Climbing Performance and Ecology in Humans, Chimpanzees, and Gorillas

This dissertation aims to establish the effects of limb proportions and body size on the climbing performance of humans, chimpanzees, and gorillas by assessing three aspects of climbing performance: 1) energetic cost, 2) fatigue, and 3) canopy access. Whether hominins were arboreal, and to what extent, is a matter of hot debate. Specifically, the relative prominence of vertical climbing in the locomotor repertoires of various hominin taxa remains a contested issue. Over the course of human evolution, both the body size and relative hindlimb length of hominins has increased. These traits are often linked to bipedality. Long forelimbs, in contrast, are hypothesized to be advantageous in vertical climbing. Apart from these statements, we know little about how size and body proportions specifically affect climbing performance. In the first study, I evaluated factors influencing the metabolic cost of locomotion (COLnet) during climbing. I assessed the effects of speed, route difficulty, experience, and anatomical variation of COLnet in rock climbers. Further, I compared the COLnet for these human climbers to that of non-human primates. I found that: 1) climbing speed is a significant predictor of climbing COLnet; 2) route difficulty is not significantly correlated to locomotor costs when controlling for speed; and 3) mass-specific locomotor costs are independent of body mass and limb proportions, both within and across species. From these results I concluded that neither changes in body mass nor limb proportions during hominin evolution are likely to have led to changes on the energetic aspect of climbing performance. In the second study, I examined the relationship between morphology, climbing grip strength (maximum voluntary contraction, MVC, of the finger flexors) and endurance (time one can maintain a grip strength greater or equal to 70% MVC) in human climbers. I showed that grip strength is negatively allometric to the function of body mass in rock climbers. Next, I tested whether strength limits tree climbing during foraging in Mbendjele foragers from the Republic of Congo. Mbendjele men’s grip strength did not significantly decrease after tree climbing. These results indicated that human foragers can accrue daily tree-climbing rates comparable to other African apes without approaching their limits of finger flexor strength or endurance. Finally, I tested whether and how humans and other apes differ in arboreal ecology, by collecting a comparative data set of positional behavior in trees of humans and sympatric chimpanzees and gorillas in northern Republic of Congo. I found that larger apes move more vertically within the core of trees and less horizontally into the periphery than smaller-bodied apes. However, despite their comparatively smaller body size, Mbendjele men spend significantly more time in the core than in the periphery of trees. In addition, humans spend significantly more time standing in trees bipedally than apes do. These results suggest that humans use trees differently than apes but are nonetheless adept in an arboreal environment and capable of foraging in the canopy. In this dissertation I combined the first investigation of sympatric human foragers and wild apes’ locomotor behavior with experimental laboratory studies. I detected no effect of limb proportions on energetic cost, fatigue, or canopy access. This suggests that fossil hominin and hominoids may have been less restricted in their locomotor repertoires than previous reconstructions predicted. I further show that modern humans are adept climbers, using a range of behaviors which allow them to access the canopy as effectively as chimpanzees and gorillas, despite humans’ anatomical specializations for terrestrial bipedalism