Gorilla knuckle-walking mechanics suggest independent evolution in African great apes

Gorillas and chimpanzees both use knuckle-walking (KW). Whether this behavior is a homologous trait inherited from their last common ancestor or the result of convergent evolution remains debated. This study investigated the biomechanics of gorilla KW by measuring ground reaction forces (GRFs) and comparing them with previously reported chimpanzee data. GRFs were recorded using two in-series force plates embedded in a horizontal beam in the outdoor enclosure of Kyoto City Zoo. Three adult gorillas voluntarily performed KW during normal activity, and 90 steady-state trials were analyzed. The results revealed interspecific differences: chimpanzees exhibited greater peak vertical GRFs in the hindlimbs, approximately 1.5 times those in the forelimbs, suggesting greater hindlimb weight support. In contrast, gorillas showed comparable peak GRFs in both limbs, with slightly higher values in the hindlimbs, indicating a more balanced weight distribution. Moreover, two individuals showed out-of-phase fluctuations in gravitational and kinetic energy of the center of mass, indicating an inverted pendulum mechanism and relatively efficient energy recovery. The distinct GRF patterns and apparent mechanical efficiency suggest that gorilla KW differs biomechanically from that of chimpanzees. These findings appear to be consistent with the hypothesis that KW in gorillas and chimpanzees evolved independently, which may suggest convergent evolution rather than shared ancestry.