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Why whales grew big

Nick Ut / AP

A new study shows how baleen whales, some of the largest animals ever known to exist on Earth, were able to get so big — and, in evolutionary terms, it's a pretty recent development.

The short answer, per NYT: Thanks to an ice age a few million years ago, whales took advantage of environmental changes to eat. A lot.

QuoteIt's like going from whales the size of minivans to longer than two school buses. Nick Pyenson, Smithsonian Insitution's National Museum of American History

How it happened: About 4.5 million years ago, ice sheets covered the Northern Hemisphere, causing large amounts of nutrients to pour into coastal waters at specific places and times. That spawned huge populations of baleen whales' favorite foods, plankton and krill. But the explosions of plankton and krill occurred pretty far apart, so only the largest whales could survive the distances between their pig-out sessions.

Bottom line: Add a few million years of evolution to the mix and blue whales are now the largest animals ever to exist on Earth, topping the scales at 380,000 pounds. "It became really advantageous if you're going to move long distances if you're big." Graham Slater, an evolutionary biologist from the University of Chicago, told NYT. In other words, Slater told NYT, if the groups of plankton and krill had been closer together the whales would have grown to a more natural, comfortable size — not the giants we see today.

Other theories: Cheng-Hsiu Tsai, from the National Museum of Nature and Science in Tokyo, told The Atlantic: "It seems to me that people tend to focus on external factors like environmental change and upwelling, but pay little attention to internal factors." For instance, some whales' skulls significantly change as they get older, while others stay the same shape, meaning some whales are genetically incapable of evolving into giant, prey-eating behemoths.

And even Pyenson acknowledged the skepticism: "We have a very spotty fossil record for baleen whales," he said.

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