New perspectives on brachiation mechanics

This review is designed to evaluate and interpret studies relevant to the locomotory mode known as brachiation, particularly as performed by the Hylobatid apes: the gibbon and siamang species. The older literature and its conclusions are evaluated against recent work performed by the author and other research groups working on brachiation models, either computer simulations or physical robots. The gibbon displays two types of brachiation: continuous contact, analogous to walking, and ricochetal, analogous to running. Both brachiation gaits display substantial pendular exchange between kinetic and potential energy. However, the fundamental feature of either of these gaits is the minimization of collisional energy loss. Collisional energy loss due to discontinuities in the trajectory of the center of mass is emerging as key in understanding locomotion using limbs in any terrestrial environment. The insight gained from this perspective applied to gibbon locomotion demonstrates that this is a critical factor in understanding many of the maneuvers employed by these animals, and can provide novel new interpretations of the morphological specializations that characterize the group. It is observed that these animals could brachiate using either totally active (muscle powered) or totally passive (nonmuscular) mechanisms. The active option would be metabolically costly, but provides substantial motion plasticity, while the passive option has the potential for profound economy, but does not allow a means to effectively contend with the inconsistencies present in the animal's natural environment. The conclusion is that the body form of brachiators and the locomotion behaviors they exhibit are a compromise between these two extremes, and these features of the gibbon's biology can only be understood by recognizing the role of collisional energy loss and evaluating both passive and active motion options together.