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Chimpanzee killings led to territorial expansion

War without weapons…?

I have always believed that humans who demonize inanimate objects (e.g., the majority of the California Legislature) such as weapons are fetishists in the true sense of the word. A fetish is defined to be “an object (as a small stone carving of an animal) believed to have magical power to protect or aid its owner; broadly : a material object regarded with superstitious or extravagant trust or reverence.” Or fear….

You can read about this story concerning lethal encounters among chimpanzees, in the LATimes online, or in its “gory” scientific form, :-) with statistics and standard deviations, in the journal article linked below. (I prefer to read the science, but “your mileage may vary [YMMV].”) :-)

In the June 22, 2010 issue of the scientific journal, Current Biology, scientists from the University of Michigan, Yale University, and the University of Arkansas present data on lethal intergroup aggression over 10 years from an unusually large chimpanzee community (about 150 individuals) at Ngogo, Kibale National Park, Uganda. Chimpanzees and bonobos are the closest living relatives to human beings. During the 10 years, the scientists observed the Ngogo chimpanzees kill or fatally wound 18 individuals from neighboring groups; the scientists inferred three additional killings on the basis of circumstantial evidence. Most victims were caught in the same region and likely belonged to the same neighboring group. The Ngogo chimpanzees now have expanded their territory and occupy the area previously occupied by some of their victims, adding about 2.5 square miles (6.4 km²) to their previous 11-square-mile (28.76 km²) domain, a 22.3% increase in territory size.

Primatologists have known for a long time that chimpanzees will silently patrol the border between two communities in single-file lines. If they come upon a male, they will attack him with their teeth, jump on his body, and pull off his testicles. If they come upon a female with a baby, they will rip the infant from her and kill it, sometimes cannibalizing the baby. Seventeen of the observed 18 lethal attacks were made by coalitions of Ngogo males on patrol. Most of the victims were infants, probably because they are easier to kill.

What scientists did not know was WHY.

Thirteen of the 21 killings (“lethal intergroup aggression” in “science speak”) occurred northeast of the Ngogo territory in an area of heavy patrol activity. Four of the 13 were adult males and nine were immature chimpanzees.

According to the scientific study (and leaving out the links to references), the mortality rates are EXTREMELY HIGH, compared with agricultural and hunter-gatherer populations of humans and with other studied chimpanzee communities:

“If its size is similar to those of chimpanzee communities studied elsewhere (X = 46.6, SD = 18.7, n = 8 communities), the 13 fatalities represent a mortality rate of 2,790 per 100,000 individuals per year. Alternatively, a rate of 867 per 100,000 individuals per year results if one assumes the northeast community is as large as Ngogo’s (150 individuals). These values are extremely high, exceeding median rates of mortality due to intergroup violence reported for humans in agricultural and hunter–gatherer populations by factors of 1.5–5 and 5–17, respectively. They are also 23–75 times higher than the median rate suffered by individuals in nine well-studied chimpanzee communities.”

You can read both the LATimes article and the journal article to answer questions that you may have, but the scientists state that their “…findings are consistent with the hypothesis that lethal intergroup aggression reduces the coalitionary strength of opponents living in adjacent groups, leading to territorial expansion by the aggressors.” Their “… findings support the hypothesis that killing neighboring conspecifics <other chimpanzees> is adaptive.” The scientists leave an unresolved question:

“An unresolved question is whether chimpanzees do so to acquire mates or resources. We cannot rule out the possibility that coalitionary attacks by Ngogo males may lead to new females joining their community, but results thus far are consistent with the resource acquisition hypothesis. By acquiring new territory through lethal coalitionary aggression, male chimpanzees improve the feeding success of individuals in their own community, which in turn can lead to increased female reproduction.”

The authors suggest future study of their “… results to address an enduring question about why humans are an unusually cooperative species….”

Well, SOME of us are, :-) those who may believe that the incidence of cooperation (or, at least, biological synergism) far exceeds that of competition in nature.

Michael Wilson, a primatologist at the University of Minnesota, who wasn’t involved with the study, said that the study supports the view that chimpanzees are competing for territory. Wilson said that such observations could help scientists figure out whether warfare could have predated the development of weapons, agriculture and ideology, which “some people in anthropology argue are the cause of war.”

In any case, it looks like unarmed higher apes had no problem killing their neighbors, without weapons, in their quest for territory or mates.

Those in Sacramento or Washington, D.C. may want to consider the facts when proposing new legislation.

Sometimes it is hard to accept really ugly truths about ourselves.

-Bill at

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