Squirrels masturbate to avoid sexually transmitted infections

By Ed Yong | September 28, 2010 6:47 pm

ground squirrels

There’s a famous picture that has probably been burned into the retinas of anyone who spends a lot of time on the internet. It’s a squirrel, standing up, with a surprisingly huge pair of testicles dangling beneath him. That’s a Cape ground squirrel and the image isn’t a fake. Males have a scrotum that’s 20% of their body length (excluding the tail) and their penis is more than twice as long.

These mighty genitals suggest that sex, and sperm in particular, is a serious business for Cape ground squirrels. To get the best odds of fathering the next generation, they need to ensure that it’s their sperm that fertilises the female’s eggs and not those of rivals. So they make a lot of it; hence, the oversized testicles.

With sperm being so important, it’s odd that some Cape ground squirrels regularly waste theirs. Yet that’s exactly what Jane Waterman saw while studying wild squirrels in Namibia. Some of them would masturbate, apparently squandering their precious sperm. What does squirrel masturbation look like? Apparently, it’s rather acrobatic. I’ll let Waterman describe it herself:

“An oral masturbation was recorded when a male sat with head lowered and an erect penis in his mouth, being stimulated with both mouth (fellatio) and forepaws (masturbation), while the lower torso moved forward and backwards in thrusting motions, finally culminating in an apparent ejaculation, after which the male appeared to consume the ejaculate.”

Many mammals masturbate including humans, other primates, rodents, and more. In every case, the same question remains – why waste the sperm? The most obvious explanation is that it’s what males do when they’re horny, but unsuccessful with it. This is the loftily named “sexual outlet hypothesis”. If it’s right, masturbation isn’t adaptive – it’s just a side effect of the intense sexual arousal generated in species where males mate with many females.

An alternative is that masturbation is actually beneficial. By flushing old sperm from the male’s testicles, it gets a higher proportion of competitive or fertile sperm ready for the next potential mating.

But Waterman thinks that both of these hypotheses are wrong, at least when it comes to Cape ground squirrels. She should know; she spent around 2000 hours spying on the animals with a pair of binoculars, noting every interaction between them, and every sexual act among the local males.

This glut of data told her that males masturbate more often when females are ready for mating. But Waterman also found that dominant males were far more likely to masturbate than subordinates, and males who had actually had sex were more likely to do it than those in dry spells. That rules out the sexual outlet hypothesis, which predicts that subordinate males and those who were spurned by females would be the most frequent masturbators. The alternative “sperm quality” hypothesis doesn’t work either, for males masturbated more often after sex than before it. It’s clearly not an act of preparation.

Waterman considered, and ruled out, the possibility that masturbation is some sort of signal. It’s unlikely that the males are in some way displaying to future mates, because they were no more likely to do it when females were close. It’s equally unlikely that they’re sending messages to rivals, advertising the fact that they’ve just had sex. After all, Waterman found that one masturbating male did nothing to put off rivals from making advances on a female.

The final explanation is that masturbation is actually a form of self-medication. By cleaning their genitals, males reduce their odds of contracting a sexually transmitted infection. It’s a new hypothesis that Waterman herself put forward, but it’s the only one that actually fits with all of her data. If it’s true, you’d expect males to masturbate more frequently after sex than before it, which they do. You’d expect them to masturbate more frequently during the time of month when females are ready to mate, which they do. And finally, you’d expect their tendency to masturbate to increase as they get more sex, which it does.

A masturbating squirrel gets cleaner genitals in two ways – it scrubs the outside bit and flushes out the inside ones. Many other rodents will groom their genitals after sex and experiments with rats have shown that this does actually help to prevent infections. This might also explain the fact that some fruit bats practice fellatio during sex.

In terms of flushing out the genital tract, some studies have suggested that this is why human men feel the need to go to urinate after sex. Cape ground squirrels, however, are a desert species and conserve water by very rarely urinating. Masturbating may be the next best thing and indeed, by eating their ejaculates afterwards, the squirrels can prevent the needless loss of water.

Reference: Waterman, J. (2010). The Adaptive Function of Masturbation in a Promiscuous African Ground Squirrel PLoS ONE, 5 (9) DOI: 10.1371/journal.pone.0013060

Image by Hans Hillewaert

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Comments (45)

  1. I think that this is an important study, but I can’t say why exactly. I think that we are coming closer to exonerating Joycelyn Elders.

  2. Keep this up and you’ll give masturbation a good name. Meanwhile, to an illustrious collection of terms for the act (waxing the bishop; snapping the carrot; polishing the porpoise; I could go on, believe me)’ you’ve brought a bright new addition:

    “Nothing. Just doing some cleaning.”

  3. “[…] after which the male appeared to consume the ejaculate”. Eeew!
    “In terms of flushing out the genital tract, some studies have suggested that this is why human men feel the need to go to urinate after sex”. Ah, how I hate that…

  4. @David – True story: a few years ago, some friends and I noticed that virtually masturbation euphemisms take the form of “X-ing the Y”, where X is a verb and Y is a noun. As we tried to come up with new and amusing ones, a history documentary was playing in the background and the narrator said: “cripping the Nazi war machine.”

  5. Also credit to John Rennie for what would have been a much better headline: “Squirrels: not just handling nuts for fun”

  6. Left_Wing_Fox

    @4: Oh my god… that’s going straight into the meme-stream.

  7. I predict that there will be an animated gif within days

  8. keon

    @6: So, “generating the meme stream,” then?

  9. keon: ‘winning the thread’

  10. Brian Too

    Might I suggest “grooming the ground squirrel”?

  11. Why don’t they catch STIs from swallowing then? Many human STIs can be transmitted that way, though I guess it’s less likely than through typical sex.

  12. How did you not link John Irving’s excellent book, “The Water Method Man” in this post?

  13. “[…] after which the male appeared to consume the ejaculate”

    Thanks for clarifying at this point that it was a *male* — it wasn’t clear from the previous sentences describing what this male was doing to its own penis. For some reason I’ve suddenly got stuck in my head the idea of Dan Brown writing natural history books…

  14. Squirrely

    Maybe the aroma of the receptive female just raises the male’s libido. ‘Nads that big could support the continued ‘activity’.

  15. Nacho

    It puzzles me that she didn’t consider the more likely possibility that males just masturbate more after having sex because they’re hornier because of all the hormones they liberated during sex. Which makes me wonder the validity (and usefulness) of a sexual habits study which doesn’t involve taking blood and/or urine samples to analyze hormone levels. I guess research grants are way easier to get nowadays, even if your methodology is flawed, if your “study” has anything to do with the environment.

  16. Ooh fighting words! You’re assuming that Waterman submitted a grant saying “I want to study squirrel masturbation”. Actually, it seems clear from the paper that this is an opportunistic study. Waterman’s area is the evolution of animal sex lives and social behaviour. It seems what happened was that she was out in the field, studying the squirrels, noticed an odd behaviour, and decided to collect some data on it.

  17. Nacho

    My snide remark wasn’t directed at the subject of her study (be it squirrel masturbation or squirrel sexual and social habits in general), but at what based on your article (I didn’t read the original paper) looks like just a paper where she shows some “unusual” behavior and her personal interpretation of what purpose it could serve, which sure sounds pretty un-scientific to me. I’m not saying to deny grants to people studying these kind of subjects, I’m just saying we’re way past the 19th century when it was enough to just observe and ponder, having so many state of the art tools and scientific equipment at our disposal.

  18. “Males do it simply to clean their genitals, reducing their odds of contracting a sexually transmitted infection.”

    We have to be careful with wording about motivation in situations like this. The males probably do NOT do it ‘to clean their genitals’, but because it feels good/they feel compelled to do it/they ‘want’ to do it. That masturbation cleans their genitals is (at least according to Waterman’s hypothesis) why the behaviour has been maintained by natural selection.

    Or I could be wrong; perhaps in their little squirrel brains, they’re thinking, ‘gosh, I feel a bit dirty down there, time to clean!’ But I doubt it. In the same way (most) humans don’t have sex ‘to pass on their genes’.

    [I’m usually a bit wary about people who are quick to read motivation into descriptions of animal behaviour, but on consideration, I think this is a valid point. Have changed that bit to read “By cleaning their genitals, males reduce their odds of contracting a sexually transmitted infection.” Thanks Karen. – Ed]

  19. Chris

    So do these squirrels have lust in their hearts?

  20. Trev

    This is yet another example of scientists “reaching” to make a conclusion they like. Next this researcher will hypothesize that frogs make their own natural condoms using mucus to avoid sexually transmitted disease.

    We know after all that sexually transmitted disease is how the dinosaurs became extinct.

  21. Trev

    Or she just likes watching them do it.

  22. Interesting and, as always, engagingly written.

  23. J

    I hope the paper is actually science as opposed to the nonsense spouted herein. Just because cleaning genitals is the only reason Waterman could come up with that fit her data doesn’t mean it’s the only one possible.

    Maybe dominant squirrels masturbate more often because that trait is linked with being dominant. Or maybe the other way around. Maybe it is also a signal to females. And I’m sure there are other reasons that I haven’t thought of in the 2 seconds I tried.

  24. I’d be curious to know more about the microbiology here. What STDs do squirrels get? Can frequent ejaculation really prevent them?

    I don’t know of any human studies about masturbation changing the behavior of STDs (except that as a sole means of sexual gratification, it is very, very low risk).

  25. Derek

    Sounds like a scientific retelling of an old semi-dirty joke.

    Q: “So why did the male Cape Ground squirrel with head lowered and an erect penis in his mouth, stimulate with both mouth (fellatio) and forepaws (masturbation), while his lower torso moved forward and backwards in thrusting motions, finally culminating in an apparent ejaculation, after which the male appeared to consume the ejaculate.”

    A: Because he can!

  26. Dan

    Best band name ever.

  27. Thanks, Ed. I’m wary too, which is why I made the point. People shouldn’t read motivation into descriptions of animal behaviour, but the thing is, they do, and so by using certain wordings over others, we can lead them away from the temptation.

    P.S. Everytime I see ‘Cape ground squirrel’ I imagine one wearing a little super-hero cape.

  28. Ah, science can never accept because they can……but, if he has the balls to do that in public, let em. Bwahahahahahaha.

  29. Marcwolf

    At least science is being brave enought to accept that sex for animals is not all instinct and solely for reproduction.
    For many years animal behavorists would have turned themselved into semantic and conceptial knots trying to justify such behaviour when possibly the simple explanation could be that the squirrel’s enjoy it. Pleasure in itself can be a very powerful motivating force.
    Most of the animal kingdom masturbate or perform some form of non-reproductive sexual activity, and surprisingly we do not have a deluge of psychotic crazed critters running around with fur on their paws (well we do but that not a cause but more of an effect :>)

    I would be interested to find out from your observations how much same sex interaction is occuring amoungst the squirrels since likewise same sex activity has been documented in many species.

    Take Care
    Marc

  30. Dredge Slug

    I can’t believe that no one thought that squirrels, like humans, might masturbate because it feels good. You’d think that Jane Waterman never rubbed one out.

  31. Sonya

    Dredge, everything that feels good evolved to feel good. The question is why.

  32. 56

    Cyndi Lauper had it right- Squirrels just wanna have fun.

  33. spacebat

    Their motivation wouldn’t be a mystery to a male researcher. That would be the shortest scientific journal entry ever.

  34. What might this tell us about masturbation as practiced by other species? (Maybe “practice” isn’t quite the right word.) Post-coitus urination seems reasonable in some cases. If selection favors masturbation, how might female masturbation be explained?

  35. The “it just feels good” hypothesis doesn’t really gel with the data that frequency of masturbation varies with dominance, time of month, whether the squirrels have just had sex and so on. You’d expect a more uniform pattern.

    And Sonya (@31) is right – if it feels good, why does it feel good?

  36. This should be easy to test.

    Shoot squirrels. One group after sex, one group after post coital masturbation and culture their urethras. Compare

  37. Gregg

    Sperm is not wasted, since it’s continuously produced. If it’s not used, the sperms die, and the body has to find a way to clear out the dead sperms so they don’t get in the way of the live viable sperms. Semen may also accumulate toxins from the body, if it’s retained (a theory based on the fact that frequent sex and masturbation help combat testicular and prostate cancer). So basically it’s not wasted, there is more than enough to go around.

  38. Keith

    Mark asks: “If selection favors masturbation, how might female masturbation be explained?”

    Is there actually any evidence to support female masturbation in other than the human species?

  39. Eli

    Keith asks “Is there actually any evidence to support female masturbation in other than the human species?”

    Ever seen a female cat in heat, and what they manage to do with chair legs?

  40. Skye

    Where can I go with binoculars to observe and record female masturbation? I don’t have any cats. Coyotes ate them all.

  41. Rob

    It’s the Circle of Life.

  42. Tor Bertin

    Keith:

    Female chimpanzees will actively rub their genitals with sticks. So, not only do they masturbate, they use sex toys.

  43. Hannah

    Did she notice if the females masturbated at all? I see some people have asked about it/mentioned it already, but I’d be curious to know about female masturbation outside of the human species.

  44. tr

    “I think that this is an important study, but I can’t say why exactly. I think that we are coming closer to exonerating Joycelyn Elders.”

    Anyone who has ever masturbated can tell you how enjoyable it is and any decent sex therapist will tell you it’s perfectly normal. The only crime poor Ms. Elders committed was being ousted from politics by sexually-repressed religious imbeciles.

    And goodness, can you imagine if most people could give themselves blowjobs? We’d never get anything done.

  45. tr

    “Dredge, everything that feels good evolved to feel good. The question is why.”

    Evolution does not work that way. While natural selection does tend to favor beneficial traits in species, this does not mean that every single thing has evolved specificially with a purpose. Sometimes, weird shit pops up in biology that seems to serve no benefit to the species. That’s life.

    The real problem with biology is that it is always assumed that humans are somehow unique to the point of superiority – for a long time, we were considered the only species with homosexual animals, the only species that had sex for pleasure, etc. The fact is, that if we enjoy having sex, then it’s no surprise that lots of other animals do it sometimes just for the pleasure (dolphins, etc).

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