How humans and chimpanzees travel towards a goal in rainforests

Travel linearity increases in humans when they travel in increasingly large groups, while chimpanzees travel more goal-directed when they are on their own

How do human-unique ranging styles, like large home range and trail use, influence the way we travel to our goals? Researchers from the Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany, investigated spatial movement patterns of the Mbendjele BaYaka people and Taï chimpanzees. Human foragers and chimpanzees travelled in similarly straight lines towards goals, but they showed clearly different patterns in how they change linearity and speed depending on group size and familiarity with an area.

The human ranging style is unique among hominoids. The Mbendjele BaYaka people move from camp to camp every few months, and thus have a large lifetime range of approximately 800 square kilometers. Human foragers collect food and take it back to their camp to process and share. Furthermore, the Mbendjele BaYaka have created a trail system and walk mostly on trails. In contrast, one of our closest living relatives, chimpanzees (Pan troglodytes), live in a relatively smaller area (25 square kilometers) and spend most of their adult lives within the same home range. Chimpanzees consume food as they encounter it and make sleeping nests at variable locations within their home range. In addition, chimpanzees (especially, the chimpanzees living in the Taï National Park, Ivory Coast, where this research was conducted) rarely use the same paths when travelling on the ground. These different ranging styles between humans and chimpanzees might shape their different spatial experience and thus the way they travel through the forest to find food in their natural habitats.

Haneul Jang and her colleagues from the Max Planck Institute for Evolutionary Anthropology set out to study how human foragers and chimpanzees compare in their spatial movement patterns in similar rainforest environments. To this aim, the researchers measured travel linearity and speed of five Mbendjele BaYaka women from the Republic of Congo and five female chimpanzees from Taï forest when they travelled off-trail areas to out-of-sight food locations during their daily search for food in the rainforest. It is the first study that recorded and analysed the ranging patterns of both humans and chimpanzees using the same data collection protocol for such an extensive amount of time (236 and 274 days, respectively).

High levels of linearity

Jang and her colleagues found that both the Mbendjele BaYaka people and Taï chimpanzees had, on average, similarly high levels of linearity when they travelled towards out-of-sight locations. However, the Mbendjele BaYaka and Taï chimpanzees clearly differed in the manner in which their travel linearity and speed changed with group size and familiarity with a foraging area. The Mbendjele BaYaka travelled with higher linearity in familiar areas compared to less familiar areas. This pattern was reversed in Taï chimpanzees - they moved with higher linearity and speed in less familiar areas compared to familiar areas.

"One possible explanation of these differences can be their different ranging style. The Mbendjele BaYaka people stay in several seasonal camps in their lifetime range, and walk 90 percent of their travel distance on human-made trails during their foraging trips in the forest. They may therefore have less experience in less familiar off-trail areas. Hence, when the Mbendjele BaYaka people forage in these areas, their travel linearity decreases due to their searching effort", says Haneul Jang, lead author of the study. On the contrary, chimpanzees decreased travel linearity and speed in more familiar areas. "Since chimpanzee groups can be highly hostile towards each other, they might need to travel more efficiently and faster in less familiar areas, where they are more likely to encounter other chimpanzee groups", says Jang.

Group size influences travel linearity

In addition, Jang and her colleagues found that travel linearity increased in the Mbendjele BaYaka people and decreased in Taï chimpanzees when they travelled in a larger as compared to a smaller foraging group. "When I followed the chimpanzees, I often saw them wait for each other and have apparent disagreements about where to go", says Karline Janmaat, the main supervisor of the study. "With more individuals joining the group, disagreement seemed to increase. It is therefore perhaps not surprising that when group size increased, their travel became less linear. For the BaYaka people we found the opposite pattern. This can have many reasons, but I like to think that it is because humans can discuss, evaluate and eventually agree about where the best food sources are, by using language, and that this helped them to travel more efficiently. Language may have facilitated an efficient evaluation of differential information."

According to the authors, this study is the first necessary step needed to compare long-range spatial movement patterns of human and chimpanzee populations in their natural habitats. "Our study provides insights into how two closely related species living in similar environments can differ in their spatial movement patterns, which possibly results from their different ranging styles", says Jang. "We hope our study can contribute to expanding comparative research on spatial movement patterns to a wide range of primate species and populations, including humans, in natural environments."

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