Fig. 2. Relationship between metabolic rate (_O2, ml min^-1) and speed (m min^-1) for a single male hopping mouse (body mass=31.5 g). The break-point of the regression relating _O2 and speed is equal to maximum aerobic speed (U_max, m min^-1). At slower speeds, the slope of the line relating _O2 and speed is equal to the net O₂ cost of transport (ml O₂ m^-1). Average _O2 at speeds greater than U_max is equal to maximum metabolic rate (_O2max, ml min^-1). N=12. Below 21.8 m min^-1, _O2=1.80+0.100U, where U is speed; above 21.8 m min^-1, _O2=3.98.