Insights into the musculature of the bonobo hand

Abstract

The human hand is well known for its unique dexterity which is largely facilitated by a highly mobile, long and powerful thumb that enables both tool manufacturing and use, a key component of human evolution. The bonobo (Pan paniscus), the closest extant relative to modern humans together with the chimpanzee (Pan troglodytes), also possesses good manipulative capabilities but with a lower level of dexterity compared with modern humans. Despite the close phylogenetic relationship between bonobos and humans, detailed quantitative data of the bonobo forelimb musculature remains largely lacking. To understand how morphology may influence dexterity, we investigated the functional anatomy of the bonobo hand using a unique sample of eight bonobo cadavers, along with one chimpanzee and one human (Homo sapiens) cadaver. We performed detailed dissections of unembalmed specimens to collect quantitative datasets of the extrinsic and intrinsic hand musculature, in addition to qualitative descriptions of the forelimb muscle configurations, allowing estimation of force-generating capacities for each functional group. Furthermore, we used medical imaging to quantify the articular surface of the trapeziometacarpal joint to estimate the intra-articular pressure. Our results show that the force-generating capacity for most functional groups of the extrinsic and intrinsic hand muscles in bonobos is largely similar to that of humans, with differences in relative importance of the extensors and rotators. The bonobo thumb musculature has a lower force-generating capacity than observed in the human specimen, but the estimated maximal intra-articular pressure is higher in bonobos. Most importantly, bonobos show a higher degree of functional coupling between the muscles of the thumb, index and lateral fingers than observed in humans. It is conceivable that differentiation and individualization of the hand muscles rather than relative muscle development explain the higher level of dexterity of humans compared with that of bonobos.

Trabecular variation in the first metacarpal and manipulation in hominids

“…TMc and McP joint movement and loading is a complex product of both bony and soft tissue morphology (van Leeuwen, Vanhoof, Kerkhof, Stevens, & Vereecke, ) and compared to humans little is known of actual loads experienced by the nonhuman great ape thumb, during locomotion or manipulation (Samuel, Nauwelaerts, Stevens, & Kivell, ). However, qualitative observations of force, which was judged by how apparently resistant objects were to the grip applied, during food processing do exist for some species (Marzke, Marchant, McGrew, & Reece, ; Neufuss, Robbins, Baeumer, Hulme, & Kivell, ).…”

“…Bonobos have a similar relative thumb length (Almécija, Smaers, & Jungers, ) and a comparable kinematic workspace (Feix et al, ) to chimpanzees. This species has well‐developed thenar musculature that can exert high pressures at the TMc joint and includes a tendon of the flexor digitorum profundus that flexes the distal phalanx (van Leeuwen, Vanhoof, et al, ), a trait that is weakly expressed or absent in chimpanzees (Susman, ; Tuttle, ). However, the bonobo thumb is not capable of the same level of force as is the human thumb (van Leeuwen, Vanhoof, et al, ).…”

“…This species has well‐developed thenar musculature that can exert high pressures at the TMc joint and includes a tendon of the flexor digitorum profundus that flexes the distal phalanx (van Leeuwen, Vanhoof, et al, ), a trait that is weakly expressed or absent in chimpanzees (Susman, ; Tuttle, ). However, the bonobo thumb is not capable of the same level of force as is the human thumb (van Leeuwen, Vanhoof, et al, ). Further, this species demonstrates a fusion of thumb and index finger musculature that may limit complex precision grips that require independent movement of these digits (van Leeuwen, Vanhoof, et al, ).…”

“…However, the bonobo thumb is not capable of the same level of force as is the human thumb (van Leeuwen, Vanhoof, et al, ). Further, this species demonstrates a fusion of thumb and index finger musculature that may limit complex precision grips that require independent movement of these digits (van Leeuwen, Vanhoof, et al, ). The shape of the bonobo TMc joint is similar to that of humans; however, unlike humans, strong volar ligaments at this joint in bonobos restrict the extension at the TMc to just 30° (van Leeuwen et al, ).…”

The Forearm Musculature of the Gray Mouse Lemur (Microcebus murinus): An Ontogenetic Study

“…Muscle mass and fascicle length data were combined to calculate the PCSA of each muscle, using the following formula modified from Schumacher (): in which a constant of 1.0564 g/cm 3 was used to represent muscle density (following Murphy and Beardsley, ). As recent publications on the forearm musculature of primates have demonstrated the negligible impact of pennation angle on muscle force (van Leeuwen et al ., ; Boettcher et al ., ), a conversion into RPCSA (as per Anapol and Barry, ) was not used.…”