The enigma of the ulna in the primate wrist

The primate wrist is a highly complex structure which is important both in manipulation and locomotion. In both contexts, it must be able to provide the required stability to withstand loading, e.g. during power grips or quadrupedal locomotion, while at the same time allowing mobility to position the hand in the three-dimensional space. Part of the complexity comes from the fact that the wrist is not a single joint, but rather a combination of different articulations that together constitute the overall mobility of the hand at the wrist. The generalized mammalian condition is a wrist joint with an articulation between the distal ulna and the midcarpal bones. However, in the hominoid lineage (e.g. bonobos and humans) there is a reduction of the distal ulna (along with other changes in morphology) leading to a reduced or absent ulnocarpal articulation. This might be linked to climbing and suspensory behaviours and allow a greater wrist mobility, particularly ulnar deviation and pro/supination. In addition, such increased wrist mobility might allow the power grips needed by humans when using tools, throwing and clubbing.
The relation between distal ulnar morphology and wrist mobility and stability in extant non-human primates, however, remains puzzling. While a reduced distal ulna has been linked to increased wrist mobility, some taxa (e.g. Macaques) with a fully elaborated ulnocarpal articulation have a greater ulnar deviation than those without (e.g. great apes and humans). In this PhD project, we specifically want to investigate the interrelationship between distal ulnar and carpal morphology and wrist mobility, with a focus on radioulnar deviation and pro/supination, in primate taxa with a different locomotor behavior and distinct phylogenetic position relative to modern humans.