The obvious answer, from a human perspective, is that they enjoy it. Or, to put it more biological terms, that the animals have a sex drive that compels them to mate with females, and, in the absence of females, they'll take the next best option. It's worth noting that humans are unusual among mammals (although not unique) in that they don't come into heat, and are willing to have sex at pretty much any time. This means that, when it comes to non-human mammals, we would expect them to masturbate more frequently at times when the female is in heat, since that's the sort of thing they find sexually exciting. Secondly, they will do it more often when they can't get at any females, perhaps because bigger males are getting there first.
But is this what really happens? A recent study looked at this, and a number of other possible explanations, to find out just why squirrels masturbate.
The squirrel family is one the largest, with about 280 recognised species. In Europe, we tend to be familiar with tree squirrels, but, in fact, most species are ground dwelling animals - including, of course, the North American prairie dogs. One such species is the Cape Ground Squirrel (Xerus inauris) of South Africa. This belongs to a group of just six species of ground squirrel, adapted to living in dry, desert-like environments in Africa and Central Asia.
Cape Ground Other Xerinine Various African Marmots,
Squirrel Squirrels Squirrels Chipmunks, etc.
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Male and female Cape ground squirrels live apart once they reach adulthood. The females live in relatively small groups in communal burrows, where they raise their young. Once the males reach adulthood, they leave the burrow to join relatively large gangs that wander across the surrounding area, on the look-out for mates. The males tend not to fight much with each other, but they do establish quite rigid hierarchies, with younger and submissive animals simply jumping back out of the way when a dominant male approaches.
Unlike some other animals, the females do not come into heat at the same time, which means that only one is available to the males at any given time. Furthermore, heat only lasts for about three hours, so the window of opportunity is fairly small. When a female does come into heat - which can happen at any time of the year - gangs of up to eighteen males will run over to burrow and rush about in a search to find her. Although the female may eventually mate with more than one of them, its the male who gets there first who is most likely to father her children, and he's most likely to be an older, dominant individual. The younger, smaller males, tend not to get any chance to mate at all.
The upshot is that this arrangement tends to result in quite a large number of sexually frustrated males. The males have, indeed, been observed to masturbate, through a combination of manual action and self-fellatio - the latter being possible because the erect penis is nearly half the length of the animal's body. As one might expect, this generally happens when the males can scent a female in heat, but that's where the "obvious" theory seems to hit a problem.
One would expect, if this activity were due to sexual frustration, that the males most likely to engage in it would be those who didn't get to mate, and especially the submissive males, who, frankly, never had much chance in the first place. But what actually happens is the exact opposite: it's the dominant males, and those who successfully mated with the female who are most likely to masturbate. So, there must be another explanation for this activity.
Perhaps the most popular theory for masturbation in mammals has been that it improves sperm quality (there is some evidence that it does exactly this in humans). That is, it flushes out dead or old sperm from the ducts, ensuring that only the healthiest sperm remains to fertilise the female. If this were the case, we would expect that the squirrels would be more likely to do it the more males the female was mating with - making it all the more important that their sperm is the best. And, indeed, this is exactly what happens. But we'd also expect that they'd do it before mating with her themselves, since there wouldn't be much point otherwise, and that turns out not to be the case: they're far more likely to do it afterwards than before (which also further scuppers the 'frustration' theory).
So what other theories remain? It could be a means of re-allocating water and nutrients from one part of the body to another (yes, they do drink their own ejaculate), but since they're no more likely to do it during the dry season than when water is plentiful, that seems unlikely. They could be showing off their virility as some sort of mating display, but the females, whether in heat or not, don't appear to take much notice, or change their behaviour, so that also doesn't seem to be the case, at least in this species.
It might even be a display to other males, effectively saying "you're too late! I got there first." That would make some sense, since, while the female will mate more than once, it's usually her first partner who fathers the pups. But, again, there doesn't seem to be much evidence that the unsuccessful males change their behaviour in response to such a display, although the scientist involved in the study concedes that that's harder to rule out.
What are we left with? well, perhaps they're trying to flush out their reproductive system, not of dead sperm, but of infectious organisms. Squirrels, like humans, can be afflicted with sexually transmitted diseases, and reducing the chance of an infection has to be a good thing. That would explain both why they do it after mating, rather than beforehand, and why they do it more often the more times they've mated. That they practice self-fellation also makes sense in this context, since that could help them clean their external genitalia. Cape ground squirrels, living in a near-desert environment, don't urinate very much, which leaves the other obvious way to flush out the tubes unavailable to them. The fact that they're such highly promiscuous animals also makes STDs more of an issue for them than it would be for many other species.
There's no direct evidence of reduced infection rates after masturbation, although that might be a difficult test to perform. But it seems to fit all the facts, and shows that, sometimes, the obvious reason isn't the correct one.
[Picture from Wikimedia Commons]
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