They are by far the closest to humans. With gorillas the separation occurred about eight million years ago, and orangutans, gibbons and monkeys that are much further out, separating as much as 30 millions years ago. Jared Diamond wrote a book called The Third Chimpanzee in which he argued we are really three sub-species of chimpanzee. The differences in the sexual behavior of chimps and bonobos are very significant to this story, because we share some qualities with bonobos that we really only share with bonobos, that are unique to the two. For example, bonobos look into each other's eyes when they have sex, having sex face-to-face which is very rare in the animal kingdom. All sorts of homosexual contacts take place among bonobos, male and female. Sex is a very important part of social life for bonobos. Bonobos have sex five or ten times a day. It doesn't mean to orgasm. They have sex like we shake hands, it's just a little rub against you and off you go. It's very casual, very friendly, and seems to be very related to releasing anxiety or social stress.
If you throw a packet of food into an enclosure of chimps, all hell breaks loose. The males fight over it. The Alpha Male and his coalition will take control of it and share it out among themselves, and maybe with some ovulating females in the vicinity, for some sexual activity. You throw the food into a bonobo enclosure, they'll look at it, have sex with each other, and then sit down a peacefully eat the food. A completely different approach to resource distribution.
Female bonobos are willing and able to have sex at any time, whether fertilization is possible or not. That's vanishingly rare in the animal kingdom. Females are only willing to have intercourse when they are ovulating. This perpetual sexual availability strongly suggests how important sexuality is in non-reproductive aspects of bonobo social life.
Leviton: Do female bonobos show when they are ovulating?
Ryan: They have sexual swellings, as chimps do, but those of bonobos are less focused. With a chimp it's a clear signal they are fertile now. Bonobos could be swelling a week after they've ovulated, or they have the sexual swelling throughout the cycle, and they'll have sex when they're not tumescent. Bonobos have no formalized rituals of dominance and submission like the status displays common to chimps, gorillas or other primates. Bonobos have never been seen to rape, kill one of their own, there's no infanticide or war between groups, none of this stuff happens with bonobos in enclosures nor in the wild.
Leviton: Obviously not the case with humans.
Ryan: Or chimps. That's the thing. You could look at chimps and bonobos as the good spirit and the bad spirit whispering in our ears.
Leviton: From another angle, it seems that bonobos and humans share "pleasure" as one of the organizing principles of sexual relations. Gorillas and most other primates only have sex when there's a chance to propagate.
Ryan: If you look at copulations-per-birth ratios, chimps and bonobos are well above 500, while gorillas might copulate ten times for every infant that's born.
Leviton: This must have been known to Darwin and others. Since humans are having way more sex than they need for the purpose of reproduction, what was the explanation for that? What is evolution and selectivity doing?
Ryan: Darwin thought primordial humans were harem-based societies like gorillas. It's called polygyny. The Darwinian view - and the scientific basis for the double standard - is that men are highly sexual and want to have sex all the time, and females are the controlling and limiting factor. Like dogs, the males will hump your leg or the furniture, but the female, unless she's ovulating, is not interested. That's what Darwin assumed humans had been like, that was how sexual selection worked. He didn't really have a lot of information about primates, it didn't exist, and he didn't have a lot of information about hunter-gatherers.
Leviton: We know now, for instance, that one of the markers for how sexual behavior will organize is the relative size of males and females in a species.
Ryan: There are several different anatomical correlates to these mating behaviors. You're talking about what's called "body size dimorphism." In a harem-based society, like gorillas, there is adaptive pressure on the size and ferocity of the male, because generally the strongest and most aggressive male is the one who vanquishes the other males, expels them from the group, and has sex with the available females. His genes for size and ferocity are passed on. You have this runaway system where the males get bigger and bigger until they reach some limiting biological factor. They are ferocious when rutting, have big canine teeth and so forth. So that's why for gorillas, or walruses or other harem-based groups, the males tend to be 150% to 200% the size of females. In monogamous systems where there's one male and one female like the gibbon, the males and the females tend to be virtually identical, almost impossible to tell the difference. (They are by the way the only monogamous ape, and they are a Lesser Ape, not a Great Ape like us.)
However, there's little to no adaptive pressure on the genitals, because the male that wins the battles is the only male having sex, there's no sperm competition. Where you do have significant sperm competition, there's less pressure causing males to get bigger and nastier, and there's more pressure on development of testicular capacity. You end up with male gorillas being twice the size of the females, but with testicles the size of kidney beans, and located inside the body rather than an external scrotum, and a very small penis.
With chimps and bonobos, with lots of sperm competition, the testicles are the size of chicken eggs, in an external scrotum where the temperature keeps them in readiness, and the males are only 15-20% bigger than the females, about the same as with humans.
Leviton: Explain these internal "sperm wars."
Ryan: The assumption, based on Darwinian theory that's carried through to today, is that competition between males occurs between individuals, and the winner has mating access. What researchers now have found is that there are several types of advantages to relocating that conflict to the woman's reproductive system. In terms of society, much like bonobos, if there's plenty of sexual opportunity for males, there's less male aggression obviously, and you've got males who are more willing to cooperate because nobody's really sure who's the father of any child, if they even think at all in terms of paternity.
Which sex act led to which kid? As we say in the book, you have lots of societies that believe the fetus is actually made from accumulated semen from several fathers, and females continue having sex with multiple partners while they are pregnant because they believe they are adding to the advantages accruing to the child. You, and me, and the other guy could all be the father of this child. On the social side, this thinking has advantages for the adults and the children. Studies have shown that children who are believed to have several fathers have a better chance of surviving to adulthood than children who don't, in those societies. There are social advantages to paternal uncertainty.
On the genetic level there are distinct advantages because one of the mistaken assumptions we have about this competitive mating system that's been assumed is that one superior male would be the best mate for every female. Between you, me and Brad Pitt, the assumption is every woman is better off with Brad, with his markers of genetic superiority, the square jaw, the symmetry and all the rest of it. But in fact what's been demonstrated is that women are often attracted to men based on the signature of their immune system, and that information is communicated through pheromones. Even though Brad may be the richest and best looking, your immune system might make the best match with her immune system.
So if your sperm is in the running, her body is able to distinguish between the different sperm cells and give an advantage to the sperm cells that are going to make the best genetic match.
We assume all this is visible in the individual and macro levels, but a lot of the really important selection takes place on the micro level we're not able to see without special equipment. Darwin couldn't see it.
Leviton: And now we know through study of female secretion during sex, and female physiology, that ejaculations from more than one male inside a single female then compete?
Ryan: Sperm is by its very nature competitive. There are 100s of millions of them, and that's a lot more than you need. All "sperm wars" are internal, all about sperm from competing men in a woman's reproductive system during one ovulatory cycle. There are evolved strategies between the sperm of different men. For instance, the first spurt of male ejaculate contains chemicals that will attack pre-existing sperm in her system, to neutralize whatever's there. The last spurt has protective chemicals that serve to block the next guy's first attacking spurt. The chemistry's complicated, and has evolved in this competitive sperm environment.
The ovum appears to be able to distinguish which sperm have the genetic markers that best suit the genetic signature of the female, to create the strongest children. One of the ways this happens is suggested by Claus Wedekind's 1995 "Sweaty T-Shirt Experiment." He was a Swiss researcher who wanted to understand why women report a man's smell is so important. If you're hanging out with guys in a bar talking about women, they never say "wow, she smells good, that's why I love her" (and if they do, it's about her perfume). But women love to talk about the particular smell of men. Women's sense of smell is much stronger than that of men, much to their discomfort sometimes. It changes with pregnancy and ovulation as well.
He wondered what the information was being picked up by women in this sense of smell, and theorized that it was something to do with men's immune system. He asked women to sniff T-shirts men had been wearing for days with no perfumes, soaps or showers, and rate the men based on how attractive they thought they must be based on smell. He found, and it's been confirmed by subsequent research, that most women were attracted to the scent of men whose major histocompatibility complex (MHC) differed from her own. MHC indicates a range of immunity to various pathogens. Children born of parents with different immunities are likely to benefit from a broader, more robust immune response themselves. The women who were deficient in one area of their immune system picked, by smell alone, as more attractive those men who supplied that missing element.
But a few women in the study seemed to be picking randomly, or even showed a preference for men with similar immunity to their own. Finally, Wedekind discovered these women were on the birth control pill. It appeared to block their ability to read this very important information in a man's smell. Look at the practical implications. How many couples got together when the woman was on the pill? They got along really well, they got married, she goes off the pill, she has a baby, and a year later the spark is gone and nobody knows why. They blame the stress of having a kid, work, or whatever. It's nobody's fault, it's just biology.
Leviton: It's another example of a pharmacological invention blocking us off from what's truly natural.
Ryan: To our chagrin, causing a lot of suffering we're blaming on ourselves, which is one of the overall themes of the book, to identify—and hopefully, interrupt—that cycle of blame.