Sunday 12 October 2014

The Mysterious Songs of the Blue Whale

The blue whale (Balaenoptera musculus) is, as is fairly known, not only the largest animal alive today, but the largest that has ever lived. Far larger, for instance, than anything that lived during the time of the dinosaurs. Yet, because, like all whales, they are not that easy to study, they remain relatively mysterious, compared with large land-based mammals.

There, however, are a number of different ways in which we can study them, and one of them is to listen to their calls. Like other whales, blue whales produce 'songs' that travel for miles through the deep, and, by listening to them, we can get at least some idea of where they are, and how numerous they are, and perhaps further information besides.

Compared with cetaceans like the humpback whale, the songs of blue whale are not particularly complex - although they remain more so than deep clicking sounds of sperm whales. For the most part, blue whale songs consist of the same element repeated over and over. This element, termed a "Z-call" because of the shape it produces on a spectrograph, has three parts: a long, deep rumble, followed by a rapid dip and then a short, musical tone at an even deeper pitch. The first part is commonly somewhere about the A four below middle-C, which is the very lowest note than can be produced on a grand piano. The last part is about three or four notes lower than that, which is generally considered below the range of normal human hearing.

(You can hear a sample here, shifted to a more human-audible pitch. There's a particularly clear call at the 2:05 mark).

There is, of course, some variation in this, both between individual whales and between consecutive songs. Moreover, studies have shown that the nature of the songs changes somewhat over time. For example, several years' worth of data now show that the pitch of blue whale song has been dropping year on year, since at least 2002. It is now typically about 1.5 Hz lower than it used to be - which, at the sort of bass pitch we're talking about, is roughly the difference between a natural and a flat note.

Quite why this should be isn't entirely clear, but it may actually be good news. It's possible that blue whales are singing slightly deeper than they used to because their populations are increasing. While they remain an endangered species, we know from other sources of evidence that their numbers are currently headed in the right direction, and have been since the end of most large-scale whaling. It might be that, because they no longer need to be heard at quite such long distances to reach others of their kind, they are singing less loudly than before. If they don't have to sing so loudly, the males, in particular, can hit the deeper notes, which may sound sexier to the females.

We're not completely sure that this is the case, but it does fit the facts.

But there's more, because the pitch of blue whale song also changes over the course of the year. Apparently, blue whales sing deeper notes in the summer than they do in the winter. It's not by much - the difference we're talking about here is only around 0.5 Hz at the most - but it's what recordings seem to show. And here, the reason is rather less obvious.

To what extent do blue whales change their behaviour through the year in other ways, that might be linked to why they would change their tunes in such a predictable manner? Here's where our lack of knowledge of blue whales and their behaviour really presents a problem, because we're not all that sure.

The studies that have shown this change in pitch were specifically of B. m. intermedia, the Antarctic subspecies. These live, as their common name suggests, in waters off the coast of Antarctica, and there are recordings of them there year round. However, sightings of them are less common in winter than at other times of the year, and it seems many of them head north at these times, being detected as far away as southern Africa and the mid Indian Ocean. Perhaps, then, they are behaving differently at these times of year, in ways that might require at least some of them to change the way that they sing?

Or, then again, perhaps it's all an illusion? The recordings that show the effect most clearly were all taken off Cape Leeuwin, the most south-westerly point of Australia. They were taken from a set of sono-buoys that were, inevitably, tethered at a fixed location. There are two different ways that that could lead to confusion in the data. Firstly, there could be something about the water itself - its temperature, for example, is obviously going to change throughout the year. However, that is far too small an effect to be causing what we're hearing here, so we can rule that one out.

Secondly, however, it might be a Doppler shift. This is the effect whereby sounds produced by objects moving towards you sound higher pitched than those from objects moving away - the usual illustration is the roar of a racing car engine as it races past you, it's pitch radically changing as it does so. If the whales migrate at particular times of the year, and, bearing in mind that this is a very small change we're talking about here, might they be more likely to be moving away/towards the location of the microphones in certain seasons? Calculations show that a blue whale probably could move fast enough to create a Doppler shift of the magnitude seen. But do they actually do so?

This possibility was recently tested when a singing blue whale swam past a sono-buoy laid by a ship off the coast of Antarctica as part of a larger research project to analyse the mystery. There was, as expected, a detectable Doppler shift in the pitch of its song as it did so, which matched that from the speed of the animal recorded from visual observations. But, it was still too small to account for the changes recorded over the year. Taken with the fact that the same project was able to record the same annual change in pitch from locations scattered widely across the southern waters, this seems to conclusively rule out that possibility. The whales can't reasonably be moving at the same speed and direction away from every buoy across the ocean, after all.

So what are we left with? There are basically two possibilities: either they're doing it on purpose for reasons of their own, or something is happening to the whales physically that affects the way they produce sound. In order for the first possibility to make sense, they would, of course, have to be able to hear the difference in the pitch themselves. It's too subtle for us to detect without electronic equipment, but, then, it is at the very edge of human hearing to start with, and it's not unreasonable that whales might be better at this sort of thing than humans are.

On the other hand, the difference is still very small compared with the changes in pitch that we know they're capable of making just in the course of a single song. If they really wanted to convey something different, you'd think they'd make a more obvious change. But it is certainly not impossible.

As for physical change, moving to and from summer feeding grounds might well have some sort of effect. One proposal has been that the change in pitch might be due to the amount of blubber the animals are carrying at different times of year. To test this, we need to know how much 'thinner' and 'fatter' whales get as the year changes. For most land-based animals, you'd just capture some, weigh them, and put them back afterwards. This is, as one might imagine, a bit tricky with a blue whale.

We can, however, use historical records from a time when whaling was more common. A paper from way back in 1929 happens to give us this information... and while there may well be a correlation with the change in blubber thickness of small blue whales throughout the year, it vanishes for the fully grown ones. (By 'small', incidentally, I mean less than about 19 metres [62 feet] long - it's all relative). Even so, it's a distinct possibility, and, of course, there could well be other physical changes - muscle mass, overall fitness and health - that enhance it in some way.

It doesn't help that we really don't know how blue whales make their sounds in the first place. It was only seven years ago, in 2007, that we discovered that baleen whales possess vocal cords, albeit ones very different from ours. We're only just now starting to get to grips with the unusually complicated mechanisms by which whale song is generated, and most of those studies are on humpback whales. Which makes sense, because they have the most complex and interesting songs, but doesn't necessarily tell us much about other species.

So, while the answer to the mysterious change in pitch probably has something to do with the physical condition of the whales at different times of year, the question is still not one that we've solved. Cracking it will likely tell us more, not only about the method of sound production in these animals, but about their overall biology, behaviour, and fitness throughout the year.

[Photo by the National Oceanic and Atmospheric Administration, in the public domain]

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