Fig. 3. Cross-correlation time shift between walking speed and CO₂ release, due to both wash-out of the respirometric chamber and possible behavioural delays in gas exchange during breathing. (A) Histogram of cross-correlation time shifts calculated from 46 animals. Mean temporal shift between speed and metabolic rate traces was 2.4 s according to the Gaussian fit curve (black line). Values below 1.2 s shift were disregarded because they were mostly produced by small chance fluctuations. (B) Sample trace illustrating the shift of the CO₂ release trace (red, Doppler-corrected trace) by the time delay (t, shaded area) obtained with the cross-correlation method (blue). The time correction results in approximately simultaneous fluctuations of CO₂ release and running speed.