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
More on animal sex:
- Male water striders summon predators to blackmail females into having sex
- “I haven’t had sex for 40 million years. Should I worry?”
- Female birds breed better in captivity if they see sexy males
- Crayfish females lure males with urine, but then play hard to get
- Pocket science – sperm races
- Holy fellatio, Batman! Fruit bats use oral sex to prolong actual sex
If the citation link isn’t working, read why here
33 Responses to “Squirrels masturbate to avoid sexually transmitted infections”
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Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his attempt to talk about the awe-inspiring, beautiful and quirky world of science to as many people as possible. He finds talking about himself in the third person strange and unsettling.
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Saturday 2 October 2010
Squirrels masturbate to avoid sexually transmitted infections | Not Exactly Rocket Science | Discover Magazine
September 28th, 2010 at 7:01 pm
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.
September 28th, 2010 at 7:04 pm
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.”
September 28th, 2010 at 7:18 pm
“[...] 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…
September 28th, 2010 at 7:20 pm
@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.”
September 28th, 2010 at 7:21 pm
Also credit to John Rennie for what would have been a much better headline: “Squirrels: not just handling nuts for fun”
September 28th, 2010 at 7:27 pm
@4: Oh my god… that’s going straight into the meme-stream.
September 28th, 2010 at 8:03 pm
I predict that there will be an animated gif within days
September 28th, 2010 at 8:08 pm
@6: So, “generating the meme stream,” then?
September 28th, 2010 at 8:33 pm
keon: ‘winning the thread’
September 28th, 2010 at 8:47 pm
Might I suggest “grooming the ground squirrel”?
September 28th, 2010 at 11:48 pm
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.
September 29th, 2010 at 12:44 am
How did you not link John Irving’s excellent book, “The Water Method Man” in this post?
September 29th, 2010 at 6:50 am
“[...] 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…
September 29th, 2010 at 8:00 am
Maybe the aroma of the receptive female just raises the male’s libido. ‘Nads that big could support the continued ‘activity’.
September 29th, 2010 at 8:19 am
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.
September 29th, 2010 at 9:22 am
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.
September 29th, 2010 at 9:46 am
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.
September 29th, 2010 at 9:51 am
“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]
September 29th, 2010 at 10:13 am
So do these squirrels have lust in their hearts?
September 29th, 2010 at 11:55 am
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.
September 29th, 2010 at 11:56 am
Or she just likes watching them do it.
September 29th, 2010 at 12:58 pm
Interesting and, as always, engagingly written.
September 29th, 2010 at 1:12 pm
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.
September 29th, 2010 at 2:44 pm
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).
September 29th, 2010 at 2:52 pm
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!
September 29th, 2010 at 4:07 pm
Best band name ever.
September 29th, 2010 at 5:41 pm
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.
September 29th, 2010 at 6:09 pm
Ah, science can never accept because they can……but, if he has the balls to do that in public, let em. Bwahahahahahaha.
September 30th, 2010 at 2:44 am
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
October 1st, 2010 at 7:12 am
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.
October 2nd, 2010 at 7:36 am
Dredge, everything that feels good evolved to feel good. The question is why.
October 2nd, 2010 at 3:19 pm
Cyndi Lauper had it right- Squirrels just wanna have fun.
October 2nd, 2010 at 3:40 pm
Their motivation wouldn’t be a mystery to a male researcher. That would be the shortest scientific journal entry ever.