Hand pressures during arboreal locomotion in captive bonobos (Pan paniscus)

Abstract

Evolution of the human hand has undergone a transition from use during locomotion to use primarily for manipulation. Previous comparative morphological and biomechanical studies have focused on potential changes in manipulative abilities during human hand evolution, but few have focused on functional signals for arboreal locomotion. Here, we provide this comparative context though the first analysis of hand loading in captive bonobos during arboreal locomotion. We quantify pressure experienced by the fingers, palm and thumb in bonobos during vertical locomotion, suspension and arboreal knuckle-walking. Results show that pressure experienced by the fingers is significantly higher during knuckle-walking compared with similar pressures experienced by the fingers and palm during suspensory and vertical locomotion. Peak pressure is most often experienced at or around the third digit in all locomotor modes. Pressure quantified for the thumb is either very low or absent, despite the thumb making contact with the substrate during all suspensory and vertical locomotor trials. Unlike chimpanzees, the bonobos do not show a rolling pattern of digit contact with the substrate during arboreal knuckle-walking but, instead, digits 3 and 4 typically touch down first and digit 5 almost always made contact with the substrate. These results have implications for interpreting extant and fossilised hand morphology; we expect bonobo (and chimpanzee) bony morphology to primarily reflect the biomechanical loading of knuckle-walking, while functional signals for arboreal locomotion in fossil hominins are most likely to appear in the fingers, particularly digit 3, and least likely to appear in the morphology of the thumb.

Trabecular variation in the first metacarpal and manipulation in hominids

“…The data support our predictions concerning subarticular RBV/TV across hominid Mc1 heads ( H1a and c ) but only partially support those relating to DA ( H1 b and c ). Average RBV/TV was higher at ulno‐distal landmarks in the Mc1 head across all nonhuman great apes, consistent with a habitually adducted thumb used in pad‐to‐side or V‐pocket grips (Bardo et al, , ; Marzke et al, ; Neufuss et al, , ) and some power grasps used in African ape locomotion ( H1a ; Neufuss et al, , Samuel et al, ). Contrary to our hypothesis, however, DA was lowest at the ulno‐distal landmarks, where RBV/TV was highest, in chimpanzees, bonobos, and orangutans ( H1a ).…”

“…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, ).…”

“…However, they are argued to be more arboreal than chimpanzees (Alison & Badrian, ; Crompton, Sellers, & Thorpe, ; Susman & Badrian, ) and engage in arboreal palmigrady more frequently (Doran, ). While the thumb is frequently observed in use during vertical climbing and suspension, it may not be meaningfully loaded (Samuel et al, ). While the thumb may be recruited in palmigrady, data on this are lacking and so it is possible the thumb is most frequently loaded during manipulative behavior in this species.…”

Metacarpal trabecular bone varies with distinct hand‐positions used in hominid locomotion

“…) and captive settings (Wunderlich & Jungers, ; Matarazzo, ,b; Samuel et al. ) can inform predictions of frequent McP joint positions and loading across the hand in different species. Although frequent McP joint postures may only reflect part of a large and varied locomotor repertoire, previous research suggests (Tsegai et al.…”

“…In a pressure study of arboreal locomotion, Samuel et al. () found that captive bonobos used ‘palm‐back’ (64%) or ‘palm‐in’ (36%) knuckle‐walking hand postures and that peak pressure was experienced by or around the third digit. However, unlike chimpanzees (Wunderlich & Jungers, ), they did not roll radially across their digits and the fifth digit always made contact with the substrate (Samuel et al.…”