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 supply and quality, temperature, activity, and other factors preclude the direct application of such data to field conditions. Recently, the doubly labeled water (DLW) technique has been successfully applied to estimate FMR in a variety of animals, but this method is expensive, requires special equipment, necessitates the sampling of blood or other body fluids, and may be unsuitable for certain species. We compared the rates of elimination of seven radionuclides with estimates of MRs (CO₂ production) from DLW measurements in the desert iguana, Dipsosaurus dorsalis. We observed temperature-dependent elimination for five of the radionuclides, and found a high correlation (r² = 0.93) between the slopes of individual 86Rb elimination curves and the MRs estimated by the DL W technique. This correlation appeared to be relatively insensitive to differences in water turnover and ingestion of the stable nutrient analog (K). The results suggest that a practical and inexpensive method for measuring FMRs of ectotherms may be derived from this relationship.

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