AEROBIC PERFORMANCE IN BELDINGS GROUND-SQUIRRELS (SPERMOPHILUS-BELDINGI) - VARIANCE, ONTOGENY, AND THE AEROBIC CAPACITY MODEL OF ENDOTHERMY

Metabolic rate is one of the most intensively studied aspects of organismal physiology, but much of he large body of data on metabolism concerns adult animals while largely, ignoring juvenile stages, and focuses on mean values of traits while neglecting to consider the significance of intraspecific variation. To study the importance of ontogenic change and individual differences in aerobic performance in a typical small mammal, we measured minimal resting oxygen consumption (Vo(2mr)) and maximal oxygen consumption (Vo(2max)) during exercise and thermogenesis in juvenile and adult Belding's ground squirrels (Spermophilus beldingi). Across a 5.5-fold body mass range, the relationship between mass and Vo(2) was best described with two-phased regressions with inflections at 212-281 g, instead of the simple linear relationship reported for other species. The reason for inflections is unclear, but they are possibly related to growth costs since they approximately correspond to body mass at the end ofthe rapid growth period in the juvenile year. We also found higher than expected intraspecific mass exponents for both Vo(2mr) and Vo(2max); this may also be related to rapid growth of juveniles. The aerobic scope (Vo(2max)/Vo(2mr)) increased from about 5.4 in newly independent juveniles to about 7.5 in adults. Interindividual variance was lowest for exercise Vo(2max) and highest for VO2mr. Residual analysis revealed that Vo(2mr) was significantly, correlated with exercise VO2max (i.e., individuals with above-average Vo(2mr) for their mass tended to have high VO2max, and vice versa). However, there was no correlation between VO2mr and thermogenic VO2max. These results provide limited support for the aerobic capacity model for the evolution of endothermy.