Category Archives: Sex


So I got a call the other day to talk to a TV reporter about kissing. Valentine’s Day was coming up, and since it was also Darwin’s Birthday, it seemed fitting to talk about the evolution of this rather odd behavior. Moreover, even though my research focuses more on fighting and war than love and kisses, spending the last year living in France had me thinking about cultural variation in kissing.

People kiss a lot in France. Friends kiss each other when saying hello and when saying goodbye. Boys kiss girls, girls kiss girls, and boys kiss boys.  All this intimacy can make a shy person raised in the Upper Midwest feel quite awkward. And it presents all sorts of challenges. How well do you need to know someone before you kiss them? Do you actually touch the lips to the cheek, or just brush cheeks and kiss the air in their general direction? (Turns out its the latter.)

Though all this kissing might suggest an easy intimacy, in other ways the French are even more reserved than Americans. As French language and lifestyle expert Géraldine Lepère says, “Do not try to hug a French person. They will freeze.”

Adding to the challenge is that the number of kisses expected varies from region to region across France. In our area, Languedoc-Roussillon, three kisses were the norm.  But in other parts of France, the norm involves anywhere from one to five kisses. Greeting friends from those other regions was always a challenge. How many times do we kiss?

Geography of French kissing.
Geography of French kissing.









Thinking about all this cultural variation in greeting kisses made me wonder about the romantic kiss. Is it a human universal, or does romantic kissing show the sort of cultural variability that we see in greeting kisses? And what about kissing in our primate cousins?

Chimpanzees don’t have romantic kisses. But then they don’t really have romance. Mating is a quick business that last seven seconds or so, and the typical mating posture doesn’t bring the lips into close proximity. Females often scream and dart away after than mating rather than staying close to kiss and cuddle (though sometimes males do a bit of grooming of their partner in the afterglow).

Chimpanzees do kiss in other contexts, though, such as greeting and reassurance. Once at Lincoln Park Zoo in Chicago I saw two female chimpanzees engaged in a leisurely lip kiss that lasted at least ten minutes. But I don’t recall seeing anything like that in the wild.

Bonobos are reported to kiss more often, and to use their tongues when kissing. Frans de Waal writes, “French-kissing’ is totally absent in the chimpanzee, which engages in rather platonic kisses. This explains why a new zookeeper familiar with chimpanzees once accepted a kiss from a male bonobo. Was he taken aback when he suddenly felt the ape’s tongue in his mouth!” (de Waal, 1998: p. 103)

Unlike chimpanzees, bonobos frequently mate face to face, which would make romantic kissing more feasible. So perhaps romantic kissing in humans has something to do with our more bonobo-like mating postures.

In The Naked Ape, Desmond Morris suggested that kissing came from sharing food between mother and offspring. This is a classic example of ethological thinking, in which some puzzling, apparently useless behavior is interpreted as a ritualized version of an older, clearly functional behavior. But I’ve never liked this explanation. Partly I suppose because it’s a bit gross. But also, this isn’t something I’ve seen either people or other primates doing. I have seen human mothers pre-chew food for their children, but they transfer the food with their fingers, not their lips. And apart from Junior High stories about kissing couples sharing their chewing gum, food sharing doesn’t seem to play much role in romantic kissing in humans.

More recently, Evolutionary Psychologists have interpreted romantic kissing as a key component of mate selection in humans. By tasting a potential mate’s lips and saliva, kissers may be able to gain useful information about their health and perhaps even genetic quality and compatibility before taking any chances on actually combining their genetic material. One survey study of American college students found that women, more so than men, are reluctant to have sex with someone they haven’t kissed, and more likely to choose not to have sex with someone if they prove to be a bad kisser (Hughes et al., 2007). This makes sense given the sex differences in parental investment that are typical of mammals. Since women rather than men carry any resulting babies inside their bodies for months and then nurse them once they are born, women have a greater evolutionary interest in making sure they they screen potential mates for genetic quality.

Hughes et al. (2007) thus suggest that kissing is an evolved strategy, and argue that romantic kissing occurs in “over 90 percent of human cultures.” Kissing seems kind of a risky way to assess someone’s health, though. After all, if they turn out to be harboring some nasty infection, sharing their saliva is maybe not the best idea. Why not just a quick sniff of their breath instead?

Moreover, thinking about all the cultural variation in kissing in other contexts made me wonder whether romantic kissing was really so invariable.

Darwin, as usual, is way ahead of us here. In his 1872 book, The Expression of the Emotions in Man and Animals, he wrote that kissing was unknown in many non-European cultures:

We Europeans are so accustomed to kissing as a mark of affection, that it might be thought to be innate in mankind; but this is not the case. Steele was mistaken when he said “Nature was its author, and it began with the first courtship.” Jemmy Button, the Fuegian, told me that this practice was unknown in his land. It is equally unknown with the New Zealanders, Tahitians, Papuans, Australians, Somals of Africa, and the Esquimaux.

Darwin’s observation that romantic kissing is not a human universal is supported by a study that will be published in American Anthropologist later this year (Jankowiak et al., in press). This study examined 88 different cultures from the Human Relations Area Files, and also consulted with people working in various non-Western societies. They found that not only was romantic kissing not a human universal, it was only found in 40% of the cultures they examined. Strikingly, they found that romantic kissing was entirely absent in hunter-gatherers, the people whose societies are widely thought to most closely resemble the conditions in which our species has lived for most of our evolutionary history.

This finding seems to me an excellent example of the importance of good old fashioned Anthropology, in which the goal is to find document and explain human variation across the planet. If our picture of human behavior is based only on the behavior of our most convenient study subjects (e.g., American college students), we will come up with a badly distorted picture of our species.

In cultures with romantic kissing, pressing the lips clearly plays a big role in mate choice.  And the evolutionary logic for women being a bit more interested in the quality of kisses than men seems sound. But because kissing occurs only in a minority of human cultures, sex differences in preference for kissing must result from some more general mechanisms, rather than having evolved as part of a specific mental module for smooching.

Works cited:

Darwin, C. (1872). The expression of the emotions in man and animals. London, Murray.

Hughes, S. M., M. A. Harrison and G. G. Gallup (2007). “Sex differences in romantic kissing among college students: An evolutionary perspective.” Evolutionary Psychology 5(3): 612-631.

Jankowiak, W. R., S. L. Volsche and J. R. Garcia (in press). “Is the Romantic/Sexual Kiss a Near Human Universal?” American Anthropologist.

Morris, D. (1967). The Naked Ape: A Zoologist’s Study of the Human Animal .Jonathan Cape.

de Waal, F. B. M. (1998). Bonobo: The Forgotten Ape, University of California Press.

Green Porno and Disgust

Isabella Rossellini is doing a show, Green Porno, in which she performs the mating behavior of a whole range of other species, such as honey bees and spiders. The show is based on a series of videos that she has been making since 2008.

Isabella Rossellini and friend.
Isabella Rossellini and friend.

Holy cow. These are awesome – and biologically accurate, in a whimsical, funny way. Rossellini clearly did her homework. But sexy? Not so much, I think. Rossellini herself says that though the bits are called “Green Porno,” “There is nothing porno about them.”

I teach a course on Sex, Evolution and Behavior, in which I occasionally illustrate key points with short video clips of the mating behavior of other species, from slugs to albatrosses. Not for titillation, mind you, but for education. In fact, while such video clips may be weird, or interesting, or comical, or gross, or some combination of these, they are pretty much never even remotely titillating. At least in my opinion. And my impression is that for most people, sex in other species is generally much more icky than sexy.

Which makes me wonder: why might this be the case?

The world is big, and has many kinds of people in it, and perhaps somewhere on the Internet there are sites for people who really do find the mating behavior of lobsters arousing. But I expect those sites are vastly outnumbered by sites depicting the mating behavior of our own species.

So why do we perceive sex in other species to be disgusting rather than sexy?

One possible explanation is that this disgust response is part of an evolved psychological mechanism that promotes mating decisions that enhance, rather than reduce, fitness. If we found slug sex really sexy, rather than disgusting, we might try mating with slugs, which (among other things) would be a real waste of mating effort.

If this hypothesis is right, that disgust at other species’ sexuality is a reproductive isolating mechanism, an adaptation to keep us from mating with the wrong species, then we should find the sexuality of species closely related to us to be particularly disgusting. After all, we probably couldn’t really mate with a slug even if we wanted to, but mating with something more closely related to us, like a chimpanzee, at least seems within the realm of possibility.

Back when the world had multiple species of humans living at the same time, such as our ancestors and the Neanderthals, interspecies mating would have been a real option. And studies of fossils and ancient DNA indicate that such matings did occur, at least on occasion. Given that hybrids can have various genetic problems, such as infertility, individuals are likely to have more offspring if they reliably mate with their own species, rather than some other species. And indeed, many aspects of animal behavior and anatomy appear to be related to species isolating mechanisms: courtship behavior, species-specific coloration patterns, and genitalia designed to fit like a lock and key, so that the parts only fit with the right partner.

So, what about this prediction, that if disgust is a reproductive isolating mechanism, we should find the sex of species closely related to us to be particularly disgusting? I don’t know of any formal studies of this, but from watching and listening to people’s reactions to monkeys at zoos, I get the strong impression that in general, people really do find monkey sex disgusting, as well as funny and embarrassing.

As a primatologist, I have seen lots of monkey sex: baboons, rhesus monkeys, chimpanzees, gelada monkeys, and so on. All these animals have sex in the open, in public, for everyone to see, so you don’t need to be a creeping pervert to see them going at it. In many cases, you would have to make a real effort to avoid it. And none of it, really, is very sexy, from a human point of view. In chimpanzees, mating is a furtive event that lasts six or seven seconds on average, which often ends with the female screaming and darting away from the male, as if she is terrified that he might beat her up. Which he might well do.

In my first field season as a primatologist, studying baboons in Kenya, I spent quite a bit of time looking at baboon bottoms. Not because I am particularly interested in baboon bottoms, but because (1) if you are following baboons, you’re going to see a lot of baboon bottoms, just that’s what you see when they’re walking in front of you, and (2) baboon bottoms display a whole lot of really valuable information about their reproductive state. You can tell whether a baboon is male or female, for example, just by looking at their bottom, even if they are just little kids. Baboons have a region of bare, tough skin going across their top of their bottoms that serves as a seating pad: the ischial callosities.

Males have a continuous seat pad surrounded by a patch of grey skin:

Back side of a male baboon
Back side of a male baboon









Whereas females have two separate pads, for the left and right cheeks, separated by the genital opening:

Female baboon at Gombe with bright red pregnancy sign.
Female baboon at Gombe with bright red pregnancy sign.







You can tell where a female is in her ovulatory cycle, as when a female is approaching her fertile time, the skin of her perineum swells up into a big pink sexual swelling, which looks extremely uncomfortable. And if a female is pregnant, the bare skin above her ischial callosities turns from grey to bright crimson.

When I was first habituating baboons (following them at a distance so they would get used to us and let us observe them at closer range), one afternoon we managed to get close enough to our group to see them resting on the face of the steep cliff where they would spend the night. Peering through my binoculars, I could see one female with what appeared to be a raw, festering sore by the base of her tail. As the weeks went by and the baboons let us get closer, I realized that this “sore” was just the bloody red pregnancy sign. It doesn’t look nearly so nasty when you are close enough to see that is not in fact a festering sore.

So during my ten months of following baboons around, I spent a lot of time looking at their bottoms, drawing pictures of their bottoms, taking notes of their bottoms to learn who was who and what their reproductive state was. At first, some features of baboon bottoms seemed kind of gross. I got used to that. And eventually I learned what sort of bottoms looked especially appealing to male baboons, by seeing who made special efforts to mate with.

Ahabazi grooming Ubergiji, who has a full sexual swelling.
Ahabazi grooming Ubergiji, who has a full sexual swelling.







But never in my loneliest days in the field did I find baboon bottoms sexy. Sorry babs, that just how it is.

On the other hand, many examples of hybrids exist. For example, as Kate Detwiler has documented, blue monkeys and redtail monkeys at Gombe and several other sites mate and produce hybrid offspring. My first experience studying primates involved a class project looking at the behavior of the hybrid offspring of a sooty mangabey and a mandrill: a mangadrill. So if disgust is part of a reproductive isolation mechanism, it clearly doesn’t always work.

Whether or not disgust is a reproductive isolating mechanism, it certainly plays a big role in regulating sexual behavior. Even within our own species, we perceive many categories of mating behavior as well, kind of gross. Or really gross. Little kids often seem to think that even kissing is pretty gross, much less the other things that grown-ups are rumored to do in private.

And much of the human disgust response makes sense from an evolutionary perspective. For example, various studies have found that women are considerably less interested in graphic depictions of mating than men are. Many men enjoy looking at graphic, impersonal depictions of anatomical function. Many women find that sort of thing disgusting. Instead, they prefer reading romance novels, or watching romantic movies, where sex occurs in the context of relationships developing between realistic characters. Or with vampires. Or werewolves. Whatever.

Is there a reproductive isolation mechanism at work here?
Is there a reproductive isolation mechanism at work here?







The emphasis is on the relationship first, anatomy second.

A recent example of this sex difference in action is the case of the politician Anthony Weiner, who sent pictures of his privates to women who were not his wife. One can imagine him thinking, “Hey, I’d love to have women send me naked pictures of themselves! Surely they would like me to do the same for them!” And maybe there are women who would like that. But I think this response by a blogger is more typical: “The truth is, guys, your ‘junk’ is one of the last things we want to see up close via digital or printed media.”

The Golden Rule, “Do unto others as you would have done unto you,” doesn’t work so well in cases where people have strong differences of opinion about what they would like to have done unto them.

And from an evolutionary perspective, this sex difference makes sense. If it is important to be choosy about mating, then being aroused by random sexual images would be counter-productive. In humans, women tend to be choosier than men. This makes sense, because men and women face much different costs from mating. A man who has sex with a random stranger might get the evolutionary “win” of impregnating her without having to do any work bringing up the baby. A woman who has sex with a random stranger may get stuck doing all the work. Thus for men, sex with strangers is an opportunity, whereas for women it is a risk, and best to be avoided. (Unless, for example, that stranger is so amazing that having his baby would be worth the trouble. Elvis, say, or Magic Johnson.) Disgust at random male junk may thus be an important mechanism guiding mating decisions that prove beneficial, at least on an evolutionary timescale.

Even for men, though, sex with strangers can be risky, if those strangers are infected with nasty diseases. So both sexes should be equally disgusted by anything that indicates such an infection, like festering sores.

In humans and certain other animals, including many bird species, sex often happens within a long-term relationship, rather than with random strangers. And just as in many birds and some mammals, like wolves, male humans do a lot of parental care. If a man has good reason to think his wife is mating only with him, and is uninterested in mating with other men, then he will have good reason to believe that her children are his, and that he should invest in them, by bringing them meat, or carrying them around, or changing their diapers, or whatever.

In contrast, among chimpanzees, males and females both mate promiscuously. Males don’t provide any parental care (apart from defending a group feeding territory), so females are always stuck doing all the work of parenting. However, males sometimes kill babies of females that they haven’t mated with. So females have a strong incentive to mate with all the males of her community, to convince them all that they might be her baby’s daddy.

Given this difference in mating strategies, if female chimpanzees used smart phones, they might like nothing better than to receive photos of male chimpanzee private parts. I suppose that’s an experiment that could be done.

Recently, disgust has gotten lots of attention as one of the key moral emotions. In college, I sat in on J. Z. Smith’s Religion in Western Civilization class.   In talking about Leviticus, the Biblical book of rules, many of which seem baffling beyond belief, Smith argued that “the complaint of the writers of Leviticus was not, ‘God, why have you made me such a sinful being.’ No, it was more like, ‘Why have you made me with so many holes?’” Humans leak all sorts of disgusting substances from their various orifices, and many of the rules of Leviticus involve proper regulation of these fluids.

Smith seemed to think that these rules involved a fairly arbitrary sense of disgust, rooted more in aesthetics than reason. For example, he argued that Jewish dietary law forbade eating pork because pigs are disgusting animals. They wallow in filth, they eat trash, we shouldn’t incorporate such filthy animals into our bodies. Smith argued that the argument that such laws protected Jews from trichinosis was an invention of 19th century Reform Jews who wanted to find more rational foundations for traditional beliefs.

More recently, Paul Rozin and others have argued that rather than being arbitrary, disgust really is rooted in evolutionary logic: many things that we find disgusting are dangerous: rotting meat, maggots, festering sores, snot, feces and the like can all transmit pathogens. If we eat such things, or even touch them, they can make us sick, or dead. From an evolutionary perspective, it makes sense to have emotional mechanisms that make us avoid such potentially dangerous substances. In this view, feelings of disgust originally based in avoidance of pathogens have expanded to provide one of the main foundations of moral feeling.

A moral emotion like disgust can guide behavior more quickly, and more reliably, than rational arguments. If we had to stop to think and debate amongst ourselves whether rotting carcasses, for example, were safe to touch, we might pick them up and expose ourselves to all sorts of pathogens before we have persuaded ourselves that maybe that’s not such a good idea. The same way with many moral actions: if people think an immediate, gut response of disgust to pedophilia, or rape, or murder, they will be inhibited from even giving it a try.

Disgust seems to involve both learned and innate components, and develops gradually as children mature. Toddlers, for example, seem to find nothing disgusting. But pretty much everyone eventually grows up to find rotting meat and vomit disgusting. People seem to have a natural disposition to develop feelings of disgust towards things that carry a strong risk of infection.

More recently, Jonathan Haidt and others have argued that disgust is at the heart of the six moral foundations or dimensions: care/harm, fairness/cheating, liberty/oppression, loyalty/betrayal, authority/subversion, and sanctity/degradation. In this view, disgust is involved mainly in the sanctity/degradation dimension.

Much of the current political debate about gay marriage involves tension between these different moral foundations. People who emphasize the first three moral dimensions see gay marriage mainly in light of care (we shouldn’t cause emotional harm to people by denouncing their private life choices as sinful), fairness (it’s only fair that people who love each other should be allowed to marry), and liberty (people should be free to be who they are and love who they love). People who emphasize the other dimensions see gay marriage mainly as an issue of authority (scripture says it is wrong) and sanctity (heterosexual marriage is a sacrament; homosexual marriage is wrong). Thus, people on both sides of the debate are morally outraged by the arguments of people on the other side.

And from an evolutionary point of view, an aversion to same-sex mating seems to make a certain amount of sense: such mating doesn’t produce offspring. But it turns out that evolution is more complicated than that. For some species, homosexual mating is not a preference or orientation for a portion of the population; it’s obligatory. For example, New Mexico whiptail lizard reproduces parthenogenetically; the species has only females, and they are all virgins. Nonetheless, even though their eggs are not fertilized, females still have go through the motions of mating, with another female, in order to produce eggs. (Incidentally, this unusual mating system seems to be a result of the hybridization between two other lizard species.)

And even in species that have both males and females, homosexual behavior occurs in a wide range of species. Among Laysan albatrosses in Hawaii, for example, there appears to be a shortage of adult males. Raising a baby albatross takes a huge amount of work. Mom and dad have to take turns flying far away to get fish to feed baby, while the other parent stays with the nest. A single parent just can’t manage. As a result, many females pair with other females. They mate with males who are paired with other females, but nest and raise chicks together with their female life partner.

Rossellini says her Green Porno doesn’t have a political agenda. It is more about educating people about what the world is like for other species. But her videos do an excellent job of illustrating that in nature, sex involves an enormous range of diversity, from the exploding kamikaze penises of honeybees to the playful, anything goes sex of dolphins.

And learning more about the sex lives of other animals is useful for getting a broader perspective for the behavior of our own species, whatever we might personally find to be disgusting. Or not.