Fig. 2. (A) Maximum aerobic capacity (_O2max), (B) carbon dioxide production, (C) respiratory exchange ratio at _O2max (RER=_CO2/_O2max) and (D) maximum running speeds, measured in S and C mice (N=35 and 24, respectively) during forced exercise on a treadmill with a hypoxic (14% O₂), normoxic (21% O₂) or hyperoxic atmosphere (30% O₂). Values are shown as adjusted means ± s.e.m. Statistical differences between hypoxic, normoxic and hyperoxic trials are shown in lowercase letters; effects of selection history for each trait at a given P_O2 are shown in Table 2; overall effects of selection on the entire dataset (i.e. across P_O2) are listed in Table 4.