Check out the post, Chimpanzee Warfare?, that accompanies this Q&A
A study published in the journal, Current Biology, this past summer found that chimpanzees kill members of neighboring groups in order to assert dominance and expand their territory. Researchers studied a chimpanzee community at the Ngogo site in Kibale National Park in Uganda over a 10-year period between 1999 and 2009. They observed male chimps participating in boundary patrols which involved incursions into neighboring territory and sometimes deadly attacks on neighboring chimps. These violent patrols eventually led the Ngogo chimpanzees to annex a portion of their neighbor’s territory. Dr. John Mitani, a primatologist at the University of Michigan and the study’s lead author, was kind enough to take the time to answer a few of my questions.
What sort of behavior typically precedes a boundary patrol? Do they often happen at a particular time of day? How often do boundary patrols occur?
I’m not sure that there’s any particular sorts of behavior that precedes patrols. Instead, large gatherings of males often precipitate these affairs. We’ve shown this in a paper that we published a few years ago. Why are large parties of males a necessary prerequisite for patrols? These are potentially dangerous activities. Male chimps appear to reduce the inherent risk of patrolling by doing so in large parties.
Patrols can happen at any time of day, but they occur more frequently in the afternoon than in the morning. Why? Patrols are energetically costly as patrollers cover a lot of ground. Easier to do this on a full stomach toward the end of the day, than when you’ve first up at the start of the day.
At Ngogo, patrols occur once on average every 10 – 14 days.
Do you think that intergroup aggression for the purpose of terretorial expansion is something that all or most chimpanzee groups do, or is it more specific to the Ngogo population?
Patrols were first documented in the 1970s at Gombe, site of Jane Goodall’s long-term research. They have been observed at other sites as well whenever chimps live side-by-side others.
Were there any observations of neighboring groups participating in boundary patrols into Ngogo territory?
We have observed encounters between neighboring chimpanzees and the Ngogo chimpanzees within the Ngogo community territory. Because we don’t follow the neighbors, we can’t say whether they were on patrol at the time.
Do you have any thoughts on what precipitates this behavior as a long-term strategy for territorial expansion? Is it related to environmental pressures or a need for increased resources or female mates?
The take home of our paper was 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.
Do you have any ideas about how targeted groups respond to high mortality rates resulting from long-term lethal aggression?
In this particular case, the targeted group ceded a large portion of their former territory to the Ngogo chimpanzees. We can’t say what other behavioral changes might have occurred in the chimpanzees living in this group as we don’t follow and observe them directly.
A lot of people who’ve read about your research think “That sounds just like human warfare,” although you noted in the Current Biology publication that human warfare varies widely, making it difficult to draw conclusions about the relationship between Ngogo behavior and human warfare. Could you respond to claims that the Ngogo behavior relates to the origins of human warfare?
It does sound like human warfare, but only in part. The thing I want to emphasize is that human warfare is quite variable. Fighting others for land is only one reason humans go to war today. Of course, different reasons may not have applied to our human ancestors and to the common ancestor of chimps, humans, and bonobos. But most of this borders on the speculative.