Unlike chimpanzees, bonobos have a reputation for being gentle and peace-loving. But new systematic observations suggest that there are similar levels of violent encounters in bonobo groups in zoos as in chimpanzees, which are considered to be more aggressive. However, the gender distribution of those involved in the conflict differs: while male animals in chimpanzees are more likely to attack female animals, aggression in the matriarchal societies of bonobos more often comes from females. New findings on wild bonobos also contradict the cliché of peaceful pygmy chimpanzees.
Chimpanzees and bonobos are the closest living relatives of humans and are often used as comparisons when inferring natural characteristics of our own species. Chimpanzees are traditionally considered aggressive and combative, while the smaller bonobos are considered gentle and peace-loving. “Some see them as a reflection of our nature or what we should be: warlike like chimpanzees or peaceful like bonobos,” says Emile Bryon from Utrecht University. “However, recent observations in the wild challenge this dichotomy between aggressive chimpanzees and peace-loving bonobos.” Aggressive interactions have also been observed in wild bonobo groups, some of which even ended fatally.
Similar conflict rates
In order to study the conflict behavior of the two great ape species in more detail, Bryon and his team observed nine chimpanzee groups with a total of 101 animals and 13 bonobo groups with a total of 88 animals in zoos. Although captive animals may exhibit different behaviors than in the wild, this approach had the advantage that environmental conditions were comparable for both species. In addition, wild bonobos are only found in the Democratic Republic of Congo, a conflict zone where nature observations are difficult.
In total, the research team recorded 3,243 aggressive interactions among the great apes, including 1,193 physical altercations such as biting, hitting and fighting, as well as 2,050 non-contact conflicts in which one monkey threatened another, chased it or threw objects at it. “Taking group size and sex ratio into account, we found no species differences in aggression rates, neither for contact aggression nor for other types of conflict,” report Bryon and his colleagues. The differences between different groups of the same species were significantly greater, with both the least conflictive and the most conflictive group consisting of bonobos.
Males as aggressors or victims
But even though the total number of aggressions in chimpanzees and bonobos was similar, the researchers discovered a striking difference: “In chimpanzees, aggression comes mainly from males and is directed against everyone. In bonobos, aggression comes from everyone, but is mainly directed against males,” reports Bryon. One explanation could be that bonobo groups are organized in a matriarchal manner. “The females are in charge,” explains Bryon. “So the males can’t afford to be aggressive toward them. And the females seem to find other ways to mediate conflicts among themselves, such as through increased sociosexual behavior that bonobos use to relieve tension. Or instead of attacking each other, they redirect their aggression toward the males.”
From the researchers’ perspective, these results also question assumptions about the development of human aggression. Until now, the so-called self-domestication hypothesis held that social selection led to a reduction in aggressiveness in both bonobos and humans. “But rather than being characterized by a general decline in aggression, the social systems of bonobos appear to redistribute aggression according to gender,” explains the research team. “Future research should continue to examine the ecological, genetic and social factors that shape aggression across pan-species, ultimately deepening our understanding of the role of aggression in the evolution of primates and, in particular, humans.”
Source: Emile Bryon (Utrecht University, The Netherlands) et al., Science Advances, doi: 10.1126/sciadv.adz2433