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Bipedalism, a defining feature of the human lineage, is thought to have evolved as forests retreated in the late Miocene-Pliocene. Chimpanzees living in analogous habitats to early hominins offer a unique opportunity to investigate the ecological drivers of bipedalism that cannot be addressed via the fossil record alone. We investigated positional behavior and terrestriality in a savanna-mosaic community of chimpanzees ( ) in the Issa Valley, Tanzania as the first test in a living ape of the hypothesis that wooded, savanna habitats were a catalyst for terrestrial bipedalism. Contrary to widely accepted hypotheses of increased terrestriality selecting for habitual bipedalism, results indicate that trees remained an essential component of the hominin adaptive niche, with bipedalism evolving in an arboreal context, likely driven by foraging strategy.

Habitual bipedal walking is unique to humans amongst primates and its associated morphological features are used to define the human clade (hominins) from other apes over the last 7 million years (Ma). Yet, the evolutionary origins of our bipedal gait remain unknown. The palaeontological record supports an adaptive hominin radiation from a closed (e.g., tropical forest) to a more open and seasonal heterogeneous environment (e.g., savannah-mosaic) as central to the emergence and evolution of terrestrial bipedalism in the human lineage [1]. However, morphological features that are advantageous for arboreal locomotion are present in the forelimbs of many hominins (early and late), generating long-standing debate around the functional significance of these features, and the link between bipedalism and terrestriality. In the absence of direct fossil evidence, extant chimpanzees that live across a habitat gradient provide ideal models to test the “savannah-landscape effect” on ape locomotor behaviour and substrate use [2-3]. Chimpanzee locomotor studies to date, however, have focused only on forest-dwelling communities [4-5], limiting our knowledge of the full range of chimpanzee locomotor behaviour and its application for modelling hominin evolution. Here, we characterize for the first time the positional behaviour and substrate-use of chimpanzees living in an open, dry habitat. Chimpanzees (P. t. schweinfurthii) of the Issa Valley, western Tanzania, live in a savannah-mosaic habitat dominated by open miombo woodland with strips of closed evergreen (riparian) forest: a mosaic that resembles the reconstructed palaeoenvironments of Pliocene hominins. To investigate the influence of an open habitat on positional behaviour and terrestriality we, 1) quantified the frequency of terrestrial and arboreal positional behaviours between the forest and woodland at Issa, and 2) compared our findings to data published on forest-dwelling chimpanzees. Specifically, we tested the hypothe

Habitat type is an important factor influencing the frequency of different locomotor behaviours among extant primates. As such, palaeohabitat reconstructions have played a prominent role in hypotheses on the emergence and evolution of hominin bipedalism. Although chimpanzees (Pan troglodytes) live in a variety of habitats ranging from forest to savannah, locomotor studies to date have focused only on forest-dwelling chimpanzees, limiting our knowledge of the full range of chimpanzee locomotor behaviour and accuracy of referential models for hominin evolution. Chimpanzees (P. t. schweinfurthii) of the Issa Valley, western Tanzania, live in a savannah-mosaic habitat consisting of miombo woodland and riparian forest; similar to reconstructed palaeoenvironments of early hominins. Here, we characterise for the first time the locomotor behaviour and substrate-use of chimpanzees living in an open, dry landscape. We quantify the frequency of terrestrial and arboreal locomotor behaviours, in comparison to previously-studied forest-dwelling chimpanzees, and between the riparian forest and miombo woodland, to investigate the influence of vegetation type on locomotor behaviour. We test the hypothesis that chimpanzees will reduce time spent moving in the trees in more open vegetation. Data was collected on 13 adults over 15 consecutive months, noting locomotor mode, activity, vegetation type, and substrate-use of focal individuals every two minutes (N = 2848 locomotor scans). Results show that Issa chimpanzees spent more time moving terrestrially in the woodland compared to the forest, but overall, they are not less arboreal than forest-dwelling chimpanzees. Considering all postural behaviour, Issa chimpanzees increase time spent travelling and reduce resting time to maintain time for feeding (predominantly arboreal) in an open habitat. Our results further demonstrate that Issa chimpanzees are just as arboreal as forest-dwelling chimpanzees, which has implications for the functi