Ngogo Chimpanzees from the Western Group Attacking the Central Group
As I have indicated in previous posts, evolutionary anthropologists studying the evolution of war and peace have been divided into Hobbesians who believe that our hunter-gatherer ancestors were naturally violent and warlike and Rousseauans who believe that hunter-gatherers were naturally peaceful. But in setting up this debate as Hobbes versus Rousseau, they have overlooked the position of Locke. This is a serious mistake, because the weight of the evidence and argumentation on this issue today is on the side of Locke's account of the state of nature as a state of peace that tended to become a state of war. It seems now that while Hobbes was partly right and partly wrong about the state of nature, Rousseau was mostly wrong, and Locke was mostly right.
THE GOMBE CHIMPANZEES
I have also indicated, how this debate extends to chimpanzees. The Hobbesians see chimpanzees as showing an evolved propensity to war, and the clearest evidence for this is the chimpanzee civil war that Jane Goodall observed in Gombe in the 1970s. At the beginning of 1973, she and her colleagues noticed that the chimps had formed two separate communities--the northern or Kasakela community, based on the valleys of the Kakombe and Kasakela rivers, and the southern or Kahama community, based on the valley of the Kahama river. By early 1974, they saw the first of a series of attacks by the Kasakela community on the Kahama community, which led over four years to the complete annihilation of the Kahama community, which allowed the Kasakela chimps to expand their territory over the area once claimed by the Kahama chimps. Since human beings are closely related to chimps phylogenetically--chimps and humans evolved from a shared common ancestor some 5-6 million years ago--chimpanzee war could show the evolutionary roots of the human propensity to war.
But the Rousseauans insist that in normal circumstances, chimpanzees are naturally peaceful, and they explain the Four-Year War in Gombe as provoked by the abnormal circumstances created by human impacts on the chimps--particularly, their being fed bananas at Goodall's camp. During her first two years at Gombe, Goodall was frustrated by her failure to closely observe the chimps because if she attempted to follow them, they ran away from her. She spent a lot of her time sitting on a hill trying to see the chimps through binoculars. But then in the summer of 1962, she discovered that if she set out bananas every day, the chimps would crowd into her camp. Soon they were eating 30, 40, or even 60 bananas per day.
Fighting over the bananas caused increased aggression among the chimps. They even threw rocks at humans who got too close. Goodall was shocked by the violence that came from competition for bananas. Eventually, she tried to lower the violence by restricting the supply of bananas. This did reduce the violence. But it also drastically reduced the nutritional benefits that the chimps had gained from the bananas. Some of Goodall's Rousseauan critics--particularly, R. Brian Ferguson (2023)--have argued that the restriction of the banana feeding created the frustration that prompted the violence of the Four Year War. When Kasakela males sought food in the southern areas of their range, Kahama males chased them away. But when Kahama chimps came into Goodall's camp, they were always fed bananas. So it's not surprising that the Kasakela chimps developed a deep animosity toward the Kahama chimps, although they had once belonged to the same community.
THE NGOGO CHIMPANZEES
Now we have a new report--in the journal Science--of a long-running chimpanzee civil war that has divided the Ngogo chimpanzees in the Kibale National Park in Uganda (Sandel et al. 2026). The scientists who have studied this group since 1995 have never fed them bananas or any other food. So artificial provisioning cannot be a factor in explaining their violence.
The Ngogo group is the largest group of chimpanzees ever studied by scientists. The size has increased from 118 individuals in 1998 to 201 in 2016. The number of mature males (over 12 years old) has fluctuated around 40 individuals.
The Ngogo chimpanzees range over a large territory, which they have expanded through war with their neighbors. From 1999 to 2008, they ranged over a territory of about 29 square kilometers (18 square miles). During this time, they engaged in intensive border patrols, particularly along their northeastern border. In 2009, they engaged in numerous lethal attacks on their neighbors to the northeast, which allowed them to expand their territory, adding about 6 square kilometers (3.7 square miles) to their territory. This seemed to confirm the "resource acquisition hypothesis" for explaining chimpanzee warfare as adaptive for territorial expansion (Mitani, Watts, and Amsler 2010).
Prior to 2015, the Ngogo chimpanzees belonged to a single group, although there was a social substructure of two to four "clusters" that could be identified each year. They were generally divided into a Western cluster and a Central cluster, but this cluster membership was fluid, because individuals could move freely between these clusters. But then on June 24, 2015, when members of the Western and Central clusters approached each other near the center of their territory, the Western chimpanzees ran away, and the Central chimpanzees chased them. For at least six weeks, the two groups avoided one another, which had never previously been observed.
In 2016, the Western males started territorial patrols directed toward the Central chimpanzees. In 2017, what was once the center of a shared territory became a border. In 2018, the Western group initiated lethal attacks on the Central group. These aggressive attacks by the Western group have continued up to the present. Remarkably, the Western group is smaller than the Central group. In 2018, the Western group had a population of 83 (including 10 adult males and 22 adult females), while the Central group had a population of 107 (including 30 adult males and 39 adult females). The death toll from the war drove the population of the Central group down to 80 in 2024.
This Ngogo civil war differs from the Gombe civil war in one striking way. At Gombe, the larger group (the Kasakela chimps) attacked and defeated the smaller group (the Kahama chimps). This led some primatologists like Richard Wrangham to develop the "imbalance-of-power hypothesis"--that larger groups (particularly, in the number of adult males) will attack smaller groups. The Ngogo civil war is evidence against that hypothesis.
Actually, the scientists who have studied the Ngogo civil war have not been able to agree on any alternative hypothesis for explaining the war. Some of them have wondered, however, whether shifting social bonds could explain the rupture that led to the war. In 2014, five adult males died, perhaps because of disease. Dr. Aaron Sandel, one of the leaders of the Ngogo Chimpanzee Project, has suggested that these five individuals might have been social bridges that held the Ngogo groups together into one community. So that the contingency of their deaths in one year created the conditions for the polarization of the community that then led to the war.
Similarly, if Abraham Lincoln had died before or shortly after the beginning of the American Civil War, the history of that war would probably have been radically altered. Or if Thomas Crooks had assassinated Donald Trump on July 13, 2024, American politics might be very different from what it is today. America might be less polarized. And perhaps there would have been no war with Iran. Such unpredictable contingencies often determine the history of war and politics.
INNER DEMONS, BETTER ANGELS, AND LOCKEAN LIBERAL NICHE CONSTRUCTION
So what, if anything, does the history of chimpanzee wars teach us about the evolutionary history of human war and peace? How we answer that question may depend a lot on whether we think humans are more closely related to chimpanzees or to bonobos.
Chimpanzees seem to be far more aggressive and violent than are bonobos. Male chimps attack females and other males. Sometimes these attacks are lethal. Male chimps also form coalitions with other males to assert a male dominance hierarchy over females and other males. These male coalitions also patrol the borders of their community, and they can launch attacks against other communities--even to the point of annihilating the whole community in war.
By contrast, bonobos have never been observed to engage in lethal attacks on other bonobos. Female bonobos seem to be dominant over males. And the females form coalitions with one another to attack males and mediate their conflicts. In contrast to chimps, bonobo communities have never been observed to go to war with one another. Bonobos from different communities can interact with one another peacefully. All of this peacemaking depends on mutually pleasurable bisexual lovemaking--rubbing their genitals together--females with other females and with males. This is why the bonobos have been dubbed the "hippie apes" who "make love not war."
Comparing human beings with these two ape species in working out the evolutionary links between the three species has provoked a debate among evolutionary biologists and social scientists. The Hobbesian scientists argue that human beings are closer to chimps, which shows that the human state of nature was a state of war. The Rousseauean scientists argue that human beings are closer to bonobos, which shows that the human state of nature was a state of peace.
In my posts on bonobos and the human state of nature, I have argued that Locke's account of the state of nature is closer to the truth than either Hobbes' or Rousseau's, and that evolved human nature combines the natural propensities of both chimps and bonobos. As Steven Pinker would say, our human nature has both Inner Demons and Better Angels. Lockean liberalism constructs a cultural niche of social institutions, mental attitudes, and moral traditions that tame the Inner Demons while eliciting the Better Angels to motivate voluntary cooperation and nonviolent relationships.
But in contrasting bonobos and chimpanzees, we should not assume that bonobos are utterly peaceful. That bonobos are often aggressive in their attacks on one another is made clear in new research by Maud Mouginot and her colleagues (Mouginot et al. 2024).
The message from this study as reported in the press--as in Carl Zimmer's report for the New York Times--is that "male bonobos commit acts of aggression nearly three times as often as male chimpanzees do." That would seem to deny the common view that chimps are far more aggressive than bonobos. But if you read the article carefully, you will see that the story is much more complicated than that.
Mouginot and her colleagues employed what scientists studying animal behavior call the "focal-animal sampling" method (Altman 1974). All occurrences of specified actions of an individual, or specified group of individuals, are recorded for a pre-determined period of time.
For their study, they had all-day focal follow data for 14 chimpanzee adult males from two communities in the Gombe National Park in Tanzania and 12 bonobo adult males from three communities in the Kokolopori Bonobo Reserve in the Democratic Republic of the Congo. They had recorded hundreds of aggressive dyadic interactions, including contact aggression (physical contact such as hit, pull, bite, kick, or jump-on) and non-contact aggression (such as charge and chase). They had also recorded whether the focal-male was the aggressor or the victim and whether he was interacting with another male or with a female.
They wanted to use this data to test the "self-domestication hypothesis" of Brian Hare and Richard Wrangham, who have suggested that bonobos evolved to be less aggressive than chimpanzees just as dogs evolved to be less aggressive than wolves. Humans selected less aggressive (or friendlier) wolves to become their companions, and over time, wolves evolved into dogs through domestication by human selection. Similarly, if female bonobos formed coalitions to punish aggressive males, and if females preferred to mate with less aggressive males, which would tend to produce less aggressive offspring, bonobos could have been self-domesticated for being less aggressive or friendlier to one another (Hare, Wobber, and Wrangham 2012).
Moreover, Hare and Wrangham have also suggested that humans could have undergone a similar process of evolution by self-domestication to be less aggressive or friendlier towards individuals within their community (Hare 2017; Hare and Woods 2020; Wrangham 2019). I have extended this idea of human self-domestication to explain the evolution of Lockean liberalism and the bourgeois virtues as symbolic niche-construction.
What Mouginot and her colleagues have found does not deny the self-domestication hypothesis of Hare and Wrangham, although it might require some refinement in the theory. They found that there was a higher rate of male-male contact aggression among bonobos than chimpanzees. And in both species, the more aggressive males had higher mating success. But they found no evidence to contradict the observation that bonobos never kill other bonobos, while chimpanzees do kill other chimpanzees in fighting both within and between communities.
One possible explanation for why bonobo males engage in more non-lethal aggression with other males than do chimpanzee males is that since bonobo females prevent males from forming coalitions, bonobo males can attack other males without suffering reprisals from male coalitions.
As Hare told Carl Zimmer, the one dramatic difference in aggressiveness between the two species remains: "Chimpanzees murder, and bonobos don't."
Frances White, one of the leading scientists studying bonobos in the wild, has explained that in the wild bonobo females serve a policing function, in that they intervene in fights to moderate conflicts through impartial mediation, because they benefit from living in a stable social order that is not disrupted by violence.
She has also observed that bonobos--like all primates--show a range of personality types, so that some individuals have more violent temperaments than others, and consequently the occurrence of violence can depend on the contingency of whether there are such violent individuals in the group. She said that many of the deaths of the males comes from "testosterone poisoning"--young males vigorously displaying their virility in the forest canopy can kill themselves by slamming into a tree.
She has also said that if dominant males are grouped together in zoos without females who can moderate their male conflicts, then nasty fighting is likely to break out. She has explained then that what the females are doing in the wild groups in pacifying conflicts is "niche construction"--behavior that creates a social environment in which stable and peaceful cooperation is adaptive.
In most of evolutionary theory, we think of organisms as carrying genes, and the inheritance of these genes by the next generation depends on organisms surviving and reproducing according to chance and natural selection in their environments. But there is another process of evolution that arises from organisms changing their environments, which modifies the natural selection pressures in their environments. This is evolutionary niche construction (Odling-Smee, Laland, and Feldman 2003). Among many animals, evolutionary niche construction includes the transmission of culturally learned traditions. And among human beings, it includes the transmission of culturally learned symbolic systems such as art, science, religion, and philosophy.
I have suggested that the history of classical liberalism is evolutionary niche construction, and that this is a big part of Pinker's argument: the history of classical liberal philosophy has created a cultural moral environment of liberalism in which peaceful cooperation and declining violence are adaptive. (This is what Deirdre McCloskey would identify as the work of rhetorical entrepreneurs in the marketplace of ideas who have used moral persuasion to create a liberal culture that honors the bourgeois virtues.)
As Pinker argues, human nature is a mixture of Inner Demons and Better Angels. Human beings are innately predisposed to violence by their Inner Demons, but the expression of those predispositions is not "hydraulic"--that is, a drive that must necessarily be satisfied--but "strategic"--that is, a propensity that is responsive to environmental triggers. Classical liberalism constructs a cultural niche of social institutions, mental attitudes, and moral traditions that tend to elicit the Better Angels to motivate voluntary cooperation and nonviolent relationships.
In this way, human nature constrains and enables but does not determine human culture and individual judgment. Within the constraints of human nature as a mixture of Inner Demons and Better Angels, classical liberalism can foster those cultural traditions and individual judgments that limit the Inner Demons and channel the Better Angels towards a system of liberty and voluntarism.
REFERENCES
Altman, Jeanne. 1974. "Observational Study of Behavior: Sampling Methods." Behaviour 48: 227-65.
Ferguson, R. Brian. 2023. Chimpanzees, War, and History: Are Men Born to Kill? Oxford, Oxford University Press.
Goodall, Jane. 1986. The Chimpanzees of Gombe. Cambridge: Harvard University Press.
Hare, Brian. 2017. "Survival of the Friendliest: Homo sapiens Evolved via Selection for Prosociality." Annual Review of Psychology 68: 155-86.
Hare, Brian, and Vanessa Woods. 2020. Survival of the Friendliest: Understanding Our Origins and Rediscovering Our Common Humanity. New York: Random House.
Hare, Brian, V. Wobber, and Richard Wrangham. 2012. "The Self-Domestication Hypothesis: Evolution of Bonobo Psychology Is Due to Selection Against Aggression." Animal Behaviour 83: 573-85.
Mitani, John C., David P. Watts, and Sylvia J. Amsler. 2010. "Lethal Intergroup Aggression Leads to Territorial Expansion in Wild Chimpanzees." Current Biology 20:R507-R508.
Odling-Smee, F., Kevin Laland, and Marcus Feldman. 2003. Niche Construction: The Neglected Process in Evolution. Princeton University Press.
Pinker, Steven. 2011. The Better Angels of Our Nature: Why Violence Has Declined. New York: Penguin Books.
Sandel, Aaron A. et al. 2026. "Lethal Conflict After Group Fission in Wild Chimpanzees." Science 392(9 April):216-220.
Wrangham, Richard. 2019. The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution. New York: Pantheon Books.
Zimmer, Carl. 2024. "No 'Hippie Ape': Bonobos Are Often Aggressive, Study Finds." The New York Times, April 12.
No comments:
Post a Comment