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 organ ismal physiology, but much of he large body of data on metabolism conc erns adult animals while largely, ignoring juvenile stages, and focuse s on mean values of traits while neglecting to consider the significan ce of intraspecific variation. To study the importance of ontogenic ch ange and individual differences in aerobic performance in a typical sm all mammal, we measured minimal resting oxygen consumption (Vo(2mr)) a nd maximal oxygen consumption (Vo(2max)) during exercise and thermogen esis in juvenile and adult Belding's ground squirrels (Spermophilus be ldingi). Across a 5.5-fold body mass range, the relationship between m ass and Vo(2) was best described with two-phased regressions with infl ections at 212-281 g, instead of the simple linear relationship report ed for other species. The reason for inflections is unclear, but they are possibly related to growth costs since they approximately correspo nd to body mass at the end ofthe rapid growth period in the juvenile y ear. We also found higher than expected intraspecific mass exponents f or 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 abo ut 5.4 in newly independent juveniles to about 7.5 in adults. Interind ividual variance was lowest for exercise Vo(2max) and highest for VO2m r. Residual analysis revealed that Vo(2mr) was significantly, correlat ed with exercise VO2max (i.e., individuals with above-average Vo(2mr) for their mass tended to have high VO2max, and vice versa). However, t here was no correlation between VO2mr and thermogenic VO2max. These re sults provide limited support for the aerobic capacity model for the e volution of endothermy.