Weighing in at 200,000 kilograms and stretching the length of a basketball court, the blue whale is the biggest animal that’s ever lived. Now, scientists have figured out why they and other baleen whales got so huge.

“It’s a cool study,” says Jakob Vinther, an evolutionary paleobologist at the University of Bristol in the United Kingdom. «I’m going to send it to my students.»

Biologists have long debated why some whales became the world’s biggest animals. Some have proposed that because water bears the animal’s weight, whales can move around more easily and gulp in enough food to sustain big appetites. Others have suggested that whales got big to fend off giant sharks and other megapredators.

Researchers have also argued about when these animals got so huge. In 2010, Graham Slater, an evolutionary biologist currently at the University of Chicago in Illinois, argued that cetaceans—a term that includes whales and dolphins—split into different-sized groups very early in their history, perhaps 30 million years ago. Dolphins remained the shrimps of the cetacean world, filter-feeding baleen whales became the giants, and predatory beaked whales stayed in the middle size-wise, with the descendants in those three groups sticking within those early established size ranges.

However, Nicholas Pyenson, a whale expert at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C., was skeptical. So a few years ago, the two decided to tap the museum’s vast cetacean fossil collection to settle the dispute. Pyenson had already surveyed living whale proportions and determined that the size of the whale correlated with the width of its cheek bones. So Pyenson measured or obtained these data from skulls of 63 extinct whale species and of 13 modern species and plotted them on a timeline that showed the whale family tree.

The data showed that whales didn’t get really big early on, as Slater had suggested. Nor did they gradually get big over time. Instead they become moderately large and stayed that way until about 4.5 million years ago, Slater, Pyenson, and Jeremy Goldbogen at Stanford University in Palo Alto, California, report today in the Proceedings of the Royal Society B. Then baleen whales went “from relatively big to ginormous,” Slater says. Blue whales today are 30 meters long, where until 4.5 million years ago, the biggest whales were 10 meters long.

“The idea … that the shift … occurred so recently—4.5 million years ago—is surprising,” says Felisa Smith, a paleoecologist at the University of New Mexico in Albuquerque, who was not involved in the work but studies body size evolution.

Next, Slater and his colleagues checked to see what was happening in the world at the time to cause the change. They found that the baleen whales’ growth spurt coincided with the beginning of the first ice ages. As glaciers expanded, spring and summer runoff poured nutrients into the coastal ocean, fueling explosive growth in krill and small animals the whales consumed, they speculate. Until that time, prey had been uniformly distributed and plentiful, but the climate change caused many fish and big sea animals to disappear, and productivity plummeted. That seasonal runoff created a new pattern of food availability: seasonal patches of very abundant food spaced far apart over the course of the year.

Goldbogen helped Slater and Pyenson understand how that change was important. He studies whale eating and diving, and his work indicates that the more concentrated the food, the more efficient the feeding, especially in whales with really, really big mouths. Furthermore, larger whales can travel faster between patches of prey. So baleen whales that got bigger, faster, thrived, whereas smaller species went extinct. And their giant size may have even promoted greater productivity, “by bringing up nutrients from deep waters as they dive and resurface,” suggests Geerat Vermeij, a paleobiologist at the University of California, Davis, who was not involved in the work.

This is not the first time researchers have decided that food has shaped whale evolution. But “the work helps us sort out the relative importance of different reasons why baleen whales got so big,” says Hans Thewissen, an anatomist who studies whales at Northeast Ohio Medical University in Rootstown and was not involved with the study. “They elegantly and explicitly analyzed the data.”

“The study provides support for when and how the largest whales evolved,” Smith agrees. However, although the timing of the shift seems to rule out that gigantism arose as a defense against megasharks and other predators prowling oceans at the time, she says, “I think there is enough uncertainty to remain somewhat skeptical” that the defense argument is dead.