Evolved to Go Down? The History of Oral Sex

Why do we go down on each other? Are we evolved for oral sex? Find out in today’s stimulating episode.

Don’t forget to subscribe, rate, and review. Support the show on Patreon at www.patreon.com/btnewberg. Research, writing, editing, and production by B. T. Newberg. Logo Design by Rachel Westhoff. Animation by Maxeem Konrardy. Additional credits, references, and more at www.historyofsexpod.com.


Humans are weird.

Not only do we mate outside of ovulation. Not only do we mate in private for some bizarre reason (see our previous episode on that). And not only do we do the horizontal mambo for a ridiculously long time, far longer than you would think would be wise in a predator-rich natural environment. Not only all that, but for some reason…

We like to stimulate each other… with our mouths.

Or at least some of us do. Oral sex has a long and complicated history, which we will get into in future episodes, but today, I want to concentrate on the most basic question possible:

Why do we go down on each other?

Because there’s no way you can pass on your genes by smoking a pole or eating a box lunch, right? Or is there? Is there some indirect, counter-intuitive way that it helps us win the game of evolution? Or are we just deliciously twisted?

That’s what we’re talking about today. I’m B. T. Newberg, and this is the History of Sex.


I’d like to thank our Patreon patron Preston Rey for making this episode possible.

Hey folks, before we get started, I want to introduce you to a great podcast called Shared History.

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Cass & Natalie are a lot of fun. They are not afraid to let the profanity rip, so if that sounds like you’re bag, check out Shared History.

Speaking of fun, obviously from the topic of today’s episode, you can tell it’s gonna be a little out there. It may not be for everybody, but for those that it is for, it’s gonna be a lot of fun. So, let’s get to it.

First, pop quiz. True or false: Humans are the only species that engages in oral sex. What do you think? True or false?

And the answer is: False. But only certain other animals do it, and that gives us an interesting clue to what might be going on with our habit for head.

As it turns out, only mammals give head. Whether we’re talking about fellatio or cunnilingus, whether we’re talking about same sex or opposite sex or however you want to cut the pie, it’s something about us being mammals.

So, the birds and the bees don’t do it. Just us. So… what’s up, mammals?

There must be some crucial difference in mammals, so what is it? Well, one possibility is our anatomy down there. A lot of other species have different genital arrangements. Birds, for example, have a cloaca, which is basically, um… just one generic all-purpose hole. You got Mr. Bird comin’ in the same door that Mrs. Bird uses to drop those little gifts on your car windshield. Researchers have suggested there might be a hygiene issue at play. That may not be the first place they wanna stick their beaks.

But if that alone could explain it, you would think any species with separate holes would figure out how to pleasure each other right quick the same way we have. But they don’t. As it turns out, it’s not all mammals, but only certain mammals that engage in oral sex.

There are three main groups of mammals, and only one of them boasts species that go down. The three main groups are monotremes, marsupials, and placentals. Care to guess which? Well, I guess have to explain them.

First, monotremes are an odd group that includes the platypus and the echidna, and believe it or not, they actually lay eggs and have a cloaca (so just one hole, not two, like birds and reptiles). So oral sex? Nope, so at least that fits with what we were saying earlier about the hygiene issue.

Second, marsupials are like, you know, koalas, kangaroos, Tazmanian devils, opossums. They do have multiple holes, or at least most of them do, so do they do it French style? Nope, not them either. Huh.

Well, that just leaves the placentals. Do they do it? Yeah, you guessed it right away, because humans have placentas, and we certainly do it. But it’s not just us. A number of placental species do in fact go down on each other. Well, maybe that has something to do with it.

Here’s one proposed reason that at least explains fellatio. It has been proposed that ingestion of male semen may help prevent the female immune system from attacking the male proteins contained in the fetus and placental wall. Basically, on this theory, oral sex is like a meet-and-greet for the cells that are going to be working together. It gets the female’s antibodies used to the male’s antigens so they don’t treat them like trespassers.

Okay, now we seem to be gettin’ somewhere. Maybe placental mammals engage in oral sex for the benefit to their placentas.

But that would only explain fellatio, not cunnilingus. And believe it or not, the more common of the two among non-human animals seems to be the latter.

Congratulations, ladies!

So, if the antigen thing can only explain fellatio, then there’s got to be more to it still.

Well, maybe we can get a clue from exactly which placental mammals do it.

Here’s a list: fruit bats, hyenas, cheetahs, lions, walruses, caribou, dwarf cavies (I guess they’re like a guinea pig?), kob antelopes, thinhorn sheep, Indian flying foxes, wolves, goats, Cape Ground squirrels, dolphins, sea otters, manatees, gibbons, macaques, gorillas, chimps, bonobos, and of course humans.

Those are the ones I could find, anyway. And these are just the species where we’ve actually observed oral sex taking place. The list may be longer, but I can only say with confidence that the list is at least this long.

And that seems pretty long. That was 22 species right there. Then again, considering that there are some 4000 different species of placental mammals out there, it’s still seems pretty rare.

So, if oral sex could be accounted for, evolutionarily speaking, by just pointing to benefits to the the placenta, you would think a lot more placental mammals would be doing it left and right. And that just doesn’t seem to be the case. The placental antibody-antigens meet-and-greet thing might be a significant factor, but it can’t be the only reason. There’s gotta be something more.

Let’s take a closer look at that list. The first one I mentioned, fruit bats, gives us a very interesting clue. What greater short-nosed fruit bats tend to do is this: when they have sex, first the female licks the male’s genitals, then they have penetrative vaginal sex, and then lastly the female licks the male’s genitals again. A study from 2009 observed oral sex in 14 out of 20 matings, and found that when they did this instead of just gettin’ right to the thrusting, the male lasted longer. A lot longer. Like seven times longer.

Researchers found that orally stimulated males spent an extra six seconds in penetration for every second of dusting off the old sombrero.

Well! That is certainly interesting, but it raises even more questions. I mean, is lasting longer a good thing for fruit bats? Does it increase chances of conception somehow? Or does it increase pleasure for the partners and thus desire to mate again? The researchers could only speculate.

There’s also that odd bit about licking him again after the deed. That presumably wouldn’t affect how long he lasts, unless it’s such a rockin’ blowjob it sends him back in time. So, how does that figure? Researchers speculate that the antiseptic properties of saliva may reduce STD transmission. So, maybe it’s a wash up before and after, and it just happens to have a fortuitous side effect.

Again, this only addresses fellatio. But there’s another kind of fruit bat, the Indian flying fox, where it goes the other way: cunnilingus instead. It seems when male flying foxes went down on the flying vixens (god, I really hope they’re flying at the time when they do it!), when they did that, the males were more often allowed to proceed to penetration. Well… yes. That makes sense! Turns out males willing to go way down south to Dixie managed to get to fourth base in 57 out of 69 cases.

How often did they make it to fourth base without going down first? The study doesn’t exactly make it clear. The authors write: “Although prolonged rituals and competition between males occurred in the remaining 12 observations, we could not follow them as it was beyond the capacity of our visual observation.”

I’m not sure exactly what that means. But it sounds like foreplay preceded every observed mating, but 12 of the couples snuck away to get a room, so we don’t know if the foreplay was universal or not.

So, that’s pretty interesting. What’s more, much the same pattern as with the short-nosed bats occurred: flying foxes spent more time in penetration after giving oral sex, and followed up with still more of it afterward, spending 2-3 times longer on the afterplay compared to the foreplay. Again, it may have something to do with washing up.

Interestingly, the males typically washed themselves up even before approaching the female – life lessons, boys! Game changer.

So, we’ve found some intriguing stuff here. I’m a little skeptical of the washing up theory, though. If oral sex meant to reduce STDs, it seems like it would only affect those transmitted from the outside of the organs, not from the primary exchange of fluids which happens inside the vaginal canal where a tongue can’t reach.

So, there’s still more explanation to be found here somewhere.

Let’s look back at that list of placental mammals again. More than a quarter of them – 6 out of 22 – are primates. Ah-ha! Curious. Gibbons, macaques, gorillas, chimps, bonobos, humans… I wonder if intelligence makes it any more likely?

Because along with intelligence comes other things, like creativity. Primates certainly do get creative with their mating. Gorillas are known to do it all kinds of different ways, including fingering, fellatio, cunnilingus, and even both at the same time in the 69 position.

Chimps have even been observed using sex toys. That’s right, sex toys! Jane Goodall observed that males would often approach a female holding a leaf over his junk. But this is no act of modesty. Rather, the male tears the leaf away bit by bit, slowing revealing his throbbing member, hoping the female will notice. He might go through several leaves in this Chimpendales striptease until she finally gets so hot she can’t help but present herself to him. Magic Mike!

Then there’s bonobos, whose social bonding sex rituals are on a whole other level. I’m talkin’ purple shag carpets, a bottle of chianti… It’s like they evolved specifically for the 70s.

For bonobos, sex goes far beyond mating. They use it to ease social tension, build relationships, or even just to say hello. And they say hello by the lovin’ spoonful! The bonobo clitoris is three times the size of a human’s, even though bonobos are otherwise smaller in stature, and there’s plenty of cunnilingus going down – both male-on-female and female-on-female.

So, maybe oral sex fills some of those other functions we talked about, like the antigen-antibody meet-and-greet, or the reduction of STD infections, but another significant may simply be: creativity. Maybe intelligent creatures just like to explore every which way they can do it, and it doesn’t take long before their playing the piccolo and yodeling in the valley.

It’s certainly not at all out of the question that oral sex in the animal kingdom may be multi-purpose, including both function and fun.

Which brings us to us.

Why do we humans go down on each other?

There are some theories we haven’t hit yet. Two of them which are odd but fun attempt to explain cunnilingus.

The first proposes that sneezing in the cabbage aids female orgasm, which in turn aids in sperm retention. Now, that’s kind of a curveball of an idea, so let me break it down for you. For most species, sperm retention isn’t a problem: gravity helps keep the male’s deposit in the bank. But for upright-walking species like ours, gravity works against us. As soon as a human female stands up, the cash tends to fall out of the ATM.

However, when a female orgasms, the muscle spasms sort of work the semen deeper into the vault, keeping it locked up. Thus, anything that produces more orgasms, like cunnilingus, may help with sperm retention, thus making conception more likely. So, for any hopeful parents out there, if you needed another reason to go deep-sea diving, there you go. It may help you conceive.

So goes the theory anyway. The evidence is a bit mixed on whether the muscle spasms actually help at all.

The other explanation for cunnilingus that’s garnered some raised eyebrows is sperm competition detection. According to this theory, a man’s unconscious motive for going down on a female is to somehow detect other men’s sperm. So, in other words, getting eaten out is like trying to pass through airport security when you’re packing more than 2 ounces of toothpaste. Now, how that detection is supposed to happen I certainly do not know. By smell, I guess? I dunno. Like I said, this one has gotten plenty of raised eyebrows, but it’s fun so there you go.

The final theory of note for today, which addresses both cunnilingus and fellatio among humans, is a much more common-sensical one. It starts from the idea that oral sex is, well, fun. And introduces variety to the bedroom. And that may keep a partner from getting bored. It may be as simple as that: you keep a mate from infidelity by keeping them satisfied. Oral sex may be one way to do that.

But there’s an even more common-sensical reason, and that’s simple pleasure. We like to have fun. Maybe we need no greater reason than that. Maybe there is no evolutionary imperative, and we and all our fellow animal species just do it because it feels good.

Of course, it’s not a question of either/or. It can be all of the reasons covered today: introducing the placenta to the male’s antigens, extending the sex act, reducing STD infections, creative exploration, sperm retention, sperm competition detection, mate retention, and finally simple pleasure. All of these things may be at play the next time that you choose to bring oral sex into your bedroom.

And that is a choice that people in the modern West are increasingly comfortable making, but it hasn’t always been that way. The history of oral sex across cultures has been varied and complex. Some cultures have been down with going down, while others have not. Why? That’s what we’re going to find out next time on the history of oral sex.

Well, that’s all I’ve got for you guys today. Thanks for listening, everybody.

If you like what we’re doing here, be sure to subscribe, rate, and review, or you can pledge on Patreon where $5 a month gets you a portrait drawn in the time period and culture of your choosing. I will draw you allegorically playing the piccolo, or yodeling in the valley. Or whatever you want, I’ll make you look awesome, I promise. Just go to www.patreon.com/btnewberg. That’s patreon.com/btnewberg.

Alright, I’ll see you next time. I’m B. T. Newberg, and this is the History of Sex.


Auguste, Annie. “The History of Fellatio.” Salon. 2000, May 4.

Baumard, Nicolas, Hyafil, Alexandre, and Boyer, Pascal. “What Changed During the Axial Age: Cognitive Styles or Reward Systems?” Communicative & Integrative Biology. 2015, Sep 25.

Borelli, Lizette. “The Evolution of Oral Sex: Does ‘Going Down’ On Your Partner Have Biological Roots?” Medical Daily. 2016, May 17.

DePierre, David. A Brief History of Oral Sex. Jefferson, NC: Exposit Books, 2017.

Green, Jonathan. “Intercourse: Oral and Other Varieties.” Timeglider. 2013-2016.

Ludden, David. “New Research: Why Religions Promote Sexual Conservatism: The Unexpected Link Between Monogamy and Trust.” Psychology Today. 2019, Jun 30.

Liu, Hui, Shen, Shannon, and Hsieh, Ning. “A National Dyadic Study of Oral Sex, Relationship Quality, and Well-Being Among Older Couples.” The Journal of Gerontology Series B: Psychological Sciences and Social Sciences. 2018, Aug 4.

Maruthupandian, Jayabalan, and Marimuthu, Ganapathy. “Cunnilingus Apparently Increases Duration of Copulation in the Indian Flying Fox, Pteropus Giganteus.PLOS One. 2013, Mar 27.

McGreal, Scott A. “Does Oral Sex Have an Evolutionary Purpose?” Psychology Today. 2013, Mar 01.

Pham, Michael. “Is Cunnilingus-Assisted Orgasm a Male Sperm-Retention Strategy?” Evolutionary Psychology. 2013, 11(2): 405-414.

Swartz, Anna. “A History of Oral Sex, From Fellatio’s Ancient Roots to the Modern Blow Job.” Mic. 2016, May 31.

Audio Credits

Podcast theme music mixed from “Gregorian Chant”, “Mystery Sax”, and “There It Is” by Kevin MacLeod. Short Shorts theme music mixed from “Gregorian Chant” by Kevin MacLeod.

Additional music and audio from:

“All Night Long” by Lionel Richie from LionelRichie.

“Channing Tatum Monologue Customers – Saturday Night Live” from Saturday Night Live.

“Fallout 4 – 2076 World Series Baseball Bat Home-run Sound Effect” from Eriq Pollard.

“Flying Fox (Fruit Bat) Sounds” from Wild Ambience

“Friends – Joey – How You Doin’ Compilation” from FRIENDS & Joey

“Let’s Do It” by Cole Porter from Renato Reyes.

“Metal Detector” from Pond5.

“Stopwatch Sound Effect” from Sound Effects.

Image Credits

Bonobo Oral Sex from Pinterest.

Censored image from ClipArt Library.

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