At speeds between the walk and the gallop, most mammals trot. Primates almost never trot, and it has been claimed that they transition directly from a walk to a gallop without any distinctive mid-speed running gait. If true, this would be another characteristic difference between the locomotion of primates and that of most other quadrupedal mammals. Presently, however, few data exist concerning the actual presence or absence of intermediate-speed gaits (i.e. gaits that are used between a walk and a gallop) in primates. Video records of running in twelve primate species reveal that, unlike most other mammals, all the primates studied almost exclusively adopt an 'amble'--an intermediate-speed running gait with no whole-body aerial phase--rather than trot. Ambling is also common in elephants and some horses, raising the question of why ambling is preferred over trotting in these diverse groups of animals. Mathematical analyses presented here show that ambling ensures continuous contact of the body with the substrate while dramatically reducing vertical oscillations of the center of mass. This may explain why ambling appears to be preferable to trotting for extremely large terrestrial mammals such as elephants and for arboreal mammals like primates that move on unstable branches. These findings allow us to better understand the mechanics of these unusual running gaits and shed new light on primate locomotor evolution.