Use of doubly-labeled water (3HH18O) for determination of H2O flux and CO2 production by a mammal in a humid environment

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Abstract
The accuracy of the doubly labeled water (3HH18O) technique for estimating H2O flux and CO2 production in pocket gophers was evaluated in laboratory experiments. Experiments were conducted under conditions of elevated humidity in order to determine the influence of unlabeled H2O vapor input on H2O flux and CO2 production rates which are determined from isotope turnover. High humidity is commonplace in burrows of many small mammals, including those of the pocket gopher, and theoretically could introduce substantial errors when determining these rates with isotopic procedures. Water influx that was estimated from tritium turnover was not significantly different from actual H2O influx which was determined from feeding rates. Also, 3HH18O techniques yielded estimates of CO2 production that were not significantly different from values determined from energy intake rates. Such close agreement was not anticipated. The unexpected accuracy probably results due to offsetting errors associated with (1) the influx of unlabeled H2O vapor through the skin and lungs of labeled pocket gophers and (2) the effects of biological fractionation of isotopes in the body H2O of labeled pocket gophers.