Hard to imagine anyone missed this, but just in case: Over at Rocket Science, Ed Yong explains thatSquirrels masturbate to avoid sexually transmitted infections — at least, that's the new hypothesis from Jane Waterman, a scientist who has been watching certain squirrels very closely.

The goal was to try to answer the mystery: Why, from an evolutionary point of view, would an animal masturbate? ("Because it can" — the punch line to an old joke — is not considered adequate.)

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.

He's not making this up. How he finds this stuff I've no idea. But do check out the whole story. It's quite fascinating.

Yong's post.

__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