Newly published, long-term research on the Issa Valley chimpanzees in Tanzania tells a riveting story. Together, these results clarify the foraging strategies of both living and extinct hominins. Joint first author Dr Rhianna Drummond-Clarke from the Max Planck Institute for Evolutionary Anthropology goes on to explain. It shows many incredible ways that chimpanzees thrive in their specialized savanna-mosaic habitat, especially through the dry season. Their results appeared in the journal Frontiers in Ecology and Evolution. They help illustrate the likely evolutionary trajectories of our early human primate ancestors.
The Issa Valley is dominated by a relatively thick forest covering the banks of the rivers and vast open woodlands. In the dry season, the chimpanzees make modifications to their foraging behavior. They are drawn to the diverse food options that flourish within these forests. The research team conducted extensive monitoring of adult chimpanzees to analyze their tree-climbing behaviors as they sought out valuable food items, primarily fruit, leaves, and flowers.
Foraging Patterns of Issa Valley Chimpanzees
Issa Valley chimpanzees are experts at tree climbing. This ability is extremely important for these animals to reach fruits located at the tips of branches. Beyond displaying their remarkable adaptability, such behavior further illustrates the importance of tree foraging to their diet. The researchers found that these chimpanzees spend significant time navigating through the tree canopy, even in an environment where trees are sparsely distributed.
Dr. Drummond-Clarke noted, “We wanted to test if something about how they foraged could explain their unexpectedly high arboreality. Savanna-mosaics are characterized by more sparsely distributed trees, so we hypothesized that adapting behavior to forage efficiently in a tree would be especially beneficial when the next tree is further away.”
The results indicate that the Issa Valley chimpanzees employ suspensory and bipedal modes of travel. These adaptive skills have developed more recently in their natural history as adaptations to their anthropogenic environment. That begs the question of how early hominins were making it through these dry spells. It implies that adaptations like these would have been key to life in then newly open habitats.
Implications for Understanding Early Hominins
In this latest study, the researchers find important parallels between the dietary habits of Issa Valley chimpanzees and Early Hominins. They explore the habitat issues each group must contend with. The behaviors we see today might give us important clues to the world of our ancestors and how they adapted to their surroundings. We show that moving through the canopy is key to survival for a big, partly tree-dwelling ape. That ability is still important, even where more open habitats occur. Adaptations to arboreal, rather than terrestrial living may have been key in shaping the early evolution of the human lineage.
The report emphasizes to what extent these results feed into larger discussions about the origins of bipedalism. For many years, it has been widely accepted that bipedalism developed as humans descended from trees to traverse open landscapes. Dr. Drummond-Clarke wants to challenge this perception. She implies that bipedal behavior would have further adapted in trees, despite emerging habitats that favored more ground dwelling species.
“For decades it was assumed that bipedalism arose because we came down from the trees and needed to walk across an open savanna.” – Dr. Rhianna C. Drummond-Clarke
Future Research Directions
Dr. Drummond-Clarke would like to stress that it is only the beginning. The study focused solely on a single community of Issa Valley chimpanzees during the dry season. This suggests that the findings may not be generalizable to other populations or other times of year.
“Importantly, this is also only one community of chimpanzees. Future studies of other chimpanzees living in such dry, open habitats will be vital to see if these patterns are truly a savanna-mosaic signal or unique to Issa.” – Dr. Rhianna C. Drummond-Clarke
The next stage of research will focus on if these same foraging patterns are maintained during wet seasons. Equally important is understanding the nutritional quality of these food sources. This knowledge will enable us to understand the impacts these behaviors have on energy efficiency across multiple settings.
“It would be interesting to investigate if these patterns remain during the wet season. Analyses of the nutritional value of foods and overall food availability are also needed to test our hypothesis that a strategy of foraging for longer in large trees on certain foods is energy-efficient in an open habitat.” – Dr. Rhianna C. Drummond-Clarke