Manual function and performance in humans, gorillas, and orangutans during the same tool use task

Abstract

This study highlights other ways in which humans' manual dexterity differs from that of other species and emphasizes the distinct manipulative function of orangutans. The differences between the species could be due to the differing muscular anatomy and morphology of the hands, with hand proportion possibly placing particular biomechanical constraints on each species. The differences between gorillas and orangutans could result from their different locomotor behaviors, and we hypothesize terrestriality facilitates the development of complex manipulation.

Trabecular variation in the first metacarpal and manipulation in hominids

“…The latter grip is reported as the most frequent and the authors suggest this may due to the fact that gorilla thumb may not be able to resist the seemingly forceful grips observed, in tip‐to‐tip position (Neufuss et al, ). However, Neufuss et al () have emphasized the great variety of grips and thumb positions used by mountain gorillas in food processing and while they did not observe precise in‐hand manipulation (sensu Landsmeer, ) in this community, it has been reported in others (Bardo et al, ; Byrne et al, ). Gorillas have also been observed engaging in tool‐use behaviors (Breuer, Ndoundou‐Hockemba, & Fishlock, ; Kinani & Zimmerman, ).…”

“…Human distal phalanges are capable of passive hyperextension as our deep flexor tendons are long compared to those of other great apes (Preuschoft, ; Tuttle, ). This movement permits full pad‐to‐pad precision grips (Napier, ) frequently used by humans to forcefully manipulate small objects, especially within the hand (Bardo, Cornette, Borel, & Pouydebat, ; Christel, ; Key et al, ; Marzke & Wullstein, ). Humans are unique among hominids in possessing an extensor pollicis brevis muscle, which is well developed and stabilizes the extended McP joint while the first interphalangeal joint is forcefully flexed, as well as abducting the thumb (Diogo, Richmond, & Wood, ; Marzke et al, ).…”

“…In captivity, orangutans do recruit the thumb in pad‐to‐side precision grips during manipulative tasks (Bardo et al, ; Christel, ). However, they far more frequently use a power‐grip, especially for larger objects (Pouydebat, Gorce, Coppens, & Bels, ), or a “V‐pocket” grip (Marzke et al, ), in which the object is held in the webbing between the full thumb and index finger (Bardo et al, ). In both grips, the orangutan thumb may provide support but is unlikely to be strongly recruited, as its relative length would make articulation of the first distal phalanx with all but the largest objects in a power hook grip challenging.…”

“…Further, this distal phalanx is difficult to articulate with an object already held by the V‐pocket at the base of this digit. Orangutans frequently reposition tools with their mouths rather than with their hand (Bardo et al, ; Christel, ). In the wild, orangutans have not yet been observed using precision grips, even during tool production and use of tools (e.g., Fox, Sitompul, & Van Schaik, ; van Schaik, Fox, & Sitompul, ).…”

“…Captive gorillas can perform “tip‐to‐tip” precision grips (Christel, ; Pouydebat, Laurin, Gorce, & Bels, ), although they also often use power grips and “interdigital brace” grips during manipulative tasks (Bardo et al, ). The latter grip threads an object between the adducted thumb and index finger as well as the palmar or dorsal aspects of the ulnar digits (Bardo et al, ; Lesnik, Sanz, & Morgan, ). Wild mountain gorillas most frequently employ precision grips that adduct the thumb during food processing, including interdigital brace, thumb wrap, and pad‐to‐side grips (Neufuss et al ()).…”