Chappell MA, Dlugosz EM (2009). Aerobic capacity and running performance across a 1.6 km altitude difference in two sciurid rodents. In press, Journal of Experimental Biology

Hypoxia at high altitudes is often assumed to constrain exercise capacity, but there have been few high- versus low-altitude comparisons of species native to a wide range of altitudes.   Such studies are ecologically realistic, as wild-caught animals tested at their native altitude are presumably maximally acclimated (via phenotypic plasticity) or adapted (by evolutionary change) to that altitude.   We compared aerobic performance, measured as maximum oxygen consumption in forced exercise (VO₂max), and voluntary wheel-running in two species of sciurid rodents captured and tested at field sites that differed in altitude by 1.6 km (2165 m versus 3800 m).   We found reduced VO₂max at 3800 m in least chipmunks (Tamias minimus), but no significant effect of altitude on VO₂max in golden-mantled ground squirrels (Spermophilus lateralis).   Individuals of both species averaged several km/day in wheels.   Most behavioral indices of voluntary running (including mean and maximum speeds, time spent running, daily running distance, and the number and duration of running bouts,) were unaffected by altitude, even in the species with reduced VO₂max at high altitude.   Metabolic rates during running and energy costs of transport differed to some extent across altitudes, but in different ways in the two species.   At both test sites, voluntary running by both species was almost exclusively at speeds well within aerobic limits.   We conclude that substantial differences in altitude do not necessarily result in differences in aerobic capacity in small mammals, and even if VO₂max is reduced at high altitude, there may be no effect on voluntary running behavior.