On the boundary of dangerous territory, a troop of about 30 individuals engaging in a border patrol climbs a rocky hill to conduct reconnaissance. Detecting the sounds of adversaries a bit too close for comfort, the squad retreats. There is no reason to risk a fight with the odds against you.
It is a scenario that has unfolded innumerable times in the history of human warfare. But in this case, it involved not people but chimpanzees in Tai National Park in southwestern Ivory Coast, West Africa's largest protected area of rainforest.
Researchers said on Thursday they have documented the tactical use of elevated terrain in warfare situations while observing on a daily basis two neighboring communities of wild western chimpanzees in Tai National Park for three years.
Information obtained during hilltop reconnaissance shaped whether the chimpanzees made forays into enemy territory, the study found, with these apes appearing more apt to do so when the risk of confrontation was lower. The study, the researchers said, records for the first time the use of this age-old human military strategy by our species' closest living relatives.
"It shows sophisticated cognitive and cooperative skills to anticipate where and when to go, and to act upon gathered information in a safe way," said University of Cambridge biological anthropologist Sylvain Lemoine, lead author of the study published in the journal PLOS Biology.
Inter-group violence is ubiquitous in chimpanzees, Lemoine said. Skirmishes occasionally occur in overlapping border areas.
"Chimpanzees compete for space, which encompasses food resources. Large territories are beneficial as it reduces within-group competition and female reproductive rates are increased in larger territories," Lemoine said.
The two neighboring groups tracked in this study were of equivalent size, between 40 and 45 individuals, with about five to six adult males and 10 to 13 adult females, the rest being adolescents, juveniles, and infants. Males are always dominant over females, the researchers said.
"Chimpanzees are extremely territorial. They undertake regular border patrols, where individuals roam in the periphery of their territory in a very coordinated and cohesive way," Lemoine said.
"They engage in inter-group encounters that are violent, dangerous, and stressful. Inter-group encounters can be vocal exchanges from a distance, visual contacts or physical contacts with fights, bites and chases. Killings are common, and victims can be from all age classes," Lemoine added.
Climbing hills does not necessarily improve the visual detection of members of a rival community, instead offering improved acoustic conditions to detect adversaries by sound.
"The tops of the hill are covered in vegetation and do not offer good lookout points," Lemoine said.
While atop border hills, the chimpanzees typically refrained from noisily eating or foraging, instead resting and listening.
They were more likely to advance into dangerous territory after descending a hill if the rival chimpanzees were further away. Such incursions occurred approximately 40% of the time when rivals were about three-tenths of a mile (500 meters) away, 50% when rivals were about six-tenths of a mile (1 km) away and 60% when rivals were about 1.9 miles (3 km) away.
Chimpanzees and the closely related bonobos are the species nearest genetically to humans, sharing about 98.8% of our DNA. The human and chimpanzee evolutionary lineages split about 6.9 million to 9 million years ago, according to research published in June.
Studying chimpanzee behavior may offer insight into our own species.
"We can better understand where we come from and what makes us human. We can better understand which kind of behaviors and adaptations were present in the last common ancestor between humans and chimpanzees, and have a better idea of the sociality and behavior of ancient hominin species," Lemoine said, referring to extinct species on the human lineage.
"It also teaches us what we have in common with our closest living relatives, how similar we are with wild animals, and that we only differ from our cousins in degree and not in nature," Lemoine added.
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