Fig. 3. The relationship between voluntary running speed and oxygen consumption (_O2) in a 21.5 g deer mouse tested at an ambient temperature of 25°C. This female ran 18.9 km during the 24 h measurement period. Data points are 60 s averages separated from other points by at least 3 min to avoid autocorrelation biases (Fig. 2). Data for speeds less than 0.034 km h^-1 (0.5 r.p.m.) were not used (see text). The broken line is equal to this animal's maximal _O2 during forced treadmill exercise (4.304 ml min^-1) and the estimated maximum aerobic speed is 4.8 km h^-1.