From a human standpoint, chimpanzees and bonobos frequently embody contrasting facets of our own nature. Chimpanzees are typically perceived as more prone to conflict, while bonobos are often hailed for their peaceful demeanor and intergroup cooperation. Recent studies delving into the behavior of these great ape species offer a more intricate understanding. Contrary to prior beliefs, bonobos exhibit higher levels of aggression within their own communities. These insights stem from a study published on April 12 in Current Biology.
"Chimpanzees and bonobos deploy aggression for distinct purposes," stated Maud Mouginot, co-author of the study and anthropologist at Boston University. "The aim is not to discredit the notion of bonobos as peaceful creatures, but rather to highlight the intricate nature of both species."
Pushing, biting, and chasing
The study delved into male aggression, often linked to reproductive behaviors. It scrutinized three bonobo communities at the Kokolopori Bonobo Reserve in the Democratic Republic of Congo, and two chimpanzee communities at Gombe National Park in Tanzania. Researchers utilized focal follows, a method involving continuous tracking of individual animals throughout a day. They recorded instances of aggressive interactions, the targets of aggression, and the physical nature of these encounters, such as pushing, biting, or chasing, among 12 bonobos and 14 chimpanzees.
"You go to their nests and wait for them to wake up, then you just follow them the entire day—from the moment they rise until they retire at night—and document their every activity," explained Mouginot.
The study revealed that male bonobos exhibited aggression more frequently than chimpanzees. In total, bonobos were involved in 2.8 times as many aggressive interactions and engaged in three times the number of physical aggressions compared to chimpanzees.
Bonobo males primarily directed their aggression towards other males, whereas chimpanzees showed a higher propensity for aggression towards females. Additionally, chimpanzees demonstrated a greater tendency to form male coalitions, with 13.2 percent of chimpanzee aggression involving such groupings, compared to only one percent in bonobos.
Altercations involving groups of males can lead to more severe injuries, and intra-community conflicts can potentially undermine the group's ability to defend against external threats from other chimpanzee groups. Bonobos, on the other hand, seem to avoid this issue as most of their disputes occur on a one-on-one basis. Instances of lethal aggression or territorial behavior are absent among bonobos, allowing their communities greater freedom to resolve conflicts internally rather than with external groups.
Male ‘coalitions’
In both species, males displaying higher levels of aggression also achieved greater mating success. The researchers found this pattern surprising in bonobos, given their co-dominant social structure, where females frequently hold higher ranks and wield significant influence in mate selection. In contrast, chimpanzees exhibit male-dominated hierarchies, where coalitions of males often coerce females into mating.
"Male bonobos exhibiting higher aggression levels secure more copulations with females, which is unexpected," stated Mouginot. "It suggests that females do not necessarily prefer more amicable males."
The researchers highlight that female bonobos and chimpanzees are not entirely passive, indicating a need for future research to explore female aggression in greater depth.
The self-domestication hypothesis
The discovery of elevated levels of male-male aggression in bonobos challenges a dominant theory in primate behavior known as the self-domestication hypothesis. Originating from the time of Charles Darwin, this hypothesis suggests that evolution has favored reduced aggression in bonobos and humans, but not in chimpanzees.
Certain aspects of the findings do align with elements of the self-domestication hypothesis, particularly concerning aggression towards females. Male bonobos exhibit lower levels of aggression towards females compared to chimpanzees. The researchers suggest that this is consistent with previous observations indicating that male bonobos seldom employ coercive mating tactics, even when physically larger than females.
The researchers were unable to evaluate the intensity of aggressive interactions in terms of inflicting wounds or injuries. They aim to gather such data in future studies, along with comparing variations in aggressive behavior across different communities and subspecies.
"I'd love to see the study supplemented with comparable data from other field sites to enhance our understanding of variation within and between species," Mouginot expressed.