Fluorescent Pink Squirrels

In visible light...

I don't normally cover stories that have been in the news recently, but this was too good to pass up, and, anyway, it wasn't that prominent so some people may have missed it. Fluorescent pink squirrels, oh yeah!

Our story begins in May 2017 with, of all things, a professor of forestry. He was conducting a preliminary survey of the plants and lichens growing in a forest in northern Wisconsin, Many kinds of lichen, and some flowers, fluoresce under UV light, so one way of finding them is to wander around in a forest at night, carrying a UV flashlight. So far, so perfectly normal for the people who study this sort of thing. But, on this occasion, the researcher discovered something entirely unexpected: when he directed his flashlight at a nearby squirrel, it fluoresced a brilliant pink.

This is not something one generally expects squirrels to do. Clearly, more research was called for.

The Sound of Squirrels

It is, of course, well known that bats and cetaceans use ultrasound to help them navigate the world using sonar. However, there is no particular reason why animals should always make sounds that happen to be in the range of human hearing (and, arguably, some good reasons why they might not want to). So, taking the usual definition of ultrasound as anything higher than 20kHz - the approximate upper limit for the hearing of a young, physically fit human - it shouldn't be too surprising that other mammals use ultrasound for purposes like communication. Or, in the case of shrews, for actual echolocation.

There has been significant laboratory research on the use of ultrasound by mice and rats since at least the 1970s, but it has also become clear that they are far from the only rodents to make sounds beyond our hearing range. Last year, I talked about the potential use of ultrasound for sexual seduction among hamsters (and, to be honest, didn't reach much of a conclusion) and we also know that, for example, baby voles call to their mothers ultrasonically if they become separated.

In the case of squirrels, though, we know rather less, and much of what we do know applies to various kinds of ground squirrel. As recently as 2013, however, it was discovered that flying squirrels also use ultrasound, presumably for social communication. Indeed, the sounds were quite complex and varied, not mere repetitive squeaks, implying that they might impart rather more information that one might think at first glance.

Stressed Out Squirrels

Southern flying squirrel

Stress is, it seems, an inevitable part of life. You just can't get away from it altogether, and that applies to animals as much as it does to humans. True, they tend to get stressed out about different things, not being able to ponder the future, although it's surely true that if we were about to be attacked by, say, a man-eating tiger, we'd probably find that as stressful as mice find the presence of cats.

In fact, stress is such an integral part of life that, at a biological level, it works in more or less the same way in all vertebrates. Stress results in the release of stress hormones, of which the most important is cortisol. In mammals, this is produced in the adrenal cortex, the outer part of the adrenal glands, which sit on top of the kidneys, and produce a whole range of different hormones of which cortisol is only one. So basic is this that all non-fish vertebrates have adrenal glands of some sort, although which bits of them make the cortisol (and, in some cases, how many there are) does vary between different groups. Even fish, which usually don't have adrenal glands as such, do have the relevant sorts of tissue somewhere in the general vicinity of the kidneys, so that they too can make cortisol. It really is that universal.

Hunky Mother Squirrels

In general, among mammals, males are larger than females. We can see this in our own species: on average, men are taller than women, and, again on average, they also tend to have more muscles and a greater overall body mass. Compared with some other species, though, the difference isn't all that great. Seals are an extreme example, with males often many times larger than females. But, in many cases, the difference is more like that with us: it's a noticeable difference, but not a dramatic one.

The reason for this is generally thought to be male-male competition. Males often have to compete for mates, and the bigger you are, the more likely you are to win that contest. It doesn't even have to be physical; the mere fact that you've been able to 'waste' calories on bulking yourself up proves that you're physically fit, and therefore an attractive mate for females. Even the fact that you've survived long enough to grow to large size is a point in your favour.

How this manifests will depend a lot on the mating system of the species concerned. In species that are monogamous, it's not such a big deal, and it's even less so if females are highly promiscuous. It's most noticeable in polygynous species, where one male monopolises as many females as he can, perhaps defending a harem from all comers. This is what happens in seals, and it's also true, for example in deer. Stags, for instance, not only are quite a bit larger than hinds, but they have huge antlers that not only let them fight off their rivals, but also further demonstrate how many calories they've been able to waste growing something huge just for the heck of it.