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If cats had Tinder (1) then a hopeful Tom might advertise “Rapid and efficient sperm intromission—ovulation guaranteed by tearing the vaginal walls. I have the spiniest penis you have ever seen”, and get lots of enthusiastic replies. Well, yowls.
But we don’t work that way. We like to take our time in sex, and pay attention to individual needs, quirks, and kinks. Those are important fact about us—although to read some sex researchers you might be forgiven for thinking that only things that exist in laboratories are real. But labs aren’t terribly sexy places, and it’s a non-accidental feature of humans that we typically have private sex where we don’t invite people in lab coats to watch and measure us while we try to break the record for “brief encounter”.
Actually, a number of other primate species don’t just “come and go” either. The technical term for hanging around with one’s primate sexual partner is called “consortship”. (2) This means something like “paying attention to one’s sexual partner before and/or after sex”.
Well, “Duh!”, you might say. But actually, there is a lot of debate about what “consortship” means in detail, and there is no doubt at all that monogamous long-term pair-bonds are not the only sexual behavior that humans exhibit. However, all that said, everywhere you find humans you find a notion that actually giving two hoots about one’s sexual partner is a nice thing to do at least once in a while.
Which brings me to penis bones.
Spend any time with a group of heterosexual women and the subject of penis morphology will come up. Guys’ anxieties about being judged are, if anything, an under-estimate—although from a sex-ed perspective it's worth pointing out that those who learn some technical skills and/or use prosthetics can short-circuit all that anxiety and satisfy their partners just fine. As Dan Savage puts it “hands don’t lose their erections”.
But—old habits die hard and it’s a good bet that penises fall under (female) sexual selection. And female selection is brutally honest in humans. When we examine our DNA most women got to be ancestors, most men didn't. (3)
An interesting new paper explores the phylogeny (history of the trait) and evolved function of the baculum—the bone in the penis. (4) “What bone?” you may ask—and that’s the point. Most primates have one, we do not. Actually, it’s even more interesting than that. Walruses, for instance, have huge bacula up to two feet in length, while the Tarsier has one less than a cm long. Why the variation?
The authors have examined the phylogenetic trees of over 5000 mammals and concluded that we had a common ancestor with a baculum and therefore lost it at some point. Other studies have suggested that mammals gained and lost the baculum nine or more times in their history. (5)
Given that heterosexual women seem interested in the size and staying power of this organ, why have humans lost the bone that artificially stiffens it?
The authors point out that species without a baculum tend not to have prolonged intromission. They then note that humans don’t make it into the “prolonged intromission” category (over three minutess) citing Kinsey’s finding that copulation in humans typically lasts two minutes or less.
I don’t want to get personal here but, two minutes? This reminded me of a recent finding that “post-copulatory affection” (e.g. “cuddling”) increases “relationship satisfaction”. As Bill Maher quipped on reporting this, “You mean doing it with my pants on and then jumping out the window immediately afterwards is wrong?”
Labs are great places for controlling variables but, apart for those with some pretty esoteric kinks, are not terribly erotic settings. Ethologists try to measure behaviour in natural settings and, in natural settings, copulation that lasts two minutes start to finish is not (we hope) terribly typical of a human sexual encounter. At least—not one that either party would rush to repeat.
There’s another evolutionary possibility that the authors of the baculum paper don’t appear to have considered—honest signalling. I think it was Richard Dawkins who first hypothesised that erections were hard to fake signals of erotic interest and vascular health and, as such, might be under sexual selection (by females) in much the same way that peacocks trains are. This would certainly be compatible with the female interest and male anxiety concerning said organ.
But it could be more interesting even than that. Human (baculum-free) penises bend inside the body of their partners, allowing interaction with internal sensitive structures, as beautifully illustrated by this scan of a couple having sex.
And what are those internal structures? Well, as I may have mentioned once or twice, the clitoris is a large and complex organ which, like an iceberg, is mostly internal. Since most people haven’t seen one I’ve gone to the trouble of having a life-sized one made up.
The honest-signalling possibility suggests a range of testable hypotheses. For example, if the hard-to-fake aspect of the male sexual organ is being measured by the complexity of the female's, do other primates show the same degree of internal complexity? Intromissal timing might not be the best measure either—given that this is often likely to be a function of privacy. For instance, if the other members of your primate troupe witness you having sex then staying inside your partner for longer minimises the access that rivals can have. But maybe the route humans took was to charm their partners in privacy. As Darwin put it "the power to charm females" is more important than the ability to "defeat other males in battle".
For humans, the mate who comes and goes is perhaps not the one to maximise chances of fertility with. On the other hand, the one who is paying detailed specific attention to you and your personality (expressed through sexual needs) and is showing had to fake (and hard to hide) arousal in specific response to you (without aid of a bone) might be just the person you want to splice genes with in a species like ours which has likes to invest so much in offspring and at least has the option of drawing the male partner into this investment sometimes. (7)
Happy Valentines Day!
References
2) Manson, J. H. (1997). Primate consortships: a critical review. Current Anthropology, 38(3), 353-374.
3) The figures are about 80%: 40%. And if this seems counter-intuitive remember that men can be wrong about paternity, women can’t be wrong about maternity. Also throughout human cultures a bit of polygyny is normal. Wilder, J. A., Mobasher, Z., & Hammer, M. F. (2004). Genetic evidence for unequal effective population sizes of human females and males. Molecular Biology and Evolution, 21(11), 2047-2057.
4) Brindle, M., & Opie, C. (2016, December). Postcopulatory sexual selection influences baculum evolution in primates and carnivores. In Proc. R. Soc. B (Vol. 283, No. 1844, p. 20161736). The Royal Society.
5) Schultz, N. G., Lough-Stevens, M., Abreu, E., Orr, T., & Dean, M. D. (2016). The baculum was gained and lost multiple times during mammalian evolution. Integrative and comparative biology, icw034.
See also Dixson AF. 2012 Primate sexuality: comparative studes of the prosimians, monkeys, apes, and human beings, 2nd edn. Oxford, UK: Oxford University Press.
And
Dixson A.F . 1987 Baculum length and copulatory behavior in primates. Am. J. Primatol. 60, 51–60.
6) Dawkins, R. (2006). The Selfish Gene: --with a new Introduction by the Author.
7) A range of papers explored these possibilities (and others) from various angles in a recent special edition of Socio-affective Neuroscience and Psychology.
Safron, A. (2016). What is orgasm? A model of sexual trance and climax via rhythmic entrainment. Socioaffective Neuroscience & Psychology, 6.
Fleischman, D. S. (2016). An evolutionary behaviorist perspective on orgasm. Socioaffective Neuroscience & Psychology, 6.
King, R., Dempsey, M., & Valentine, K. A. (2016). Measuring sperm backflow following female orgasm: a new method. Socioaffective Neuroscience & Psychology, 6.
