The stability-mobility conflict in the primate thumb

Primates use their thumb both in manipulation and locomotion leading to conflicting mechanical demands. High thumb mobility is required for manipulative skills while stability and strength are important in locomotion. In this study, we want to investigate how the anatomy of the primate thumb is adapted to this stability-mobility conflict. We focus on two highly dexterous catarrhines, the bonobo (Pan paniscus) and the olive baboon (Papio anubis), with a distinct locomotor, postural and prehensile behavior leading to a different thumb use and load. We obtained fresh-frozen cadaveric hand and forearm specimens via collaboration with the Royal Zoological Society of Antwerp (RZSA), Belgium (5 bonobos; Bonobo Morphology Initiative) and the CNRS, France (3 baboons). A detailed dissection was performed of each specimen with quantification of soft-tissue parameters (e.g. muscle mass and length, fascicle length, ligament dimensions). Each specimen was CT scanned and 3D surface models were created for the trapezium and first metacarpal (MC1) using Mimics software to assess the geometry of the trapeziometacarpal (TMC) joint. Bonobos and olive baboons have a fully opposable thumb, which is reflected in the well-developed thumb musculature. Bonobos have a saddle-shaped TMC joint allowing a wide range of motion, while the prominent volar beak and high joint curvature provide stability. In addition, five ligaments surround the TMC joint, acting as passive stabilizers. We believe that this anatomical configuration offers the required stability for forceful gripping during climbing and suspensory locomotion. Thumb loading is relatively low in baboons, being restricted to occasional climbing and palmigrady. Despite the cylindrical-shaped TMC joint, opposability is maintained by the relatively long length of the thumb. We want to thank Drs. Nauwelaerts, Stevens, Pereboom (RZSA), and Berillon (CNRS) for giving us access to the primate specimens.