Gait mechanics of lemurid primates on terrestrial and arboreal substrates

Aspects of gait mechanics of two lemurid species were explored experimentally. Substrate reaction forces were recorded for three animals each of L. catta and E. fulvus walking and running at voluntary speeds either on a wooden runway with an integrated force platform or on elevated pole supports with a section attached to the force platform. The average height of the back over these substrates and fluctuations in this height were evaluated using video-analysis. Animals preferred walking gaits and lower speeds on the poles, and gallops and higher speeds on the ground. At overlapping speeds, few adjustments to substrate types were identified. Hind limb peak forces are usually lower on the poles than on the ground, and the caudal back is closer to the substrate. This suggests that greater hind limb flexion and reduced limb stiffness occurred on the poles. The support phases for both limbs at higher speeds are slightly elongated on the poles. Forelimb peak forces are not lower, and the trajectory of the caudal back does not follow a smoother path, i.e., not all elements of a compliant gait are present on the simulated arboreal substrates. The horizontal, rigid poles, offered as substitutes for branchlike supports in the natural habitat, may not pose enough of a challenge to require more substantial gait adjustments. Across substrates, forelimb peak forces are generally lower than hind limb peak forces. The interlimb force distribution is similar to that of most other primates with more even limb lengths. Walking gaits present a greater divergence in fore- and hind limb forces than galloping gaits, which are associated with higher forces. The more arboreal E. fulvus has higher forelimb forces than the more terrestrial L. catta, unlike some anthropoid species in which the arborealists have lower forelimb forces than the terrestrialists. As in other primate and nonprimate quadrupeds, the major propulsive thrust comes from the hind limbs in both lemurs. While our data confirm certain aspects of primate gait mechanics (e.g., generally higher hind limb forces), they do not fully support the notion of greater limb compliance. Neither a compliant forelimb on branchlike supports, nor a negative correlation of forelimb force magnitudes with degree of arboreality were observed. Increasing forelimb-to-hind-limb-force-ratios with increasing speed and force magnitudes are also not expected under this paradigm.