SI dataset S1 - knuckle-walking biomechanical strategies

African apes engage in a distinct form of locomotion called
knuckle-walking, but there is much ambiguity as to when and how this
locomotor behaviour evolved. This study aims to elucidate potential
differences in knuckle-walking elbow posture and loading in African apes
through the study of trabecular bone. Using a whole-epiphysis approach, we
quantified variation in trabecular structure of the distal humerus of
chimpanzees, western lowland gorillas, and mountain gorillas in comparison
to orang-utans, siamangs and a sample of Old and New World monkeys.
Results demonstrate differences in the distribution of trabecular bone
within the distal humerus that are consistent across taxa that habitually
use a flexed-elbow posture in comparison to those that use an
extended-elbow during locomotion. Western lowland gorillas show an
extended-elbow pattern consistent with the straight forelimb position
during knuckle-walking, whereas chimpanzees show a flexed-elbow pattern.
Unexpectedly, mountain gorillas show an intermediate pattern between their
western counterparts and chimpanzees. The differences found in elbow joint
posture between chimpanzees and gorillas, and between gorilla species,
point to diversification in the knuckle-walking biomechanical strategies
among African apes, which has implications in the debate regarding the
locomotor behaviour from which human bipedalism arose.