Olecranon and trochlear notch orientation is related to extended limb postures during locomotion

Most anthropoids have anteriorly facing trochlear notches and proximally directed olecranons, as they utilize crouched postures during arboreal locomotion. African apes have posteriorly oriented olecranons and proximally angled trochlear notches, which have been argued to be suspensory and quadrupedal adaptations, respectively. However, baboons converge on this phenotype, which suggests a possible association with either body size or reliance on extended forelimb locomotor behaviors rather than forelimb suspension. To test the association with size and locomotor mode, we measured olecranon and trochlear notch angles and compared them to olecranon and distal ulna lengths, controlling for body size by use of both a geometric mean of limb joint skeletal dimensions and species body masses, in a broad sample of anthropoids. We found strong correlations between olecranon orientation and trochlear notch angle. Both had relatively strong negative correlations with olecranon size and weaker correlations with ulna length. This relationship appears to be largely driven by larger-bodied hominoids having more posteriorly oriented olecranons and proximally positioned trochlear notches than smaller-bodied monkeys. However, both gibbons and baboons have a more posterior olecranon and proximal trochlear notch relative to monkeys of similar size. The strongest correlations of olecranon and trochlear notch orientations, however, were to body size (both for the geometric mean and body mass). These results suggest that increasingly posterior olecranon orientation and proximal trochlear notch orientation may be associated with increased reliance upon extended forelimb postures, either during suspension or terrestrial quadrupedal locomotion.