Welcome To The Monkey House.


I went to the San Francisco Zoo a couple weeks ago.  One of the nice things about going to the zoo on a weekday in January is that there aren’t too many people about, and the animals engage in behavior that they might otherwise be too riled up or embarrassed to exhibit.  Case in point:  I wandered up to the lion-tailed macaque exhibit just in time to see the little monkey masturbate furiously, ejaculate (with a disconcertingly human-like expression on his face), scoop the evidence up from the wooden plank on which he was lounging, and eat it.

This may have been regular behavior for the macaque, but as a frequent visitor to the zoo, I can confirm it is a rather unique thing to witness and the experience has affected me accordingly.  Specifically, it has led to a sort of furtive, bemused interest in animal sexuality, and the ways in which it differs from our own.  (Not that the above behavior is entirely out of the question for human beings.)

All of which is a roundabout way of explaining how, last night, I found myself quietly absorbed in “Sperm Competition And The Function Of Masturbation in Japanese Macaques”, a dissertation by a researcher for the University of Munich named Ruth Thomsen.

Thomsen spent the better part of two years hiding in the forests of Yakushima Island, stealing ejaculatory fluid from macaques before they had a chance to eat it.  (This behavior is apparently pretty universal – among macaques, not researchers.)  The thesis undoubtedly had a long and embarrassing road to publication, but Thomsen’s findings are interesting – and have, dare I say, important implications on the subject of human masturbation.

What Thomsen discovered was that during the mating season (roughly February – September) masturbation was nearly universal among macaques, and whether through mating or masturbation the monkeys ejaculated on average once a day.  On first glance the frequent masturbation doesn’t seem to make much evolutionary sense – ejaculate itself is finite, of course, and there didn’t seem to be much reason for the males to be wasting it on what amounted to a recreational activity.  It was only by analyzing the makeup of the fluid itself that Thomsen was able to arrive at the answer.

It turns out that male macaques can be roughly separated into two groups, which Thomsen calls “guarders” and “sneakers”.  Guarders are dominant males; they mate with females on a very regular basis and used their social stature to block other males from mating.  They masturbate infrequently and at no regular time of day, and so semen builds up in their testes.  When they do masturbate, they produce a large volume of ejaculate, but it’s chock-full of dead or malformed sperm.

Sneakers, on the other hand, are second-class males who almost never get the opportunity to mate with females; when they do so it is done quickly and in secret.  They masturbate very frequently, and produce less seminal fluid per ejaculation.  But!  When Thomsen analyzed their ejaculate, she found that it was far more robust than that of the guarders.  By masturbating on such a regular basis, the sneakers were clearing the tubes (so to speak) to make room for newer, healthier ejaculate, and were ensuring that when they did have a chance to mate, their sperm would be in much better shape to make the journey upstream (so to speak).


What implications does Thomsen’s research have for us?  Well, first of all, Thomsen fairly convincingly lays to rest the idea that masturbation in primates – including humans – is somehow unnatural or caused by mental illness.  ”Masturbation occurs in non-human primates at an astonishingly high frequency of 65.4 %, in 34 of 52 investigated species,” she writes. “As this study is the first in which data concerning masturbation in wild living primates has been systematically collected, I wish to emphasise that the common perception of masturbation as a mainly pathological or abnormal behaviour in primates can now be considered defunct.”

Second, Thomsen noticed much higher rates of masturbation in primate groups who live in multi-male / multi-female social groups (as opposed to something like multi-female, single-male, like gorillas; or single-female-with-offspring, like chimpanzees).  The theory is that males in this social system face much more sperm competition, and the likelihood that they are mating with a female who has already mated with another male is higher, so they want to keep their sperm more vital.

Considering that most humans live in multi-male / multi-female groups , there’s no real reason not to assume that this instinct is at least partially responsible for human male masturbation.  So, men: next time you feel bad about masturbating, just remember that you’re really just fulfilling your evolutionary desire to keep your sperm strong and lively.

Lastly, it’s tempting to note that most human males could probably be classified either a guarder (high frequency of mating, relatively low frequency of masturbation) or a sneaker (low frequency of mating, extremely frequent masturbation).  But that’s probably taking the comparison too far.