Some way better blogging of this paper can be found at Discover Magazine. My take on it has way more philosophizing and way less “omg masturbating squirrels!”.
The background: The adorable Cape ground squirrel (Xerus inauris) lives in small groups segregated by sex. Females live in burrow systems and are somewhat site-specific while groups of males roam through the home ranges of multiple female groups. Females come into oestrus about 4 times per year, asynchronously, so males move from burrow system to burrow system checking for oestrous females. When they find one, the most dominant males get the most opportunities to mate; some subordinate males may not mate at all. Males are under intense selection not only to get mating opportunities, but also to out-compete the sperm of other males who mate with the same female. This can best be done by producing a whole lot of sperm.
Which is why Dr. Waterman was surprised when she observed male squirrels masturbating (more specifically, autofellating!)—isn’t that a waste? Turns out, it might not be; masturbation could be flushing out old and/or dead sperm—this apparently is the case in humans. If this is true, you’d expect the squirrels to masturbate before mating, and to do so more often when there are females in oestrus around.
But that’s not the only possibility. In fact, the paper has a whole table of hypotheses for why squirrels masturbate, predictions for each hypothesis, and whether the predictions were met in ground squirrels. The table formatting is hideous, but it’s very nice to have the hypotheses and predictions laid out so clearly. To run through them quickly: the squirrels could masturbate to improve sperm quality, to work out the frustration of not mating if they’re subordinate, to regain energy (by eating the ejaculate), to advertise to both potential mates and rivals, or to clean up afterwards (and reduce STI risk). Or a combination of any of the above.
Dr. Waterman quickly rejected the energy hypothesis, since males masturbate regardless of ambient food/water supply, and the advertising hypotheses. She found that masturbation frequency increases, rather than decreases, with social dominance, so the sexual tension release hypothesis is also out. The remaining hypotheses (increased sperm quality and STI reduction) are both supported by the observations that masturbation increases with number of mates and when oestrous females are present. One key fact differentiates between the two hypotheses, however: male squirrels masturbate after, rather than before, copulation. So it’s more likely that they engage in masturbation as a form of genital grooming, rather than to increase ejaculate quality.
Yes, this is a mildly titillating study with no practical applications. But I think it demonstrates a few interesting things. First, for all the plethora of studies on sexual selection, intrasexual competition, mate choice, and other things relating to sex in the animal kingdom, masturbation has received very little research attention even though it’s not necessarily rare in many species (awkward dog moments, anyone?). There’s not even much theory for why masturbation might have evolved, if indeed it did. In fact, til recently most biologists might have been inclined to dismiss masturbation as non- or maladaptive, and to see it as a byproduct of, well, animals that can’t get any wanting to feel better (that’s the sexual release hypothesis). But the fact is, this hypothesis wasn’t tested often before (look at the paper’s bibliography). And few alternatives were proposed—in fact, Dr. Waterman came up with most of the alternative hypotheses in this paper herself.
Which brings me to another point: there are lots of hypotheses being tested here, and this is how we should test for adaptation: by looking for the best explanation among many, rather than asking “is my adaptive hypothesis better than a null, non-adaptive one?”. Suppose Dr. Waterman hadn’t thought of STI reduction as a possible explanation. There was still pretty good support for the sperm quality hypothesis, so that might have been the paper’s conclusion. Or suppose she had only asked “Do males who masturbate more have higher fitness?” The answer would be yes, because it’s dominant males who masturbate the most, because they also mate the most, but then we’d have no idea why there was a link between masturbation and fitness. It sounds a bit ludicrous, but there are tons of papers that do just this—and then offer up an untested, hand-waving, often soundbite-like explanation which they never follow up.
Finally, although STI reduction seems to be the reason for the origin of masturbation in these ground squirrels, the energy hypothesis may still be at play. The Discover Magazine blog post I linked to above puts it best:
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.
The squirrels aren’t getting a huge amount of water from their ejaculates—not enough that it’s sustaining them through the dry season—but they aren’t going to let it go to waste.