The number of different sounds an animal can make depends on a number of factors. Many of these are physical, due to the way that their larynx and vocal cords are set up, and to their ability to modulate the sounds it produces with their mouth, lips, and so on. For instance, while there has been at least some success in getting chimps and gorillas to use human sign language, they can't actually speak because of purely physical limitations in their upper respiratory tract. Added to this is the matter of just how much complexity they need to get across anyway.
Broadly speaking, the more sociable an animal is, the more need it has for complex communication. If you rarely come across other members of your own species, you probably don't need to say much when you do. A simple "go away" is probably about as much as you need, and you can co-opt the same threat against hostile members of other species, too. Beyond that, you may need some kind of mating call, and a means for mothers to find their offspring, and you're pretty well sorted.
But, if you live in a society, things become more complicated. and we would expect social animals to, on average, have a larger social repertoire than those that habitually live alone. And, indeed, there seems to be some fairly strong evidence that this is the case. Many of these studies have been conducted on primates, but there have been some on other mammals, and even on birds.
Earlier this year, I talked about a study about communication in Asian small-clawed otters, and, a few days ago, a similar study was published on giant otters (Pteronura brasiliensis), another social species. Observing both wild and captive members of the species, the researchers identified no less than 22 different sounds produced by adults. Cubs, perhaps due to a combination of physical immaturity and a different social context, managed only 11 of these, but this does remind us that the details of calls can change during an animal's life.
Most of these call types fell into one of three broad categories. In accordance with a theory first proposed in the 1970s, calls with a negative connotation tended to be deeper and harsher than those that were more positive. For example, growls, snorts, and harsh screams were used as aggressive warnings to other otters, and also as alarm calls. Contact calls, used to keep the group together, to locate cubs that had wandered off, or just while playing with or soothing one another tended to be higher pitched, and more musical; terms here included coos, hums, whistles, and whines. The third category, begging calls, were used to solicit food from others (not always with any success, it has to be said), and were also more musical, and were generally various kinds of squeak, whine, and high-pitched scream.
This sounds somewhat anthropomorphic - that we ourselves would interpret growls as more negative than a cooing sound. But, then, we're animals, too, and if this is, as hypothesised, a general trend among birds and mammals, there's no reason that we would be any exception to that. Only two types of call fell outside the three main categories, one of them a sort of gurgling hum used by happy cubs while they suckled. The other was only recorded once, a high-pitched growl emitted by a female while she was mating. (You can listen to all these call types, bar the last one, in the zip file here).
We can determine what these calls mean by observing what the otters are doing as they make them. But what are we to make of sounds made while an animal doesn't appear to be doing anything at all? Here, we turn to another study, published, as it happens, on the same day in the journal PLoS One. This concerns the Asian house shrew (Suncus murinus).
Asian house shrews are widespread in southern Asia, being found from India and southern China to the islands of western Indonesia. They are not regarded as very social animals, although the fact remains that their social lives have not been studied in great detail, so they may be less solitary than we think. For example, the related greater white-toothed shrew nests communally during winter, and it's at least possible to house families of Asian house shrew together in captivity without them becoming violent.
Even so, that's hardly likely to represent the sort of social complexity found in otters, and there's likely less need for detailed communication. Like many other shrews, however, this species is highly vocal, making rather a lot of noise as it goes about its daily routine. Some of these calls are echolocation calls, similar to those in bats, and common among shrews. But up to seventeen other call types have been identified, which would at least suggest that they need to communicate something, and quite regularly. After all, if you make too much noise for no good reason, and you're an animal the size of a shrew, something's going to find and eat you.
And yet, earlier this year, it was reported that these animals make a repetitive, low, squeaking noise while lying still in their nests doing absolutely nothing. Indeed, they're frequently on their own while they're doing this, which makes the sound's purpose doubly puzzling. (You can listen to it here).
Now, anyone who has owned a dog knows that animals do sometimes make noises in their sleep, apparently associated with dreaming. Granted, we can't ask them to check, but that certainly seems the most likely explanation, as, for example, one report on the sounds made by bush-babies suggests. Here, however, it's a repetitive, long-lasting sound, which makes that unlikely - although we can't rule it out, given that we don't really know how much REM sleep the shrews get, let alone what they might dream about when they do.
The detailed analysis of the calls conducted in the study showed that they were slightly more complex than thought, being interrupted on occasion by a less tonal, noiser, and longer-lasting sound. Each regular call was also preceded by a brief click, too short for humans to identify in sound recordings, and which it isn't even clear how the shrews make. Interestingly, the authors conclude that each shrew makes a slightly different sound while resting, which it's possible that others shrews could use to distinguish them from their fellows. It's nothing like as individual as the differences between the calls of, say, bats, but it might not need to be. The very largest a group is ever going to be is a mother with a maximally-sized litter, and that's only nine individuals. If they really do need to tell each other apart, that's the greatest number of differences they're ever going to need to identify.
Is it a sound used to help bond groups together, but that is so habitual, that they even do it when they're on their own? Might it, perhaps, be used to signal discomfort, perhaps through the amount and distribution of the "noisier" sounds among the more musical ones? Is it something to do with the fact that the animals studied were all in captivity, and not in their native habitat? That much remains a mystery, and shows us that, especially with some of the less studied groups of mammal, there is still much to learn.
[Photo by L. Shyamal, from Wikimedia Commons]
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