1. Repeated measurements of daily field metabolic rate (FMR) were made on 11 Meadow Voles (Microtus pennsylvanicus Ord.) by means of the doubly labelled water technique. The objective was to quantify the individual consistency of FMR by calculating the repeatability of successive measures on the same individuals. One more general goal was to test whether FMRs are sufficiently repeatable to be convenient for field studies of natural selection. 2. Voles were all non-reproductive females, 5-7-months old. They were maintained individually in outdoor enclosures 25 m(2) and were injected 1-3 times with doubly labelled water. Two to six measures of daily FMR were thus obtained per individual (x=4.09, SD=1.51, n=11) over a period of 42 days in July and August 1994. 3. Body mass and rate of change in body mass accounted for 31.6% of the observed variation in FMR; ground-temperature variations had no significant effect on FMR, probably owing to the small range of temperatures experienced by voles. 4. Repeatability of mass-corrected log(10) FMR measurements was low (r=0.261). This low repeatability was very consistent with the results of a previous study on consistency of mass-specific daily energy expenditures of caged pouched mice. 5. A partition of the total variance observed in mass-corrected FMR showed that 63.0% of the variance originated from differences within individuals, 27.6% from differences among individuals, and 9.4% from measurement error. FMR was not consistently higher or lower on the first or second day of measurement, indicating no clear effect of handling stress. 6. A practical implication of these results is that single measurements of daily field metabolic rates are not necessarily a good predictor of the average 24 h energy budget for a given individual. On a more theoretical basis, these results indicate that, although the ability of an animal to manage its energy economics may largely determine its selective value, daily FMR seems to be a poor candidate variable to relate this ability to individual fitness in experimental studies. 7. We suggest that future studies of repeatability of FMR should concentrate on energetically constraining periods. This would allow further evaluation of the potential role of interindividual differences in FMR as a tool for understanding evolutionary pathways that shaped energy economics of animals.
