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.

The great majority of flying squirrel species (there are over 40, and we keep finding more) live in Asia. As I'd imagine most readers of this blog will be aware, they don't actually fly, instead gliding from tree to tree using a broad membrane stretched between their limbs as something more akin to a paraglider than a true wing. (And I say "paraglider", rather than "parachute", because they can, in fact, gain height while gliding). Perhaps surprisingly, the flying squirrels are a genuine biological group, which is to say that the gliding membrane did, in fact, only evolve once among squirrels.

Which is not to say that it only evolved once among mammals. There are a number of different kinds of gliding possum, not all of which are directly related, and the so-called "flying lemurs", which can be more accurately described as "gliding-animals-that-kind-of-look-like-lemurs-but-aren't-really." In fact, gliding as even evolved twice among rodents, since there are a bunch of animals in Africa called "anomalures" that do look remarkably like flying squirrels, but which belong to an entirely separate branch of the rodent family tree.

At some point, however, a population of true flying squirrels snuck across from Asia to North America, where just two species survive today, known helpfully as the northern and southern flying squirrels. (One mostly in Canada and Alaska, one mostly in the contiguous US, although there's a fair degree of overlap around the Great Lakes and New England/Nova Scotia). It's in these two species that ultrasound use was discovered, although there's no particular reason to suppose that their Asian relatives don't do the same thing.

Southern flying squirrels (Glaucomys volans) are found across almost the whole of the eastern US, as far across as Minnesota and Texas, with some populations as far south as Honduras. This is a huge region, and the two 2013 studies that examined their calls were conducted in quite different parts of that range - one in Ontario and one in Missouri and Alabama. Despite the great distances involved, one might expect that, since the calls are presumably instinctual in nature, they'd be pretty much the same in the two locations. In fact, however, they were quite radically different, and this naturally raises the question as to why that might be.

To try and get to the bottom of that, a third study has now been conducted, aiming to produce a detailed library of southern flying squirrel calls, and crucially to do so from a point in between that of the other two studies - specifically, woodland near West Point, Indiana.

Now, it's not mentioned in the paper, but I did wonder at this point whether or not the southern flying squirrels of Ontario and Alabama were generally considered to belong to the same subspecies. It turns out that they're not; those in Canada and Missouri belong to the nominate subspecies (G. volans volans), while the Alabama sort are G. volans saturatus. Having said that, defining subspecies can be a subjective matter, especially in the absence of hard genetic evidence. The two subspecies were first distinguished in 1915, and, so far as I can tell, there hasn't been a significant review of the relevant taxonomy since 1961... so there might well be more genetic variation than we're aware of.

On the other hand, nobody has had cause to challenge the scheme, either. And, if we assume that out current understanding is correct, the squirrels in Indiana (and, indeed, pretty well everywhere north of Arkansas and South Carolina) are the same subspecies as those in Ontario. If that's what's causing the difference, it should just be the Alabama squirrels that are outliers. Which is not what the study found.

In all, the new study looked at over 300 different calls recorded from wild squirrels visiting prepared feeding stations. Comparing their pitch and duration enabled the researchers to identify eleven different calls, three of which were at least partially ultrasonic. That the squirrels were able to make eleven different sounds suggests a fair degree of complexity in their communication, even if, with only sound recordings to go on, we can't be sure exactly what they all mean.

Significantly, seven of these calls had never been recorded before, and only one had been heard at both previous study sites. This, as it happens, was the highest pitched of them all, a rapid trill at 25 kHz (about seven-and-a-half octaves above middle C, if you're wondering). Presumably, this call is quite important, since it's the only one that doesn't seem to change across the full width of their range. It's also quite similar to a call known to be made by northern flying squirrels during their breeding season. Since this new study was also conducted during the southern species' breeding season, there's a good chance that it has something to do with reproduction.

Furthermore, the fact that so many of the other calls were different from those heard in the other studies (regardless of subspecies) is interesting. Some of this may simply be due to this including a far larger sample of wild, rather than laboratory, animals. However, since the new sounds included many of the most common calls recorded, it's unlikely that this is just down to luck - some of the calls are likely to be genuinely different.

The authors point out that they might have caught some calls only used between juveniles and adults, which the lab studies certainly wouldn't have picked up on. However, especially since none of the new calls were especially high pitched (as you'd expect if some of them were made by juveniles) it seems more likely that at a fair number of them are down to simple geographic variation. In other words, even within the same subspecies, southern flying squirrels have local "accents".

There have been similar findings with squirrels before. Both red squirrels and prairie dogs (which are a kind of ground squirrel) have been recorded to have local dialects; in the latter case they even vary from colony to colony in the same general area, possibly as a means of identification. Flying squirrels are very social animals, sharing communal nests during the winter, and often hanging out in large groups during the summer, perhaps as a means of improving their overall foraging success, or just to stay close to potential mates. And it would seem, with so many different calls to choose from and a great deal of regional flexibility, they have rather a lot to say to one another.

Now if only we knew what all those sounds meant...

[Photo by Ken Thomas, in the public domain.]