Scientists think they've figured out why the biggest whales—those of the baleen variety, including blue whales—got so big. As they explain in a study in Proceedings of the Royal Society B, the researchers found that these behemoths didn't really become behemoths until about 4.5 million years ago. Their theory is that climate changes around that time created the perfect feeding conditions in the ocean for them to thrive. Specifically, glacial sheets forming in the northern hemisphere led to nutrient-rich runoffs in coastal waters, reports NPR. In addition, upwelling currents brought even more nutrients up from the depths, resulting in great patches of krill and fish (whale prey) gorging on them, notes the Atlantic. Another factor played a role: Whales had to travel great distances to feed on these easy meals, and larger baleen ones thus had an advantage over their smaller cousins.
“All of a sudden—'boom'—we see them get very big, like blue whales,” Nick Pyenson of the Smithsonian Institution’s National Museum of Natural History tells the New York Times. “It’s like going from whales the size of minivans to longer than two school buses.” Blue whales and their ilk had even more going for them. Because of their baleens—mouth filters made up of the same stuff as human fingernails—the whales could take big gulps and spit water back out without losing any fish in the process. Researchers believe this is why fossils show whales of about 30 feet in length began to balloon in size around this time, while smaller whales disappeared. Over a few million years, large whales increased from 10 tons in mass to 100 tons. (Humpback whales are surprising researchers.)