Sunday, 12 May 2013
Going for a Song
So how can we study what they're up to? For land animals, there are a number of options. You could, of course, just watch them, and, for many purposes, that's enough. But if you want to know where they go on a 24-hour basis, that's not really going to work. For one thing, it's impractical, and, for another, you'll likely annoy the animal, so that (even leaving ethics aside) it isn't going to act normally. So, if you want to study an animal's movements over a long period of time, your best bet is to catch the animal, and fit it with a GPS tracking device. Then, having released it, you wait a while for it to calm down and get used to this collar thing around it's neck, and then see what it does.
The tracking device will often have a transmitter built into it, giving you real-time data, but it's also possible to just log the data and collect it when you're done - the collars are often self-detaching, or can be remotely triggered to fall off, so you don't necessarily have to catch your animal again. These devices are not cheap; the starting price I found from one of the major suppliers was $1700. But still, they are undeniably very useful.
It's rather trickier doing the same thing for aquatic mammals. For one thing, your device has to be waterproof at depth, and, if you want it back when you're done with it, having it fall off isn't much of a solution: you are going to have to find and catch your animal again. As a result, transmitters of this sort cost about twice as much as the land-based kind. Given the shape and size of most aquatic mammals, a collar isn't a great solution, either, so they don't come with one - you'll probably have to physically glue it to your animal. And that's after the trivial matter of humanely capturing and anaesthetising a half-ton sea lion (or whatever) in the first place.
Now, don't get me wrong: these sorts of things are still useful. If you really want to know what aquatic mammals are doing 24-7, they're likely your best bet. Put the right sort of sensor in there, alongside the GPS, and you'll know when and how far they're diving, too. Yes, they're expensive, and awkward to deploy, but if you wanted an easy life, you should probably have stuck to studying hedgehogs in your back garden.
In fact, we can do some great science this way. For example, a couple of recent studies by Elaine Leung and co-workers used these sorts of devices to monitor the behaviour of New Zealand sea lions (Phocarctos hookeri). They showed that male sea lions off the coast of the Auckland Islands travel further afield in search of food than the females do. This is important for conservation, since it means that the females are more likely to be found in the areas where fishing boats are, potentially putting them at greater risk. Similarly, young sea lions off the Auckland Islands have to spend more time looking for food than those living close to New Zealand's South Island, suggesting that the former is a tougher habitat for them.
However, for some species, there is at least one other option. Humpback whales (Megaptera novaeangliae) are famed for their habit of singing, producing sounds that travel for great distances underwater. So one way that you can track their movements is simply to listen for them.
For this, you'll need an array of hydrophones spread across a wide area, so that you can triangulate the whales' positions from multiple recordings. Although the sort of hydrophones that are best for this aren't particularly cheap to buy, place, and retrieve, they will monitor every whale that passes by, rather than needing one device per animal. And, of course, once they're in place, you can use them for all sorts of other things as well, not just checking up on whales. There's also the advantage that the whales will have no clue that you're doing anything at all, ensuring that they act naturally and don't get stressed.
On the other hand, there are a couple of disadvantages, too. Firstly, the whale does have to be wherever you put the hydrophones - you can't just follow them wherever else they go. And, of course, your whale does actually have to be singing for you to find him.
'Him'? Well, yes, because it's really only the males that this works for. It's not quite that females don't sing at all, but when they do, the songs are much shorter, and less suitable for tracking their movements for any length of time. Males, on the other hand, can spend hours at a time singing, albeit with pauses between individual songs.
Although we don't know exactly why the whales are singing, the fact that only males do it suggests that it may have something to do with mating. They're not necessarily attracting mates, although that's certainly one possibility. Another is that it could be some kind of social message emphasising their virility. A rather less violent equivalent of stags or antelopes clashing antlers or horns, in other words. Most likely, it's a combination of different reasons, depending on the exact nature of the song.
Still, because we assume that it has something to do with sex, most such studies have been done on humpback whales during the breeding season, for example, off Hawaii. However, we do know that they sing at other times of year, too, especially during their annual migrations between warm and cold waters.
So Joy Stanistreet and co-workers used an array of hydrophones in the Stellwagen Bank Marine Sanctuary, off the coast of Massachusetts, to monitor humpback whale activity round the year. It's a preliminary study, aimed at least in part at proving that this sort of thing works, but even so, it reveals some interesting pieces of information.
In the year of monitoring, the whales began singing on the 14th of March, and continued, averaging seven and a half hours a day between them, until 11th of June. Then they fell silent, only to start up again in late August, continuing almost until the end of December before silence reigned once more. So why do they only sing in spring and autumn?
The reason for their silence between January and mid-March is clear: they aren't there. This is the humpback whale breeding season, when the whales of the north-west Atlantic head down to the Caribbean for a few months in search of sun, sea, and sex. However, during the summer, they certainly are around, feeding on the local sand lance, among other fish.
Perhaps they're too busy eating to bother with singing. Perhaps their mating season actually lasts longer than we think it does, and some individuals are either still trying it on after they've left the Caribbean, or trying to get in early before the southward migration starts. Or perhaps, if its main purpose is to advertise the whale's masculinity, they're signalling to potential rivals either side of the mating season. That we don't know what the females are up to may make it difficult to decide between some of these possibilities.
On the one occasion where the researchers heard two males singing at the same time, and within earshot of one another, they kept a wide berth, and seemed somewhat wary of one another, resuming their normal behaviour once one of them had stopped. But, with only one example, its hard to know what that really means - especially if different songs can convey different information that we haven't yet deciphered.
So it's a start, rather than an end to the research. It does help us get some kind of estimate of the local population density (it's lower than when they're at their breeding grounds, apparently) and of their speed (fast enough to show they aren't just drifting about aimlessly, but not as fast as when they're migrating). It shows us where the males, at least, travel, and how often and for how long they're calling, if not exactly why they're doing it.
And all without the whales being any the wiser that they're being observed.
[Picture by Whit Welles, from Wikimedia Commons]