Sunday 6 May 2018

Diving for Your Dinner

Cetaceans (whales, dolphins, and porpoises) are the best adapted of all mammals to life underwater. Their food is often found far below the waves, requiring them to dive deep to find it. But this, of course, is countered by the fact that, not being fish, they also have to return to the surface to breathe. The deeper the desirable food happens to be, the longer they will have to dive for just to reach it, and the longer they will have to spend recovering on the surface afterwards - even if the actual feeding time remains the same.

Such pay-offs are arguably particularly important for the huge rorqual whales, which feed by lunging at great masses of krill or other small prey and gulping them down. For them, it really matters that wherever they are diving is rich in food, so that they can find enough to offset the effort required to catch it. Quite how they strike that balance should depend on how good they are at diving, which relates to things such as their lung capacity and how much oxygen they need to sustain their bodies.

That's something that will clearly vary with age, and perhaps even sex (especially if the sexes are different sizes), but will also vary between different species of rorqual. Even if they live in the exact same area, therefore, we should expect different species of whale to behave differently when it comes to diving, and how they vary will, in turn, tell us something about their respective biologies.

These sorts of studies have been performed in diving birds, but hardly at all for cetaceans. The problem is, of course, that whales dive much further than birds do, so studying what they're doing at all when they're down where you can't see them is hard enough, without having to get a large enough sample size to make meaningful comparisons between different groups.

Fortunately, over the last ten years or so, there have been significant improvements in the sort of monitoring technology that we need. It's still a fairly expensive and difficult process, though, so most studies have focused on demonstrating that, yes, diving whales do modify their behaviour depending on where they expect the food to be. But now we have the first large-scale study of how two different species respond to the situation in a single environment.

The study was conducted over a two-year period in the Kitimat Fjord System, a series of narrow inlets about half way along the coast of British Columbia. Rather than use expensive tagging systems, the researchers simply watched the whales from a boat, measuring how long they stayed at the surface between dives and how they behaved, coupled with the use of echosounding to locate the krill on which they would have been feeding.

(If you've ever wondered, krill are closely related to shrimp, but aren't the same thing. Differences include the arrangement of the gills, the number of limbs, and the shape of the tail. Krill also tend to be transparent, with luminescent organs on their abdomen).

The two species of rorqual that live in the fjord are humpback whales (Megaptera novaeangliae), famous for the sophistication of their songs, and fin whales (Balaenoptera physalus) which narrowly avoid being famous, on the grounds that they are merely the second largest animal on Earth, after blue whales.

The results of the study show that, despite living in the same area and preying on essentially the same food, the two kinds of whale do show different diving behaviours. The fin whales dove, on average, for longer than the humpback whales, typically spending about six-and-a-half, rather than four-and-a-half, minutes beneath the waves. Despite this, they did not spend any longer at the surface, apparently taking just as long to refill their lungs and gather their energy for another dive.

Six-and-a-half minutes is, of course, far less time than either species is capable of holding their breath for should they really have to. So there's certainly no indication that the whales returned to the surface only at the last moment, instead spending what they presumably felt was a comfortable and profitable time at depth. The reason that the humpback whales didn't dive for so long, then, seems to be that (again, on average) they didn't dive so deep.

Of course, these whales were in a fjord, not the deep ocean, so there was only so far down they could go. The depths in question were perfectly within reach of the humpbacks, and, in fact, while the average may have been significantly less, the deepest estimated dive made by a humpback in the study was quite a bit further down than the deepest estimate fin whale dive. So, if they could make really deep dives, why didn't they do so more often?

A large part of the reason may be do to do with the maneuverability. We know, from a number of previous studies that fin whales can swim faster than humpbacks - diving at about twice the speed. This means that, while the latter are perfectly capable of diving to extreme depths, it takes them a long time to get down that far, leaving less time to search for and engulf any food that's down there. From the estimates in this latest study, there's a cut-off of around 150 metres (490 feet) below which the pay-off isn't usually worth the trouble for humpbacks, but that isn't much of a concern for the larger, faster, fin whales.

Of course, krill at shallower depths would be easier for fin whales to feed on, too. They may be able to feed deeper down without difficulty, but that doesn't mean they have to. What may be happening here is that the fin whales deliberately dive deeper to avoid the humpbacks, and feed on the krill that the latter can't easily reach. That way, there's more for everybody. The fin whales don't have to worry about somebody else getting there first, and the humpbacks don't have to overly exert themselves, ceding the deeper waters to those more happy to use them.

Naturally, there is overlap, and the authors of the new study are quick to point out a number of limitations of their (relatively cheap) study. For instance, because they were watching what the whales were doing, they could only get information during the day, and don't know how the whales might have been behaving during the hours of darkness.

But what this does show is that, at least some of the time, two different kinds of whale can feed on the same food, in the same place, without having to clash with one another, purely due to the differences in their physique and stamina. That they can do so in remote fjords little troubled by human visitors is likely quite a bonus.

[Photo by Uwe Kils, from Wikimedia Commons.]

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