1. The repeatability of a trait is a critical factor in determining how that trait is affected by natural selection. We examined the repeatability of a key physiological trait, maximum oxygen consumption (VO2max), in a wild population of Belding’s Ground Squirrels, Spermophilus beldingi. VO2max is an integrated measure of organismal metabolic performance. It can be intuitively related to fitness because it sets an upper limit to sustainable power output during ecologically important activites such as locomotion and thermoregulatory heat production.
2. We used respirometry to determine VO2max during exercise and thermogenesis. Exercise VO2max was elicited in an enclosed running wheel. Thermogenic VO2max was obtained with acute cold exposure in a helium-oxygen gas mixture.
3. Repeatability of both exercise and thermogenic VO2max was high over 2 h intervals but declined over longer test periods (6-18 days and 1-2 years). In general, repeatability was higher for exercise VO2max than for thermogenic VO2max.
4. We found no repeatability for animals tested initially as juveniles and then 1 or 2 years later as adults; evidently there is sufficient plasticity in VO2max to decouple aerobic performance between these life stages. A small number of adults tested in successive years showed significant repeatability of exercise VO2max but no repeatability of thermogenic VO2max
