Locomotor diversity has meant many different things depending on the subject area or investigator. While no concrete definition of locomotor diversity is currently available, this has not stopped researchers from making a number of assertions about the underlying mechanisms that determine whether a species will have high or low locomotor diversity. Some of these claims include that: (1) arboreal primates demonstrate higher locomotor diversity than terrestrial species; (2) anatomically generalized and small‐bodied primates demonstrate higher locomotor diversity than anatomically specialized and large‐bodied taxa; and (3) primates demonstrate higher locomotor diversity compared to non‐primate mammals. None of these claims have been tested in any sort of comparative framework. Using previously published locomotor repertoire data from 110 mammalian species, this study co‐opted the Shannon–Wiener diversity index to calculate a singular measure of locomotor diversity. Phylogenetic analyses of these locomotor diversity indices reveal that within primates, small‐bodied species demonstrate greater locomotor diversity than large‐bodied taxa. However, this effect becomes non‐significant when accounting for differences in substrate‐use (i.e. arboreal vs. terrestrial). Anatomical specialization has no effect on locomotor diversity in primates. Furthermore, primates do not inherently have greater locomotor diversity than other mammalian taxa. Findings from this study suggest that movement on arboreal substrates requires all mammals, regardless of taxonomic affiliation, to demonstrate high locomotor diversity. These data further highlight the challenges faced by arboreal animals and raise the possibility that being able to switch fluidly and frequently between many different locomotor modes may be advantageous for animals moving in an arboreal milieu.

During quadrupedal walking in most animals, the forelimbs play a net braking role while the hindlimbs are net propulsive. However, the mechanism by which this differentiation occurs remains unclear. Here we test two models to explain this pattern using primates and felines: (1) the Horizontal Strut Effect (in which limbs are modeled as independent struts), and (2) the Linked Strut Model (in which limbs are modeled as linked struts with a center of mass in between). Video-recordings were used to determine point of contact, mid-stance, and limb protraction/retraction duration. Single limb forces were used to calculate contact time, impulses, and the proportion of the stride at which the braking to propulsive transition (BP) occurred for each limb. We found no association of the occurrence of the BP and mid-stance, little influence of protraction and retraction duration on the braking-propulsive function of a limb, and a causative relationship between vertical force distribution between limbs and the patterns of horizontal forces. These findings reject the Horizontal Strut Effect, and provide some support for the Linked Strut Model, although predictions were not perfectly matched. We suggest that the position of the center of mass relative to limb contact points is a very important but not the only factor driving functional differentiation of the braking/propulsive roles of the limbs in quadrupeds. It was also found that primates have greater differences in horizontal impulse between their limbs compared to felines, a pattern that may reflect a fundamental arboreal adaptation in primates.

Speed-related gait transitions occur in many animals, but it remains unclear what factors trigger gait changes. While the most widely accepted function of gait transitions is that they reduce locomotor costs, there is no obvious metabolic trigger signalling animals when to switch gaits. An alternative approach suggests that gait transitions serve to reduce locomotor instability. While there is evidence supporting this in humans, similar research has not been conducted in other species. This study explores energetics and stride variability during the walk–run transition in mammals and birds. Across nine species, energy savings do not predict the occurrence of a gait transition. Instead, our findings suggest that animals trigger gait transitions to maintain high locomotor rhythmicity and reduce unstable states. Metabolic efficiency is an important benefit of gait transitions, but the reduction in dynamic instability may be the proximate trigger determining when those transitions occur.

During quadrupedal walking, most primates utilize diagonal sequence diagonal couplet gaits, large limb excursions, and hindlimb-biased limb-loading. These gait characteristics are thought to be basal to the Order, but the selective pressure underlying these gait changes remains unknown. Some researchers have examined these characteristics during vertical climbing and propose that primate quadrupedal gait characteristics may have arisen due to the mechanical challenges of moving on vertical supports. Unfortunately, these studies are usually limited in scope and do not account for varying strategies based on body size or phylogeny. Here, we test the hypothesis that the spatiotemporal gait characteristics that are used during horizontal walking in primates are also present during vertical climbing irrespective of body size and phylogeny. We examined footfall patterns, diagonality, speed, and stride length in eight species of primates across a range of body masses. We found that during vertical climbing primates slow down, keep more limbs in contact with the substrate at any one time, and increase the frequency of lateral sequence gaits compared to horizontal walking. Taken together these characteristics are assumed to increase stability during locomotion. Phylogenetic relatedness and body size differences have little influence on locomotor patterns observed across species. These data reject the idea that the suite of spatiotemporal gait features observed in primates during horizontal walking are in some way evolutionarily linked to selective pressures associated with mechanical requirements of vertical climbing. These results also highlight the importance of behavioral flexibility for negotiating the challenges of locomotion in an arboreal environment.