A New Experimental Protocol for Assessing Hominoid Assisted Arboreal Bipedalism

Arboreal bipedalism is suggested as a precursor and adaptive locomotor mode for the immediate ancestor of hominin terrestrial bipedalism, yet detailed investigation of its locomotor biomechanics is hindered by its low frequency and observation difficulties in free-ranging hominoids. Further difficulties are faced in the creation and installation of a suitable experimental setup in natural settings. Captive studies may potentially reduce logistical issues, but data on arboreal bipedalism are scarce. We present an experimental design and protocol for collecting video data on arboreal bipedalism in captive primates, from which qualitative and quantitative gait data can be extracted. Our protocol increases the frequency of this rare behavior. Data were collected on six adult chimpanzees (three males, three females) at La Vallée des Singes, Romagne, France. The chimpanzees voluntarily engaged with a simulated arboreal foraging scenario consisting of two parallel PVC tubes and a high-value food reward. Five GoPro cameras recorded interactions with the experimental equipment. For validation of the effectiveness of our experimental design, protocol interactions were identified as successful (activity completed) or unsuccessful. All age and sex classes had successful interactions. Full strides were observed alongside the identification of two forms of arboreal bipedalism, forward-facing and sideways. This highlights the variation within the arboreal bipedalism locomotor category and the capacity for our experimental design to provide suitable data for gait parameter analysis and interspecies comparisons. Our protocol thus permits detailed investigation of arboreal bipedalism's role in the evolution of hominin bipedalism.