Ds Gaits Fine Branches Versus Flat Surfaces

The inferiority of DS gaits lies chiefly in their narrow support polygons. But as Prost noted in 1969, this disadvantage disappears on a narrow support. As footfalls become collinear, the areas of all support polygons shrink and converge on the contact surfaces of the feet, no matter whether the gaits are DS or LS. On a narrow support like a branch or a pole, stability depends mainly on having the line of gravity fall somewhere between the contact point in front and the one in back. A DS gait can meet that criterion well enough if the support is narrow (Figure 3B).

But even if we accept that DS gaits lose most of their disadvantages on sufficiently narrow supports, it remains puzzling that even such highly terrestrial primates as chimpanzees and baboons retain DS gaits on the ground, rather than reverting to the LS walking gaits preferred by other terrestrial mammals. Hildebrand (1967) reported that many primates—lemurs, ceboids, cerco-pithecoids, and pongids—walking on the ground typically walk somewhat crabwise, especially in faster walks, with the trunk turned at an angle to the direction of travel and both hindlimbs overstriding the ipsilateral forelimbs on the side of the body that the trunk is turned away from. (This overstriding is evident in Figure 4.) Larson and Stern (1987) conjectured that chimpanzees do this to mitigate the risk, common to all diagonal-couplets gaits, of having the hindlimb hit the forelimb on the same side when stride length increases with speed. We conjecture further that this crabwise walking may also address a fundamental shortcoming of DS gaits, by increasing the sizes of the walking animal's support polygons. If the body is turned to one side while the limbs continue to swing back and forth in the direction of movement and stride length is increased, then the contact points of the ipsilateral feet (Figure 3B, no. 7, "unilateral bipod") will move apart from each other, and the areas of the triangles of support will be correspondingly increased (Figure 9). The size of the increase will depend on body proportions, the precise placement of the feet, and the magnitude of the angle between the body axis and the direction of movement (angle a in Figure 9). The extent to which primates walking on the ground actually do enlarge their support polygons in this way remains to be determined experimentally; but in principle, unilateral overstriding would be expected to improve the stability of DS walking gaits on flat surfaces.

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