Locomotor behaviour of Chimpanzees living in the mosaic-savannah environment of Issa Valley, Tanzania

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 functional significance of arboreal adaptations in early hominins and conservation strategies of savannah-mosaic habitats.