Chimpanzee violence:
We used to say, “Ask another radiologist – get another diagnosis.”  I often added, “Go back to the first radiologist and get a third diagnosis.”  It’s kind of a joke in radiology, but it seems to be the case in science.

I thought I knew all about chimpanzee violence.  First, there was a lot of it.  It was about as common as violence in undeveloped societies, which is more than what is found in developed societies, even the United States even including warfare.  I don’t know where they put prison or capital punishment when they are adding up violence.  And the numbers might not be all so comparable.  Whereas murders in developed societies tend to be – but of course are not always – acts of individuals, chimpanzees generally kill each other by setting out silent in single file looking for a victim.  Second, what I understood was that there would be a band of chimpanzees that had been getting along well enough.  Then a segment of that band would begin to become isolated from the rest of the band and eventually be driven out.  Then there would be a campaign of systematic panicide (that’s “homicide” among chips, since their genus is named Pan not Homo) with patrols setting out from the main band and singling out and killing the exiles until they had been exterminated.

It all made perfectly good if rather depressing sense.  Lacking any understanding of the relationship between population size and fertility, the chimpanzees nonetheless must control the size of their social group.  Their lamentable behavior in killing each other does accomplish this. 

But now more evidence is on the table.  (Killer Instincts ECONOMIST vol. 395 no. 8688 June 26, 2010 page 83 reviewing work published in Current Biology led by John Mitani and others)  The question that researchers addressed was whether raids against other bands were for the purpose of securing more land or carrying off females.  The obvious answer is none of the above. Accomplishing either would increase the mating pool size of the attackers and if carried too far destroy them with infertility.  But of course the researchers can have had no way of knowing that.  So they narrowed their focus to land and females.

Sure enough, although females were frequently attacked and their young offspring killed, they did not see females joining with the attackers.  I guess not.  Would you?  So it had to be land.  And indeed a bit of particularly desirable land fell to the use of the most aggressive band after a long campaign, but there was no indication that this was the direction of all the attacks.

Well the triumphant and particularly nasty band they had watched might enjoy their ill gotten gains.  If they push it too far, they will pay a terrible price.

So given the weight of evidence now, territorial expansion seems to be the inciter of group violence.  Of course it is blamed for wars among humans as well.  I have my reservations in both cases, but for now I stand corrected.

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