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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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