Chimpanzees climb up hills to stealthily gather information about rival groups and plan safe routes into enemy territory.
“Securing high ground to spy on nearby groups is an essential military strategy for humans,” says Sylvain Lemoine at the University of Cambridge. Lemoine and his colleagues found that two neighbouring groups of western chimpanzees (Pan troglodytes verus) that live in Taï National Park, Ivory Coast, use similar tactics.
To study their behaviour, the team observed both chimpanzee communities, each consisting of between 30 and 37 individuals, every day for three years.
“Chimpanzees are extremely territorial,” says Lemoine. “We saw regular border patrols in the periphery of their territories for both groups.”
These patrol units would repeatedly stop to rest on hilltops when they approached rival land. “High ground favours good acoustic conditions,” says Lemoine, who says that this behaviour suggests that chimpanzees pause to listen out for enemies.
The chimpanzees on watch then used any sounds they could hear, which included calls and drumming, to judge how risky it was to enter the other group’s territory. The closer the enemy seemed, the less likely the chimpanzees were to advance onto enemy land. For example, the chimpanzees proceeded into enemy territory 60 per cent of the time when their rivals were 3 kilometres away, but just 40 per cent of the time when their rivals were 50 metres away.
“The key point is that chimpanzees do not take risks,” says Lemoine. “They are weighing the costs and benefits of engagement.”
When there is a low risk of conflict, such as when there are fewer rival chimpanzees or they are further away, the patrol groups feel more confident to expand their territory. Having more land means the group has access to more resources, says Lemoine, which means they can have more offspring that survive to adulthood.
“Chimpanzees are actually using much more sophisticated territorial strategies than we thought,” he says.