A New Study of Chimpanzee Warfare: Hobbes Versus Rousseau

 A chimpanzee war party of males on patrol typically moves in a single-file pattern.

I have written a series of posts on the debate over chimpanzee warfare, which can be found here and here. 

As I have indicated, this is ultimately a debate in political philosophy between Hobbesians who argue that war is rooted in human nature and Rousseaueans who argue that war is a purely cultural trait and that the original state of nature was peaceful.  Since chimpanzees and bonobos are the living primates most closely related to human beings, the dispute over whether chimps and bonobos are naturally warlike or peaceful becomes part of the dispute over the evolutionary roots of human warfare.

A new article favoring the Hobbesian position has just been published in Nature:  Michael L. Wilson, et al., "Lethal Aggression in Pan Is Better Explained by Adaptive Strategies than Human Impacts," Nature 513 (18 September 2014): 414-417.  Here's the abstract:

"Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing.  Two kinds of hypotheses have been proposed.  Lethal violence is sometimes concluded to be the result of adaptive strategies, such that killers ultimately gain fitness benefits by increasing their access to resources such as food or mates.  Alternatively, it could be a non-adaptive result of human impacts, such as habitat change or food provisioning.  To discriminate between these hypotheses, we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades.  Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos.  We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio).  Variation in killing rates was unrelated to measures of human impacts.  Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported."

This is impressive evidence for the Hobbesian argument.  But as indicated by a news story in The New York Times and a  blog post by Marc Bekoff, the Rousseaueans are not ready to surrender.  Four points need to be made here.

First of all, notice that after 50 years of studying 22 communities, there have been only 58 directly observed killings and 41 inferred.  Even if one concedes that intraspecific killing occurs, one can see that it is remarkably rare, and generally these animals are peaceful and cooperative.  The Hobbesians have to admit this.

The second point here for the Rousseaueans is that the bonobos still look even more peaceful than the chimps.  And that's why the hippie bonobos are so loved by the Rousseaueans.

The third point is that Rousseaueans like Brian Ferguson will argue that this study does not go deeply enough into the long history of human disturbance of chimpanzee habitats to see its effects.  Ferguson is writing a book--Chimpanzees and War--that will elaborate his case for the Rousseauean position.  Meanwhile, Ferguson has written a good response to the Nature article.

The final point is that even if lethal coalitional raiding is a natural adaptation for chimpanzees and human hunter-gatherers, complex human warfare as it arose first with bureaucratic states is a largely cultural invention that can be changed through cultural evolution.  Richard Wrangham, one of the senior authors of the Nature article, has stressed this.  And, indeed, if Steven Pinker is correct, the cultural history of recent centuries has shown declining violence.