Gorillas In Trees

Researchers show that even large silverback gorillas spend a substantial amount of time in trees

To the point

• Gorilla arboreality varies: The study shows that Bwindi mountain gorillas in Uganda and western lowland gorillas in Gabon spend significantly more time in the trees than the highly terrestrial Virunga mountain gorillas.
• Even the largest gorillas are arboreal: Contrary to traditional assumptions, arboreality in gorillas is not driven solely by the frequency of fruit eating or limited by their large body size, but instead even large silverbacks spend substantial time in the trees to forage on leaves as well as fruit.
• Ape and human evolution: Recognising the importance of arboreality in gorillas has important implications for how we understand their anatomy, their ecology and the evolutionary inferences we can make from gorilla-like morphology in the fossil record.

Scientists at the Max Planck Institute for Evolutionary Anthropology, as well as Rocky Vista University (USA), show that gorillas spend much more time in the trees than previously thought. Gorillas are typically considered the most terrestrial of the great apes due to their large body size (males average up to 170 kg or 375 lbs) and a diet comprising primarily terrestrial vegetation. However, new research shows that some gorilla populations can spend just as much time in the trees as some chimpanzee populations. Even large adults can average up to 20 to30 percent of their time in the trees to forage on leaves and fruit. This broader understanding of gorilla behaviour has important implications for how we interpret gorilla anatomy, their adaptive ecology, and gorilla-like features in our ape and human fossil relatives.

Since the 1974 discovery of 'Lucy' in Ethiopia with her bipedal legs and ape-like arms, there has been ongoing debate about the importance of tree climbing in human evolution. Additional fossil discoveries over the last decades have revealed large differences in body size, diet and the palaeoenvironments in which our fossil human relatives lived. Therefore, understanding what drives arboreality (spending time in trees) among living apes today is critical for interpreting how important arboreality was for our fossil human ancestors.

Gorillas are typically considered the most terrestrial of the great apes (orangutans, gorillas, chimpanzees and bonobos), often depicted in popular media sitting on the forest floor eating leafy vegetation. Adult females range between 70 and 100 kilograms (155-220 lbs) while silverbacks (adult males) can be nearly twice as a big, between 160 and 170 kilograms (350-375 lbs). Although all gorillas are still very capable tree climbers when needed, such as for building nests to sleep at night or accessing fruit, we typically consider them too large and cumbersome to move in the trees like smaller, more agile chimpanzees and bonobos, for example. The perception of gorillas being primarily terrestrial also stems from the seminal studies of the Virunga mountain gorillas (i.e., the gorillas of Dian Fossey), which live in high-altitude montane forest where little fruit grows. These gorillas, indeed, spend up to 98 percent of their time on the ground.

Gorillas spend more time in trees than thought

There are two species of gorilla: western gorillas (divided into western lowland and Cross River subspecies) and eastern gorillas (divided into mountain and Grauer subspecies), living in a variety of habitats across central Africa. Considering this diversity, are the behaviours of the Virunga mountain gorillas representative of all gorillas? To address this question, research led by scientists at the Max Planck Institute for Evolutionary Anthropology used long-term data collected at two gorilla research sites lead by first-author, Martha Robbins: the mountain gorillas of the Bwindi Impenetrable National Park in Uganda, which is only around 30 kilometres away from the Virunga mountain gorilla population, and western lowland gorillas of the Loango National Park in Gabon.

By analysing observational data collected over the past decade on multiple gorilla individuals and groups, the team found that both Bwindi mountain gorillas and Loango lowland gorillas spent much more time in the trees each day than documented in the Virunga gorillas. Although smaller-bodied, younger individuals, such as infants and juveniles, were more arboreal than larger-bodied adults, even the huge silverbacks spent a substantial amount of time in the trees. "Adult females spent 20 to 30 percent of their time in the trees, while silverbacks spent 18 to 20 percent of their time being arboreal, which is much more than two to seven percent found previously in the Virunga mountain gorillas" says lead author Robbins. "This demonstrates that the large body size of gorillas is not necessarily a limiting factor to tree climbing and that gorillas can be just as arboreal as some chimpanzee populations", says co-author Rhianna Drummond-Clarke.

Reinterpreting gorilla anatomy and adaptive ecology

Even more surprising, the authors found that, contrary to previous assumptions, the frequency of arboreality was not being driven solely by the need to access fruit in trees. In fact, "almost half of Loango gorilla fruit-eating was done on the ground, after it had fallen from the tree - a behaviour that requires further exploration," says Robbins. Instead, most of the time that both Bwindi and Loango gorillas were in the trees, they were eating non-fruit items like tree leaves.

Although scientists have long recognised that gorillas are capable tree-climbers, their behaviour and anatomy is often interpreted in the context of their terrestrial behaviours, particularly knuckle-walking. "The fact that some gorillas spend much of their daily life in the trees and that their large body size is not a hindrance to this arboreality has important implications for how we interpret gorilla anatomy. Furthermore, this work is critical to the inferences we make from fossils that have morphology that is gorilla-like or indicates a large body size; these are features that can imply frequent arboreality, not just terrestriality", says senior-author Tracy Kivell. These results will influence our understanding of ape and human evolution.