Peering into the behaviour of wild bonobos has revealed the surprising personalities of our closest cousins
Female bonobos are a feisty bunch. They have dominant, high-status roles within bonobo society, in stark contrast to the more marginalised role that females play in chimpanzee groups.
It might therefore not surprise you to find that female bonobos show higher levels of irritability than male bonobos, who in turn are more introverted.
These are among the findings of a detailed personality test on bonobos. It is the first time such a test has been undertaken in the wild.
Bonobos are our lesser-known cousin in the animal kingdom. Along with chimpanzees, they are our closest living relatives.
They share many similarities with chimps and were once considered the same species, known as the "pygmy chimpanzee".
Older bonobos were less friendly and more aggressive than the youngsters
We now know this is far from the case. While chimpanzees tend to be more aggressive and manipulative, bonobos are much more gentle. In fact they are so gentle they often express their affections towards many members of the group with sex, the so-called bonobo handshake.
In the latest insights, published in the American Journal of Primatology, researchers peered into the behaviour of a group of 16 wild bonobos from the Luo Scientific Reserve in the Democratic Republic of Congo.
To do so, Congolese trackers – along with Japanese field researchers – observed the bonobos over six months and rated them using the "Hominoid Personality Questionnaire".
This test teases out behavioural traits such as irritability, jealousy and levels of affection.
The team found that five factors dominated their trait analysis, which they grouped into the following categories: Unemotionality, Friendliness, Aggressiveness, Irritability, and Activity.
The less strict a social culture is, the more space we give to personalities instead of social roles
Some of the findings were expected. For instance, bonobo personalities were starkly different from chimp personalities that have been previously studied.
Along with the fact that male bonobos were more introverted than females, they also discovered that older bonobos were less friendly and more aggressive than the youngsters.
"These sex differences differ from those of the chimpanzees in a way that reflects the differences in social systems of the two species," explains lead author Cintia Garai of the Lukuru Wildlife Research Foundation in the Democratic Republic of Congo.
What was more surprising is that there were no sex differences in aggressiveness between females and males.
This could be explained by the fact that bonobo societies are less hierarchical compared to chimpanzees, says Garai. This equality could also enable bonobos to better express their diverse personalities, she adds.
Bonobos are an equally important species to consider if we want to understand our last common ancestor
"The less strict a social culture is, the more space we give to personalities instead of social roles."
Chimpanzees are very different. In chimp societies it is males who act with most aggression. They are even known to purposely kill other members of their group.
Another surprising finding was the lack of any neurotic traits in the bonobos, even though these are widely found in other ape species. Additionally, there were no traces of dominance or openness, which are both present in chimpanzees.
Although these findings are unlikely to tell us anything about human behaviour as such, the results do give us more clues in our efforts to understand the behavioural changes that happened along the messy path of evolution that led to us.
Primatologists often look to chimpanzees to give us greater insights into our last common ancestor with chimps and bonobos, which is believed to have existed over six million years ago.
We can no longer look at levels of chimp aggression and suggest that these behaviours are in-built in humans
But bonobos are an equally important species to consider if we want to understand our last common ancestor. They were often overlooked in the past, in part because there are far fewer bonobos than chimpanzees, which makes them harder to study.
Many researchers are therefore keen to stress that these stark differences between bonobo and chimpanzee behaviour should change our understanding of some human traits once considered innate, such as male aggression and dominance.
We can no longer look at levels of chimp aggression and suggest that these behaviours are in-built in humans too.
"It tells us there are different ways to solve similar problems," says Alexander Piel of Liverpool John Moores University in the UK, who was not involved with the study. "We tend to look at evolutionary behaviours as more or less beneficial than others. But the idea is that every species is solving problems in its own way."
That is, chimps and bonobos have different social strategies, and both are effective.
Each went down their very own evolutionary path over one million years ago. "So it could be that neither of them represent much about their common ancestor," says Piel.
The next step will be to understand if these personality traits are consistent over time and if similar trends exist in other bonobo communities.
"I think it's phenomenal that personality is showing up and we can study it," says Piel. "If anything I'd be surprised if it wasn't there, how can there be such differences in how groups behave, but not in personality? It's great."
Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.
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