Fig. 4. Number of shuttles at different oxygen concentrations. Plotted values are mean values of 12 lizards ±s.d. during a 7.5 h period inside an electronically controlled temperature-choice shuttle box. ^*Significant difference vs 21% oxygen conditions. For comparison, the filled circles show the average number of times a lizard shuttled during the 7.5 h in the absence of a thermal stimulus (constant 34.5°C at 21% O₂) (Cadena and Tattersall, 2009). The open square denotes the average number of shuttles (error bars not shown; see text for details) in the extreme temperatures shuttle box trials at 21% and 4% O₂ (offset slightly for clarity).