Crafty chimps use hilltops to spy on rival groups, reveals new research.
They use high ground to conduct “reconnaissance” – often before making forays into enemy territory at times when there is reduced risk of confrontation, say scientists.
Tactical use of elevated terrain in warfare situations had been considered unique to humans.
Now, for the first time, one of the oldest military strategies has been observed in our closest evolutionary relatives.
Researchers conducted a three-year study of two neighboring 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 sometimes took place.
The findings, published in the journal PLoS Biology, showed that chimps were more than twice as likely to climb hills when heading towards the contested frontier (58 percent) as when they were traveling into the heart of their own territory (25 percent).
The researchers also discovered that while atop border hills, chimps were more likely to refrain from noisily eating or foraging and spend time quietly resting – enabling them to hear distant sounds of rival groups.
The further away the location of hostile chimpanzees, the greater the likelihood of an advance into dangerous territory upon descending the hill.
Researchers say that suggests that chimps on high ground gauge the distance of rivals, and act accordingly to make incursions while avoiding costly fights.
Other mammal species – such as meerkats – use high ground to keep watch for predators or call to mates.
However, the research team says this is the first evidence for an animal other than humans making strategic use of elevation to assess the risks of “intergroup conflict.”
Study lead author Dr. Sylvain Lemoine, a biological anthropologist at Cambridge University, said: “Tactical warfare is considered a driver of human evolution.
“This chimpanzee behavior requires complex cognitive abilities that help to defend or expand their territories, and would be favored by natural selection.
“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.”
The study, published in the journal PLOS Biology, was conducted at the Taï Chimpanzee Project.
Teams of researchers spend up to 12 hours a day following four groups that are “habituated” to the presence of humans.
It is one of the few sites where data is collected simultaneously on multiple communities of wild chimpanzees.
The project researchers have GPS trackers, through which the study authors were able to reproduce maps of two chimp territories that border each other, including elevation data.
Each group consisted of 30 to 40 adults at any one time.
The study used more than 21,000 hours of track logs from a total of 58 animals recorded between 2013 and 2016.
Dr. Lemoine explained that to establish and protect their territory, chimpanzees perform regular tours of the periphery that form a sort of “border patrol”.
He said: “Patrols are often conducted in subgroups that stay close and limit noise.
“As an observer, you get a sense that patrolling has begun. They move and stop at the same time, a bit like a hunt.”
The type of hills near the border used for reconnaissance are known as “inselbergs” – isolated rocky outcrops that break up the forest canopy.
Chimps repeatedly returned to some of the inselbergs, where time on the summit was passed in a more muted state.
Dr. Lemoine said: “These aren’t so much lookout points as listen-out points.
“Chimpanzees drum on tree trunks and make excitable vocalizations called pant-hoots to communicate with group members or assert their territory.
“These sounds can be heard over a kilometer 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.”
The research team analyzed tactical movements in the 30 minutes after a stop longer than five minutes on a hill near the border and compared it to movements after stops in low-lying border areas.
Following a hilltop recce, the likelihood of advancing into enemy territory increased from 40 percent when rivals were 500 meters (1640.42 feet) away, to 50 percent when rivals were at 1000 meters (3280.84 feet) , to 60 percent when rivals were at 3000 meters (9842.52 feet) .
Dr. Lemoine said: “Chimpanzees often expand their territory by encroaching and patrolling in that of their neighbors.
“Hilltop information-gathering will help them to do this while reducing risks of encountering any enemies.
“The border zone between the two groups was in a state of flux.”
He says more territory can boost food provision and mating chances.
His previous work suggests that larger chimpanzee groups live in bigger territories with reduced pressure from rivals, which in turn increases birth rates within communities.
He said the new study suggests that chimps use hilltop reconnaissance to avoid confrontation, and violence is relatively rare.
But he said fights, and even kidnappings and killings, did occur between rival group members.
Dr. Lemoine added: “Occasionally, raiding parties of two or three males venture deep into enemy territory, which can lead to fighting.
“Confrontations between rival chimpanzees are extremely noisy.
“The animals go into an intimidating frenzy, screaming and defecating and gripping each other’s genitals.”
Produced in association with SWNS Talker
