Why Do Knuckle‐Walking African Apes Knuckle‐Walk?

Abstract

Among living mammals, only the African apes and some anteaters adopt knuckle-walking as their primary locomotor behavior. That Pan and Gorilla both knuckle-walk has been cited as evidence of their common ancestry and a primitive condition for a combined Homo, Pan, and Gorilla clade. Recent research on forelimb ontogeny and anatomy, in addition to recently described hominin fossils, indicate that knuckle-walking was independently acquired after divergence of the Pan and Gorilla lineages. Although the large-bodied, largely suspensory orangutan shares some aspects of the African ape bauplan, it does not regularly knuckle-walk when terrestrial. While many anatomical correlates of knuckle-walking have been identified, a functional explanation of this unusual locomotor pattern has yet to be proposed. Here, we argue that it was adopted by African apes as a means of ameliorating the consequences of repetitive impact loadings on the soft and hard tissues of the forelimb by employing isometric and/or eccentric contraction of antebrachial musculature during terrestrial locomotion. Evidence of this adaptation can be found in the differential size and fiber geometry of the forearm musculature, and differences in torso shape between the knuckle-walking and non-knuckle-walking apes (including humans). We also argue that some osteological features of the carpus and metacarpus that have been identified as adaptations to knuckle-walking are consequences of cartilage remodeling during ontogeny rather than traits limiting motion in the hand and wrist. An understanding of the functional basis of knuckle-walking provides an explanation of the locomotor parallelisms in modern Pan and Gorilla. Anat Rec, 301:496-514, 2018. © 2018 Wiley Periodicals, Inc.

Insights into the musculature of the bonobo hand

“…Additionally, the recruitment of wrist flexors as shock absorbers during knuckle‐walking (Simpson et al. ) may further reinforce the prominence of flexor muscles in the forearm of knuckle‐walkers. Another example that may reflect differences in locomotion is found in the proportion of intrinsic hand muscle PCSA.…”

The Functional Anatomy of the Carpometacarpal Complex in Anthropoids and Its Implications for the Evolution of the Hominoid Hand

“…Both would have substantially reduced the effective distance available for force dissipation, thereby requiring increased force to achieve the same dissipation of energy during eccentric contraction. These same authors argue that as the wrist and MCP joints enter into dorsiflexion during locomotion, muscles crossing these joints, such as wrist and long digital flexors, have become the principle means of performing protective negative work (Lovejoy et al, ; Simpson et al, ). Straus () noted that juvenile chimpanzees could walk with a palmigrade hand posture, and that their digital flexors became relatively shortened only after they began to KW.…”

The biomechanical importance of the scaphoid-centrale fusion during simulated knuckle-walking and its implications for human locomotor evolution

“…It has been argued that knuckle walking was adopted by the African apes as a way to ameliorate the consequences of repetitive impact loadings on both soft and hard tissues of the forelimb via isometric and/or eccentric contraction of the antebrachial muscles during terrestrial locomotion 36 . Our obtained results are consistent with this interpretation, since they show that the scaphoid-centrale fusion plays a role in reducing the experienced loads during knuckle-walking, which suggests that high loads due to quadrupedal locomotion were indeed a problem and a skeletal trait that contributes to better withstand the mechanical loads from knuckle-walking would therefore be adaptive.…”