Fig. 3. Mean times (± S.E.M.) spent gaping in lizards at three different levels of oxygen – 21% (filled circles), 10% (open circles) and 6% O₂ (filled triangles) – during 15 min periods of observations of lizards that were in thermal equilibrium with different environmental temperatures (30–40°C). The broken lines represent Hill equations (see equation 1) fitted through the mean data. The vertical dotted lines represent the mean time at which lizards spent 50% of their time gaping (ET₅₀) at each level of oxygen inspired, as calculated from each lizard. The inset graph refers to the ET₅₀ values for male (filled circles) and female lizards (open circles) at 21, 10 and 6% O₂.