|In visible light...|
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.
I should start, however, by briefly explaining what fluorescence is. For a start, it's not the case that the squirrels literally glowed in the dark. That would be bioluminescence, and not the sort of thing we would have been likely to miss. Instead, what happens in fluorescence is that a substance absorbs electromagnetic radiation, and then immediately emits it again as visible light - usually of a lower wavelength, and therefore energy, than the initial source.
The classic example, and the one that's relevant here, is shining invisible UV light on the substance and having it glow visibly in response. The substance only glows when illuminated in this manner, and stops as soon as the UV source is switched off - if it didn't, and continued to glow using energy it had stored up somehow, that would be phosphorescence, which, again, is something slightly different.
So, basically, what we're saying here is that the squirrels glow bright pink if, and only if, you shine a UV source onto them.
As I've mentioned above, it's not so unusual for plants to be fluorescent - we have known this since the 19th century. It has also been observed in various invertebrates, such as coral and butterflies, and even in a few vertebrates, where it can be seen in, among other things, some turtle shells and bird feathers. It's rather less common in mammals.
In fact, the only fluorescent mammals we previously knew about were tropical opossums. Even then, in most cases, it's a transient phenomenon, and not terribly impressive, although Emelia's short-tailed opossum (Monodelphis emiliae), an animal that lives along the banks of the Amazon River and some of its main tributaries, is said to have a particularly bright red fluorescence.
As it happens, though, fluorescence has previously been observed in squirrels, but it's quite a different sort of phenomenon. In this case what's happening is that a number of fox squirrels (Sciurus niger), a widespread species in the eastern US, suffer from a particular form of the genetic condition porphyria, which (among other things) causes a fluorescent compound to build up in the bones and teeth. However, the squirrels themselves aren't visibly fluorescent; the effect only becomes apparent when you have their skeletons to look at. Plus, of course, it's a disease, albeit one that seems surprisingly common in this particular species.
The squirrels in Wisconsin, however, were visibly glowing pink when illuminated in UV. Moreover, they were a different species: the southern flying squirrel (Glaucomys volans).
The great majority of "flying" squirrels are native to Asia, but, at some point, some of them managed to cross over the Bering land bridge into North America, giving rise to the three species living on that continent today. (Indeed, the closest relative of the American flying squirrels is probably a species native to Kashmir in northern India/Pakistan, from which they seem to have diverged around 14 million years ago, during the Middle Miocene).
The southern flying squirrel lives across pretty much the entire eastern half of the US, along with south-eastern Canada, parts of Mexico, and even Guatemala and Honduras. Until recently, it was thought that the only other species of flying squirrel in North America was, appropriately enough, the northern flying squirrel (G. sabrinus), native to those parts of Canada and Alaska warm enough to have trees (there's a significant area, around New England and Nova Scotia, where both are found together).
However, in 2017, it was discovered that one population of the northern species was genetically distinct enough to be named as a different species. Bravely avoiding the temptation to call it the "western flying squirrel", scientists instead named it Humboldt's flying squirrel (G. oregonensis) and determined that it lives along the west coast of North America, from British Columbia to California.
Since all three species are reasonably common, this meant that, when the researchers decided to check whether or not they had been imagining things, and that southern flying squirrels really were fluorescent, they could also check the other two species at the same time. To get really clear and unambiguous evidence, the way that they did this was to use the skins of flying squirrels kept in natural history museums, photographing them in normal light, and then again under UV light. They also did the same for the skins of three other common American species: fox squirrels, eastern grey squirrels (Sciurus carolinensis), and American red squirrels (Tamasciurus hudsonicus). How widespread would the phenomenon be?
All but one of the 109 flying squirrel skins that they examined did, indeed, fluoresce a brilliant pink under UV light. This means that all three species show the same effect, and it isn't just something unique to Wisconsin. Similarly, males fluoresce just as much as females. On the other hand, the three kinds of non-flying squirrel examined showed nothing at all.
The question, obviously, is why? With the phenomenon only just having been discovered, that's a question it is much harder to answer. However, while squirrels don't go around waving UV flashlights at each other, if their fur is fluorescent, it will be much more visible to others of their kind - if they can see into the ultraviolet part of the spectrum.
That has never been specifically tested, but we do know of two key differences between American flying squirrels and the kinds of regular tree squirrel whose fur failed the fluorescence test. One is that all of the tree squirrels are active primarily during the day, when the light is bright, but that the flying squirrels are active at night, and particularly during dawn and dusk, when UV illumination from the sky can be at its most apparent.
The other is that, while we may not know how well, or even if, flying squirrels can see UV light, we do know that tree squirrels (and ground squirrels) can't. That's because, at least in the species examined, the corneas of their eyes block such radiation, while that of American flying squirrels is entirely transparent in that part of the spectrum.
The fluorescence is strongest on the fur of the animal's undersides and tails, and these parts are probably quite visible to other flying squirrels as they jump from tree to tree, especially around dawn and dusk. They may, therefore, be used for signalling; one proposal, for instance, is that the brightness of the effect illustrates how healthy the squirrel is, something that might be used in identifying a suitable mate. Another possibility is that it helps the squirrels camouflage themselves against the local fluorescent lichen
It would be interesting to know how far this extends through the squirrel family tree. All three American flying squirrels show the effect, and to essentially the same degree, while the local tree squirrels don't. But do those flying squirrels in Kashmir, let alone the various other Asian species, do the same? It's far too early to know the answer to that, or to know exactly what's going on even with these three species that we do know about.
There is surely much more to learn, and that is part of the point of science.
[Photo by "MimiMia", from Wikimedia Commons.]