Blue whales are the largest animals ever to have lived. But they also pose a quandary: How did they get so big? It turns out that their gigantic size may have been driven by the polar ice caps.

Reaching up to 200,000 kilograms (441,000 pounds) in weight and measuring in at 30 meters (nearly 100 feet) long, blue whales are huge. But why they are so big has confounded marine biologists for a long time. Some have suggested that the support from water enabled them to balloon, while others thought that maybe size was an adaptation due to massive mega-predators like megalodon.

While it has previously been suggested that whales evolved their gigantic size not long after they appeared in the oceans some 30 million years ago, the researchers found that this was probably not the case. Their giganticism is actually a relatively recent trait and was not present for most of their history, only having evolved within the last 4.5 million years and independently in multiple branches of the whale’s family tree, suggesting there was some sort of advantage to large size.

“We might imagine that whales just gradually got bigger over time, as if by chance, and perhaps that could explain how these whales became so massive,” explains Graham Slater, who co-authored the study published in Proceedings of the Royal Society B. “But our analyses show that this idea doesn't hold up – the only way that you can explain baleen whales becoming the giants they are today is if something changed in the recent past that created an incentive to be a giant and made it disadvantageous to be small.”

The researchers decided to have a look at what was going on in the oceans at the time when the whales had their growth spurt. They found that the shift in size corresponded with the start of the first Ice Age, in which the polar caps spread and glaciers grew.

Before the Ice Age, food in the oceans was plentiful but widely distributed. It is thought that after glaciers and sea ice grew, significant runoff from the ice washed nutrients into the oceans creating seasonal boons in food such as krill. This concentrated food in smaller areas particularly suited to large filter feeding animals such as baleen whales, which become more efficient at scooping up krill the larger their mouths get.

Not only that, but even though these seasonally available dense patches of food were farther apart, the gigantic sizes of the whales made it more efficient for them to travel thousands of kilometers from one pocket to the next, meaning that the bigger the whales were more likely to outcompete their smaller relatives.