USE OF FECES TO ESTIMATE ISOTOPIC ABUNDANCE IN DOUBLY LABELED WATER STUDIES IN REINDEER IN SUMMER AND WINTER

The reliance on samples of blood or urine to estimate isotopic abundan ce in studies of energy metabolism using the doubly labeled water meth od has restricted application of the technique to animals that are eit her tame or easy to catch. This is generally not the case with large, free-ranging wild mammals. The use of feces as a source of body water in which to measure the concentration of isotopic markers was investig ated in four female reindeer in summer and in winter. (H2O)-H-2 and (H 2O)-O-18 were injected to similar to 160 parts per million excess. Sam ples of plasma and feces were then collected simultaneously for up to 456 h. Both isotopes were equilibrated with body water at 8 h postdose . There were no significant differences by animal between dilution spa ces, rate constants, rates of CO2 production, and total energy expendi ture (TEE) calculated based on samples of plasma or feces in any trial . Mean TEE was 3.557 W/kg (SD 0.907, n = 4) in summer and 1.865 W/kg ( SD 0.16 6, n = 4) in winter.