Figure 1

Duty Factor (DF) plotted against speed (U) for walking (red), grounded running (white) and aerial running (blue) gaits. (a) Includes data points for trials over all snow conditions and (b) includes data points for ptarmigan exclusively moving over a very hard snow surface only. Inlay figures in (a) represent the horizontal kinetic energy E_kh(J), and potential plus vertical, E_p + E_vk (J) energy plots of fluctuations of in the Centre of Mass (CoM) for each gait from one bird; walking (W), grounded running (G–R), aerial running (A–R). On the inlay figures the solid black line within each box are represent the kinetic energy, whereas the dashed black line represents the potential gravitational energy fluctuations. In the main figure the lines of best fit describing the linear regression for wild ptarmigans freely moving in the field are shown in black. The solid red lines represent the lines of best fit for the laboratory treadmill data. The vertical dashed line denotes the maximum sustainable speed from the treadmill data⁷. To the right of the vertical line at 2.0 ms⁻¹, the red line becomes dashed to denote projected speed beyond that sustainable in the laboratory. The horizontal dotted line represents the threshold duty factor of 0.5.