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Why Chimpanzees Kill

The views expressed are those of the author and are not necessarily those of Scientific American.

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male chimpanzee

Image: Jenny Varley via Flickr

Chimpanzees are mostly peaceable creatures, spending much of their time foraging for food and grooming each other. But occasionally they kill their own kind. Why they engage in these lethal bouts of aggression has been uncertain. One theory holds that killing is an evolved strategy for reducing competition for resources; another posits that human disturbance—including hunting and deforestation—has triggered the behavior. Now a large study of killings in chimp communities across Africa has cast new light on the dark side of our closest living relatives.

Michael Wilson of the University of Minnesota and his colleagues amassed data on chimps in 17 communities at 10 sites in west, central and east Africa whose members had long been observed by researchers. For each community the team obtained data on demography, population size, ranging behavior of the group, and so forth. The team also compiled data on bonobos from four communities, to determine whether bonobos really are less aggressive than chimps, as is commonly asserted. Wilson presented the study results on April 12 at the annual meeting of the American Association of Physical Anthropologists in Portland, OR.

Focusing on 86 cases in which scientists had observed the killings or had inferred them based on compelling evidence, Wilson and his collaborators found that kills occurred in most of the chimpanzee communities and that victims tended to be infant and adult males outside the killer’s social group. Most of the killings were conducted by groups of males.

Interestingly, human disturbance did not appear to be a factor in the kills. Indeed the community with the most kills—a group at the site of Ngogo in Uganda’s Kibale National Park—had the least human disturbance. Neither was the ratio of males to females in a group a factor. What did appear to be a factor was the number of males in a group: the higher the number of males in a group, the higher the number of kills. Ngogo, the community with the highest rate of kills per year, also had the highest number of adult males.

“The number of males is important because the more males there are, the more competition there is for mates in the community,” Wilson explains. The number of males also equals the community’s fighting strength for defending their territory and the food resources in it. Males in communities with more males can afford to be more aggressive because they have backup. The researchers did not identify a particular number of males that triggered killing. Rather, Wilson offers, the key may be the relative numbers of males in neighboring communities—that is, the balance of power.

“This tells us something about human evolution,” Wilson comments. He notes that although scientists do not know whether humans and chimps inherited their capacity for lethal aggression from a common ancestor or whether it arose in both species through convergent evolution, “lethal aggression is related to power asymmetries where members of one group can kill others with low cost.”

As for the bonobos, this study bolsters the claim that they are less aggressive than chimpanzees: there were no clear-cut homicides in any of the bonobo communities. Another presentation given at the meeting provided a possible clue to the apparent absence of male aggression among these apes: Victoria Wobber of Harvard University and her colleagues studied testosterone levels in chimpanzees and bonobos from infancy to adulthood and found that whereas chimpanzee testosterone levels surged during adolescence (particularly among males), bonobo testosterone production remained consistent over the course of development.


Kate Wong About the Author: Kate Wong is an editor and writer at Scientific American covering paleontology, archaeology and life sciences. Follow on Twitter @katewong.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. hogansherrow 11:42 am 04/19/2012

    A nice piece covering a very good study and talk presented at the AAPA’s in Portland last week. Unfortunately, the study, talk and this piece don’t actually address the question of WHY chimpanzees kill. Rather this study presents the conditions under which chimps kill. It’s a subtle, but important difference. Also, the differences in T between chimps and bonobos may be related, but it’s a stretch to say that the Harvard study is somehow related.

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  2. 2. Dolmance 12:18 pm 04/19/2012

    It’s an interesting question, but I’m far more interested in knowing why they have this Imperial Byzantium like impulse to bite off faces, hands, feet and genitals.

    I understand there are chimpanzees that have vocabularies consisting of hundreds of words. Perhaps some scientist might consider simply asking them.

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  3. 3. naya8 1:48 pm 04/19/2012

    If we follow a mathematical sense that is:
    chimpanzees + Humans = the most related organisms
    male chimpanzees kills and male humans kills
    and the tow have the same concequences for killing; then:
    chimpanzees + humans = the same reason for killing
    and also:
    chimpanzeez + humans = innate predisposition for killing
    Please tell this to JOHN HORGAN
    Nihaya khateb

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  4. 4. Extremophile 2:52 pm 04/19/2012

    One thing is strange: The study considers “hunting and deforestation” as cases of human disturbance, but not the presence and work of researchers.

    If one asks what the influence of researchers is on Chimp behavior, the answer may be completely different.

    I wonder especially, how often scientists have given medication to chimps, out of the (understandable) wish to not lose their object of studies, and possibly out of an (understandable) feeling of compassion in a health crisis.

    At least one such case has been documented in the year 2000.

    The assumption that the presence of researchers has no influence on chimp behavior would need to be verified before conclusions are based on it.

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  5. 5. kclancy 2:52 pm 04/19/2012

    I missed the Wilson talk, but saw the Wobber. That dataset was really very cool. And if I remember correctly, there was higher intra- and interindividual variation in T among chimps compared to bonobos, AND bonobos had more stable T concentrations through development (rather than increasing over time like chimps). Looking forward to reading the paper!

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  6. 6. jtdwyer 4:17 pm 04/19/2012

    I’m just passing through, but testosterone and adrenaline are very important to human aggression…

    Also, I seem to recall seeing something that indicated that male chimp groups often gang up on an individual whose assertiveness oversteps his social position?

    Humans sometimes behave similarly…

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  7. 7. jhorgan 4:27 pm 04/19/2012

    Why do chimps and not bonobos serve as models for ancestral humans Wilson, Wrangham and others say chimp violence is significant and bonobo nonviolence insignificant, but this emphasis reveals more about their bias in favor of the war-is-in-our-genes thesis than it does about human evolution.

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  8. 8. naya8 1:36 am 04/21/2012

    To jhorgan:
    Chimps do not serve as models for ancestral humans other than bonobos,they and humans share more than 98% of their DNA .Chimps are our closest relatives. You can not separate between the tow issues: war-is-in-our-genes thesis and human evolution, they goes together.

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  9. 9. dudeinhammock 5:57 pm 04/21/2012

    If m/f ratio wasn’t a factor, how exactly does the greater number of males lead to increased competition for mates? I’d also be interested in knowing whether these groups of chimps are currently or have ever been provisioned. Interesting research.

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  10. 10. dudeinhammock 5:59 pm 04/21/2012

    Naya8: Unlike you, jhorgan understands that chimps and bonobos are equally closely related to humans. He also knows how to spell the number two.

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  11. 11. naya8 12:47 pm 04/22/2012

    dudeinhammock:How you are so sure that chimps and bonobos are equally closely related to humans in their microstructure of their basic brain? the fact that they are closely related in their DNA,could not tell nothing about sharing the same fragment of DNA responsible for the phenotype of killing.for your information:even between humans who share 99% of their DNA, you could find one human more closely related to chimps in that character of killing than to other human. For your second note I will not respond, it’s not worthy.

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  12. 12. Extremophile 5:42 pm 04/22/2012

    @ naya8: “You can not separate between the tow issues: war-is-in-our-genes thesis and human evolution, they goes together.”

    We don’t even know whether war is in the genes of chimps. How can you be sure that it is in ours?

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  13. 13. naya8 1:15 am 04/23/2012

    @Extremophile: My observations through real life give a strong indication for genetic basis for behavior of organisms.Complicated organisms like humans, their behavior is modified by environment, but still the geneom is the base.As long as humans are primitive they behave with instinct to kill in certain cercumstances.Dveloped brain and well-evolved,makes its interpretation to avoid aggressive behavior in all its shapes including killing and war, then after hundreds of years behaving without aggression humans became peaceful creatures, becuaes genes that are not used get to be dormant.All over the history we have been wittnessing this character of killing in all the cultures of human-beeing. My own theory claims that our brain is an organ like all other organs in our body;then genes are responsible to it’s structure.However, the role of genes is ended at this point of involvement.Depending on the quality of the micro-structure of brain , that brain could respond to any environmet in its way according to that structure that he inherited from his parents exactly like inheriting the shape and structure of any organ in his body.The bottom line is:Our brain-structure in some aspects (the region responsible for killing)is identecal to that structure in chimp’s brain.

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  14. 14. trafficproducts 11:07 pm 04/23/2012

    We humans are to protect chimpanzees

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  15. 15. Bops 9:52 pm 04/27/2012

    I think Chimps are small nasty animals that enjoy fighting for almost any reason.

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