The tactic is so effective when the enemy is near that Sun Tzu recommends it in The Art of War: “He who occupies the high ground,” the Chinese general declared in the ancient military treatise, “will fight to advantage”.
But soldiers are not alone in having hit on the idea. Troops of chimpanzees in Ivory Coast have taken up the same strategy, researchers say, scaling hilltops for recce missions and advancing if the enemy is distant or outnumbered.
“These military tactics that we see in humans – the importance of high ground – is maybe something that’s deeply rooted in our evolutionary past,” said Sylvain Lemoine, a primatologist at the University of Cambridge and lead author on the study.
“The chimps go up the hill, stop there, and, based on the information they gather, either carry on or retreat,” he said. “The high ground enables them to get information on their neighbours, especially the number, and how far away they are.”
Lemoine and his colleagues tracked two neighbouring groups of chimpanzees, each consisting of 30 to 40 adults, in the Taï national park in Ivory Coast between 2013 and 2016. The groups have separate territories, roughly 5km (3 miles) by 7km, but they overlap, and the troops are constantly trying to expand the area under their control.
Rather than moving as a whole, the groups often break into smaller units that patrol the borders and carry out scouting missions in case a swift land-grab is on the cards. From more than 20,000 hours of recordings, the researchers saw that chimps were most likely to climb hills when they reached the edge of their territory and more likely to rest quietly at the top than on hills deep within their territory. This would make it easier to hear the calls of any nearby neighbours, and work out how well defended the land was, the scientists say.
The researchers found more evidence that the hilltop stops were a reconnaissance tactic. When the chimps moved on from their vantage points, they took a route that minimised the risk of a scrap with their neighbours. If the other group was far away, or outnumbered, the chimps tended to push on into the rival territory. But if the neighbours were near, or present in numbers, the recce unit retreated.
After a hilltop recce, there was a 40% chance of advancing into enemy territory when rivals were 500 metres away, a 50% chance when they were a kilometre away, and a 60% chance when the neighbours were 3km away. The work is published in Plos Biology.
While meerkats and many other animals take to higher ground to keep watch for predators, and sound the alarm if needed, the chimps’ tactics are more complex. They appear to anticipate where conflicts may occur, use higher ground to assess the risk, and make collective decisions on how to proceed.
“They’re going to the hills to get information they don’t have and that is quite sophisticated,” said Lemoine. By pushing into their rivals’ territory, the whole troop fares better, as there is less competition for food and other resources. “It’s all about securing space and increasing their territories,” Lemoine said.