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Don't Blame The Chimps! Who's Responsible For The Lethal Aggression In Sierra Leone's Chimpanzees?
Robert Sussman
Robert Sussman
is Professor of Physical Anthropology at Washington University in St. Louis. He is most recently the author of The Myth of Race: The Troublesome Persistence of An Unscientific Idea.

In the center of Sierra Leone, three male chimpanzees emerge from a forest fragment into the crop field of a small village. In silence, they approach several mango trees. Each chimpanzee ascends a different tree and proceeds to collect every ripe mango, leaving the trees barren. Once on the ground, they return to the forest with their arms full of fruit. The non-native, cultivated fruit has become a staple in their diet and a preferred food source – making up for the limited wild resources within the forest fragment. As they eat the mangoes, they disperse the seeds throughout the forest. Their long-term adaptation is to use human-introduced crops.

Like most chimpanzees across West Africa, these chimpanzees do not live in a protected area or deep in the heart of an expansive forest reserve. Instead, they live scattered around human villages and make use of a habitat that was formed as a result of widespread deforestation. Human impacts dominate the ecology of this population of chimpanzees, from the high population density to the preferred food sources available in their habitat. Some of these impacts may be obvious, while others may be hidden and insidious. Regardless, chimpanzees must adapt to these realities. A high density, a limited core range, an introduced food source, and competition with a neighboring species, will affect the behavioral adaptations of these chimpanzees.

The dynamics of the interactions between this population of chimpanzees and the human communities that they live among are multi-faceted and complex. Everything from habitat destruction, to the types of crops being raided and dispersed by the chimpanzees, to the degree of chimpanzee poaching by humans, carries with it a long and unique history specific to this area. The chimpanzees are adapting behaviorally based on these realities. So, when one reads the paper “Lethal aggression in Pan is better explained by adaptive strategies than human impacts” by Wilson et al. in Nature, we respectfully disagree. The question is: has natural selection favored the evolution of lethal intergroup aggression in chimpanzees and humans as a means of enhancing resources, such as food and mates (Silk 2014)?

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Chimpanzees are animals that, regardless of whether they’re male or female, normally do not have a propensity to kill. In normal, non-threatening situations, they are peaceful animals. As Jane Goodall once said, they are “far more peaceable than humans.” This can be seen in populations that have not been greatly mucked about by humans, such as Goualougo and Fongoli, two reservations in the Congo and Senegal. However, chimpanzees are also very intelligent and sensitive animals and, once their environment is stressed, either directly by humans or indirectly by humans, can develop many behavioral symptoms.  These symptoms have been explored by Margaret Clark (1991). When the population is too large for the area, when there are too many males per female or in one place, when the animals’ normal pattern of dispersion is disturbed, etc., animals, especially males, are able and do kill one another. The specific reasons for these behaviors are likely to be different in different situations and can be seen particularly at the Gombe and Ngogo sites.

Many of the chimpanzee sites are undergoing massive changes. Jane Goodall’s 2003 National Geographic article shows that these animals are losing habitat by conversion of land for agriculture, firewood, commercial logging and mining, etc. This is often created by the loss of once pristine forest. The chimpanzees are being hunted for bushmeat and illegal exotic pet trade. They are stressed by infectious disease, such as Ebola fever. They are stressed by armed conflict. The laws in place to protect apes are weakly enforced.  Other factors include human population growth, the staggering scale of human poverty and disease in populations surrounding chimpanzees, lack of human economic opportunity, political indifference and corruption, general conflict, and weak community involvement in the management of natural resources. For example, in the past the Gombe Park was a very large forest, connected to other forests. Yet, in 1972, “there was little difference between what was inside the park and what was outside” (quote from Lilian Pintea in Goodall 2003). Now Gombe is a 13.5 square mile patch of isolated forest that holds 100 chimpanzees. In some ways, chimpanzees are more like domestic cats since they do not have language or culture. You can’t tell a cat that it can’t go outside because it will get eaten by coyotes. Likewise, you cannot tell chimpanzees why their environment is degraded and how this might affect their behavior. The chimpanzee has to deal with the changes without future knowledge or foresight.

When one looks at the Nature paper (Wilson et al. 2014), one can understand some of the things that are now disturbing chimpanzees and how humans have had a great deal to do with creating the environment that is shaping chimpanzee behavior. However, there are a number of points in this paper that are not clear. First, the authors lump together observed, inferred and suspected killings. This is not good because, as we have seen in past research on infanticide (Bartlett et al. 1993), many of the inferred and suspected killings are often wrong. For example, the suspected killings of chimpanzee males at Maheli were not observed. Upon the Japanese researcher’s return to Maheli, one male was found at another site and predation by lions was highly suspected (Tsukahara 1993). Second, since the Nature article authors are trying to show a high incidence of killing by males, the mix up of male and female killings is confusing. They also seem to mix up whether the animals killed were from within or outside of the group – surely important for their ultimate hypothesis. They also mix up weaned vs. unweaned individuals (adult killings vs. infanticides), which again has little to do with their original hypothesis that males kill other males to gain some reproductive advantage.

Chimpanzees are more complicated than the broad brush they are often painted with –especially when used to reach broad evolutionary conclusions about the nature of humans. With chimpanzee behavior, we see a complex interplay among biological, ecological, and social factors, with many behaviors uniquely suited to specific environments and influenced by the particular variables within those environments (Lanjouw 2002). Making chimpanzees even more complex is the fact that they can learn these behaviors through social influences (Humle 2002). This mix of ecological pressures influencing social transmission can be seen in chimpanzees adapting to specific environments by 1. learning to make tools (McGrew 1992); 2. which medicinal plants in their environment to consume (Hoffman et al. 2004); and even 3. predation techniques (Schöning et al. 2008).

It is also well documented that anthropogenic effects influence chimpanzee behavior. From influencing food choices in cultivated landscapes (Hockings et al. 2009) to chimpanzees maintaining vocal silence in areas where they have been hunted (Halloran et al. 2013), there are numerous examples of chimpanzees responding to human presence through behavioral means.

It is against this backdrop of the near limitless factors that may shape chimpanzee behaviors and the infinite varieties of human influences on those behaviors that the Nature study commits its most profound oversimplification. The study considers 3 variables of human impacts: “provisioning”, “area”, and “disturbance.” According to the methods, it assigns a “human disturbance” score for each included community designating a score from 1-4 on 5 possible categories of human disturbance: (1) disturbance to habitat; (2) degree of harassment of study animals by people; (3) amount of hunting of study animals; (4) degree of habituation to human observers at beginning of studies; and (5) whether major predators have been eliminated. It is not clear in the paper what parameters were used in each category to assign these scores.   In this, the score relies on subjective ratings of disturbances that are immediately apparent in the present.  However, it ignores more insidious disturbances that may be removed from the present time and place.   At the same time, it universalizes the highly variable nature of “human disturbances.”

The Tonkolili Chimpanzee Site in Sierra Leone is a case in point as to how human disturbances can be unique, multifaceted, and difficult to rate on a universal scale. It is unclear how the situation at the Tonkolili Site would rate on the human disturbance scale. A “standing crop nest” count of the site reveals a staggering population density of 2 chimps per square kilometer (Halloran unpublished raw data). It is likely that such density may be attributable to large-scale deforestation across Sierra Leone brought on by mining interests and biofuel plantations, some of which are over 100 km from the site. The chimpanzees at this site also have a long history of interacting with humans in a variety of ways.   Because the density is so high in the area, the chimpanzees rely on crop raiding to sustain themselves.   In defense of these crops, the villagers have taken to hunting the chimpanzees. In addition, the horrendously brutal Sierra Leonean Civil War decimated both human and chimpanzee populations across the region. The variables at the Tonkolili Site beg the following questions: Would the deforestation that is distant but possibly influencing the population density of the chimpanzees count on the “disturbance to habitat” scale? Would the crop-raiding, essential to sustain the population, count as “provisioning”? What “degree of harassment” was a war, years ago, scored as, or was it?

The Tonkolili Site showcases the fact that human disturbances are too specific and variable to be boiled down to three variables and an arbitrary human disturbance scale. Chimpanzees adapt to ecological pressures through learned behaviors that may continue to be socially influenced after the immediate pressures may no longer be apparent. Their behaviors are too elaborate to be generalized. Such a generalization creates a false and misleading characterization of the species. The use of this characterization to make any sort of statement about the nature of the human species is highly questionable, and we believe is wrong.

Now if we look at figure 2 of the Nature paper we see how this figure can support different hypotheses: for example, that chimpanzees are normal animals who under certain situations, usually related to human interference, can be put in situations that lead to killing. If one looks at the three highest cases of killing, 59% of all intergroup killings of grown individuals come from just 9 years of the 426 years of observations.   Gombe (Kahama and Kasakela) and Ngogo have a rate of 1.9 killings per year (Brian Ferguson in Horgan 2014). These sites have seen intense killings occur over a short period of time. We know that these sites have been under a great deal of human disturbance, for all kinds of reasons, that has affected the population and its demography. Chimpanzees usually live in quite dispersed populations, where animals rarely mix and move together. Under normal situations, males do not usually move in groups. In the other 417 observation years, killings calculate to .03 per year or one animal “killed” every 33 years (Ferguson in Horgan 2014). The exceptionally bloody 9 years are true outliers, with a weaned male kill rate over 40 times higher than the rest of the record.

We would argue that, under certain circumstances usually caused by some kind of human interference, chimpanzee populations undergo extreme stress, which leads to unusual situations. These seem to be related to unusual population demography and intense interactions. Otherwise, we suspect that chimpanzees live generally normal lives, with extremely few aggressive interactions.

The Tonkolili Chimpanzees is a very small population of chimpanzees. However, they reveal an important point: human impacts are incredibly variable. Until a few years ago, like most chimpanzees in the world, they remained unstudied by primatologists. Given that recent surveys have found more chimpanzees living in fragments outside of protected areas than within (Brcnic 2010), we are left to imagine all the other chimpanzee groups living with their own unique circumstances dominated by human impacts – unstudied by primatologists and unrepresented in this paper.

Robert Sussman is Professor of Physical Anthropology at Washington University in St. Louis. He is most recently the author of The Myth of Race: The Troublesome Persistence of An Unscientific Idea.

A.R. Halloran is a primatologist at Lynn University who specializes in primate vocal communication. He has extensively studied chimpanzee calls and is the author of The Song of the Ape.


Bartlett, TQ., Sussman, RW, Cheverud, JM, & (1993). Infant killing in primates: a review of observed cases with specific reference to the sexual selection hypothesis. American Anthropologist. 95:958-990.

Brncic, T. M., Amarasekaran, B., & McKenna, A. (2010). Final report of the Sierra Leone National Chimpanzee Census Project. Tacugama Chimpanzee Sanctuary, Freetown, Sierra Leone, 118.

Goodall, J. (1986). The chimpanzees of Gombe: patterns of behavior. Cambridge, BelKnap Press.

Goodall, J. (2003). What’s new at Gombe. National Geographic. 203:76-89.

Halloran, A. R., Cloutier, C. T., & Sesay, P. B. (2013). A previously undiscovered group of chimpanzees (Pan troglodytes verus) is observed living in the Tonkolili District of Sierra Leone. American journal of primatology, 75 :519-523.

Hockings, K. J., Anderson, J. R., & Matsuzawa, T. (2009). Use of wild and cultivated foods by chimpanzees at Bossou, Republic of Guinea: feeding dynamics in a human‐influenced environment. American Journal of Primatology, 71: 636-646.

Horgan, J. (2014). Anthropologist Brian Ferguson challenges claim that chimp violence is adaptive. September 18. Scientific American Blog.

Huffman, M. A., & Hirata, S. (2004). An experimental study of leaf swallowing in captive chimpanzees: insights into the origin of a self-medicative behavior and the role of social learning. Primates, 45: 113-118.

Humle, T., & Matsuzawa, T. (2002). Ant‐dipping among the chimpanzees of Bossou, Guinea, and some comparisons with other sites. American Journal of Primatology, 58:133-148.

Lanjouw, A. (2002). Behavioural adaptations to water scarcity in Tongo chimpanzees. Behavioural diversity in chimpanzees and bonobos. Cambridge, Cambridge University Press, 52-60.

McGrew, W. C. (1992). Chimpanzee material culture: implications for human evolution. Cambridge, Cambridge University Press.

Power, M. (1991). The egalitarians human and chimpanzee: an anthropogical view of social organization. Cambridge: Cambridge University Press.

Schöning, C., Humle, T., Möbius, Y., & McGrew, W. C. (2008). The nature of culture: technological variation in chimpanzee predation on army ants revisited. Journal of Human Evolution, 55:48-59.

Silk, Joan, B. (2014). Animal behaviour: The evolutionary roots of lethal conflict. Nature, 513:321-322.

Tsukahara, T. (1993). Lions eat chimpanzees: the first evidence of predation by lions on wild chimpanzees. American Journal of Primatology, 29:1-11.

Wilson, et al. (2014). Lethal aggression in Pan is better explained by adaptive strategies than human impacts. Nature, 513:414–417.



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  1. David Sloan Wilson says:

    Thanks to Robert Sussman and A.R. Halloran for this in-depth critique of the Wilson (not me!) et. al. article. One purpose of TVOL is to showcase science as a process of constructive disagreement and this article provides a good example.

    The following points are helpful in thinking about the innateness of aggression.
    1) All species have evolved to be phenotypically plastic, calibrating their behaviors to their environmental context.
    2) Aggression is an adaptive behavior for some environmental contexts.
    3) Many species have therefore evolved to be aggressive in appropriate contexts.

    Given these propositions, imagine that chimp populations are typically at a low density and that this is a context that doesn’t warrant aggression. Now imagine that human disturbance creates unusually dense chimp populations and that this context does warrant aggression. Chimps become aggressive because of human disturbance, but aggression is also part of their evolved repertoire.

    Learning can be understood in a similar way. It is an evolved mechanism for adapting to one’s current environment in an open-ended fashion, and this includes aggression. It is natural and a response to a novel environment at the same time.

  2. Joost van Schijndel says:

    The instance described in link to occurred a few years before I spent time in Loango. The habitat consists of large tracts of coastal savannah/forest, swamps, etc., with an incredibly low human population density. I was in one camp with about 5 others, the other research camp was 10km away with perhaps 8 people, no villages within 10-20km, no agriculture in the forest, practically no poaching.