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.

Medtool 4.2 (www.dr-pahr.at) was used to quantify trabecular bone throughout the distal epiphysis of the humerus using a Holistic Morphometric Analysis (HMA) following the methods described in Gross et al. (2014) and Tsegai et al. (2013). Briefly, cortical bone is separated from the trabeculae (SI Fig. S2) by casting rays at different angles from the outer cortical shell and terminating them on contact with background voxels. The inner structure is then closed with a spherical kernel the size of the average trabecular thickness in that bone (Pahr and Zysset, 2009), and the 3D edge of this inner structure defines the boundary between subchondral trabecular and cortical bone (Gross et al., 2014). Bone volume fraction (BV/TV), quantified as a ratio of bone volume to total volume, and degree of anisotropy (DA), quantified following the mean intercept length (MIL) method and bounded between 0 (isotropy) and 1 (anisotropy), were measured throughout the distal epiphysis using a sampling sphere with a 5-mm diameter on a 2.5- mm background grid.

The data used in this article are vector (categorical variables) and numerical (measures) values. There are no missing values. There is a comprehensive README xls file with detaild explanations on the data collected and calculated used as the basis of the article to be downloaded with the dataset.