Fig. 4. Vertical displacement of the center of mass across a range of different diagonality and duty factor combinations, computed from a mathematical model of the vertical component of the force exerted by the foot on the ground (McNeill Alexander and Jayes, 1978). Vertical-displacement data are presented for diagonalities of 50-75% and duty factors of 50-25%. Empirical data (black dots) are plotted at the actual diagonality: duty factor combinations used by the primates in our sample. Ambling results in smaller vertical oscillations of the center of mass because the individual limb forces are more uniformly distributed throughout the stride, which reduces the peak force acting on the body.