production) from DLW measurements in the desert iguana, Dipsosaurus dorsalis. We observed temperature-dependent elimination for five of th e radionuclides, and found a high correlation (r(2) = 0.93) between th e slopes of individual Rb-86 elimination curves and the MRs estimated by the DLW technique. This correlation appeared to be relatively insen sitive to differences in water turnover and ingestion of the stable nu trient analog (K). The results suggest that a practical and inexpensiv e method for measuring FMRs of ectotherms may be derived from this rel ationship.