HQ Team
November 8, 2023: Chimpanzees spy on rival groups and confront them when there is a reduced risk, a study finds.
Researchers at Cambridge’s Department of Archeology said high-ground reconnaissance, one of the oldest military strategies, was observed in our closest evolutionary relatives.
“Tactical warfare is considered a driver of human evolution,” said Dr Sylvain Lemoine, a biological anthropologist from the University of Cambridge’s Department of Archaeology, and lead author of the study.
“This chimpanzee behaviour requires complex cognitive abilities that help to defend or expand their territories, and would be favoured by natural selection,” he said in a statement.
Territorial control
“Exploiting the landscape for territorial control is deeply rooted in our evolutionary history.
“In this use of war-like strategy by chimpanzees we are perhaps seeing traces of the small-scale proto-warfare that probably existed in prehistoric hunter-gatherer populations.”
Researchers conducted a three-year study of two neighbouring chimpanzee groups in the West African forests of Côte d’Ivoire, tracking the primates as they traversed their respective territories, including an overlapping border area where skirmishes occasionally took place.
The team found that chimpanzees were more than twice as likely to climb hills when heading towards this contested frontier as when they were travelling into the heart of their territory.
While atop border hills, chimpanzees were more likely to refrain from noisily eating or foraging and spend time quietly resting – enabling them to hear distant sounds of rival groups.
Avoiding costly fights
The further the location of hostile chimpanzees, the greater the likelihood of an advance into dangerous territory upon descending the hill.
“This suggests that chimpanzees on high ground gauge the distance of rivals, and act accordingly to make incursions while avoiding costly fights,” according to the researchers.
This is the first evidence of an animal other than humans making strategic use of elevation to assess the risks of “intergroup conflict,” they said.
The research team spent between eight to 12 hours a day following four groups. They used GPS trackers and each group consisted of 30-40 adult chimpanzees.
The study used more than 21,000 hours of track logs from a total of 58 animals recorded between 2013 and 2016.
To establish and protect their territory, chimpanzees perform regular tours of the periphery that form a sort of “border patrol”, said Dr Lemoine.
‘Inselbergs’
The type of hills near the border used for reconnaissance are known as “inselbergs,” or isolated rocky outcrops that break up the forest canopy.
Chimpanzees repeatedly returned to some of these inselbergs, where time on the summit was passed in a more muted state.
“These aren’t so much lookout points as listen-out points,” said Dr Lemoine.
“Chimpanzees drum on tree trunks and make excitable vocalisations called pant-hoots to communicate with group members or assert their territory. These sounds can be heard over a kilometre away, even in dense forest.”
“It may be that chimpanzees climb hilltops near the edge of their territory when they have yet to hear signs of rival groups.
“Resting quietly on an elevated rock formation is an ideal condition for the auditory detection of distant adversaries.”
Following a hilltop recce, the likelihood of advancing into enemy territory increased from 40% when rivals were 500 metres away, to 50% when rivals were at 1000m, to 60% when rivals were at 3000m.
Food, mating chances
“Chimpanzees often expand their territory by encroaching and patrolling in that of their neighbours. Hilltop information-gathering will help them to do this while reducing risks of encountering any enemies,” said Dr Lemoine.
Additional territory can boost food provision and mating chances, he said. The latest research suggests that chimpanzees use hilltop reconnaissance to avoid confrontation, and violence is relatively rare.
“But fights, and even kidnappings and killings, did occur between rival group members.”
“Confrontations between rival chimpanzees are extremely noisy. The animals go into an intimidating frenzy, screaming and defecating and gripping each other’s genitals.”