Animal IQ

We Finally Know Why Whales Are The Biggest Animals Ever To Roam The Earth

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How much do you know about whales? You probably know that they're mammals, not fish. Maybe you know that most of them travel in pods. Perhaps you're aware that there are two main kinds, toothed and baleen. But one thing you certainly know about them is that they're big. Now, scientists finally have a pretty good idea of why that is.

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A Family Resemblance

Let's go back to the two main types of whales for a minute. Toothed whales, such as sperm whales and orcas, use their sharp, pointed teeth to catch and eat fish and other aquatic animals. Like dolphins and porpoises—the species that make up the majority of the toothed whales—most whales of these types are about the size of a human being, though the massive sperm whale reaches lengths in excess of 60 feet. But even that is small fry compared to baleen whales. The smallest of them reach about 20 feet long, while the largest, the blue whale, is the planet's biggest animal ever at 200 tons. What makes this kind of a head-scratcher is the baleen—instead of teeth, these whales have a fine, comb-like substance through which they filter seawater and subsist on the tiny krill within. It takes an awful lot of krill to keep 200 tons of whale happy, so why are the baleen whales the giant ones?

Fortunately, there are a lot of whale fossils to shine a light on the subject, and a 2017 paper published in the Proceedings of the Royal Society B used 140 of them to come to some fascinating conclusions. According to co-author Nick Pyenson at the Smithsonian National Museum of Natural History, the record shows that early baleen whales averaged between 16 and 32 feet long—pretty comparable in size to their toothy cousins. Tellingly, when they began to grow, they all grew at once. That is, the giant whales of today aren't all descended from a single lineage of giants, but from a variety of prehistoric baleen whales that each independently reached tremendous sizes. That suggests that there was some sort of external effect that drove this change, and that whatever it was, it didn't affect toothed whales in the same way. Are you thinking the answer might be in the krill? You'd be right.

Ice Krill Sundaes

The embiggening of the whale started about 4.5 million years ago, more than 2 million years before the first human ancestors climbed down from the trees. An ice age was freezing vast sheets of water on earth, freezing glaciers and icebergs in place and altering the currents of the ocean. As a result of the new water patterns, nutrients began to concentrate in certain areas of extremely cold water. In turn, the krill gathered in these cold spots, and the whales followed suit. Of course, that dense concentration meant that when the whales had eaten their fill, they'd have a long way to go until they got to their next meal. So it was the biggest whales, the ones with the largest fat reserves, that could make the journey. It just goes to show the importance of loading up before a big trip.

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