Seasonal and interindividual variation in field water metabolism of femalemeadow voles Microtus pennsylvanicus

We analyzed variations in water flux rates on a large sample of meadow vole s (Microtus pennsylvanicus) to quantify the effect of season on water metab olism of individuals and to examine patterns of intra- and interindividual variability. Voles were nonreproductive females maintained in outdoor enclo sures where they fed on natural vegetation. They were injected one to three times with doubly labeled water, which resulted in one to six measures of daily water flux rate per individual. Summer water flux rates of voles were 258% of the predicted values for herbivorous eutherian mammals of similar size. To date, very few studies have focused on mammals with such high wate r flux rates. Body water volume of individuals was higher in summer than in winter (75.6% vs. 72.5%), and water flux rate of animals was 12.5% higher in the winter season (0.99 vs. 0.88 mL H2O g(-1) d(-1)). Between-season dif ferences in water fluxes were proportional to differences in energy expendi tures, hence the water economy index remained constant across seasons (0.30 mL H2O kJ(-1)). Intraindividual variability of water flux rate was high co mpared to interindividual variability (repeatability, r < 0.30), which will make it difficult to study natural selection of water metabolism in a micr oevolutionary framework, at least in meadow voles.