The most obvious example is, perhaps, the sonar of bats, but bats are not the only mammals to make ultrasonic calls. We know, for example, of fifty species of rodent that use ultrasonics, and, since such studies are still largely in their infancy, and there are over 2,000 species of rodent to pick from, it's a fair bet that there are far, far more we don't know about yet. Obviously, though, the rodents aren't using these calls for sonar (at least, not that we can tell) so what's the point of it?
It's likely just that there's no particular reason why a rodent should want to make sounds that humans happen to be able to hear. So far as they're concerned, their ultrasonic calls aren't really any different from all of their others, and they're using them for much the same sorts of purpose. For the most part, they have been observed during courtship, suggesting that they're basically mating calls. That's probably not the only thing they're used for, and there's evidence that mice, for example, can encode information for quite complex social interactions in their calls, but it does seem to be a common one.
Not that mating calls themselves are necessarily lacking in complex information. It would be useful, for instance, to have some idea of the age and maturity of a prospective partner. And, of course, which sex they are. The latter is generally obvious in the calls of larger mammals, that we can hear, and it would be surprising were it not also true in the ultrasonic calls of rodents. Early studies seemed to show that, for most rodents, only the male emitted these kinds of noise. We now know that that isn't true, but even so, for many species, it does seem to be the male that uses ultrasound the most.
The majority of these studies have been done on house mice, not least because they're common lab animals. However, we would expect that other rodents, especially if they have lifestyles different from those of house mice, will be different in how they use ultrasonics. For instance, one study on California mice (that's a species, by the way, not just mice that happen to be from California) showed that, contrary to expectations, males and females make pretty much the same sounds. This is possibly because they're monogamous, and, being already familiar with their partner, really don't need to be reminded which one of them is the male.
The golden hamster (Mesocricetus auratus) is a familiar rodent. There are at least twenty different species of hamster, and this by no means the most common. Indeed, it is a threatened species, in the wild found only along one relatively short stretch of the Syrian/Turkish border north of Aleppo. But it just so happens to be the main one that we've domesticated, so the odds are that any hamster you may have seen was a golden hamster. (Unless it was some kind of "dwarf hamster"; those are something else).
Left to themselves, golden hamsters are solitary animals, each with their own burrow that they don't let anyone else into. Most of the time, a female hamster will beat up any male that tries to approach her. If she's in heat it's a different matter, although even then, he'd best not push his luck by staying around too long after the deed. Even so, it's clearly going to be useful to him to get some advance warning of how she's going to react if he pops into the burrow. Obviously, there are signals other than ultrasound that can give him a clue here - scent being an obvious one - but it's at least possible that ultrasonics are also relevant.
To test this, researchers tried placing pairs of hamsters in adjacent cages and recording the ultrasound they produced. The females in the study were all in heat, and since they were able to see and smell one another, but not interact physically, the males would certainly have been aware of any signals they were producing. Similarly, most of the females adopted the "lordosis" posture, lowering their forelimbs and curving their back to present their rump. (If you're a hamster, or, indeed, a cat, this is a sexually seductive pose. If you're a human, it probably means you have a back problem).
The first thing to say is that this certainly seems to work. Both males and females produced more ultrasonic squeaks when presented with a member of the opposite sex, whereas pairs of males completely ignored each other. Paired females, on the other hand, did squeak at one another a little, possibly because, being in heat, it didn't take a lot to excite them, but even they were obviously more interested when it came to males.
The noises were, for the most part, single-note squeaks, although they were clearly capable of producing more complex multi-note sounds, and did so on occasion. But there was no sign of the surprisingly complex "songs" that we know male mice can produce. In fact, almost half of the recorded calls were "noisy" rather than musical. Such atonal noises - although not at the same pitch - have also been observed when hamster pups call to their mother, and could be a sort of general greeting call used between friendly hamsters. Another possibility is that they're just shouting really loudly, something that inevitably leads to a loss of fine control of anything other than volume. The pitch of the more musical calls, peaking at 33-37 kHz, is also lower than that in mice, and also, perhaps more surprisingly, than that of rats. (For comparison, the highest note a healthy young human can hear is about 20 kHz).
But can the hamsters use ultrasound to distinguish males from females? The researchers found that, on average, male calls were more musical, and more likely to contain multiple notes than females ones. However, not all members of a given sex sounded the same, and the differences in the average sound obscures the fact that there's quite a lot of overlap. In short, the sound might give the other animal a hint, but not a reliable one... which would rather defeat the point. Most likely, since hamsters are more than capable of telling one another apart by scent alone, there really isn't any point in making the calls noticeably different as well.
Another apparent difference may be equally irrelevant to the hamsters themselves. When separated from their potential partner, females continued calling out to them for some time, but males gave up almost immediately. The researchers, however, point out that female hamsters don't call while adopting the sexually suggestive lordosis posture. This means that, by the time they were separated, the males had been calling for three minutes solid without any useful response, and may well have gotten bored, or simply been tired. The females, having been silent, were still getting into the swing of things, and more eager to advertise their frustration.
So, it may be that sexual identity isn't really encoded in the calls after all. But that doesn't mean that nothing is. After all, that wide variation between individuals may mean something to the hamsters that isn't clear to us. If the harsher, more atonal sounds, aren't simply the result of shouting, perhaps they're attention-getting, as we know is the case with male deer? Even if they are, does the ability to produce them sound sexy?
We aren't entirely clear how the hamsters make these sounds in the first place, although, as one might expect, it clearly has something to do with the muscles around the larynx or "voice box". But it's entirely plausible that fitter, more attractive, individuals can produce more of a particular kind of sound than might otherwise be the case. It's certainly common enough in other animals. Even if all it shows off is your stamina, determination, and level of downright sexual frustration, that's probably pretty useful information.
If you're a hamster looking for love.
[Photo by Andreas Hein, from Wikimedia Commons]
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