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.


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.


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