The positional behaviour of chimpanzees in the open and dry habitat of Issa Valley, Tanzania, and implications for the emergence of hominin bipedalism

Bipedalism (and its associated morphological features) defines the human lineage (hominins) from other apes (hominoids) since our split ~7 million years ago. A porous fossil record however leaves uncertainty around the context of its emergence and evolution. The palaeontological record supports an adaptive hominin radiation from closed (e.g., tropical forest) to more open and dry environments (e.g., savannah-mosaic) as central to the emergence and evolution of bipedalism. However, the persistence of morphology considered adaptive for arboreal locomotion in hominins (e.g., long forelimbs, curved phalanges) has raised questions over the functional importance of ‘arboreal’ morphology and the selective niche in which bipedalism evolved. Chimpanzees that live in savannah-mosaic habitat provide valuable models to test the selective pressures that may act on a large-bodied, semi-arboreal ape in open habitat. This thesis presents the first quantification of chimpanzee positional behaviour and substrate use in an open and dry habitat, Issa Valley, Tanzania. With comparisons to published data on forest-dwelling chimpanzees, I test palaeoanthropological hypotheses of selective pressures acting on the positional behaviour of a large-bodied, semi-arboreal ape in an open habitat. Data were collected (1) over 15 months focusing on the positional behaviour of adults of the Issa Valley chimpanzee community and (2) over 5 months focusing on Issa chimpanzee arboreality in relation to their foraging strategy and foraging tree structure. The results are presented over three publications (Chapters 2, 3 and 4). Chapter 5 brings these works together to discuss the overall implications for chimpanzee behavioural plasticity, reconstructions of ape behaviour from the fossil record, the potential selective pressures hominins faced in a savannah mosaic habitat, and ultimately the evolutionary context in which habitual bipedalism emerged in the hominin lineage.