The Origin of Big, Carl Zimmer

On this happy anniversary–the 150th anniversary of the Origin of Species–let us contemplate one of evolution’s great works: the origin of giants.

Whales are the biggest animals to ever live. Blue whales can get up to 160 tons–about as heavy as 2000 grown men. They are trailed in the rankings by the fin whale and a few other related species of whales. There are no lobsters in their ranks, no clams, no rodents. All these giants feed in much the same way. They swallow up water and filter it through fronds in their mouths called baleen. Most of the food they eat is tiny stuff, like krill and other small invertebrates. So some scientists have wondered how big whales manage to put enough tiny bits of food in their bodies to get to such huge sizes.

Unfortunately, whales dine out of sight, so scientists have had to tackle those questions with indirect clues. Jeremy Goldbogen, a biologist now at the University of British Columbia, has gathered all the clues he can, from data recorders carried by diving fin whales to video of baleen whales feeding near the ocean surface. To make sense of that data, he has worked with zoologist Robert Shadwick and ad paleontologist Nick Pyenson, also of UBC, as well as Jean Potvin, a physicist whose speciality is parachutes.

Yes. Parachutes. Let me explain.

Potvin (left, with an accelerometer on his head) has been helping the biologists build a sophisticated physical model of a feeding fin whale. They set out to build a model that would produce the kinds of behavior that real whales do. For example, a fin whale will dive hundreds of feet down in search of food. Once it gets deep enough, it speeds up dramatically, and then abruptly slows down, almost stopping. Yet even as it slows, its tail is still moving up and down, generating tremendous thrust. Then, about half a minute later, it speeds up and slows down again. What’s going on?

According to the scientists, this pattern occurs when the whales lunge into a cloud of krill and drop open their jaws. Pleats under the lower jaw open up, engulfing huge amounts of water. The whale slows down because of the drag. It behaves, in other words, a lot like a parachute.

I first wrote about this whale work two years ago for the New York Times when Goldbogen and his colleagues published some of their first results. Since then, Potvin has brought his expertise on parachute physics to these parachuting whales. He and the other scientists have developed a sophisticated new model that tracks the incoming water more carefully. It’s a lot of water, the scientists have found: in one lunge, a fin whale can momentarily double its weight.

If a whale simply let the water come rushing in, there would be a tremendous collision–more than a whale could handle. Instead, the scientists argue, the whales actively cradle their titanic gulp. As the water rushes in, the whales contract muscles in their lower jaw. The water slows down and then reverses direction, so that it’s moving with the whale. (It just so happens that fin whales do have sheets of muscle and pressure-sensing nerve endings in their lower jaw. Before now, nobody quite knew before what they were for.) Once the water is moving forward inside the whale it can then close its mouth and give an extra squeeze to filter the water through its baleen.

This bizarre strategy may be the secret to the huge size of some whales. A fin whale can get 20 pounds of krill in a single gulp, but it can gulp every 30 seconds. Because krill live in gigantic swarms, they can keep gulping and get enough food in four hours to fuel their bodies for an entire day.

In order to make lunge-feeding work, you have to have a really big mouth to capture enough water in one gulp. But in order to have a big mouth, you need a big body. And in order to keep that big body running, you need to get a lot of food. And in the very act of getting that food–diving deep, lunging open-mouthed, and then pushing a school-bus-sized volume of water forwards–requires a lot of energy on its own.

Goldbogen and his colleagues wondered what sort of trade-off lunge-feeding whales faced between the costs and the benefits of eating like a parachute. To find out, they took advantage of measurements scientists made of hundreds of fin whales at whaling stations in the 1920s. Goldbogen punched the measurements into a computer and then analyzed the proportions of fin whale bodies of different sizes. From those calculations, the scientists also figured out how much water whales of different sizes could engulf.

It’s a general rule among living things that big organisms are not just magnified versions of little ones. The proportions of parts changes along the way. In some cases, these changes are the result of natural selection acting on different physical forces. For example, an elephant can’t stand on the delicate legs of a horse. They need legs shaped like stout pedestals to hold their bodies up against gravity. In other cases, the different proportions are just a result of development–some parts grow faster than others. This scaling exists not just between species, but within a single species. For example, a human pygmy is, in some ways, a scaled down version of a taller person. But their heads do not scale down the same way, because their brains are still about the same size as a taller person’s.

As they report today in the Proceedings of the Royal Society, Goldbogen and his colleagues found that big fin whales are not just scaled-up versions of little fin whales. Instead, as their bodies get bigger, their mouths get much bigger. Small fin whales can swallow up about 90% of their own body weight. Very big ones can gulp 160%. In other words, big fin whales need more and more energy to handle the bigger slugs of water they gulp. As their body increases in size, the energy their bodies demand rises faster than the extra energy they can get from their food.

This scaling may explain some of the weird diving patterns found in lunge-feeding whales. Blue whales are twice as big as humpback whales, for example, but both species dive for the same period of time (about eight minutes) and to the same depth (148 meters). All things being equal, you’d expect that blue whales would be able to dive deeper and longer, because they could store more oxygen in their bigger bodies. Blue whales also make fewer lunges than humpback whales (6 versus 15). It’s possible that the gigantic blue whales are hard up against a size limit. They need so much energy for their lunges that they can’t afford to hold their breath longer, and they can only manage to make a few lunges before they run out of reserves and have to head for the surface.

If the scientists are right, they may have discovered one of the big ironies in evolution. Lunge-feeding may have allowed whales to become the biggest animals ever to roam the planet. But this was not an open-ended invitation. Once whales got large enough, lunge feeding itself became so costly it prevented them from getting any bigger. Perhaps some day another animal will evolve a new strategy that will let it get even bigger than a blue whale. But for the animal kingdom as we know it, we may be sharing the planet with the biggest species it can offer.

Photo credits: Blue whale, Wikipedia; Potvin, Gary Peek; lunge diagram, Nick Pyenson]

[Update, 11/25: Did I say 200? One of my zeroes walked away when I wasn’t looking. Also, I’ve fixed the link to the new paper and added photo credits.]