Fig. 2. CO₂ release during locomotor activity in a single ant. (A) The measured amounts of CO₂ (black line) are confounded by the Doppler effect, due to the ant's movement in the air current of the respirometric chamber. Doppler time shifts were corrected according to the momentary position of the ant in the chamber (red line; details in Materials and methods). The red line is also shifted by a fixed offset, compensating for a small constant delay produced by the tubing which connected respirometric chamber and gas analyser. (B) Running speed of the ant, as calculated from the position (video) recording above. Shaded areas indicate that the ant was running against the direction of air flow during flow-through respirometry. Asterisks mark turns of the animal at the ends of the respirometric chamber. The broken line indicates the maximum running speed considered for linear regression in Fig. 4. (C) To correct time delays produced by chamber washout, and possible behavioural delays in CO₂ release, we estimated the time shift between the CO₂ release and the speed traces using cross-correlation. The cross-correlation shift was calculated in a sliding data window (75 data points, or 15 s), and tested for temporal shifts of up to 10 s between the two traces. Mean value for cross correlation is 2.4 s (broken line); see Fig. 3A.