Autoeroticism, fellatio, autofellatio; a compendium of examples of non-reproductive sex could go on and on. The wild party of sexual behaviours in the natural world defies our imagination, and while marvelling in this panoply is fun, the point is that sex has evolved to be much more than simply an act of reproduction for most animals, and that includes us. That is not to say that the myriad purposes of these acts are the same, nor that similar acts are rooted in the same evolutionary origin. It appears that some of them, notably the many autoerotic acts, may simply, like in us, exist because they are pleasurable. We should not make the mistake of assuming that all behaviours have some specific evolved function: animals can enjoy sensory stimulation too. Rats enjoy being tickled, cats purring, and the Croatian fellating brown bears certainly appear to be enjoying themselves.
Humans engage in a huge range of sexual behaviours, most of which are not reproductive, and some of these are seen selectively in animals. There is little dispute that a healthy sex life between people helps pair bonding and stability in relationships, which may or may not be homo- or heterosexual, monogamous or polyamorous, or other combinations I haven’t thought of. So while the pleasure of sex accounts for the sheer volume, in many circumstances, a secondary function is in the reinforcement of social bonding, mostly between couples. Aside from us, only one animal engages in quite such a large sexual repertoire with comparable enthusiasm, and so the question for ethologists and psychologists is whether they do it like they do it for similar reasons. The bonobo, Pan paniscus, is the fifth member of the remaining great apes, alongside us, gorillas (Gorilla gorilla), orangutans (Pongo pygmaeus) and the chimpanzee (Pan troglodytes). In fact, bonobos resemble the chimp so much so that they used to be called pygmy chimps and were only designated a separate species in the 1950s. They are not significantly smaller than their genus cousins. They are morphologically different, though not by much: bonobos are exclusively arboreal, living in small groups in only one forested region by the Congo River in the Democratic Republic of Congo, where there are fewer than 10,000 remaining. They tend to be less muscular than chimps with narrower shoulders, and slightly longer arms, more gracile. They have pink-red lips and dark faces, and often sport a neat centre parting on their tufty head hair.
Like all great apes, their society is highly structured. Unusually, bonobos operate within a matriarchy. Dominant females reign over social groups, and bestow male status depending on their relationship with the senior females. They form tight-knit groups and exert control over males, especially with regards to aggression and mating requests. Also unusually, for primates, as females mature, they move away from their natal groups and set up in a new clan, if welcomed by the ruling matriarchs.
One of the ways that females express bonding with each other is via vigorous genital-to-genital contact (in the scientific literature, this is referred to as ‘GG rubbing’). Two females approach each other and rhythmically rub together what we presume are their clitorises, for up to a minute. Their clitorises become engorged, and sometimes the participants shriek. Frequencies vary, but some observations indicate that they do this about every two hours. In bonobo culture, this female–female sexual interaction is far from unusual. It’s also one of the main ways that females ingratiate themselves with new social groups.
Bonobos are surely the horniest species in all creation. GG rubbing isn’t limited to females. It occurs in every possible combination, regardless of sex, age or even sexual maturity. Females also do it with males, males do it with other males, both do it with infants. Males tend to do GG in a mounting position, that is, not face-to-face, with their tumescent penises touching. Sometimes they will fence face-to-face with their erections, typically while hanging from branches on a tree.
Statistics for human sexual behaviour involves a fair degree of guesswork, but I think it’s a reasonable assumption that anyone who has sexual contact with multiple people many times a day is unusual. Yet for the average bonobo, that’s par for the course.
Yet female bonobos get pregnant and have offspring at about the same rate as chimpanzees – one child every five or six years. A rough calculation: assuming ten sexual encounters a day for five years (which is well within observed behaviour), and one child in the same time period, means that about one in 18,250 sexual acts results in a baby. This is not quite the same statistic as the one quoted earlier, that only 1 in 1,000 sexual encounters in human apes that could result in a baby actually does – we’re working with incomplete data-sets here. It does indicate though that we share a pattern of behaviour with our nearest cousins that our fictional alien scientist might eventually spot: we have clearly separated sex and reproduction.
Much has been made of the sex lives of bonobos, understandably, as they are close evolutionary relatives, and they do have sex in ways that are perhaps more comparable to our own sex lives than, say, fruit bats or mole rats. Various claims have been made about their living in a ‘make love, not war’ hippy commune based upon the sheer rate at which they orgasm. This pleasant sentiment has been made in contrast with chimpanzee culture, which is patriarchal, violent and murderous. As ever, the truth is somewhat more complicated.
Male chimpanzees physically fight for status and kill to reinforce it. This has never been observed in bonobos, where females dominate, and male status is in relation to the status of their mothers, whom they stay close to and depend upon all their lives. It’s not quite right, though, to suggest that bonobos are a peace-loving ape for whom sex is the gentle answer to everything. Lethal aggression has been observed in wild bonobos, and lots of the ethology of bonobos has been done in the unnatural habitats of zoos, an artifice which may skew the results. These environments sometimes seem to create artificially super-dominant females, and they can be ultra-violent in conflicts. Some male bonobos in zoos are often without a full complement of fingers or toes, and one in Stuttgart Zoo had his penis bitten in half by two superior females.
It is inviting to apply human interpretations to animal behaviour, and it is similarly tempting to suggest that the presence of these non-reproductive sexual acts in us relates to our evolutionary origins. But the evidence is not compelling. It is problematic to draw any strong conclusions that these are evolutionary in origin, and derived from similar roots to what we observe in bonobos, monkeys, dolphins, otters or tegu lizards (as we shall soon see). Bonobos are not our ancestors, and neither are chimpanzees.
Often, when studies of our closest evolutionary cousins are discussed, the implication is that behaviours seen in these species explains our own. Great apes are more closely related to each other than they are to, say, otters, but are not evolved from one another. The three of us – chimps, bonobos and humans – have a shared common ancestor. What is truly fascinating about the bonobos is their evolutionary history. The Congo is a vast river that snakes through central Africa. Bonobos live exclusively on the left bank. Only recently have we begun to work out how they got there. We know that the branch that became the genus Homo – the humans – and the one that gave rise to the genus Pan – chimps and bonobos – separated six or seven million years ago, somewhere in Africa. There are meagre fossils from this time and place but one reasonable candidate for last common ancestor would be the creature Sahelanthropus tchadensis, a hominin much more chimp-like than human. This is a messy time in ape evolutionary history, and there isn’t a scientific consensus on quite how, where and when our lines diverged, nor indeed how clean the break was.
After a time though, our genealogical lines had truly split, and the chimps and bonobos would form a distinct branch. Just as we have reconstructed human population histories using DNA, we can work out who mated with whom – and when – using genetics, by comparing the DNA of living chimps and bonobos. It reveals that there has been no gene flow between chimps and bonobos for at least 1.5 million years – ‘gene flow’ being a scientific euphemism for successful reproductive sex. Sediment analysis from the banks of the Congo River suggest that it is something like thirty-four million years old, and it is mighty enough to act as an impermeable barrier to most terrestrial animals and any proposed gene flow. It seems that natural fluctuations in its high and low tides during the ebb and flow of an ever-changing climate meant that around two million years ago, the tide was low enough that a small founding population crossed the Congo. These pilgrims were then forever isolated on the far shore, and in the time since they were transpontine,1 all of the characteristics specific to bonobos emerged.
This is how many speciating events occur: a small troop splits off from a larger group, but is not necessarily representative of the overall variation seen in the total population. Any species can be isolated in behaviour – one group starts feeding off a tree that fruits at a different time – or in space – a one-way ticket across an otherwise uncrossable river. Once separated, they breed, and the gene pool from which this new population is founded is free to go off in its own direction. It’s not difficult to conceive of slight differences in the first ancestors of the bonobos that gave rise to their sexual liberation. In chimps, oestrous displays, including brightly and hi-vis swollen genitals, very clearly match when they are most fertile. In bonobos, females appear to be at peak fertility for much longer than they actually are. For humans, there are no convincing visible signs of periods of high fertility, which typically reaches its peak a few days after menstruation finishes.2 The fact that bonobos have extended the cues of fertility beyond the obvious signals is a clue to our sex lives. It is conceivable that natural genetic variation that influences oestrus could have been amplified by natural selection in a founding population of the bonobos’ ancestors.
Though I am cautious about over-interpretation of these sorts of similarities, this is crucial in thinking about our own evolution. We have characteristics in common with both species in the genus Pan, with whom we share a common ancestor long before either Pan or Homo evolved. They have diverged from each other, both genetically and behaviourally. The genetics of the bonobos indicates that perhaps only a few small changes in a founding population seeded a radical change in behaviour, and a totally different population structure – Pan paniscus are less violent than Pan troglodytes and use sexual encounters rather than violence to settle disputes and establish social hierarchy.
We do neither. Bonobos are fascinating, but they are also effectively an island species, and island species are often evolutionary oddities. For reasons of geographical isolation, they can be both genetically and behaviourally weird. That doesn’t mean that the lives they live are irrelevant to understanding our own, but let’s face it, bonobo sex lives are very different from ours, or even how we might wish them to be – it sounds exhausting. Sexual contact in bonobos serves a very different function in them, compared to us. Even though the rates of non-reproductive sex might be comparable, and may share a similar genetic basis in inception, the motivation is different, and the evolutionary histories are different. We don’t touch each other’s genitals to resolve conflicts, nor as a collegial greeting, nor in anticipation of a decent meal, at least not in polite society. Demystifying our own sexual preferences, peccadilloes and predilections is worthy of our scrutiny, but again, it may just be that it feels good.
1 ‘Transpontine’ is an unnecessarily fancy word for ‘across a river or bridge’. It emerged in the nineteenth century as a pejorative term for the types of sensational, sometimes saucy melodramas that were typical of theatres on the south bank of the Thames in London at that time. Bearing that in mind, I’m happy with the bonobos being transpontine.
2 Though many people have claimed there are physical and behavioural signs: these have included breast symmetry, face flushing, smell, gait, clothing choices and more. In almost all cases these studies have low sample sizes, or are methodologically flawed or questionable. One of the most famous, which generated a million unquestioning headlines, was that female lap-dancers in strip clubs received more tips when they were ovulating than at any other time during their menstrual cycle. Science relies on numbers to graduate anecdote to data, but this study featured merely eighteen self-reporting dancers over a period of about two menstrual cycles, which any scientist worth their salt would decry as woefully underpowered.