Reputation: Bonobos are miniature, sharing, caring chimps, living in hippie communes with no aggression and lots of sex.

Reality: Not really. Bonobos are roughly the same size as chimps, can be aggressive and use sex in very specific contexts.

Bonobos (Pan paniscus) used to be known as "pygmy chimpanzees", a designation that served to distinguish them from regular chimps (Pan troglodytes). But the difference in body size is small – only a matter of a few kilograms – and it certainly is not the most interesting difference between the species.

Takayoshi Kano was one of the first to document the central position of females in bonobo society

“Bonobos have more style,” wrote primatologist Frans de Waal in Scientific American in 1995. “The bonobo, with its long legs and smallish head atop narrow shoulders, has a more gracile [slender] build than does a chimpanzee.”

A typical bonobo has red lips, neat little ears and a distinctive hairdo, which de Waal described as “an attractive coiffure with long, fine, black hair neatly parted in the middle.” Some individuals even have a whiff of a comb-over.

But their appearance is not the thing that really sets bonobos apart from chimps. The most striking difference is the status and dominance of females.

In the mid-1970s, Japanese primatologist Takayoshi Kano was one of the first to document the central position of females in bonobo society. This contrasts with chimpanzees, where females tend to spend a lot of time marginalised at the edge of the community.

Kano also observed lots of unusual sexual behaviour.

There is this perception that they have sex all the time, that they are like nymphomaniacs

“Genito-genital rubbing”, for instance, usually began with female A approaching and staring into the face of female B. The pair would then embrace “and begin to rub each other’s genitals together (probably clitoris) rhythmically and rapidly,” he wrote in 1980 in the Journal of Human Evolution. This typically lasted for less than 20 seconds, and occasionally for over a minute.

When males and females copulated, Kano recorded that in around one-third of cases, the pair would adopt the missionary position. In a few instance, he saw females mating with different males and sometimes with juveniles or infants.

This is all true, but the public fascination with these behaviours has given rise to a view of bonobos that is a little extreme, says Zanna Clay of the University of Birmingham in the UK, who has spent years studying wild bonobos. “There is this perception that they have sex all the time, that they are like nymphomaniacs.”

The reality is more nuanced. The frequency of copulation in bonobos is not as high as most people assume, she says. “In terms of reproduction they are not more sexually active than chimps.”

Females will also often use genital rubbing to defuse tension between two rival groups

The genital rubbing and touching is very common, but it only happens in very specific contexts, often ones that are not obviously sexual.

“People think they just do it for fun but it’s not really to do with that. It’s to do with uncertain social situations,” she says, summarizing the conclusions she and de Waal came to in a paper published in the journal Behaviour in 2014.

For instance, when a group arrives at a new feeding tree, there is tension over who is going to make the richest pickings. “The females will have lots of genital contact with each other and that will relieve the apprehension of this feeding competition,” says Clay. “Once they are calm they can actually feed together in the tree and be quite peaceful.”

Females will also often use genital rubbing to defuse tension between two rival groups, avoiding the kinds of violence seen in chimp wars. But this does not mean that bonobos are incapable of aggression.

“One of the reasons they have this genital touching is because they need to relieve tension after they’ve had fights.”

There is also the occasional bit of pseudo-copulation, testicle touching and even sometimes “penis fencing”

Things can get particularly nasty in zoos, where the artificial set-up can let females assume more power than they normally would in the wild. These super-dominant females can be pretty violent towards males, says Clay.

“There are lots of males in zoos that are missing digits. There’s a male bonobo that’s actually missing the tip of his penis because the female has bitten it off,” she says. “This isn’t quite [in line] with the stereotype of them being peaceful.”

Although females rule, the role that males play in bonobo society is also often underestimated.

If a dominant female has a son, he will benefit from her position in society. As a result, “you can get some males that are more dominant than low-ranking females in the group,” says Clay.

When the Congo River formed around 34 million years ago, all the apes were on the right bank

In 2010, researchers found that mothers helped their sons to get closer to and more matings with estrus females.

Finally, males also engage in sex-like behaviours, roughly analogous to the genital rubbing of females. Kano noted male-male mountings and anus-to-anus contact, and there is also the occasional bit of pseudo-copulation (back to back, scrotum against buttocks), testicle touching and even sometimes “penis fencing”.

This stuff is a doctoral study just waiting to happen.

Nobody is quite sure how bonobos wound up so different to chimpanzees, especially as a 2012 genetic analysis suggests the two species have only been charting distinct evolutionary pathways for less than one million years.

However, a cursory inspection of the distribution of chimpanzees and bonobos across Africa strongly suggests a role for the Congo River. If you were to sit on a raft and drift downriver towards the Atlantic Ocean, you would find chimpanzees occupying the right bank and bonobos on the left.

There’s a male bonobo that’s actually missing the tip of his penis because the female has bitten it off

Takeshi Furuichi studied under Kano and is now a professor at Kyoto University in Japan. In 2015, he and his colleagues, writing in Evolutionary Anthropology, proposed that when the Congo River formed around 34 million years ago, all the apes were on the right bank. But sediments suggest that the flow of water was much reduced around one million years ago, so a pioneering band of apes could have reached the left bank.

The social and sexual differences between chimps and bonobos might have their origin in this moment, says Furuichi.

“In bonobos, females show estrus and have sexual intercourse with males even during the period they can’t conceive,” he says. This is very different from the relatively limited sexuality of female chimps, but could have arisen as a result of just a few genetic changes in that founding population, says Furuichi.

With many females sexually active at once, there would have been less and less competition between males, until eventually the females took control. The rest, as they say, is evolutionary history.

If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.