A group of Issa Valley chimpanzees navigate an open woodland crown to forage on new leaves. Credit: Rhianna C. Drummond-Clarke/Greater Mahale Ecosystem Research and Conservation (GMERC).
New research on chimpanzees might explain why early hominins, despite being able to walk upright, kept their tree-climbing adaptations.
The study published in Frontiers in Ecology and Evolution is based on observations of chimpanzees in the Issa Valley, Tanzania. The researchers aimed to gain insight into why human ancestors who started walking on 2 legs continued to climb trees.
“For decades it was assumed that bipedalism arose because we came down from the trees and needed to walk across an open savannah,” says Dr Rhianna Drummond-Clarke from the Max Planck Institute for Evolutionary Anthropology in Germany.
Drummond-Clarke’s previous research suggested that this wasn’t the case, instead showing chimpanzees in woodland environments spent just as much time climbing trees as chimpanzees in the forest.
Unfortunately, there is not much fossil evidence to help scientists understand how early hominins lived.
The Issa Valley is an open woodland habitat surrounded by riverbanks and small, thick forest. The diet and habitat of chimpanzees living there is similar to early hominins.
Observing the behaviour of these chimpanzees may provide insight into how extinct hominins lived.
“We wanted to test if something about how they foraged could explain their unexpectedly high arboreality,” says Drummond-Clarke.
Arboreality refers to an animal’s ability to live and spend their time in trees.
“Savannah-mosaics are characterised by more sparsely distributed trees, so we hypothesised that adapting behaviour to forage efficiently in a tree would be especially beneficial when the next tree is further away,” Drummond-Clarke says.
The researchers observed the chimpanzees during the dry season, recording what they ate and how they forged their food. They also took note of the size and height of the trees within the forest.
The chimpanzees were commonly found munching on fruit, leaves and flowers. All these foods are often located at the end of branches, meaning chimpanzees needed to be able to climb to reach them safely.
A young male chimp feeds on woodland seeds. Credit: Rhianna C. Drummond-Clarke/Greater Mahale Ecosystem Research and Conservation (GMERC).
“Here we show that safely and effectively navigating the canopy can remain very important for a large, semi-arboreal ape, even in open habitat,” says Drummond-Clarke. “Adaptations to arboreal, rather than terrestrial, living may have been key in shaping the early evolution of the human lineage.”
Often some of the food was found on thin branches. These branches were too thin for the chimpanzees to climb on due to their size. Instead, the chimps either hang under them or stand upright and hold nearby branches with their hands.
“We suggest our bipedal gait continued to evolve in the trees even after the shift to an open habitat,” says Drummond-Clarke. “Observational studies of great apes demonstrate they can walk on the ground for a few steps, but most often use bipedalism in the trees. It’s logical that our early hominin relatives also engaged in this kind of bipedalism, where they can hold onto branches for extra balance.”
There isn’t enough fossil evidence to make any conclusions and the researchers call for more studies on different aspects of chimpanzee foraging.
“This study only looked at foraging behaviour during the dry season,” says Drummond-Clarke. “Future studies of other chimpanzees living in such dry, open habitats will be vital to see if these patterns are truly a savannah-mosaic signal or unique to Issa.”