The biggest issue I have with this paper is that it doesn't seem to properly consider the possibility that this behaviour isn't an evolutionary adaptation at al

The biggest issue I have with this paper is that it doesn't seem to properly consider the possibility that this behaviour isn't an evolutionary adaptation at all. I mean, this same behaviour is also exhibited in humans, where it is apparently not an evolutionary adaptation, but rather an unintended consequence of the twin evolutionary benefits of high sex drive and manual dexterity. Why should squirrels be any different? I suppose the thing to do would be to get a large population of squirrels and somehow prevent some of them from masturbating, and then see whether the rate of STIs was lower in one group than the other. Not sure how you could stop them, though, without attaching some kind of device that might impact negatively (or positively) upon their mating opportunities (nobody wants to mate with a squirrel with its arms taped up, or do they?)