Fig. 4. Simplified representation of mean age-related changes in body mass (top), whole-animal maximal oxygen consumption (middle) and mass-specific maximal oxygen consumption (bottom) in male (broken lines) and female (solid lines) deer mice. For maximal oxygen consumption, heavy lines indicate _O2max (exercise) and thin lines indicate _O2sum (cold exposure). Lines are derived from `breakpoint' (piecewise) regressions based on data in Fig. 2.