Chimpanzees do boundary patrolling

The Chimpanzees who live at the Ngogo site deep within Uganda’s Kibale National Park spend their days foraging and feeding, wrestling and playing, grooming and socializing. But every 10 to 14 days a group of males gathers and moves away from the rest of the group. They form a single-file line as they walk purposefully toward the edge of their territory, eventually striking out into the territory of a neighboring group of chimpanzees. They move in atypical silence, scanning the underbrush and listening for any sign of other chimps. If they encounter a large group of neighboring chimps, and are outnumbered, they flee back to their territory. But if they come across a single chimp from a neighboring group, they attack – surrounding, beating, and jumping on the victim. Some victims are killed outright, others manage to escape, broken, bleeding, and unlikely to survive. Infants are often torn away from female chimpanzees and are killed and cannibalized.

This behavior, known as boundary patrolling, was first observed by Jane Goodall in the 1970’s among chimpanzees at Gombe National Park in Tanzania and has since been observed in many chimpanzee groups that live side-by-side with others, including the Ngogo group. These border patrols have captured the interest of primatologists and the public, not only because of their shocking brutality, but also because they bear a striking resemblance to human warfare. Until recently, primatologists have been unable to definitively explain why chimpanzees participate in border patrols and lethal attacks against their neighbors. But after observing the Ngogo chimpanzee community for 10 years between 1999 and 2009, primatologist John Mitani and a group of researchers have discovered the answer: chimpanzees attack their neighbors in order to expand their territory.

During the 10-year observation period, researchers witnessed the killings of 18 neighboring chimpanzees at the hands of Ngogo males on patrol, and found circumstantial evidence for three more killings. The majority of the Ngogo border patrols, and 13 of the killings, occurred in an area to the northeast of Ngogo territory. These killings represented a staggering loss for the chimpanzee community living in this area. The death rate experienced by these chimpanzees during the 10-year observation period was 23 to 75 times higher than the median death rates experienced by nine other well-studied chimpanzee communities.

In 2009 the Ngogo chimpanzees incorporated this northeastern region into their territory. While they had previously only entered this region during boundary patrols, large parties of Ngogo chimps began to feed and socialize in the region, and neighboring chimps were no longer observed in the area. The newly acquired land represented a significant 22% increase in territory for the Ngogo community. This new land provided them with increased access to resources including food and safety, and may allow them to attract more female mates. Dr. Mitani, lead author of the Ngogo study, noted that these observations demonstrate that “patrolling seems to be part of a long-term strategy to dominate neighbors. If successful, chimpanzees can acquire more land. All chimpanzees will do this if they can. The Ngogo chimpanzees have been particularly successful because they have an in built competitive advantage. That advantage is due to the extremely large size of the community.”

The violent boundary patrols and annexation of neighboring territory observed at Ngogo may sound a lot like human warfare. After all, humans have fought over access to land and resources for centuries. But what can this research tell us about the origins of human warfare? Some scientists, most notably primatologist Richard Wrangham, see this chimpanzee behavior as evidence that a propensity for warfare stretches deep into our evolutionary past. He reasons that similarities between human and chimpanzee warfare indicate that warfare has been shaped by evolution in both cases, arguing that, “there has been selection for a male psyche that, in certain circumstances, seeks opportunities to carry out low-cost attacks on unsuspecting neighbors.” And perhaps this psychological disposition gives rise to warfare among chimps and humans.

But Dr. Mitani, lead author of the Ngogo study, urges caution, emphasizing “that human warfare is quite variable. Fighting others for land is only one reason humans go to war today.” The problem is that human warfare is so incredibly complex – there are as many reasons for war as there have been wars. The chimpanzee warfare observed at Ngogo most closely parallels the territorial raiding seen among contemporary humans living in traditional hunter-gatherer groups. But did our ancient ancestors practice this kind of warfare? Did this type of warfare give rise to the way we fight wars today? In this case it’s difficult to make direct comparisons between human warfare and the behavior of the Ngogo chimps, and Dr. Mitani warns that answering these questions may be based largely on speculation.
Original Research:

Mitani, J., Watts, D., & Amsler, S. (2010). Lethal intergroup aggression leads to territorial expansion in wild chimpanzees Current Biology, 20 (12) DOI: 10.1016/j.cub.2010.04.021
Story Credit here and to read the Q&A with primatologist John Mitani that accompanies this post.