The blue whale, which uses baleen to filter its prey from ocean water and can reach lengths of over 100 feet, is the largest vertebrate animal that has ever lived. On the list of the planet’s most massive living creatures, the blue whale shares the top ranks with most other species of baleen whales alive today. According to new research from scientists at the University of Chicago, Stanford University and the Smithsonian’s National Museum of Natural History, however, it was only recently in whale’s evolutionary past that they became so enormous.
In a study reported May 24 in Proceedings of the Royal Society B, Graham Slater, assistant professor in geophysical sciences at the University of Chicago; Nicholas Pyenson, the museum’s curator of fossil marine mammals; and Jeremy Goldbogen at Stanford University traced the evolution of whale size through more than 30 million years of history and found that very large whales appeared along several branches of the family tree about 2 to 3 million years ago. Increasing ice sheets in the Northern Hemisphere during this period likely altered the way whales’ food was distributed in the oceans and enhanced the benefits of a large body size, the scientists say.
“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,” said Slater, a former Peter Buck postdoctoral fellow at the Smithsonian museum. “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.”
How and why whales got so big has remained a mystery until now, in part because of the challenges of interpreting an incomplete fossil record. “We haven’t had the right data,” Pyenson said. “How do you measure the total length of a whale that’s represented by a chunk of fossil?” Recently, however, Pyenson established that the width of a whale’s skull is a good indicator of its overall body size. With that advance, the time was right to address the long-standing question.