16 November 2010

Is female orgasm adaptive? Let's ask the clitoris.

Correction, 22 December 2010: Vincent Lynch, author of the second paper discussed in this post, notes in the comments that he didn't actually conclude that female orgasm was an adaptation. I've corrected the post accordingly.

ResearchBlogging.orgWhether or not a trait is an adaptation, shaped by natural selection for a specific function, can be a surprisingly contentious question in evolutionary biology. When the trait in question belongs to human beings, though, "contentious" reaches a whole new level—because when evolutionary biologists consider humans, their conclusions get personal.
Erotic sculpture on temple wall, Khajuraho, India. Photo by Abhishek Singh aka Bailoo.
Among the myriad traits and behaviors of Homo sapiens evolutionary biologists might choose to study, few can be as personal as the female orgasm. The adaptive function of male orgasm is about as clear-cut as possible—it's a mechanistic necessity for uniting a sperm with an egg. But while female orgasm is enjoyable (or so I am told; this is as lousy a point as any to admit that my expertise in this phenomenon is purely academic), it isn't necessary for fertilization. No man can be a father without having had at least one orgasm, but a woman could conceivably give birth to a huge family without having any.

To explain the existence of female orgasm in an evolutionary context, then, biologists have two options: (1) discover a way in which female orgasm shapes reproductive success indirectly, or (2) conclude that female orgasm isn't an adaptation. Possibilities advanced for the first option range from the benefits of closer bonding with a mate—sex is, after all, about more than mere reproduction—to suppositions that the contractions associated with orgasm help draw semen into a woman's reproductive tract.

The argument in support of non-adaptive female orgasm takes a developmental perspective: that female orgasm is really male orgasm, as experienced in a female developmental context. That is, women have orgasms for the same reason men have nipples—because the anatomies of both sexes are constrained by their origins in the same underlying developmental program. If this is the case, natural selection would work to optimize male orgasm, without necessarily affecting female orgasm—and that suggests a way to test whether female orgasm is an adaptation.

Natural selection removes less-fit versions of traits from a population—making that trait less variable within the population under selection. Traits that don't affect survival or reproductive success, on the other hand, are free to accumulate variation via mutation. So non-adaptive traits can be identified by comparing their variation to traits with known adaptive functions.

Who cares what natural selection thinks, anyway? Photo by JorgeMiente.es.
Psychologist Kim Wallen and philosopher of science Elisabeth Lloyd (who had advanced the hypothesis that female orgasm is non-adaptive in a 2005 book) made just such a comparison in a 2008 study. Variation in female orgasm would be challenging to measure, so they used the clitoris as an anatomic proxy. This let them use the penis—which shares a developmental origin with the clitoris and is presumably under natural selection associated with male sexual function—as an adaptive standard for comparison. In comparison to (flaccid) penis length, Wallen and Lloyd found that clitoris length was indeed more variable [$a]. As a second control, the authors also compared variation in clitoris and penis length to variation in the length of women's vaginas, understanding that this trait, unlike the clitoris, is important for female reproductive success. Vaginal length turned out to be about as variable as penis length, and much less so than clitoris length.

There are several objections to be made to Wallen and Lloyd's analysis, and many were made in a response [$a] by evolutionary biologist Vincent Lynch. Lynch objected to the use of length as the focal measure for the size of the clitoris, and showed that clitoral volume was about as variable as penile volume. (I would add that the study of social insects Wallen and Lloyd cite as a precedent for their analysis isn't actually focused on variation, but on the symmetry of traits under consideration, which is not quite the same thing.) More critically, though, Lynch points out that there isn't any known relationship between clitoral size and ability to achieve orgasm—so the data don't have the bearing on the question that Wallen and Lloyd assigned in the first place. Lynch concluded that female orgasm is an adaptation after all—a more conservative interpretation of his result is that we can't answer the question by measuring clitorises.

Understanding the evolution of human sexual behaviors can help us to figure out how best to navigate the tricky business of a sexual relationship with another person—an approach most recently exemplified in the book Sex at Dawn. But we also tend to view evidence that natural selection favors a particular trait or behavior as a kind of approval, or as evidence of what is "natural." That's silly. Whether or not they help to make more babies, orgasms are fun, and they're a wonderful part of our most intimate expression of affection and love. In some respects, that's all we need to know.

References

Crespi, B., & Vanderkist, B. (1997). Fluctuating asymmetry in vestigial and functional traits of a haplodiploid insect. Heredity, 79 (6), 624-30 DOI: 10.1038/hdy.1997.208

Lynch, V. (2008). Clitoral and penile size variability are not significantly different: lack of evidence for the byproduct theory of the female orgasm. Evolution & Development, 10 (4), 396-7 DOI: 10.1111/j.1525-142X.2008.00248.x

Wallen K, & Lloyd EA (2008). Clitoral variability compared with penile variability supports nonadaptation of female orgasm. Evolution & development, 10 (1), 1-2 DOI: 10.1111/j.1525-142X.2007.00207.x

11 comments:

  1. I'm rather surprised that Wallen and Lloyd actually got away with publishing these conclusions in the first place, honestly, it's well established that lack of a clitoris does not preclude orgasm (in the same way that lack of a pudendal nerve ALSO does not preclude orgasm).

    Also, to let you know, the results as to whether female orgasm actually aids fertilization have been pretty contradictory, some have found influence, while others (which I tend to think are better studies) have not.

    Me, I'm inclined to think it's more about emotional pair bonding and behavioral influences. But that's just me.

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  2. Thanks, Sci! I wasn't aware that orgasm was possible without a clitoris at all. The lack of association between clitoral size and ability to achieve orgasm is also paralleled in men, actually—men with "abnormally" small penises (and their partners) report about the same sexual satisfaction as the rest of us.

    I'm inclined toward the pair-bonding function for orgasm myself, too. It's just too obvious that for humans sex is as much (or more) about emotional connection as it is about making babies.

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  3. Oh, and I suppose I should've mentioned, this is my contribution to the next Carnal Carnival.

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  4. No man can be a father without having had at least one orgasm

    If you define "orgasm" as "ejaculation", sure. One can ejaculate without orgasming, though.

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  5. Yeah, I indulged in a little dramatic license on that one—there are also "multiply orgasmic" men who can experience orgasm without ejaculation.

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  6. this should be an interesting comment thread :0)

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  7. How about having an orgasm makes a woman more likely to engage in sex, and thus increases her reproductive output? I would think that alone would make orgasms "adaptive". (Before the onset of birth control, anyhow)

    An interesting question is that, presuming the hypothetical hermaphroditic animal that eventually split into two sexes had to orgasm to reproduce sexually, isn't it possible the female orgasm is an evolutionary remnant?

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  8. Christie, that certainly seems like it should be at least one part of the adaptive value for orgasm—for men and women, actually.

    The "evolutionary remnant" idea is an interesting hypothesis, too. I think, though, that sex for pleasure and/or social bonding is mostly known only in apes, which suggests that orgasm as we know it is a pretty recent innovation. But then, identifying orgasm in more distantly-related species would be a real challenge!

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  9. I'm with Scicurious on this one. There is a fairly large literature about the importance of semi-permanent and long-standing pair bonding in the success of humans as a species. Anything that would increase that pair bonding would increase reproductive fitness.

    Also, if I'm not mistaken, there is nothing resembling an orgasm in other species. In my opinion, that means it's an evolutionary remnant of sorts that turned out to be very, very useful.

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  10. Hi,

    Nice blog and treatment of the Wallen and Llyod paper. I am the Vincent Lynch who responded, and while I think you have characterized my response to Wallen and Llyod correctly for the most part. I'd like to point out that I did not conclude that "that female orgasm is an adaptation after all". I concluded that, given the rational and data presented by Wallen and Llyod, one could analyze that data in such a way to reach the opposite conclusion they did; thus, this kind of data can not be used either to support the by-product hypothesis not to refute it.

    My final point was that there is no real data either way.

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  11. Hi, Vincent! Thanks for clarifying—I'm sorry if I mis-characterized your intended conclusion, and I'll make a correction to the post. It's actually rather comforting to know we agree on the interpretation of your analysis.

    (You're also the first author to respond to something I've written about his or her work in the comments, which I find kinda cool.)

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