All age groups gained mass during the active season, but mass-gain of adult females was delayed during lactation. 2. The relationship of body mass to metabolic rate varied widely; when the relationship was significant, R2 varied from 10.3 to 72.6%. Body mass affects VO2 more during lactation than at any other period. 3. Mean VO2 of adult males was higher in June than that of adult, non-lactating females. 4. VO2 of reproductive females was significantly higher during lactation than during gestation or postlactation because specific VO2 varied. Specific VO2 of non-reproductive females declined over the active season. 5. Specific VO2 of all age groups declined between the premolt and postmolt periods. The reduced maintenance costs can contribute 20-46% to daily growth. 6. Observed VO2 was lower than the value predicted from intraspecific or interspecific Bm:M regressions. 7. VO2 of wild-caught marmots was lower than that of marmots maintained in the laboratory, probably because of dietary differences. 8. Because basal metabolism is a stage on a food-deprivation curve, we suggest that basal metabolic rate is not an appropriate measure of the metabolic activity of free-ranging animals.
