Why squirrels masturbate, or Some thoughts on nuts

This image has not been digitally altered.

It's a Cape ground squirrel, writes sci-blogger Ed Yong, and they all (or, anyway, all the dude squirrels) look like that. And if you think this is impressive, you should see their penis*.

Perhaps somewhat unsurprisingly, Cape ground squirrels are known for masturbating.

Ed quotes from the observations of researcher Jane Waterman:

"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."

With that kind of flexibility, the question of why squirrels masturbate might seem superfluous, but Waterman's research suggests Cape ground squirrel behavior could form the basis for a new theory of male masturbation. In general, there are two competing hypotheses: Either masturbation is just a fun outlet with no adaptive benefit, or it's a useful tool (of sorts) for flushing older sperm out of the pipes and keeping what's used on the ladies fresh, and ready for fertilization. Waterman spent 2000 hours watching Cape ground squirrels, and recording every single sex act the males took part in**. She thinks there's a third explanation …

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.

If she's right, Ed says, the theory could also explain why so many species of male animals groom their genitals after sex, and why human males are prone to post-coital peeing. This could even make a connection back to everybody's favorite Internet science sensation: Fruit bat fellatio.

What I want to know now, though: Does it actually work? Somebody needs to get out there and run a little squirrel STD testing clinic to find out whether the male squirrels who masturbate most are any more STD-free than their less-enthusiastic brethren. And, as far as human applications go, I'd love to know whether urination really does anything to "tidy up", as it were. Suffice to say: Interesting theory. Needs moar data.

Not Exactly Rocket Science: Squirrels masturbate to avoid sexually transmitted infections