They hunt in groups, share food and work together: Who are they?
by Paige Brown, Nature Research Center Blogger-in-Residence
Intelligent. Strong. Social.
These characteristics are probably some of the traits we can all associate with chimpanzees, especially male chimpanzees, at least from what most of us know about them in movies, the media and museums. Those of us who know a bit more about the primate kingdom might know that chimpanzees are apes, that they form dominance hierarchies in their communities, and they are known to make use of plants and sticks as tools to fish for termites in termite mounds, for example.
Hunting. Cooperation. Sharing. Protection. Warfare.
These chimpanzee behavior characteristics might not be so familiar to us, precisely because these behaviors have rather puzzling origins. However, according to Ian Gilby, co-director of the Jane Goodall Institute Research Institute at Duke University and wild chimpanzee behavior specialist, group hunting and meat-sharing among chimpanzees have indeed been confirmed by data collected at Gombe, Tanzania, where Jane Goodall started observing chimpanzees over 50 years ago. Gilby spoke at the Daily Planet Café on Thursday night during a Science Café at the Nature Research Center.
“I’ve always been interested in how animals interact with one another,” Gilby said during his Science Café talk. “Cooperation is particularly interesting, because it is a puzzling thing. A lot of people might be surprised to know that chimpanzees actually hunt monkeys, eat them, and unfortunately tear the carcasses apart and share the meat with other members of their community.”
But why would chimpanzees help each other? Why would they share meat from a monkey kill? Why do they hunt in groups, and why would multiple chimps participate in a risky group hunt if they could simply bum meat off of other individual chimpanzee hunters?
“A group hunt is a risky, energetically expensive thing, to climb into the trees and catch a money that is trying to get away, is big, has canines and will bite you,” Gilby said. “If you could somehow get meat from one of your pals, without actually taking a risk yourself, why would you do it?”
In order to investigate this question, Gilby and colleagues have been in the process of collecting years’ worth of observational data in the field, following chimpanzees around in the wild to observe their behaviors. Gilby has collected more than 4 years of observational data on chimpanzee hunting behavior in the field.
A key finding from his long-term data collection, Gilby says, has been the finding that while hunting among chimpanzees is a group effort, key males, known as “impact hunters,” are highly influential within the group.
“We are finding with the chimps that there is a real variation in how cooperative individuals are, and how prone to risk-taking they are,” Gilby said. “What I think is one of my most exciting discoveries is what I call impact hunters; these are certain males who are just really gung ho hunters. For some reason, they have a different threshold, and they go after the monkeys before anyone else does. But when they do that, what they are doing is actually changing the equation for all the other chimps.”
Gilby’s discovery of an impact hunter phenomenon in chimpanzee behavior was featured in a 2008 Harvard press release and a 2008 Animal Behavior journal article. According to this research, chimpanzee hunting behavior is significantly predicted by individual variation in hunting motivation, with hunts rarely occurring in the absence of “impact” males. These gung ho males act as a catalyst to cooperative hunting. In other words, less gung ho chimpanzees are more likely to participate in a hunt once the impact male has jumped into the trees and started attacking a group of monkeys. This is because at this point, the chance of a second, third or fourth chimpanzee catching a confused and distraught monkey is improved.
“These individual differences have a cascading effect on whether or not other individuals hunt,” Gilby said. “It is not necessarily a coordinated action.”
So, chimpanzees may cooperate in a hunt as a result of mutual benefits, but critical impact male individuals play a role in creating low-cost opportunities for others to beneﬁt by joining a hunt in progress.
“When a party of chimpanzees encounters a troop of red colobus [monkeys], the impact hunters tend to be the first to hunt,” Ian Gilby said in the 2008 Harvard press release. “By doing so, they dilute the prey’s defenses, thereby reducing the costs of hunting for other chimpanzees.”
So what next? Gilby is now interested in studying the genetics that might lie behind some male chimpanzees being more risk-taking, more gung ho hunters than other chimpanzees. Just as in humans, individual differences in chimpanzees’ genes, in their DNA, could account for complex group behaviors.
So are chimpanzees being nice when they help others in hunts and share pieces of meat? While the jury is still out on the origins of cooperation and social behavior, many chimpanzees may participate in what looks like cooperative behaviors out of self-interest: they are more likely to succeed once impact hunters have made the first move.
Want to learn more about cooperative chimpanzee behavior? Visit the Jane Goodall Institute Research Center website, and the Visual World Investigate Lab at the Nature Research Center in the NC Museum of Natural Sciences, where you can watch videos of chimpanzees hunting in the wild.