Chimpanzee Aggression Study

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A new Arizona State University study shows that deadly aggression among chimpanzees cannot be blamed on human disturbances. ASU anthropologist Ian Gilby will discuss his study.

Ted Simons: A new ASU study that looks at the impact of humans on deadly aggression among chimpanzees is also raising questions on whether or not humans are predisposed to violence. ASU anthropologist Ian Gilby joins us to discuss his study. Good to have you here. Thanks for -- What exactly did you look at out there?

Ian Gilby: Thanks for having me, first of all. Well, chimpanzees are unusual among mammals, among animals in general, in that they practice coalitionary lethal aggression. So meaning they will essentially team up with one another and kill other chimpanzees.

Ted Simons: Yes.

Ian Gilby: So we're -- We were interested in trying to explain that very unusual behavior.

Ted Simons: And what you're looking at is whether or not human interaction or just the slightest impact of human existence is changing chimp behavior?

Ian Gilby: Right. There's really two alternative explanations for -- To explain this aggression. One is that it's an adaptive behavior. That chimps get some kind of benefit, either by -- Achieving more mating success, more food, something like that, from the aggression. Or the alternative, that -- And it's sort of a small relatively vocal group that advocates for this position, suggests that it's aberrant behavior that's a byproduct of human disturbance of some kind.

Ted Simons: So you try to figure out what was correct.

Ian Gilby: Exactly.

Ted Simons: What did you find?

Ian Gilby: By "we" in this case I mean 30 authors on the giant study, it's actually an impressive collaboration, headed up by Michael Wilson at the University of Minnesota and Richard at Harvard, and we together pooled data from 18 different chimpanzee sites, 426 years of data, to address whether or not chimpanzee violence or specifically chimpanzee killings were affected by ecological factors or human disturbance.

Ted Simons: What did you find?

Ian Gilby: We found that it's ecological factors and not human disturbance.

Ted Simons: Basically occasionally a chimp is going to kill a chimp.

Ian Gilby: Yeah.

Ted Simons: And there may not be -- Will it be adaptive, will it be because of territory --

Ian Gilby: There's a lot of evidence, actually, that it is adaptive. That territories increase after a -- A group killing has occurred. We know also that with larger territories female chimpanzees reproduce more quickly, they have more surviving offspring. They're even heavier when they have a larger range. So there are definite advantages to having a larger territory.

Ted Simons: Are there triggers that set off these lethal attacks?

Ian Gilby: That's a great question. They go on patrol, so they go out to the edge of their range, groups of males quietly stealthily traveling along -- Apparently searching for members of the neighboring community. But it doesn't seem -- Sometimes they're brought together by rich food source, for example, so in there's a nice fruiting tree at the edge of the range that two communities will be drawn to, then you may have an intergroup attack.

Ted Simons: Again, was this studied -- You said a lot of years and a lot of chimps were looked at. Was it basically one general location?

Ian Gilby: East Africa through central Kafka to West Africa.

Ted Simons: We can't just say it's one group chimps.

Ian Gilby: Exactly.

Ted Simons: You also studied Bonobos?

Ian Gilby: There were four Bonobo studies.

Ted Simons: They aren't killing each other?

Ian Gilby: There was one case. In I think 97 years of observation.

Ted Simons: Are they just more peaceful animals?

Ian Gilby: They certainly seem to be. It seems to be -- They seem to be very divergent common chimpanzees.

Ted Simons: A little more laid back.

Ian Gilby: Yeah.

Ted Simons: Take things as they lay?

Ian Gilby: Exactly.

Ted Simons: What do we take from this study, especially as it involves human evolution and our predisposition to violence, war, killing, etc.?

Ian Gilby: It's something we have to be very cautious about. And -- But it is interesting to note I said at the beginning, that chimpanzees are very unusual in that they practice coalitionary lethal aggression. Humans do it too, right? So that's a very interesting connection. As anthropologists, we are trying to understand what it means to be human and particularly how we became human. So we're talking about lots of different traits. Big brains, behavior, life history, and so on. A lot of those traits, the fossil record helps us with. Lucy, discovered by Don Johansson, we know she was bipedal because of her pelvis. One of the tricks is to use living prime mates as a referential model for what our earliest ancestors might have been like.

Ted Simons: See what they're doing now and look and compare to fossils.

Ian Gilby: Chimpanzees are our closest levels relatives, and there's more and more evidence that the last common ancestor of apes and humans was very chimp like. So we're not saying that this is the way our -- The last common ancestor was, but their current information suggests, it just gives us clues toward what we were starting from.

Ted Simons: Before I let you go, anything about the study surprise you?

Ian Gilby: No.

Ted Simons: You basically thought that it wasn't human interaction it was basically adaptive behavior.

Ian Gilby: Exactly. Yep. There was no -- For most of us, it was not a surprise.

Ted Simons: All right. Must be fun studying chimps.

Ian Gilby: It's exciting. Put it that way.

Ted Simons: Never a dull moment. Thank you so much for being here.

Ian Gilby: No problem. Thank you.

Ian Gilby:Anthropologist, Arizona State University;

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