New research from West Africa suggests our closest evolutionary relatives, chimpanzees, use a battlefield tactic previously thought to be unique to humans. Scientists have discovered that chimpanzees will climb hills near territorial borders to surveil rival groups in a strategic manner similar to ancient human military scouts.

Researchers studied two neighboring chimpanzee communities living in forests in Ivory Coast, West Africa over three years. They tracked the primates as they traveled within their respective territories, including an overlapping border zone where skirmishes occasionally occurred.

The team found chimpanzees were over twice as likely to ascend hills when traveling to this disputed border area compared to within the core of their own territory.

When atop border hills, chimpanzees were more likely to abstain from noisy food foraging and rest quietly, allowing them to listen for sounds of rival groups further away.

Reconstructed topography (meters above sea level) across the Taï chimpanzee territories of the South group (left) and East group (right), based on the accumulation of 21,186 h of focal chimpanzee footprint tracking (4,817.8 km for the South, 5,168.9 km for the East) between December 2013 and October 2016 from a total of 58 chimpanzees. The red line represents 95% of the kernel distribution for the Southern group, the blue line 95% of the kernel distribution for the Eastern group. Dashed black lines represent the peripheral area, with peripheral hills above 230 m used only by the Southern group in dark red, used only by the Eastern group in blue, and used by both groups in black, during this time period | photo Sylvain R. T. Lemoine et al.

The further the location of hostile chimpanzees, the more likely the surveilling chimpanzees were to subsequently advance into dangerous territory upon descending the hill. This suggests chimpanzees gauge distance to rivals from hilltops and act accordingly to conduct raids while avoiding costly fights.

Other mammal species like suricates use heights to spot predators or call mates but researchers say this is the first evidence an animal other than humans makes strategic use of elevation to assess “intergroup conflict” risks.

The team argues this behavior requires complex cognitive abilities that help defend or expand territories and would be favored by natural selection, much like how tactical warfare drove human evolution.

The research was conducted at the long-running Taï Chimpanzee Project where teams spend 8-12 hours daily following four habituated communities, providing rare data on multiple wild chimpanzee societies.

Chimpanzees | photo Acrumpacker on Wikimedia Commons

Researchers used GPS trackers to reconstruct elevation maps of the two bordering chimpanzee territories studied, corroborating terrain with old French colonial maps. Over 21,000 hours of tracking data from 58 chimpanzees between 2013-2016 were analyzed.

To establish and protect their lands, chimpanzees conduct regular patrols along peripheral areas forming a kind of border patrol. Patrols typically occur in sub-groups that remain close and limit noise.

From an observer’s perspective, it seems the chimpanzees have started patrolling as they silently move and stop together semi-coordinated, a bit like a hunt, explains project researcher Sylvain Lemoine.

The type of hills near the border used for surveilling are known as “inselbergs”—isolated rocky outcrops breaking the forest canopy. Chimpanzees repeatedly returned to certain inselbergs where they spent time in a more subdued state atop them.

Lemoine notes these areas served more as listening posts than true lookouts. Through careful use of elevated terrain for scouting rival communities, chimpanzees may be exhibiting remnants of small-scale proto-warfare employed by ancestral human hunter-gatherer populations.

Sources

University of Cambridge | Lemoine SRT, Samuni L, Crockford C, Wittig RM (2023) Chimpanzees make tactical use of high elevation in territorial contexts. PLoS Biol 21(11): e3002350. doi.org/10.1371/journal.pbio.3002350

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