Fig. 2. (A) Straight line velocity (µm s^–1 VSL) and (B) beat cross frequency (Hz; BCF) responses of boar spermatozoa to the addition of 15 mmol l^–1 bicarbonate/CO₂. Data are means (± s.e.m.) calculated from treatment means for eight individual boars. The points at 22 min represent the NaCl-control samples, which had not been exposed to bicarbonate/CO₂. ***Bicarbonate/CO₂ vs no bicarbonate/CO₂ planned contrasts; VSL effect (F_1,87=61.9), P<0.001 and BCF effect (F_1,87=49.9), P<0.001. Changes in mean frequencies (± s.e.m. from eight individual boars) of group 1 (fast and linear) spermatozoa plotted against the time elapsed after bicarbonate/CO₂ addition. Points at 22 min represent the NaCl-control samples. (C,D) The difference between the absence and presence of sAPM, respectively.