Sunday 9 March 2014

Clicking Clans

There are two broad types of whale alive today: those with teeth and those without. Generally speaking, it's the members of the latter group that are physically larger: these are the great filter-feeding, krill-slurping, baleen whales. The toothed whales, in contrast, tend to be much smaller. The biological group of toothed "whales", after all, includes dolphins, porpoises, and all their strange fresh-water relatives. Even the big ones, such as killer whales, are smaller than the great majority of baleen whales.

But there is an exception, and its by far and away the largest of all the toothed whales. This is, of course, the sperm whale (Physeter macrocephalus), and, while it's merely average when compared with the great baleen whales, at up to 50 tons in weight, it's nothing to be sniffed at. (The second-largest, incidentally, is probably the relatively obscure Baird's beaked whale, at 12 tons, which is itself about half again the weight of the animal in the #3 spot).

In one respect, however, sperm whales do fall behind their toothless kin, and that's with respect to the sounds that they make. The "whale song" that you're most likely to hear on commercial recordings is typically that of the humpback whale (Megaptera novaeangliae), which is especially complex and sophisticated. But it's by no means unique, and all baleen whales seem to do something similar.

Toothed whales can also be pretty vocal, even when they aren't sending out sonar clicks to find food and the like. The sounds are quite different from what we normally think of as "whale song", but they too seem to be used by whales and dolphins to communicate with one another. This is especially true of belugas, which have an impressive repertoire of chirps and whistles, but, again, they're hardly alone.

Sperm whales, in contrast, don't produce much that we can really call song at all. This is partly due to their anatomy, which seems to lack some of the structures that dolphins and so on use to make their noises. Still, that doesn't mean that they can't make sounds, or that, when they do, it's only ever for echolocation. Sperm whales do use sound to communicate, and when they do, it's by a series of rapid clicks, referred to as a "coda".

Like other whales, they don't make these sounds in quite the same way that we do. If a human being tried to sing underwater, it's fairly easy to see what would go wrong, and a whale would be no different if it tried the same technique. Indeed, they don't even appear to have vocal cords, which would presumably be of no use to them. Instead, they have a set of echo chambers inside their nasal cavity, complete with a pair of lips inside each of their nostrils. (Which is to say, under the blowhole). It's this bit that's less complex in sperm whales than it is in dolphins, but it's certainly still there, and the end result is that the whales can make sounds without drowning themselves.

To make sound - whether for echolocation or communication - a sperm whale blows air through its right nostril. The nasal cavity runs from just in front of the eyes right up to the animal's snout; if you look at a picture of sperm whale, you'll get some idea of just how far that is, and how long an echo chamber that gives them. Moreover, the right cavity passes between two huge fluid filled sacs, containing the valuable spermaceti for which the animal is named, and which focus and modulate the sound.

From there, the air passes through the lips into a second, smaller chamber, round underneath the (closed) blowhole, and back down the left nostril - which, in the sperm whale, does not have its own set of lips. Behind all of this is another chamber lying over what would be the forehead if the snout wasn't so enormous, and which is shaped so as to reflect any sound back into the organs in front of it.

This produces a very loud, high pitched sound. In the case of the clicks making up a coda, these are roughly a minor third above the highest note on a standard piano (so, D#7, if that helps) although it does vary quite a bit depending on the age and sex of the individual. High pitched, then, but not "ultrasound". It's also loud enough that it can be heard over a mile away.

A coda consists of a burst of very short clicks, usually no more than ten, but occasionally up to thirty. Although they pause for varying amounts of time between each click, the total burst is still over rapidly, with no more than one or two seconds being required to get out even a lengthy series with relatively long pauses.

The clicks themselves don't seem to vary for a given individual, so whatever information the sperm whales are passing among themselves  must be encoded in their pattern. That is, "clickclick clickclick click" might mean something different from "clickclickclick (pause) click". Having said which, it's unlikely that they're anything too sophisticated, because the signals just aren't complex enough for that. Even so, because of the way they respond to one another, it's likely that they're at least communicating something.

A large part of that is likely social information. Even without the pattern, the exact tone of the clicks will at least convey the size of the individual calling. Males are quite a lot larger than females, so that at least should be apparent, as should the animal's age. At least one other thing they're conveying though, is their relationship. That's because the exact pattern of codas varies, not between individuals, but between groups. In effect, by listening in on sperm whales calling to one another, we can detect a number of different "dialects".

There are, by our best guess, a few hundred thousand sperm whales across all of the world's oceans combined. But they do travel quite widely, mixing with one another as they do so, so these groups are very large and extensive. Such groups, with an identifiable dialect, are called "clans", and each contains several thousand individuals spread over a vast area. For example, there are five clans of sperm whale in the South Pacific, and there's considerable overlap between the waters that they inhabit - although, apparently, its different in the Atlantic.

Sperm whale clans are clearly not family units. They're far too big for that, and sperm whales typically travel with about a dozen or so relatives, forming a stable social unit that changes only gradually. These family groups are, as with most mammals, linked together by common descent through the female line, and the fact that they do mix, of only slowly, may indicate that the same is true for the clans as well, albeit at a greater remove. Most likely, the dialects that we can discern in different clans are spread through social learning, passed down over the generations, much as languages are among humans.

We don't even know how many sperm whale clans there are worldwide. We have a pretty good picture of their distribution in the South Pacific, and some idea of how they differ in the North Atlantic, but very little about them elsewhere. Just this month, a study was published identifying two distinct clans living off the coast of Japan and the Bonin Islands in the North Pacific.

Interestingly, these two regions have quite a distinct oceanographic environment, with one being close to a strong current, and the other in much more stable, warm waters. The two clans showed different patterns of diving throughout the day, suggesting the use of different hunting strategies suited to each environment. It could be that those, too, are culturally learned behaviours, since, so far as we can tell, the genetic difference between the two groups is minimal at best.

This information isn't just interesting for its own sake; it can also be useful from a conservation standpoint. Perhaps the genetic variation isn't that great, but isn't it just as worthwhile preserving these unique clans and their lengthy and distinct heritages?

[Picture by Gabriel Barathieu, from Wikimedia Commons]

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