Why chimpanzees attack and kill each other

June 21, 2010, University of Michigan
Common chimpanzee in the Leipzig Zoo. Image credit: Thomas Lersch, via Wikipedia.

Bands of chimpanzees violently kill individuals from neighboring groups in order to expand their own territory, according to a 10-year study of a chimp community in Uganda that provides the first definitive evidence for this long-suspected function of this behavior.

University of Michigan primate behavioral ecologist John Mitani's findings are published in the June 22 issue of Current Biology.

During a decade of study, the researchers witnessed 18 fatal attacks and found signs of three others perpetrated by members of a large community of about 150 chimps at Ngogo, Kibale National Park.

Then in the summer of 2009, the Ngogo chimpanzees began to use the area where two-thirds of these events occurred, expanding their territory by 22 percent. They traveled, socialized and fed on their favorite fruits in the new region.

"When they started to move into this area, it didn't take much time to realize that they had killed a lot of other chimpanzees there," Mitani said. "Our observations help to resolve long-standing questions about the function of lethal intergroup aggression in chimpanzees."

Mitani is the James N. Spuhler Collegiate Professor in the Department of Anthropology. His co-authors are David Watts, an anthropology professor at Yale University, and Sylvia Amsler, a lecturer in anthropology at the University of Arkansas at Little Rock. Amsler worked on this project as a graduate student at U-M.

Chimpanzees (along with bonobos) are humans' closest living relatives. Anthropologists have long known that they kill their neighbors, and they suspected that they did so to seize their land.

"Although some previous observations appear to support that hypothesis, until now, we have lacked clear-cut evidence," Mitani said.

The bouts occurred when the primates were on routine, stealth "boundary patrols" into neighboring territory. Amsler, who conducted field work on this project described one of the attacks she witnessed far to the northwest of the Ngogo territory. She and a colleague were following 27 adult and adolescent males and one adult female.

"They had been on patrol outside of their territory for more than two hours when they surprised a small group of females from the community to the northwest," Amsler said. "Almost immediately upon making contact, the adult males in the patrol party began attacking the unknown females, two of whom were carrying dependent infants."

The Ngogo patrollers seized and killed one of the infants fairly quickly. They fought for 30 minutes to wrestle the other from its mother, but unsuccessfully. The Ngogo chimpanzees then rested for an hour, holding the female and her infant captive. Then they resumed their attack.

"Though they were never successful in grabbing the infant from its mother, the infant was obviously very badly injured, and we don't believe it could have survived," Amsler said.

In most of the attacks in this study, chimpanzee infants were killed. Mitani believes this might be because infants are easier targets than adult chimpanzees.

Scientists are still not sure if the chimpanzees' ultimate motive is resources or mates. They haven't ruled out the possibility that the attacks could attract new females to the Ngogo community.

Mitani says these findings disprove suggestions that the aggression is due to human intervention. Lethal attacks were first described by renowned primatologist Jane Goodall who, along with other human observers, used food to gain the chimps' trust. Some researchers posited that feeding the animals might have affected their behavior. The Michigan researchers didn't use food.

He cautions against drawing any connections to human warfare and suggests instead that the findings could speak to the origins of teamwork.

"Warfare in the human sense occurs for lots of different reasons," Mitani said. "I'm just not convinced we're talking about the same thing.

"What we've done at the end of our paper is to turn the issue on its head by suggesting our results might provide some insight into why we as a species are so unusually cooperative. The lethal intergroup aggression that we have witnessed is cooperative in nature, insofar as it involves coalitions of males attacking others. In the process, our chimpanzees have acquired more land and resources that are then redistributed to others in the group."

The paper is titled "Lethal intergroup aggression leads to territorial expansion in wild chimpanzees." The research is funded by the Detroit Zoological Institute, the Little Rock Zoo, the L.S.B. Leakey Foundation, the National Geographic Society, the National Science Foundation, the University of Michigan, the Wenner-Gren Foundation for Anthropological Research, and Yale University.

Explore further: Chimps: More males means boundary fights

More information: Amsler et al.: “Lethal intergroup aggression leads to territorial expansion in wild chimpanzees.” Publishing in Current Biology 20, 12, June 22, 2010. www.current-biology.com

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35 comments

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Display comments: newest first

blyster
Jun 21, 2010
This comment has been removed by a moderator.
Skeptic_Heretic
3 / 5 (2) Jun 21, 2010
And they say we don't have a common ancestor. LOL at creationism.
jselin
5 / 5 (2) Jun 21, 2010
I hate to say it but this sounds identical to gang activity...
92ef55af
Jun 21, 2010
This comment has been removed by a moderator.
trekgeek1
5 / 5 (6) Jun 21, 2010
I hate humans. Can you all kill yourselves?


You go first, I promise we'll follow.
Skeptic_Heretic
3.7 / 5 (3) Jun 21, 2010
I hate to say it but this sounds identical to gang activity...

I'd say it sounds identical to global politics.
jselin
5 / 5 (2) Jun 21, 2010
I hate to say it but this sounds identical to gang activity...

I'd say it sounds identical to global politics.

Agreed in general but the description of the chimps out on patrol and killing the rival "gang" members when they see them is literally identical.
pierluigi_rotundo
1 / 5 (2) Jun 21, 2010
I think that is the big difference between humans and animals

Thx
Pierluigi Rotundo
dtxx
1 / 5 (1) Jun 21, 2010
It must suck returning to camp with nothing but a mangled baby and a black eye. How did the other chimps not hear anything for over an hour?
zslewis91
3 / 5 (2) Jun 21, 2010
They attack neighbors who are intent upon doing the same thing to them. Chimps initially establish territory they can reasonably sustain,

Angriff ist die beste Verteidigung.

you sir..are no expert, your crude and faulse logic means nothing here, not a thing. all you "know it alls" should get into the fields of study,,,,,publish papers and so on...stop presenting your opinons as facts. for they are not
92ef55af
2.6 / 5 (5) Jun 22, 2010
"I hate humans. Can you all kill yourselves? "

"You go first, I promise we'll follow."

Done!

Now it is your turn.
92ef55af
1 / 5 (7) Jun 22, 2010
Rectification: only the americans need to kill themselves.
probes
Jun 22, 2010
This comment has been removed by a moderator.
Skeptic_Heretic
2.3 / 5 (3) Jun 22, 2010
Rectification: only the americans need to kill themselves.

Right, because with no one tending our farmland the rest of you will starve to death all on your own. Don't worry, the GPS system will be completely out of sync in about 15 minutes so all that lovely technology you have will be worth nothing as well. Your economy will cease to be because, well, we buy everything.

In short, I laugh at you, heartily. Lose the concept of nationalism and grow up, one race, one society.
probes
1 / 5 (2) Jun 22, 2010
These chimps are highly intelligent, they don't need GPS. Heck, they can build their own VASIMR engines in 3.7 hours.
ralph_wiggum
Jun 22, 2010
This comment has been removed by a moderator.
Skeptic_Heretic
3.7 / 5 (3) Jun 22, 2010
China over the years has aborted close to 1/3 the size of their present population.

Fine example of atheist technocracy!

Because China just screams technologically advanced to you, doesn't it.
Wayfarer
5 / 5 (1) Jun 22, 2010
Its interesting how the males seem to concentrate on killing the infants,this puts me in mind of how adult male lions will kill cubs when trying to take over a pride.
Skeptic_Heretic
2.3 / 5 (3) Jun 22, 2010
Its interesting how the males seem to concentrate on killing the infants,this puts me in mind of how adult male lions will kill cubs when trying to take over a pride.

Actually most social mammals will do this. It appears to be yet another part of our shared lineage.
ralph_wiggum
5 / 5 (4) Jun 22, 2010
Impromptu poll of Physorg's moderating efforts...

My post that read "I love the smell of bananas in the morning" (with apologies to "Apocalypse Now") got deleted by the moderator for being "OFF TOPIC", while a gem like "Rectification: only the americans need to kill themselves" was preserved for posterity.

Please rate this a 1 if I'm being a crybaby or a 5 if the moderator should be more consistent in his/her efforts.

P.S. Moderator, please don't ban me, I love Physorg :)
Skeptic_Heretic
2.3 / 5 (3) Jun 22, 2010
Our biology will have to change in fundamental ways- the urge to reproduce, the intellect to plan for the future and live within our means, the requirement that no child be subjected to damage either before or after birth and so have the chance to grow up happy and healthy and thereby end the cycle.
The problem here is that without legal restrictions akin to China, or a genocidal event, those of lower ethical imperative outbreed those who would be rational and reasoned in how to accomplish this. It is a problem that social evolution won't readily solve.
Skeptic_Heretic
1 / 5 (2) Jun 22, 2010
Of course you don't want to denounce all families with more than two children as having "lower ethical imperatives".
Not by any means, I figured that post would be misconstrued and I'd have to answer it.
There is an easy solution which will be difficult to establish. The function of reproduction has to be separated from the function of raising. Raising children will be a function for certified parents only. Biological parentship has to be replaced by qualified parentship. This agenda will solve a lot of problems.
Well that steps back into my statement of Chinese style legal restriction.
Ethelred
5 / 5 (3) Jun 23, 2010
Please rate this a 1 if I'm being a crybaby or a 5 if the moderator should be more consistent in his/her efforts.


That was OFF TOPIC. So off with your head.

So is this post.

Whereas the Imoderators are never off topic since they NEVER explain just what amazingly bizarre excuse for logic caused them to act as they do.

Heck they don't even try to make sense.

My best guess is that one idiot with a Phd. wanders around and rolls a 20 sided die. Posts that make their saving throw are retained. Despite the claims on the site that they are British they act just as rational as the guys at a Dungeons and Beavers get together at Cal Tech. Thus I suspect that they really aren't British.

They definitely are neither consistent nor rational.

Even some my complaints about the lousy way they handle things stay and others disappear.

Ethelred
Skeptic_Heretic
3.2 / 5 (5) Jun 23, 2010
Even some my complaints about the lousy way they handle things stay and others disappear.
I'd be happy enough if they could simply address the rampant sockpuppetry of Alizee and clan, or maybe jsut abolish the YEC posts so I don't have to re-explain free market failure, abiogenesis, and evolution over and over and over.
Ethelred
5 / 5 (3) Jun 24, 2010
I have the impression that Alizee The Multi-Named Crank has been banned a few times. Can't think of another reason for no longer posting, after all this time, as Alizee.

I don't mind the YEC posts IF the poster wants to engage in REAL debate. Hit and run posts like Mabarker and some others engage in are intellectually dishonest and cowardly which annoys me. Constant evasion like Marjon does has reached the point that I wouldn't mind seeing him go away.

An honest debate with him would be a vast improvement.

Ethelred
Skeptic_Heretic
5 / 5 (2) Jun 24, 2010
An honest debate with him would be a vast improvement.
You can't have an honest debate with a YEC. By the very nature of their argument they must be dishonest either intellectually, or overtly.
Ethelred
5 / 5 (3) Jun 24, 2010
Maybe you can't but I can and have. Not all YECs are aggressively ignorant. Many are just plain ignorant, never having been exposed to the truth.

Ignorance is curable.

I am not claiming that I have made realists out of believers. However I have convinced at least a few that maybe they don't know everything because the Bible told them so.

Patience and self-control are needed to get anywhere in such a discussion. Even with that far more scarper off then actually begin to think.

Ethelred
Skeptic_Heretic
5 / 5 (1) Jun 24, 2010
Maybe you can't but I can and have. Not all YECs are aggressively ignorant. Many are just plain ignorant, never having been exposed to the truth.
Unfortunately the only ones I've spoken with are willfully ignorant, or make utterly insane statements, akin to kevinrtrs, a common poster here.
Au-Pu
not rated yet Jun 26, 2010
The lead in said that chimps killed other chimps "violently".
Did they really expect them to kill them any other way?
The word "Violently" was superfluous.
Incidentally, how did the foregoing posts drift so far from the topic?
Should there not be some relevance?
Ethelred
5 / 5 (2) Jun 26, 2010
Perhaps the method of killing is a bit more savage then the author expected. Limb from limb for instance.

Or maybe it was authorial overkill with extreme prejudice.

Should there not be some relevance?


You aren't from around here are you?

Ethelred
Ethelred
1 / 5 (1) Jun 28, 2010
I don't understand his giving you a 1 either. Maybe he is just pissed at you and doesn't care what you wrote. So I gave you a four and am not sure it shouldn't have been a five.

Ethelred
Ethelred
not rated yet Jun 28, 2010
For a male chimp to join a foreign group it will be inpossible.A fimale to flip it is more possible but still it will be hard.


Not hard for the females. Chimp bands are made up of brothers but NOT sisters. The females leave the band they were born in to join others.

Ethelred
probes
not rated yet Jun 28, 2010
Heck these chimps are so intelligent they don't even try to make VASIMR engines in 2.8 seconds.
Ethelred
not rated yet Jun 28, 2010
There was no scorning of victims in Otto's post. He seemed rather disgusted by the whole situation. Which I suspect is a bit atypical of Pan Troglodyte as it is atypical of Homo Sapien. But similar things have happened with humans.

Keep in mind that the adult female chimp is not surrounded by sisters and therefor any alliances are based on need, not family as the case for the males.

Ethelred
Ethelred
5 / 5 (1) Jun 29, 2010
What's the suitable word for attributing a feeling of "gladness" to a victim?
Otto is not responsible for the emotions of victims. Haven't you heard of Stockholm Syndrome?

http://en.wikiped...syndrome

You want to say that they are able to feel "gladness", but not able to feel "sorrow" and "fear"?
Excuse me but just where the hell did I say anything like that? You seem to have personal issues on this that is causing you to impugn motives that you don't like for the writings of me and Otto.

Ethelred

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