Why chimpanzees risk tree food in barren landscapes

Chimpanzees in the Issa Valley of Tanzania are defying long-standing ideas about our ancestors leaving trees.

Despite living in open Savannah woodlands, these chimpanzees spend most of their time in trees, risking awkward climbs and acrobatic movements, reaching fruits, seeds and leaves, perched on thin branches. A new study suggests that their high-risk tree foraging may help explain why early human ancestors remained rock climbing characteristics even after evolving to walking upright.

Chimpanzees climb in public places

The Issa Valley has mottled forests and dry woodlands, which are believed to be similar to the environment in which early humans lived. The researchers observed that these chimpanzees still climb extensively, like some fossil cousins. They often forage in broad crowns, hanging them, or walking upright along branches to get food.

“For decades, it was believed that genderism emerged because we fell from trees and needed to go through an open Savannah,” said Dr. Rhianna Drummond-Clarke of the Max Planck Institute for Evolutionary Anthropology. “Here, we show that safe and efficient navigation of canopy canopy may still be very important for large semi-aluminum apes, even in open habitats.”

Foraging in difficult conditions

During the dry season, Isa Chimpanzees shift their focus to woodland foods, especially fruits, seeds, and even parasitic flowers, called pilostyles, which grow along the end branches. To collect these resources, chimpanzees must carefully pass through sparse trees with less open space between them.

The researchers measured the size and structure of more than 200 trees and analyzed 301 foraging sessions. They found that chimpanzees spent more time foraging on larger, food-rich trees and had open crown-shaped shapes such as umbrellas or upside-down cones.

Key findings include:

• Longer foraging times are associated with trees with abundant food and larger trunks.
• Chimpanzees use suspension behavior – hanging under branches – especially in open trees.
• Immature fruits and difficult-to-treat seeds are associated with longer foraging sessions.
• Even in sparse vegetation areas, it is more likely to be suspended in shaped trees.

These behaviors challenged the idea that early humans left trees simply because forests disappeared. Instead, tree-based foraging remains crucial even in more open habitats – possibly enough to keep natural selection favorable for climbing adaptation.

Lessons from human evolution

These findings are based on growing fossil evidence that early bipeds such as Ardipithecus and Australopithecus retained the characteristics of rock climbing: curved fingers, long arms and strong shoulders. Fossils alone cannot reveal behavior, but these living chimpanzees may provide a useful window for how semi-planet apes survive the landscape of trees and grass.

“If the Isa Valley chimpanzees could be considered as a suitable model, then suspension and two behaviors might be crucial for a large, fruit-eating translucent human being.”

Still only one group

While fascinating, the study authors acknowledge limitations. Data are from the single chimpanzee community during the dry season. More work is needed to test whether other Savannah-based chimpanzees show similar patterns throughout the year and to better understand how food availability and tree types shape rock climbing behavior.

Still, the news is clear: If you want something good on the treetop, you need tools to get there, including some very human-like movements.

Magazine: The Frontiers of Ecology and Evolution
doi: 10.3389/fevo.2025.1561078

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