Figure 1.

Published duty factors of avian bipeds (a) and quadruped forelimbs (b) relating to Froude number. Larger animals (above 10 kg in red) have lower duty factors than smaller animals (below 10 kg in blue) at equivalent Froude numbers. Regressions shown as bold lines. Quadruped regressions with 95% CIs (bounding thin curves) are for eight large species and three small. Mice are assumed to have leg lengths of between 2 and 3 cm (resulting in uncertainty in Froude number, denoted by the horizontal lines). See text for definition and discussion of Froude number.