Fig. 4. Example trace of a typical burrowing trial (female mouse, 36.1 g, excurrent airflow rate=1.6 l min^-1). The mouse was placed in the chamber at the point indicated on the graph. It then explored the chamber and scratched at the soil for 3 min, rested for 1 min and burrowed for 35 min. The washout after the mouse was placed in the chamber suggests an equilibration time of approximately 5 min. F_O2, fractional oxygen concentration.