1Department of Biological Sciences, Dartmouth College, 2Department of Anthropology, Dartmouth College, 3Department of Human Evolutionary Biology, Harvard University
Saturday 10:15-10:30, Galleria South
Honey is an important resource for hunter-gatherers in both rainforest and savanna habitats. It is a high-calorie food and commodity, but it can be dangerous to acquire, often necessitating extraordinary tree-climbing abilities. Yet the mechanics and energetic costs of human tree climbing have received little attention. Here we use portable respirometry (Oxycon Mobile) to evaluate the energetic costs of tree climbing by Twa (pygmy) hunter-gatherers living in southwest Uganda. We found that the energetic cost of vertical climbing was sevenfold higher than resting and threefold greater than walking through dense undergrowth. To explore how the high cost of climbing might be mitigated anatomically, we examined ankle joint kinematics during climbing and used portable ultrasound (MicroMaxx) to estimate the physiological cross-sectional area (PCSA) of the gastrocnemius muscle. We found an average ankle dorsiflexion of 36.4 (+/-2.18) degrees. The maximum observed values (>46 degrees) exceed the limits at which soft tissue failure is thought to occur. Such extreme dorsiflexion appears to enable a climber to position his mass closer to a tree and thus reduce his energy expenditure. We also found that the relative PCSA of the gastrocnemius was significantly lower (p<.05) among the Twa than their Bantu neighbors, indicating a force-limited but more excursive muscle. Taken together, these findings can be interpreted as energy-saving adaptations. A lower PCSA is expected to reduce metabolic costs as well as the mechanical costs of extreme dorsiflexion. Our results implicate tree climbing as a selective pressure for some derived aspects of the human pygmy phenotype.
VVV and AJC are supported by NSF Graduate Research Fellowships.