1Department of Human Evolutionary Biology, Harvard University, 2Department of Anthropology, University of California Santa Barbara
April 14, 2018 3:45, Foothills Ballroom II
Convergent evolution of the human pygmy phenotype in tropical rainforests likely reflects adaptation, yet evidence for this phenotype’s selective benefits remains lacking. Here we propose and test a model of how a rainforest environment can alter gait kinematics such that short stature becomes energetically advantageous. Specifically, we hypothesize that constraints on step length imposed by dense vegetation result in a stature-dependent tradeoff between walking speed and economy. According to this model, in a dense habitat taller individuals are expected to walk more slowly or less economically due to their inability to achieve preferred step lengths. We tested predictions from this model with experimental field data from two relatively short-statured populations that regularly forage in the rainforest: the Batek of Peninsular Malaysia, and the Tsimane of the Bolivian Amazon. First, we tested the prediction that step length is constrained in a rainforest environment. Second, we tested the prediction that shorter stature promotes walking at relatively higher preferred velocities in rainforests. Both predictions were supported, providing the first empirical evidence for the human pygmy phenotype as a locomotor adaptation. This model also has taxonomic relevance beyond humans, as directional selection for short stature should occur whenever environmental constraints cause biologically significant reductions in locomotor capacity in the context of a species’ ranging and foraging ecology. This study highlights the value of applying laboratory-derived biomechanical models in field settings to test evolutionary hypotheses.