The blue whale may be the largest animal to have ever existed, but new research has found that the species only claimed the title relatively recently. By comparing the bones of modern whales to fossils from extinct species, a team of scientists from Stanford, the University of Chicago and the Smithsonian has traced the growth spurt of baleen whales to about 4.5 million years ago, when climate change increased the food supply.
For most of their 30-million year evolutionary history, baleen whales have been relatively small, capping out at about 10 m (33 ft) in length. The researchers found that whales as big as today's behemoths, which can measure up to 30 m (100 ft) in the case of the blue whale, began appearing fairly rapidly about 3 million years ago, and they wanted to know why.
The first challenge was to find a way to accurately determine the size of extinct species, given the somewhat patchy fossil record. Working with more complete modern skeletons, the researchers found that the width of a whale's skull could be a good starting point to calculate the length of its body. Using this system, they examined skulls from the Smithsonian's collection and worked out the length of 63 extinct whale species.
According to the team's findings, the uptick in whale sizes really took off about 4.5 million years ago. Around that time, something shifted in the ecosystem that drove a two-pronged path towards gigantism, causing bigger animals to start appearing in different lineages, while the smaller species started dying off.
"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," says Graham Slater, co-author of the study. "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 change seems to line up with the beginning of an Ice Age, and the researchers suggest that the changing climate would have benefitted baleen whales' feeding habits of filtering krill out of water. As new ice caps formed, nutrients would have seasonally been washed into coastal waters, increasing the food supply. According to previous research on modern whales, filter-feeding is even more efficient in larger species, so nature would have favored the bigger baleens at the time.
As an added bonus, the larger animals would be better equipped for thousand-mile migrations, to take advantage of seasonal shifts in the food supply.
"An animal's size determines so much about its ecological role," says Nicholas Pyenson, co-author of the study. "Our research sheds light on why today's oceans and climate can support Earth's most massive vertebrates. But today's oceans and climate are changing at geological scales in the course of human lifetimes. With these rapid changes, does the ocean have the capacity to sustain several billion people and the world's largest whales? The clues to answer this question lie in our ability to learn from Earth's deep past — the crucible of our present world — embedded in the fossil record."
The study was published in the journal Proceedings of the Royal Society B.