Birds are by no means the only animals to make lengthy annual migrations. While most annual migrations by mammals are of the relatively short-distance kind - moving down the side of a mountain as winter approaches, say - some can be surprisingly impressive. To travel a truly long distance, walking is generally not your best bet, so most mammals that do so are not the kind that would travel overland. Given their parallels with birds, perhaps the most obvious mammalian examples are bats. While most sleep away the insect-free winter in a cave, some bats do travel long distances. As just one example, straw-coloured fruit bats (Eidolon helvum) have been reported to travel up to 2,500 km between Congo-Kinshasa and Zambia in search of the best ripe fruit in any given season.
Which isn't up there with the most impressive of the migrating birds, by any means, but isn't exactly a short hop, either. And, if you can't fly, swimming is another option. Indeed, one of the mammalian species that migrates the furthest is the humpback whale (Megaptera novaeangliae), with migrations of up to 10,000 km having been reported.
Humpback whales are, in fact, found just about everywhere on Earth that's covered by seawater, excepting only the Arctic Ocean and some smaller seas with restricted physical access (such as the Mediterranean). Different populations, therefore, can migrate along very different routes. But the pattern is generally the same: they spend the summer in cold waters, where they stuff themselves with krill, and then head towards warmer climes as the winter sets in, giving birth and raising their young over the summer before returning to their feeding grounds.
The thing is, though, that when we describe those colder waters as "feeding grounds", that's literally what we mean. While insect-eating birds migrate because they don't want to starve through the winter, humpback whales do go pretty much the entire winter without feeding. They do all of their eating in summer, at high latitudes, and then fast for the other half of the year. There are some exceptions to this - there's a population of humpback whales in Arabian Sea that don't migrate - but, in general that's the pattern.
Or so goes the conventional wisdom. In recent years, there has been increasing evidence that humpbacks do, in fact, feed in tropical waters. This could be a recent artefact, rather than something that's always happened (at least, on a regular basis) since there is also some evidence that krill stocks are falling as the world warms, something that's obviously very worrying for an animal that eats little else. Since humpbacks are large even by the standards of whales, they need to eat a lot to keep them going through the winter, and any drop in their food supply could have a significant effect.
But how common is this winter feeding? Is it just an occasional snack when food happens to present itself, or something more common? And how able are humpbacks to adapt to a changing world in this manner? A recent study looked at the feeding habits of an Australian population of the animals to get some idea of how flexible they might be.
The population in questions spends the winter off the eastern and western coasts of Australia, then migrates more or less directly south in the summer, feeding off the Antarctic coast. But how do you tell what they've been eating? It's hard, after all, to watch them continuously through the course of a year. Well, it turns out that there is a difference between food eaten in Antarctic waters and that eaten in the tropics, and it's one that leaves a signature in the body.
Specifically, organic matter in the Antarctic has lower levels of carbon-13 relative to carbon-12 than that found further north. Most of this organic matter is in the form of phytoplankton, which is eaten by the krill that are eaten by the whales, so that the signal should propagate up the chain. Compare the levels of these two isotopes in a whale, and we should know where it did its feeding. Similarly, the levels of differing nitrogen isotopes can give an idea of what the animal has been eating, since fish (for example) contain higher levels of nitrogen-15 than krill. To get a proper picture, though, we need to look at some part of the animal that grows slowly but steadily, so that we can be sure that the signal hasn't been blurred over the many years of its life.
In the case of whales, the likely best bet are the baleen plates. These are the long plates of material than hang down inside the mouths of humpbacks, and of most other large whales, in place of where their teeth would be. Lined with hair-like fringes, they filter the krill out from the water, allowing the animal to feed. They are composed primarily of keratin, the same substance that makes up hair and fingernails, and, like hair, they grow continuously as the animal ages, being steadily worn down as they do so, so that they don't become too long.
Of course, in order to examine this, you do need a chunk of baleen from the whale in question. The only realistic way of doing this, short of killing the whales, is to examine those that are already dead - in this case using museum samples from stranded whales dating back to 1940. While hard to avoid, this does give one limitation to the study; since all the whales were dead, it's reasonable to assume that they hadn't been very well when they last fed, and it's entirely possible that ill whales don't behave in the same way as those that are perfectly healthy.
With that in mind, though, the study was able to find evidence of cyclic changes in the isotopic composition of baleen as it grew - effectively annual growth rings, although the fact that the baleen constantly wears down means that they won't last for the whole of the animal's lifespan. Aside from confirming that the general technique seems to work, this also adds another piece of information we didn't previously have: the baleen plates in humpback whales apparently grow at an average rate of 16 cm (6 inches) per year, slowing as the animal ages. This, it turns out, is about what we'd expect from other, closely related species.
Quite how accurate the resulting interpretations of feeding behaviour are could be questioned, since it is not clear that high levels of nitrogen-15 mean that the animal was fasting, as the study's authors assume. Nonetheless, they did identify what appear to be three different feeding strategies among the twenty whales they were able to examine.
Ten of the whales seemed to behave exactly as expected. Assuming that the isotopic signals mean what we think they mean, these animals ate in the summer, and largely starved themselves during the winter breeding and calving season. Almost as many, however, diverged slightly from the behaviour that we'd expect. These seven whales supplemented their summer diet of krill with other food sources, either by eating fish alongside the krill, or by taking occasional meals of whatever was available through the winter period. Interestingly, all but one of these died off the east coast of Australia, suggesting that the population living off the west may be more 'traditional' in its feeding habits. This could be because the eastern population travels through the fertile waters off New Zealand and Tasmania on its way north in the summer, thus extending their feeding time... although, then again, it could be just a coincidence, due to the small sample size.
The remaining three whales (all from the eastern coast) were completely different, apparently not having fed in Antarctic waters at all during the last year of their life. One can't help but think that this might have something to do with whatever eventually killed them - perhaps they were just too ill to make the trip south.
At any rate, what this appears to show is that humpback whales are at least capable of switching both their times of feeding and the nature of exactly what they eat, being able to consume fish as well as krill. Whether they're happy about doing it, or whether it's something forced on them due to ill health or whatever, is another question, but at least they can. If krill stocks continue to drop, they may have little choice.
[Photo by Marina C. Vinhal, from Wikimedia Commons.]