As opposing armies headed along the dusty roads converging on Gettysburg, Pennsylvania, Union Brigadier General John Buford recognized and seized the high ground. On the morning of July 1, his cavalry held it against very heavy odds until reinforcements arrived. From those advantageous ridges and hills Union troops later defeated attacking Confederate forces, turning the tide of the legendary battle and the changing course of America’s Civil War.
Buford’s heroic initiative and actions followed a guiding principle as old as warfare itself. Hilltop castles and fortifications around the world testify to the topographical advantages long recognized by human strategists. The heights are easier places to defend, and they enable wide views for gathering crucial information about your opponents: exactly where they are, in what numbers and what they are doing.
Now, research published Thursday in PLOS Biology suggests that humans aren’t the only ones to make tactical use of elevation in times of group conflict. Groups of chimpanzees in the forests of Ivory Coast’s Taï National Park also seek out the high ground as an advantageous position to gather intelligence about rival groups. The chimps climb hills not to give them an edge in territorial battles but instead to seek out the doings of other groups to avoid coming into conflict with them at all. They go high to keep the peace.
“As a group, they are anticipating potential risks and gathering information to reduce the risk,” says primatologist and study co-author Sylvain Lemoine, of the University of Cambridge and Max Planck Institute for Evolutionary Anthropology. “It’s a form of pre-emptive behavior before a conflict occurs and shows a much higher degree of cognition than just reacting directly to a fight.”
The chimps seem able to reflect on their own knowledge; they are aware of what they don’t know, and they act to increase the information that they need about their rivals. Lemoine and colleagues suggest it’s possible that this cooperative cognitive behavior, key in human societies, arose because it offers an evolutionary advantage. Bands that use it may gain a competitive edge, because avoiding dangerous conflicts helps keep the chimps alive and fit for reproduction.
At the Ivory Coast’s Taï Chimpanzee Project, generations of chimps, living in four neighboring communities, have been studied consistently for 45 years. For the new study, an international team used data from three years of arduous fieldwork by many different researchers, who observed the daily behaviors and ranging patterns of two neighboring groups of chimps between 2013 and 2016.
Numerous isolated rocky hilltops poke above the canopy in the national park’s forests, offering listening posts for curious chimps. The study found that when groups of chimps traveled toward a territorial border, where they were likely to contact a rival group, they much more frequently took to the hills—the better to observe and gather information about the whereabouts of the other group. When the chimps left those same hilltops, observations show that they moved in ways that would avoid the other groups, diffusing conflicts before they began.
But when the chimps were moving towards the center of their territory, away from other groups, they took to hilltops only half as often. “If they were just using the hills for any purpose, like feeding, we wouldn’t expect any difference,” Lemoine says.
Co-author Roman Wittig, director of the Ape Social Mind Lab at the Institute of Cognitive Science in Lyon, France, says studies have shown that chimps understand their landscapes and choose the easiest routes when ranging between food sources. That makes theses chimps’ treks to elevation especially notable. “Climbing these hills is quite energy-consuming,” Wittig says. “They would not frequently climb the hill in order to climb the hill. There is some purpose behind this.”
Observations of chimp behaviors while on various hilltops revealed that purpose. In the center of territories and farther away from neighboring groups, the animals tended to move around more and to feed more—both noisy activities. But when at high elevation near the border of a territory, they sat quietly and rested, allowing them to listen carefully, detect neighboring groups and estimate how far away they are. Chimp noises, including pant-hoots and buttress drums, can commonly be heard from two-thirds of a mile away, and acoustics may travel even farther at elevation above the forest canopy.
“It is remarkable to see such clear evidence of strategic decision-making in a group-level context,” says primatologist James Brooks, of the Kyoto University Institute for Advanced Study in Japan, who wasn't involved in the study. “If chimpanzees are integrating their knowledge of the local topology into their territorial conflicts, it also raises questions about what other complex strategies they may be employing.”
The study’s novel approach combined geographic data with direct observation. Three years of GPS ranging data simultaneously mapped the movement of two chimp groups who share overlapping territories. This information showed how the animals traversed the landscape, offering larger insights into when and how they used the hilltops and how they moved to encounter or avoid each other. This big-picture data was paired with detailed behavioral observations from the ground. Human observers followed 58 individual chimps daily, for between 8 to 12 hours at a time. Over the three-year period, that resulted in well over 10,000 observation hours for each of the two groups.
“The data were so uniquely well suited to be able to answer this question, that probably nobody else could actually answer, in a really compelling and comprehensive way,” says animal behaviorist Malini Suchak, of Canisius University, who wasn’t involved in the research. The group’s detailed movement patterns suggested how the animals were gathering and using information, she notes. Pairing that data with detailed observations of how the chimps were actually behaving at the same time reveals the intentionality behind their actions.
Chimpanzees (Pan troglodytes) are cooperative animals. They hunt together, patrol the borders of their territories together and even form “political” alliances to determine group power structure.
Lemoine, Wittig and others working in Taï National Park have been exploring the roots of this group cooperation among the chimps. One hypothesis suggests that members of a group work better together when faced with an external enemy.
Wittig previously co-published a study showing that when preparing for a likely conflict with other groups, chimps consciously interact and bond more closely together, building levels of the “love hormone” oxytocin that help keep their group together and prevent defections when trouble begins. When groups of chimps clash, the conflicts can range from vocal shouting matches at a relatively safe distance to violent conflicts. “If you get into an intergroup encounter, and you might be separated from your group because the group is fleeing, you can be killed,” Wittig says.
Other research shows that conflicts can harm chimpanzee communities in less obvious ways. A previous study by Lemoine and many of the same authors found that when pressures are high between neighboring groups, chimps don’t have babies as frequently, and offspring survival also decreases. “These conflicts can have detrimental long-term consequences,” Lemoine says.
If group conflict so seriously hinders reproductive success, the authors theorize, it might have driven important evolutionary adaptations that favored minimizing such conflicts. Those adaptations may include complex abilities to think and work together—like using hilltops as intelligence-gathering listening posts.
Just as we often see among humans, competition with outside groups might have forced chimps to cooperate more closely together, Wittig explains. “The bigger the outside threat, the more a society comes together,” he says. “With the chimps, we’ve seen a very similar reaction.”