The blue whale can swallow half a million calories in a single mouthful. When it spots its prey—shrimp-like creatures called krill—it lunges forward, accelerating rapidly and opening its jaws to an almost right angle. Its mouth expands, its tongue inverts, and it engulfs around 110 tonnes of water—about the same mass as another small blue whale. Over the next minute, it pushes the water through sieve-like plates, filters out the krill, and swallows. Then, having reloaded its face, it’s ready for another attack.
This sequence, known as lunge-feeding, is the signature move of the rorquals—the family of giant whales to which the blue belongs. It allows these animals to grow to huge sizes by feasting on huge volumes of miniscule prey.
But lunge-feeding didn’t evolve among titans. The fossil record tells us that the technique first arose in species in the size class of the modern minke whales—the smallest of the rorquals at a mere (!!) 5 metres long. To understand how and why lunge-feeding evolved, we need to understand how the minkes do it.
That should be easy, since minkes are the most common whales in the Antarctic. But they are also among the most elusive. Large rorquals like the blues will spend a reasonable amount of time at the surface recharging their lungs, allowing scientists to easily stick tags and recorders to them. But with minkes, “you see them for a breath or two and then they’re gone,” says Jeremy Goldbogen, a whale researcher at Stanford University. “You have a second to get it in the right spot.” No one had been able to tag one, and not for lack of trying.
That changed in February 2013. While trying to study humpbacks in Antarctica, a team led by Ari Friedlander at the Oregon State University found big groups of minkes, up to 40-strong, feeding around large floes of ice. They seemed to be socialising, and they were certainly relaxed enough for the team to sneak up in an inflatable boat. Friedlander sat on a pulpit at the front, carrying a long carbon-fibre pole with a suction-cup tag at the end. He waited, and eventually, one of the whales surfaced at just the right spot for him to stick the tag on. It was a first.
For 18 hours, the tag stayed on and recorded pressure, temperature and acceleration. A few days later, the team managed to tag a second animal, for a further 8 hours of data.
And what data! During that time, the two animals managed 2,831 lunges over 649 dives. While they were feeding, they lunged around 100 times every hour. That’s just over half a minute to accelerate a five ton body through water, swallow the equivalent of a king-size bed, filter out all the food within it, and be ready to go again. And again. And again.
The team also found that the minkes did something that other rorquals don’t: they made long, shallow dives under large floes of floating ice to feed on the swarms of krill beneath. No other predator can exploit this resource so efficiently. Blue, fin and humpback whales are too big, and they feed in more open, ice-free waters. Penguins and seals can dive under the ice, but they’re limited to picking off one prey at a time. The minke, by combining manoeuvrability with lunge-feeding, can consume the food beneath the ice in bulk.
Their feeding strategy is similar in kind to that of a blue whale, but very different in degree. Since they’re smaller, they can’t engulf as much water, but they also expend less energy on lunging. For a blue, lunging is a massively draining affair that pays off only because it guarantees the world’s most calorific mouthfuls. For a minke, lunging costs barely more energy than steady swimming. So while blues make a few gigantic gulps, minkes can take small gulps at a much faster rate. And that provides a clue about how lunge-feeding evolved.
Goldbogen thinks that the strategy helped early small rorquals to eat more efficiently, by swallowing large amounts of prey at once. That allowed some of them to become truly enormous, by using the same behaviour to consume vast quantities of prey in the open ocean. Others, like the minke, kept to a small size to exploit patchier swarms that their giant cousins couldn’t reach. “As body size evolved to bigger size classes, you got this niche partitioning,” says Goldbogen. “That’s our leading hypothesis.”
The study is a small victory for the team, not least because “Japan is still whaling minkes in the Antarctic,” says Goldbogen. “They claim it’s for scientific purposes but I don’t think anyone was fooled by that claim. Here we show that you can study whales without killing them. That was obvious before but providing some published accounts is important too.”
There’s more to come. The team has just developed a new class of tag that has cameras as well as the usual sensors. They’ve deployed it on five humpbacks and two blues, and they’re about to try it on some minkes in the coming months. “It’s pretty much a full flight recorder on top of a whale,” says Goldbogen. “We’ve been having a lot of fun.”
Reference: Friedlander, Goldbogen, Nowacek, Read, Johnston & Gales. 2014. Feeding rates and under-ice foraging strategies of the smallest lunge filter feeder, the Antarctic minke whale (Balaenoptera bonaerensis). Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.106682