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Chaos or collaboration?

ASU researcher looks at how humans learned to cooperate


October 28, 2015

How and why did human beings evolve to cooperate with unrelated individuals or even strangers? Consider National Public Radio — donors could listen for free, yet thousands of people donate, collectively supporting a public good.

To understand how this cooperative inclination developed in humans over the last millennia, scientists look to our primate cousins to see if they can observe glimmers of behavior that will help us understand the nature of human collective action. Chimpanzees, together with bonobos, are our closest living relatives, with whom we share at least 96 percent of our genome and a common ancestor that lived some five to seven million years ago. Ian Gilby Ian Gilby in the field. Download Full Image

ASU researcher Ian Gilby has been studying and observing chimpanzees in the wild since 1997. Gilby is an assistant professor in the School of Human Evolution and Social Change and a research affiliate with the Institute of Human Origins. He is also the codirector of the Jane Goodall Institute Research Center database that is the repository of detailed demographic and behavioral data on two communities of chimpanzees collected for over 70 years at Gombe National Park, Tanzania. Using these records and other long-term observational data from the Kibale Chimpanzee Project studying the Kanyawara chimpanzees of Kibale National Park, Uganda, Gilby and colleagues investigated the dynamics of chimpanzee group hunts of red colobus monkeys.

Chimpanzees hunting red colobus monkeys

In this study, which is the first to analyze hunting data from three chimpanzee communities, the researchers found that hunting was more likely to occur if certain key individuals were present when a chimpanzee group encountered red colobus monkeys. They hypothesized that these “impact hunters” catalyzed hunting by being the first to attack, thereby creating opportunities for others to catch prey in the ensuing chaos. Indeed, as predicted, impact hunters were more likely to hunt first than expected by chance, and importantly, affected long-term hunting patterns. At both Gombe and Kanyawara, overall hunting rates decreased after the death or “retirement” of impact hunters.

These results suggest that rather than hunting in a coordinated fashion, these chimpanzees follow an “every chimpanzee for himself” strategy. Hunting in groups is promoted because an individual’s selfish attempts to catch prey incidentally benefit others.

“These results are particularly exciting,” says Gilby, “because they demonstrate the importance of systematic individual differences for understanding the evolution of cooperation. The next step in our research is to investigate the source of this variation.”

"'Impact hunters’ catalyse cooperative hunting in two wild chimpanzee communities," written by Ian C. Gilby (Arizona State University), Zarin P. Machanda (Harvard University), Deus C. Mjungu (The Jane Goodall Institute, Gombe Stream Research Centre), Jeremiah Rosen (Harvard University), Martin N. Muller (University of New Mexico), Anne E. Pusey (Duke University) and Richard W. Wrangham (Harvard University) is published online in the Philosophical Transactions of the Royal Society B.

Julie Russ

Assistant director, Institute of Human Origins

480-727-6571

Aggression in male chimpanzees leads to mating success


November 13, 2014

In the animal kingdom, the battle of the sexes often truly becomes a battle. Among chimpanzees, males may violently attack females, sometimes resulting in serious wounds. While unpleasant to watch, the frequent occurrence of such violence at several East African field sites suggests that aggression toward females functions as a form of sexual coercion.

Previous research from the Kanyawara chimpanzee community in Kibale National Park, Uganda, has supported the sexual coercion hypothesis. Males who directed aggression at certain females mated more often with those females than did other males. Moreover, these aggressive males were actively solicited for mating by those females at the time of peak fertility. Critically, aggression over the long term had a greater effect than violence in the immediate context of mating. Ian Gilby in the field Download Full Image

However, until now, it has been unclear whether aggression toward females increases male chimpanzee reproductive success.

A new study published this week in Current Biology by Feldblum et al., including ASU scientist Ian Gilby, a senior author on the study, provides strong evidence that male aggression toward females is indeed adaptive. The authors analyzed 17 years of observations of the Kasekela chimpanzee community in Gombe National Park, Tanzania.

Researchers used DNA obtained from fecal material to determine the paternity of 31 infants born during the study period. This is the first study to present genetic evidence of long-term sexual coercion as an adaptive strategy in a social mammal.

The rate at which a male directed aggression at a female not only increased the pair’s mating frequency, but also significantly increased the probability that he sired her offspring. This strategy was most effective for high-ranking males, and aggression toward females outside of their periods of sexual receptivity was the best predictor of paternity. Male aggression was not used to force sexual encounters either during or immediately following aggression.

“This indicates that males, particularly those of high rank, successfully employ a strategy of long-term sexual intimidation,” says Ian Gilby, an assistant professor in the School of Human Evolution and Social Change and research affiliate with the Institute of Human Origins at Arizona State University.

Although there could be female preference for dominant males, the fact that aggression increased paternity likelihood for high-ranking males indicates that the patterns of paternity did not arise as a result of female choice but rather from mate guarding by strong alpha males.

Gilby cautions that while these results may provide clues about the origins of sexual violence in humans, he says, “We should be careful not to jump to conclusions. Chimpanzees are one of our closest living relatives, but 7 million years of evolution separate us, and our mating systems are very different. Nevertheless, recognizing the adaptive value of male-female aggression in chimpanzees may ultimately help us to understand, and hopefully prevent, similar behavior among humans.”

Julie Russ

Assistant director, Institute of Human Origins

480-727-6571