Ulna Curvature in Arboreal and Terrestrial Primates

In terrestrial quadrupeds where the forelimbs are habitually used to push against the substrate, the ulna is subjected to cranial bending and develops a caudal curvature. During arboreal locomotion, the forelimbs are habitually used to pull on the substrate during climbing and clinging, and the ulna responds to this bending load by developing a cranial curvature. This phenomenon is explored in a sample of ninety-four primate species that were each assigned to one of seven locomotor categories that characterize their forelimb use. The ulnae were photographed from the medial aspect, and nine landmarks and twelve semilandmarks were digitized using tpsDig. Measures of curvature, robusticity, and relative olecranon length were derived from these landmarks. There is a strong association between ulna curvature and locomotion, with arboreal species having cranially curved ulnae and more terrestrial species having caudally curved ulnae. This association remains true after accounting for phylogenetic relatedness, length of the ulna, and variation in body mass. The ulnae of brachiating species have less curvature than expected, which may relate to the fact that the bone is subjected to tensile loading. Large-bodied arboreal and terrestrial species tend to have more slender ulnae compared to smaller species, regardless of locomotor mode or phylogeny. This finding implies that curvature serves as an effective mechanism for mitigating habitual loading on the ulna in large-bodied species, thus allowing osseous mass to be minimized. In smaller primates, an unpredictable loading environment means that extra bone mass must be employed to manage bone stiffness.