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Does Oral Sex Confer An Evolutionary Advantage? Evidence From Bats

The views expressed are those of the author and are not necessarily those of Scientific American.


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ResearchBlogging.orgRegular readers of this blog know that while I think studying animal cognition, behavior, and communication is (sometimes) fun and (always) interesting, the real importance – the why should I care about this – is because by understanding animals, we can attempt to learn more about ourselves.
I’ve written about this before. Here are the relevant excerpts:

When human adults show complex, possibly culture-specific skills, they emerge from a set of psychological (and thus neural) mechanisms which have two properties:
(1) they evolved early in the timecourse of evolution and are shared with other animals, and,
(2) they emerge early in human development, and can be found in infants and children, as well as adults.

Three questions necessitate a comparative evolutionary approach (or, minimally, are enriched by such an approach):
(1) Is a given trait unique to humans?
(2) Does the acquisition of a given trait depend on uniquely human abilities?
(3) What functional problem does a given trait solve, and did it evolve for this particular function?

That the first question necessitates a comparative approach should be obvious. If comparative data indicate that even only one other species possesses the trait in question, then the question shifts a bit, and we have to determine whether the trait is homologous (depending on the same mechanisms), or homoplastic (depending on distinct mechanisms that presumably evolved independently). How can we distinguish homology from homoplasy? We look for signatures, or common features. For example, face processing in humans shows behavioral signatures (e.g. degradation when faces are inverted) and neural signatures (localized cortical activations). Those same features have been found in various monkey species that have been tested in face processing tasks, and this provides one piece of evidence for homology.

The third question distinguishes among the original function of a trait and the way it is currently used. Language, for example, allows us to recombine a finite set of elements in essentially infinite patterns to create meaning. Did this capacity evolve to facilitate communication, or for some other purpose? Assume that chimpanzees, for example, do not show evidence of this mechanism in their communication, but DO exhibit this mechanism for arithmetic computation. This might suggest that this ability evolved for number, and was then “re-purposed” by humans for communication. Of course, it is also possible that this capacity evolved independently in chimpanzees and in humans, but this seems less likely given the relatedness of our two species.

I used cognitive examples above, but of course these questions and methods of investigation apply to behavior more generally (especially since cognition and behavior are only different by virtue of different levels of analysis).
If oral sex offends you, the time to click away is now. Otherwise, read on.


The function – in evolutionary terms – of oral sex has been debated for some time. Certainly it is pleasurable for most people and is fairly commonplace among humans, but what is its adaptive value? How did it arise in evolution, and how does it impact on an animal’s chances of survival and reproduction? Keep in mind that the way something functions in humans today may not represent its original function, or the way it may have functioned earlier in evolution, as in the example above with respect to language.
Some might argue that oral sex does not have adaptive value. How could it increase reproductive fitness when engaging in oral sex does not seem to allow sperm to meet egg? If sperm and egg don’t have a rendezvous, someone could be super duper awesome but an adaptive loser. After all, the point of increasing reproductive fitness is, well, reproduction. But maybe the way oral sex works in humans is not the way it originally worked when that behavior first emerged.
So let’s look at oral sex in animals. There is some evidence that bonobos engage in fellatio, but this is infrequent, and usually among juveniles and considered part of play (see this article (PDF) or this paper). This might lead you to believe that fellatio is primarily a human activity or emerged as a part of play (as opposed to a part of sex) – in either case, not a convincing evolutionarily adaptive explanation. Doesn’t contribute to adaptive fitness. And you know what THAT means. It means oral sex is BAD.
But several months ago, a very interesting paper was published in PLoS ONE, titled “Fellatio by Fruit Bats Prolongs Copulation Time.”
Aha! A potential evolutionary explanation for oral sex? That would ruin the assertion that oral sex is an invention of human culture!
Before we get any farther, here’s the bat porn you’ve all been waiting for:

Video 1: I’m pretty sure all the Batman/Batgirl fellatio jokes have already been made.

So the man short-nosed fruit bat (Cynopterus sphinx) cozies up to the girl bat at the bat bar, and after some awkward flirting and bad pick-up lines (“Wanna see my bat signal?”) he takes her back to his place. They decide to “watch a movie” and before you know it, things are getting hot and heavy.
bat signal.jpg

Figure 1: Okay, maybe one Batman joke.

So the man bat enters the girl bat from behind, and while they’re getting it on, the girl bat bends over and licks his penis. There was a very high, very significant correlation between the duration of licking and the duration of intercourse.
batsex1.jpg

Figure 2: Correlation of 0.828, p < 0.001. Each second of licking prolongs intercourse by six seconds.

There was also a group difference, with those matings in which there was licking lasting significantly longer than those matings in which there was no licking.
batsex2.jpg

Figure 3: Lickers have sex significantly longer than non-lickers. Also, a cartoon of bat fellatio, for your viewing pleasure.

So why is this research important? First, it is the first example of fellatio found in adult animals other than humans and found in the context of sex (in the bonobos it was always in juveniles, and in the context of play). Second, the male short-nosed fruit bat penis is morphologically and physiologically similar to the primate and human penis; both contain erectile tissue (corpus cavernosa and corpus spongiosum; read more about them at Neurotopia). The authors note that if the erectile tissue is stimulated during copulation, then the penis’s rigidity will increase and the erection will remain longer (and by longer, I mean for a greater amount of time). So they speculate that the female “licks the male penis to increase penile stimulation, stiffening the penis and maintaining the male’s erection. At the same time, the female’s saliva may increase lubrication, thus facilitating intromission and thrusting. In combination, these features may prolong copulation in C. sphinx.” This makes sense.
They further speculate that the prolonged copulation might assist the sperm in their journey down (up?) to the oviduct (here’s the background on the girl parts of reproduction, courtesy of blog bff Scicurious). Another possibility is that the prolonged intercourse stimulates the secretions of the pituitary gland, increasing the likelihood of successful fertilization. Yet another possibility is that fellatio may reduce the risk of sexually transmitted diseases: saliva functions as an antibacterial, in addition to having antifungal, antichlamydial, and antiviral properties. Further evidence supporting this speculation comes from the fact that in these bats, males also regularly lick their penises following intercourse. All of this could lead to increased reproductive fitness and provide an evolutionary account for the function of oral sex. While humans aren’t typically able to simultaneously engage in oral and vaginal sex (barring on the occasional porn set), all of these explanations could still explain the perseverance of oral sex in humans, either because they still provide similar benefits, or because they are sort of evolutionary leftovers.
It should be acknowledged that these are speculations, and they will need to be subjected to further experimentation and research. However, they lay out some very testable hypotheses and clear predictions. And that, after all, is how science is done.
Yeah, further research in BATS, to start with. Sorry guys. “Does Fellatio Simultaneously Prolong Erection and Reduce The Risk of STDs in Humans? A Field Study” Riiiiiiiiight. Good luck getting that past the IRB.
Now, here’s the public service message portion of this show. Dylan Evans, a lecturer at University College Cork, in an argument about the uniqueness of human behavior, brought this article up, and his opponent shut him down by accusing him of harassment, triggering a formal investigation. He was exonerated by the HR folks, but the university president sanctioned him anyway. His application for tenure could even be denied on the basis of this issue. This is a potential infringement on academic freedom. (Though this case is likely more complex than it seems.) Go read about it at Pharyngula, and if you’d like, sign the petition.
You can also find snippets of the relevant documents (and more links) at NeuroDojo.
Tan M, Jones G, Zhu G, Ye J, Hong T, Zhou S, Zhang S, & Zhang L (2009). Fellatio by fruit bats prolongs copulation time. PLoS ONE, 4 (10). PMID: 19862320

Jason G. Goldman About the Author: Dr. Jason G. Goldman received his Ph.D. in Developmental Psychology at the University of Southern California, where he studied the evolutionary and developmental origins of the mind in humans and non-human animals. Jason is also an editor at ScienceSeeker and Editor of Open Lab 2010. He lives in Los Angeles, CA. Follow on . Follow on Twitter @jgold85.

The views expressed are those of the author and are not necessarily those of Scientific American.



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  1. 1. razib 7:59 am 05/19/2010

    great post dawg. great post.

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  2. 2. fernery 9:15 am 05/19/2010

    If the only link between ‘man’ and bat is Batman, than the hypothesis that fellatio would offer any evolutionary benefit to ‘man’ is shaky at best, sexual harassment at worst: http://www.washingtoncitypaper.com/blogs/sexist/2010/05/19/the-morning-after-offensive-bat-fellatio-edition/
    Sounds like something Frank Miller might use in lieu of plot though.

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  3. 3. Sven DiMilo 10:06 am 05/19/2010

    Dylan Evans, a lecturer at University College Cork, in an argument about the uniqueness of human behavior, brought this article up, and his opponent shut him down by accusing him of harassment, triggering a formal investigation. He was exonerated by the HR folks, but the university president sanctioned him anyway. His application for tenure could even be denied on the basis of this issue. This is a serious infringement on academic freedom.

    This account turns out to be completely wrong. Evans’s case has precisely nothing to do with academic freedom. Read the comments at Pharyngula for why.

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  4. 4. hectocotyli 10:58 am 05/19/2010

    Of course, from the point of view of the amorous bats it probably just feels good.
    Which is just an admittedly glib way of saying that regardless of any evolutionary fitness, the bats have their own topical motivations. I bet there’s something about the copulating bat penis that tastes particularly yummy to the female. I leave the controlled taste-test study to others, though.
    But here’s my other thought, agreeing with @fernery: Oral sex in bats may not speak a whole lot about oral sex in humans. Hidden ovulation (not 100%, but enough) in humans has enormous implications, including a heavy selective pressure on frequent & easily (& broadly)-triggered arousal. The resultant landscape of human sexuality is almost infinitely diverse: from masturbation, to oral, to shoeware, to, um, safety pins (though that one might have been the tumor’s fault. See current Radiolab).
    And a final thought on the Dylan Evans issue. I think it’s potentially very dangerous to spin a defense against a sexual harassment claim as one of ‘academic freedom’. None of us (other than the involved parties) are qualified to speak to the particular dynamics of what transpired. There’s a pretty miserable history of overt and covert silencing of victims of sexual harassment, and the public nature of this case makes me awful uneasy.

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  5. 5. Jason G. Goldman 1:40 pm 05/19/2010

    @4: You are right. This is more complex than we know about (and I have amended the post above, to reflect that). My intention here was to bring up the question, not to take a stance, and I see that I let some of my own opinions (formed of course on the basis of what has been reported) leak through.
    @2 and @4: And so the issue of adaptive benefit, surely the bats aren’t consciously aware that they are increasing copulation time and increasing evolutionary fitness. But that doesn’t mean they aren’t. The interesting part is that this is the first example of oral sex in an adult mammal and as part of sex, not as part of play. While the link between bat fellatio and human fellatio is speculative at this point, it lays out some clear hypotheses that can be tested, in both species.

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  6. 6. hectocotyli 4:12 pm 05/19/2010

    No disagreement from me about the conscious motivation/reproductive fitness split. But aside from the topical similarities (fruit bats suck dick just like us!) I see a lot of pitfalls with drawing such direct hypothetical lines between bat & man.
    The main issue is that the circumstances of fruit bat fellatio are very specific, and only within the context of copulatory sex. This makes structuring testable hypotheses relatively straight-forward (as these things go).
    Humans…well, not so much. There’s an amazing range of non-reproductive sex that people indulge in, and I’m not sure that oral sex holds any more privileged a spot than any other. The most promising fitness parallel with the bats may be in the context of heterosexual “foreplay” (hilarious term, if you think about it) but it gets more tenuous from there.
    None of which detracts at all from the sheer awesomeitude of fruit bat fellatio.
    & I’ll lay good money that a copulating bat penis haz a flavor.

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  7. 7. darwinsdog 10:41 am 05/20/2010

    …the real importance – the why should I care about this – is because by understanding animals, we can attempt to learn more about ourselves.

    Understanding animals is interesting & important in its own right. Anthropocentrism is as bad as racism, sexism, or nationalism. Get over it. And while you’re at it, get over anthropomorphizing chiropteran behavior.

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  8. 8. daedalus2u 2:41 pm 05/20/2010

    Darwin’s dog is completely correct. My interpretation of the data is different. Yes there is a correlation of copulation time with licking, but correlation does not mean causation. Since male bats do not do any child care, there is no plausible reason for sexual activity to promote any sort of bonding.
    I suspect that licking serves to accelerate copulation, that in the absence of licking release of semen might take even longer, or might not happen at all. The licking serves to shorten copulation times. The longer copulation is taking, the more licking is needed to accelerate it.
    I think comparing this to human behaviors is pure projection.

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  9. 9. Jason G. Goldman 3:28 pm 05/20/2010

    @8 and @7: If you go back and read what I’ve written you will see no claims about causation, or anything like it. In fact, I use the word “possible” (or variants of it) 4 times, “potential” once, and variants of “speculation” 4 times. I use terms like “hypothesis” and “testable predictions.”
    I also never mention pair bonding, even once. I mention these possible functions of oral sex: (1) to increase the possibility of conception; (2) to decrease the possibility of contracting at STD.
    As far as comparing it to human behavior, yes, it is speculative, as I wrote above:

    While humans aren’t typically able to simultaneously engage in oral and vaginal sex (barring on the occasional porn set), all of these explanations could still explain the perseverance of oral sex in humans, either because they still provide similar benefits, or because they are sort of evolutionary leftovers…It should be acknowledged that these are speculations, and they will need to be subjected to further experimentation and research. However, they lay out some very testable hypotheses and clear predictions. And that, after all, is how science is done.

    It is possible that oral sex emerged separately in bats and in humans, and it is also possible that they are related. Even if they emerged separately, they could still serve similar functions. Or they might not. In this post, I review a really interesting study of oral sex in bats, the first species we’ve observed engaging in oral sex as adults and in the context of sex, and consider the possibility that oral sex in humans may serve similar functions – or may be based on behaviors that originally emerged to serve similar functions – and say “we need to do more research.”
    I don’t think this is “pure projection;” it’s “considered projection.” The business of science is in generating testable hypotheses… which is a fancy way of saying “making projections and seeing if they hold up to experimentation.”
    As for anthropomorphosis, I’m not anthropomorphizing at all, other than as a literary device or for humor (e.g. “They decide to “watch a movie” and before you know it, things are getting hot and heavy”). Any reasonable reader knows that this functions as a joke.

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  10. 10. animalia 10:21 am 05/21/2010

    The study says that licking did not occur in some instances probably because the female had been forced to mate and so copulation duration could be shrtened. (They also mention earlier that some females were forced to mate.)
    This suggests to me some connection between the licking and the greater likelihood of fertilization, which may be a cause and effect relationship originating from some sort of grooming function.
    From what I can find there seems to be a female-biased sex ratio due to the earlier maturation of females and resource-defence polygyny which means few males to fertilize the eggs and the likelihood that sperm is a limited resource over which females will compete therefore prolonging copulation gains a female fertilization advantage.
    I also expect human females could take advantage of the way this is being described as being like human fellatio and maybe from now on a few licks of the base of the penis is all human females will be subjected to LOL
    I think the particular morphology of the bat is being overlooked – hanging by the feet, no hands, licking very much a part of grooming and social interaction – all this accidently coming together to enable females to gain success from getting enough sperm, probably.

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  11. 11. SC (Salty Current) 10:47 am 05/21/2010

    It is possible that oral sex emerged separately in bats and in humans, and it is also possible that they are related. Even if they emerged separately, they could still serve similar functions. Or they might not.

    This data on a particular species of fruit bat suggests nothing about humans that shouldn’t be immediately obvious from observation of humans alone. Many humans engage in oral sex (fellatio and cunnilingus) as part of the sex act. (The article itself suggests that an orangutan was observed doing something very similar.) So you’d have to be silly to speculate and develop hypotheses about humans on the basis of bonobos alone (and this line between sex and play seems fairly problematic in any case) without taking into account human behavior itself. It’s pretty obvious that oral sex – among a zillion other things – can have an arousal function, and probably a lubrication effect, in humans. That is a very long way from an “evolutionary explanation” for oral sex in humans. This study tells us zip about any purported adaptive advantage of oral sex as practiced by humans, who are not fruit bats.

    Aha! A potential evolutionary explanation for oral sex?

    In a species of fruit bat. And again, if that’s a potential evolutionary explanation for oral sex in humans, it should have been obvious long ago from our knowledge of humans.

    Some might argue that oral sex does not have adaptive value.

    [citation and clarification needed]

    the assertion that oral sex is an invention of human culture!

    [citation needed]

    So why is this research important? First, it is the first example of fellatio found in adult animals other than humans and found in the context of sex (in the bonobos it was always in juveniles, and in the context of play).

    This is incorrect. The article itself mentions orangutans.

    “Fellatio by Fruit Bats Prolongs Copulation Time.”

    This is a fairly bold title when what they have is a correlation and speculation regarding a mechanism. Could be spurious. I’m not saying it is, but the possibility exists.

    I mention these possible functions of oral sex: (1) to increase the possibility of conception; (2) to decrease the possibility of contracting at STD.

    It’s rather irresponsible to mention this possible STD-related function when you’re talking about humans in the same context without noting that oral sex in humans is also a means of transmitting STDs.

    Now, here’s the public service message portion of this show.

    Totally irresponsible. You should go read about it at Pharyngula, including the entire comment thread.

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  12. 12. SC (Salty Current) 11:12 am 05/21/2010

    What is the evidence concerning the relationship between the duration of intercourse and the likelihood of conception in humans (or fruit bats, for that matter)?
    By the way, the article also notes:

    Both male and female Livingstone’s fruit bats (Pteropus livingstonii) also licked the genitals of their partners during heterosexual interactions [24], [25].

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  13. 13. Mike Olson 1:42 am 05/22/2010

    Okay, from a strictly non-involved observational point of view this is both the funniest and most interesting blog I’ve seen in a long time. Don’t get me wrong other posts have merit, other posts have more depth or more humor…but not both humor and validity at the same time…fantastic…absolutely fantastic….

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  14. 14. animalia 4:09 am 05/22/2010

    I wonder, what is the adaptive value of finding animal (including the human animal)sexual behaviour funny????
    The laughing reactions people have to these types of studies is all the evidence we need that bringing up these types of studies in certain situations needs to be handled with caution.
    As for the adaptive nature of the reactions? I think it is partly an anxiety reaction for numerous reasons, one of them is the recognition that for males it is all too easy to be left out of passing on DNA and so there is the competitive nature of sex for men expecially. Also while we pretend sex is about cooperation between the sexes it is often about conflict between the sexes with males competing to get something from females and often with strong emotions involved due to the exploitation, harm, and danger that often accompanies the coming together of two genetic strangers in actual or potential reproduction.
    Being honest is something that at least subconsciously we know needs to be hidden – hiding the real motives and selfishness behind sexual behaviour. Hence the automatic joking can be the only reaction to distract us from facing what is really going on in each sex and between the sexes.

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  15. 15. Jason G. Goldman 5:03 am 05/22/2010

    @14: The function of humor, in this case, is to interest people. To keep things light. Science is a hard sell. If we keep it light, make in engaging, use humor effectively, and don’t skimp on the details, I think we can do a better job of selling it. Also there’s the part where I do this blogging thing to amuse myself. Better than playing internet flash games.
    @11: This is a blog, not a peer-reviewed journal article. Its (intended) function for the reader is education and amusement. Its function for me is education, amusement, and lots and lots of writing practice, of a different sort than they train us for in grad school. While I strive to keep things accurate and detailed, I do not cite every statement I make, nor do I think this is necessary. This is not the venue for that. I reserve that writing style for academic writing. Sometimes, blog posts don’t even get a spell check, but that’s part of why blogs are so fantastic: they facilitate the speedy transmission of thoughts and ideas. Compare that to a manuscript I submitted to a journal 3 months ago, which is still under review.

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  16. 16. animalia 7:34 am 05/22/2010

    I was not meaning your blog specifically but more the reactions of people generally who see anything to do with sex and react with ‘tittering’.
    I grew up when rape was the subject of enormous humour so I think there is far more to humour about sex than getting people interested. I don’t think humour would go down very well with most people, especially women, as a way to interest people in studies of these types of common behaviours across species. And there are plenty of jokes about cuckolded men, infidelity, promiscuity etc etc.
    Sex certainly sells (and I love Olivia Judson’s ‘Dr Tatiana’s sex advice to all creation’) but I – as someone who studies animal sexual behaviour – am curious about why people find sex funny, and how humourous reactions can actually stop people understanding what is really going on – the serious side of sex, if you like, where selfish genes are very much at work. The laughter seems to stem any further interest into behaviours rather than have people keen to understand and face what is actually going on.
    All that most people seem to have learned from this study of fruit bats is the total falsehood that fruit bat females give blow jobs. The humour and anthropomorphism and male sexual fantasies seem to have got in the way of the science/knowledge of what might really be going on.
    Don’t forget that when a male’s genitals are activated his brain switches off. (Humour….or is it?)

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  17. 17. SC (Salty Current) 9:18 am 05/22/2010

    @11: This is a blog, not a peer-reviewed journal article. Its (intended) function for the reader is education and amusement. Its function for me is education, amusement, and lots and lots of writing practice, of a different sort than they train us for in grad school. While I strive to keep things accurate and detailed, I do not cite every statement I make, nor do I think this is necessary. This is not the venue for that. I reserve that writing style for academic writing.

    Please. The reason I requested a citation (the reference was to Wikipedia, not peer-reviewed articles) for those particular claims is that I’ve seen several people argue that this paper somehow refutes claims made by people about humans, but they never seem to point to examples of these alleged claims (not that it refutes even the most strawmannish of them in any novel way, but that’s another issue). It’s not about the nature or purpose of blogs, but about supporting your arguments – if you say you’re arguing with someone or that the paper is intervening in an ongoing debate, you should point to, y’know, the actual arguments people have made. Otherwise, the educational value of your blog is compromised.

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  18. 18. windy 5:39 pm 05/22/2010

    It is possible that oral sex emerged separately in bats and in humans, and it is also possible that they are related.

    If it didn’t emerge separately, that would mean it was present in the common ancestor of the majority of currently living mammals more than 70 million years ago and was subsequently lost in all but a few species… I suppose anything is ‘possible’, but do you really find this possibility worth speculating about?

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  19. 19. ntrygg 1:15 pm 05/23/2010

    interesting – I would think that oral sex also played an important role in relationships
    years ago, I read an article in Ms magazine of all things that basically said that in early human times, the females who engaged most freely in sex with males likely got more food, protection and other benefits from the males, than the females who didn’t.
    So, in the days before humans connected sex to babies, oral would have been as good a method as any other

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  20. 20. Robyn 3:55 pm 05/23/2010

    The thing about human evolution is that speculation can run rife in all directions and contrary directions.
    In chimpanzees and bonobos we do see females to a small degree exchanging sex for food but mostly they provision themselves. Chimpanzees females seem to mate to avoid a beating rather than to gain something positive from the males. Both chimpanzees and bonobos mate promiscuously and this is why the males have large testes. Human males have much smaller testes and this is because human evolution went in the direction of females reducing their number of mates and therefore the reduction in sperm competition. Testes size is the only robust evidence we have of mating systems.
    Which also brings up the question of ejaculation outside the vagina. Even in bonobos that do engage in brief sexual interactions of various kinds, ejaculation, from what I can find, still seems to be limited to vaginal sex.
    With the reduced sperm production in human males even with reduced sperm competition it would seem that males that preferentially ejaculated vaginally would have greater fitness than those who had alternative preferences. But as foreplay oral stimulation, among other things, may have enabled females to gain ejaculations from their preferred male or males and reduce the likelihood of the male mating with other females. Depleting a preferred male of his sperm store as well as other resources is one way females can out compete other females.
    Personally I cannot see any connection between the penis-licking bat and human fellatio. Apart, perhaps, for female competition over preferred sperm which is limited in supply.
    And that’s pretty tenuous.

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  21. 21. Jason G. Goldman 12:55 am 05/24/2010

    Is the issue here that, for some people, it is not clear why studying animals can tell us something about humans? Or is the issue here that (some) people are uncomfortable considering the notion that human sexuality may be related to a set of behaviors we share with other animals? Or that oral sex may have emerged and persevered due to evolutionary selection pressures? What’s the sticking point?

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  22. 22. windy 3:58 am 05/24/2010

    Or is the issue here that (some) people are uncomfortable considering the notion that human sexuality may be related to a set of behaviors we share with other animals?
    I am only uncomfortable with extremely unparsimonious suggestions to that effect being thrown around casually, like the idea that oral sex in bats and humans could be ‘related’, if by that you mean homologous.

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  23. 23. Robyn 5:29 am 05/24/2010

    Firstly, I’m certain that no evolutionary biologist would suggest there is any homology between this genital licking in bats and human fellatio.
    But the main point is that any behaviour in animals can in no way tell us as humans what is acceptable or unacceptable behaviour. They simply cannot be used in that way. They may help to understand aspects of our behaviour but nothing more.
    Take, for example, infanticide by males. This is a behaviour across many species (too many to name but eg lions, gorillas, hippos, tarsiers, dolphins, through to spiders) which also occurs in humans. It is clearly adaptive in that a male killing another male’s offspring means the mother becomes fertile again and can be mated and made to invest in his offspring instead. It is a behaviour that brings a male reproductive advantage.
    What it broadly tells us fits with other things we know about reproduction such as the reproductive asymmetry of the sexes and conflict between the sexes which all traces back to the asymmetry of the sex cells.
    We also know that in some species (eg langurs) females mate with multiple males to try to confuse paternity and avoid infanticide. So in langurs, for example, the new male that takes over a group can sometimes be ‘cuckolded’ and not kill an offspring which is not his.
    Cuckoldry is another animal behaviour that is common where females improve their reproductive fitness by tricking males into providing for offspring that are not in fact theirs.
    It is obvious with these cases that to understand these behaviours within the context of adaptive reproductive behaviours and to note their existence in humans cannot be confused with any ‘right’ or ‘wrong’.
    In the case of fruit bats and oral sex there is the implication from the talk about it that this behaviour can be used to promote oral sex in humans as acceptable. That is as ridiculous as the evidence re. infanticide or cuckoldry being used to argue that these behaviours are therefore acceptable in humans.
    What is clear about the evolution of the sexes and sexual reproduction is the degree of conflict of interests between the sexes. And just like the crass ‘monogamous females/promiscious males’ favours male self-interest, anything that favours the self-interests of one human sex over those of the other needs to be treated with caution – and investigated and inderstood in much greater depth within a much wider context. That far more males now want and expect fellatio (without having to pay) means they will have their self-interests boosted thanks to something totally unconnected in fuit bats. Not that some human females are not freely choosing to perform fellatio,that is clearly so, only to note that the facts of sex and reproduction are as much about conflicts, manipulation, exploitation etc – across species – so this context, more than anything, must always be the one in which sexual behaviours viewed. What other species tell us about sex is that ‘fun’ is often absent – and especially for females. Sad but true.
    The bottom line is undertanding – yes, acceptable or not – no.

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  24. 24. SC (Salty Current) 6:31 am 05/24/2010

    Is the issue here that, for some people, it is not clear why studying animals can tell us something about humans? Or is the issue here that (some) people are uncomfortable considering the notion that human sexuality may be related to a set of behaviors we share with other animals? Or that oral sex may have emerged and persevered due to evolutionary selection pressures? What’s the sticking point?

    This is completely ridiculous. There’s no “sticking point.” People have made specific criticisms of your specific post. If you’re not going to respond to people’s concrete comments but instead flail around about others’ alleged discomfort or lack of understanding of broader questions, there’s no point in engaging with you.

    …for some people, it is not clear why studying animals can tell us something about humans?

    I think you’re confused about what this study tells us or doesn’t tell us about anything, quite frankly. But by all means continue to believe others are resisting your subtle understanding out of discomfort.

    Link to this
  25. 25. Jason G. Goldman 4:27 pm 05/24/2010

    @22 and @23: I’m with you on this one. Proving homology is very hard, even in very closely related species, like humans and chimps or bonobos. But there is still something to be learned by looking at similarities. As I’ve said before, even if they’re not homologous, two similar behaviors in different species could still be similarly adaptive. This possibility is what I think could be interesting, here.
    @23: I have tried not to conflate understanding the potential origins or functions of a behavior and whether or not such a behavior is morally or culturally acceptable or not. While I don’t have any problems with oral sex in humans, this is not the argument I’m trying to make, and I can see where this might not be clear. There are certainly things, like racism, that have biological and evolutionary roots that are certainly not acceptable in our society. I think that by understanding the origins of these behavioral or cognitive pheonmena we can better understand how to approach and engage with them.
    @24: I believe what you say to be blatantly untrue; in each of my comments I’ve addressed specific comments by name or number. And I will do it again, for each of your points.
    The point of the comments section here is conversation. And if, while engaged in conversation, one becomes uncertain about something, it is normal to ask for clarification. Have you never said to someone, in conversation, “what do you mean by that?” or “could you please clarify your position?”
    That said, I am a busy person. I’m a graduate student with two lines of research, and teaching and administrative responsibilities. I also do things like read books, see movies, cook, watch TV, see friends, and go out. I have many things to do, many of which are more important or time-sensitive than responding to comments. This is what I do in my free time, despite the fact that I consider it an important for professionalization and outreach. I do it where and when I can, where and when I feel like it, or where and when it interests me. If I don’t respond to each and every comment or question, that’s sort of just how it goes.
    To your second point: I might indeed be confused; I’ve never claimed to be the world’s authority on anything. Not yet, at least. I happen to know a lot about certain things, so I can speak about them with some level of expertise and inisght. If you read some of my archives, you will see I have no qualms about saying things like “this confuses me” (e.g. today’s post on dog personality) or “this isn’t my area of expertise, but the way I interpret this is…”

    Link to this
  26. 26. Practically Uninformed 10:09 pm 06/10/2010

    While I am intrigued that this behavior is exhibited in creatures as distant from humans as bats, it…unnerves me a bit.
    I don’t know why, really. It’s just odd and vaguely disturbing to hear that non-human species engage in some of the kinky stuff that we like to engage in.
    …suddenly, I wonder if there is any evidence of S&M in non-human species…

    Link to this
  27. 27. JWE 6:40 pm 04/20/2011

    #26: “…suddenly I wonder if there is any evidence of S&M in non-human species…”
    Well, cows do wear leather.

    Link to this
  28. 28. wGraves 10:44 pm 04/20/2011

    Chinchillas do it better. The length of the male apparatus is such that, when erect, the males don’t have to bend down to clean themselves up after the act. I was quite astonished the first time I witnessed the behavior, right after mating.

    Link to this

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