Bonobos are perhaps best known for their promiscuity: sexual acts both within and between the sexes are a common means of greeting, resolving conflicts, or reconciling after conflicts. The researchers made the discovery that these free-loving primates also hunt and kill other primates while they were studying a bonobo population living in LuiKotale, Salonga National Park, in the Democratic Republic of Congo.
They had been observing the bonobos there for the last five years, which is what made the new observations possible. Although Hohmann's team did have prior evidence for monkey hunting by bonobos, it came exclusively from indirect studies of fresh fecal samples—one of which contained the digit of a black mangabey. Yet, in the absence of direct behavioral observations, it was not entirely clear whether the bonobos had hunted the mangabey themselves or had taken it from another predator.
The researchers have now seen three instances of successful hunts in which bonobos captured and ate their primate prey. In two other cases, the bonobo hunting attempts failed. The data from LuiKotale showed that both bonobo sexes play active roles in pursuing and hunting monkeys. The involvement of adult females in the hunts which is not seen in chimps may reflect social patterns such as alliance formation and cooperation among adult females, they said.
Overall, the discovery challenges the theory that male dominance and aggression must be causally linked to hunting behavior, an idea held by earlier models of the evolution of aggression in human and non-human primates. Future work on the bonobos of LuiKotale may shed light on the social and ecological conditions that encourage their monkey-hunting expeditions, yielding insight into the evolutionary significance and causes of aggression, hunting, and meat eating in bonobos, chimpanzees, and ourselves.
The second difference was mentioned earlier: bonobos have a lot of sex. Group sex, hand sex, oral sex, genital on genital rubbing, and good old vaginal sex are all in the bonobos' repertoire, and they exercise their sexual abilities frequently, so much that Hare refers to sex as the "bonobo handshake.
The young will have sex with the old, and males and females frequently engage in same-sex activity. Chimps, on the other hand, are much more reserved, mating primarily for procreation.
For bonobos, sex seems to fulfill the role that competition plays in chimpanzees. For example, when a chimpanzee group stumbles upon a food source, the most aggressive males will often eat their fill first, frightening off all others, and leaving only scraps.
When a group of bonobos discovers a cache of food, they often have an orgy, and then everyone shares. Bonobos are actually physically built for this sexual problem solving. The clitorises of female bonobos are extremely large for the animals' size and are likely very sensitive.
While more modest in size, males' genitalia may also be similarly sensitive and thus primed to deliver "good feelings. More striking are the differences in how chimps' and bonobos' brains are wired. A study showed that bonobo brains are more developed in regions associated with empathy.
Moreover, bonobo brains feature a thick connection between the amygdala -- the brain's fear center -- and the ventral anterior cingulate cortex, a region associated with rational functions like decision-making and impulse control. Chimps lack this developed connection, meaning that they are likely less in control of their fear and aggression. This includes 58 that were directly observed by researchers; the rest were counted based on detective work - tell-tale injuries or other circumstances surrounding an animal's death or disappearance.
Interestingly, the team also compiled the figures for bonobos, with strikingly different results: just a single suspected killing from 92 combined years of observation at four different sites. This is consistent with the established view of bonobos as a less violent species of ape. The researchers' global compilation of chimp violent crime statistics allowed them to consider what conditions in a community produce a higher murder rate. Chimpanzees live in well-defined colonies, and groups of males patrol the borders of each colony's territory.
This is where violent conflicts are known to arise, particularly if a patrol encounters a single chimp from a neighbouring community - but never before has this much data on the lethality of those interactions been combined in a single study.
When the scientists compared the figures across chimpanzee research sites, they found that the level of human interference e. Instead, it was basic characteristics of each community that made the biggest difference: the number of males within it, and the overall population density of the area. These parameters link the violence to natural selection: killing competitors improves a male chimp's access to resources like food and territory - and crucially, it will happen more frequently when there is greater competition from neighbouring groups, and when the males can patrol in large numbers, with less risk to their own survival.
Dr Shultz was not involved in the study, but told BBC News the scale of the collected data was impressive. In an accompanying commentary for the journal Nature, Prof Joan Silk from Arizona State University said the results "should finally put an end to the idea" that violence in wild chimpanzees was a product of human interference. She suggested that our perceptions of our evolutionary cousins can sometimes be distorted, because we want to believe that it is the nice behaviours, not the nasty ones, which have deep evolutionary roots.
There is no need to cling to such ideas, Prof Silk argues: "Humans are not destined to be warlike because chimpanzees sometimes kill their neighbours.
Prof John Mitani, a behavioural ecologist at the University of Michigan and one of the study's authors, agrees. We have the ability to shape and alter our behaviour in ways that they can't.
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