Gorillas are arboreal apes

There is debate over the importance of the arboreal niche in hominid evolution.^1,2,3,4,5,6 Gorillas are considered to be primarily terrestrial,^7,8,9,10 which influences interpretations of their anatomy, evolution, and the inferences drawn from ape and human fossils.^{11,12,13,14,15} This perception of gorilla terrestriality stems from their large body size,¹⁶ their highly terrestrial herbaceous diet,¹⁷ and results of the only study of gorilla terrestriality versus arboreality, that of the Virunga mountain gorillas (Gorilla beringei beringei).^8,9 However, Virunga gorillas live in an ecological extreme and their behavior may not be representative of all gorillas.^{18,19,20,21,22,23,24,25,26} Here, we investigate arboreality in frugivorous populations: the mountain gorillas (G. b. beringei) of Bwindi Impenetrable National Park, Uganda, and western gorillas (Gorilla gorilla gorilla) of Loango National Park, Gabon. Although arboreality is negatively correlated with body size, both Bwindi and Loango gorillas are more arboreal than Virunga gorillas (adult females 21% and 34%, respectively, versus 7% for Virunga gorillas; adult males 18.1% and 18.6%, respectively, versus 2% for Virunga gorillas). Moreover, increased frugivory is not the only driver of arboreality; nearly half of Loango gorilla fruit-eating was on the ground and adults of both populations spent most of their arboreal feeding time consuming nonfruit items such as tree leaves (Bwindi: 63%; Loango: 70%). These results demonstrate that gorillas are more arboreal than previously reported and that arboreality is not always driven by fruit consumption nor limited by body size. These findings have important implications for understanding ecomorphology in extant and fossil great apes.^27,28.