Recently, an infant chimpanzee at the L.A. Zoo was killed by an adult member of the troop in full view of zoo visitors. A few days later, a student volunteering at the Chimp Eden sanctuary in South Africa was pulled into an enclosure and attacked by two adult male chimpanzees. Both incidents served as startling reminders of the capacity for violence in our closest relatives and have left many people wondering what makes chimpanzees commit such severe acts of aggression.
Unfortunately, while incidents like these are rare, they are not abnormal. Put simply, violence is a fact of life in chimpanzee society. While males typically grab all the attention with their aggressive dominance struggles and their lethal intergroup raids, females also kill on occasion, with infants and other adult females being their most likely victims. In chimpanzee communities, severe aggression can be a means to reduce or prevent resource competition. This can result in the killing of members of other communities, as well as immigrant females and even infants within the troop. In some cases, the killing of an infant can increase mating opportunities for males. For instance, if a female gives birth, she will not enter estrus for another four to five years while she nurses and raises her new child. If that child dies, however, she will quickly become receptive again. Thus, there can be an incentive for a male who is not the parent to kill the infant so that he can mate with the mother (this is one reason primatologists believe that females may try to confuse paternity).
Violent behavior can serve many functions in chimpanzee society. What functions did these incidents at the L.A. Zoo and Chimp Eden serve? Honestly, we don’t know. It’s much easier to offer an evolutionary explanation for why violence exists in general than it is to explain specific acts.
As caregivers for captive chimpanzees, we witness aggressive behavior on a daily basis. Sometimes the motivation behind it is clear; other times we are left scratching our heads. The way I think about it is this: evolution has endowed chimpanzees with certain tendencies for aggressive behavior, but it does not control how those tendencies are applied. Aggression towards non-group members in the wild can help chimpanzees defend territory and the resources located therein. But that same aggressive tendency can also result in an attack on the very people trying to care for them in captivity.
All we really know is that violence in chimpanzees is not an aberration, nor is it all they are capable of. In fact, one of the reasons why we might be uncomfortable with chimpanzee violence is that it hits a little too close to home. To be sure, aggression in chimpanzees is shocking in its sheer physicality – teeth and fists instead of knives and guns. But even though chimpanzees exhibit higher rates of aggression overall, rates of lethal violence in chimpanzees are similar to those in some human societies. In some ways, we are more alike than we’d like to believe.