This is Part 1 of a series, Why We Fight, about the origins of warfare.

Check out the Q&A with primatologist John Mitani that accompanies this post.

This post was chosen as an Editor's Selection for ResearchBlogging.orgThe 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 beat a hasty retreat 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.

Jane Goodall first observed this behavior, known as boundary patrolling, in the 1970’s among chimpanzees at Gombe National Park in Tanzania. Goodall was initially shocked when one of her colleagues saw a group of male chimps viciously beat a neighboring female chimp, killing and partially eating her 18-month-old infant in the process. Goodall had developed a close relationship with the male chimps, so at first she was reluctant to believe that their brutality was anything more than a ”one-time occurence, a bizarre aberration.” But mounting evidence to the contrary eventually led her to lament that, “Sadly, the ‘noble ape’ was as mythical as the ‘noble savage…’”

In fact, since the 1970′s other primatologists have seen boundary patrolling in many chimpanzee groups that live side-by-side with others, including the Ngogo group. Boundary patrols have fascinated primatologists and the public, not only because they are shockingly brutal, but also because they bear a striking resemblance to human warfare. For years primatologists have been unable to definitively explain why chimpanzees participate in boundary patrols and deadly attacks against their neighbors. But after following 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 as a way 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. Most of the Ngogo boundary 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 between 1999 and 2009 was 23 to 75 times higher than the median death rates experienced by nine other well-studied chimpanzee communities.

In 2009 the Ngogo chimpanzees took over this northeastern region. 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 seen in the area. The newly acquired land expanded the Ngogo community’s territory by 22% and provided them with increased access to food and safety, and may allow them to attract more female mates. Dr. Mitani, lead author of the Ngogo study, noted that these observations show 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 seen 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, he concludes, this psychological disposition gave rise to warfare among chimps and humans.

But Dr. Mitani 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 seen at Ngogo most closely resembles 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

Further Reading:

Read about Dr. Richard Wrangham’s theories on the origin of violence and warfare in humans. [PDF]

Check out the Q&A with primatologist John Mitani that accompanies this post.

  12 Responses to “Chimpanzee Warfare?”

  1. [...] This post was mentioned on Twitter by and others. said: Chimpanzee Warfare? [...]

  2. [...] From a relatively new blog, Smells Like Science, an interesting take on chimpanzee warfare. [...]

  3. [...] From a relatively new blog, Smells Like Science, an interesting take on chimpanzee warfare. [...]

  4. I will make it very clear up front that I am not a biologist, so bear with me if my question is ignorant. I have one concern or point of confusion with the article:

    The conclusion is presented as very definitive: …Chimpanzees attack their neighbors as a way to expand their territory. The article, however, doesn’t make it clear why this is so. How is it established that this is the reason for boundary patrols, rather than merely an admittedly advantageous, but incidental consequence?

    For instance, if boundary patrolling were “really” about protecting existing territory from encroachment by another group, or simple out-group hostility, or a mechanism to further within-group dominance (e.g. a male shows off his strength by beating outsiders), or [insert loose speculation here], we might still see this territorial expansion as an eventual outcome. (I’ve no idea whether those speculations qualify as good hypotheses compatible with other data, but I’m not trying to propose a serious alternative, here!)

    Whether or not it makes sense to say that patrolling is for expansion might depend on the evolutionary origin of chimp warfare. If the behaviour evolved under conditions where territorial expansion were a likely outcome of boundary patrolling, that fits. However, if the common evolutionary condition sees groups of equal strength and size, where boundary patrols are likely to inflict equal attrition on both sides, then it’s hard to see how territorial expansion could lend positive pressure for this as an adaptation.

    Now, maybe the paper already excluded alternative hypotheses, or maybe my concern is misguided and based on ignorance. Either way I should be very gratified to have it clarified!

    • This is a very good question.

      What the primatologists who conducted this research know is that over a 10 year period the Ngogo chimpanzee boundary patrols, and consequently killings of neighboring chimps, were focused in a particular area. The neighboring group living in this area experienced an extremely high mortality rate during this time and eventually vacated the area, allowing the Ngogo community to use the area as they would any other part of their territory. The researchers assumed that heavy boundary patrolling of the area had led the neighboring chimps to cede it to the Ngogo chimps.

      But I think part of what you’re asking is: what motivates individual chimps to participate in boundary patrols? Unfortunately we can only speculate on the answer to this question since we can’t communicate with wild chimps. But this brings up another interesting question – why do individuals participate in potentially dangerous boundary patrols when patrols primarily benefit the group as a whole and offer limited benefit for the individual? One somewhat controversial answer is group-level selection, where behaviors and traits evolve in social animals to benefit the group. Another possible answer is that this is (in part) a cultural adaptation, meaning it is a behavior that is learned and passed down to future generations. Although the fact that boundary patrolling has been seen in almost every chimpanzee group that has been observed suggests that it is characteristic of the species and has something to do with biological evolution.

      You also asked about the conditions under which chimpanzee ‘warfare’ evolved. Answering this question is also somewhat speculative, but primatologists have thought about it. The leading explanation for how this sort of ‘warfare’ evolved is called the “Imbalance of Power Hypothesis.” Dr. Richard Wrangam has written about this hypothesis in detail here [PDF]:

      But the idea is that this sort of behavior will emerge when there is intergroup hostility and a power imbalance such that one group can attack another with few repercussions. Wrangam argues that this idea explains the development of warfare in humans and chimpanzees.

      So after all of that I guess I haven’t given you a direct answer. It seems that most primatologists now agree that boundary patrolling and intergroup violence provide a way to exert dominance over neighbors. This behavior can lead to territorial expansion in some cases. Whether or not this is the primary purpose of boundary patrols in an evolutionary or individual sense remains an open question.

  5. [...] From a relatively new blog, Smells Like Science, an interesting take on chimpanzee warfare. [...]

  6. It sounds bad but since we have already seen that they share our desire for war, what would happen if we dropped Neolithic weapons in their midsts? A carved stone hand-axe or a stone tipped spear? Would they be able to see the killing potential and replicate the weapons?

  7. [...] is largely based on evidence of violence in the animal world, particularly observations of violent behavior among chimpanzees, our closest animal relatives. But other social scientists have instead studied modern humans in an [...]

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