Fig. 4. Forced and voluntary running energetics in Mongolian gerbils (pooled data from 40 individuals). The open circles and broken regression line show values from forced treadmill exercise, with each point representing one individual at one speed. The solid circles and solid regression line show values from voluntary exercise. Regression coefficients and r² values are shown as averages of individual means (bottom) and from pooled data (top). Points from voluntary exercise are 1 min averages separated by 3 min intervals to avoid autocorrelation (see text). For comparison, the maximum power output in forced exercise (i.e. _O2max) is about 196 kJ kg^–1 h^–1.