The blue whale, which uses baleen to filter its quarry from ocean water and may reach lengths of over 100 feet, is the greatest vertebrate animal that’s ever lived. On the record of the most massive living creatures in the planet’s, the blue whale shares the top ranks with most other species of baleen whales alive now. Based on new research from scientists at the Smithsonian’s National Museum of Natural History, nevertheless, 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, Nicholas Pyenson, the museum’s curator of fossil marine mammals, and collaborators Graham Slater at the University of Chicago and Jeremy Goldbogen at Stanford University, traced the evolution of whale size through more than 30 million years of history and found that quite large whales appeared along several branches of the family tree about 2 to 3 million years ago. Increasing ice sheets in the Northern Hemisphere in this period likely altered the manner of the whales’ food and enriched the benefits of a big body size that was distributed in the oceans, the scientists say.
How and why whales got so huge remained a puzzle until now, in part because of the challenges of interpreting an incomplete fossil record, how and why whales got so large. “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?” Lately, however, Pyenson established that the width of a whale’s skull is a good indicator of its own entire body size. With that improvement, the time was appropriate to address the long standing question.
The Smithsonian holds the greatest and richest skull groups for both living and extinct baleen whales, and the museum was among the few places that housed a collection which could provide the raw data needed to analyze the evolutionary relationships between whales of different sizes. Pyenson and his colleagues measured a wide range of fossil skulls from the National Museum of Natural History’s collections and used those measurements, together with published data on additional specimens, to estimate the span of 63 extinct whale species. The fossils included in the investigation represented species dating back to the earliest baleen whales, which lived more than 30 million years ago. The team used the fossil data, together with data on 13 species of modern whales, to examine the evolutionary connections between whales of different sizes. Their data clearly showed that the large whales that exist today were not present for most of whales’ history. “We live in a time of giants,” Goldbogen said. “Baleen whales have never been this big, ever.”
The research team tracked the discrepancy back to a shift in the manner in which body size evolved that occurred about 4.5 million years ago. Not only did whales with bodies longer than ten meters (approximately 33 feet) begin to evolve around this time, but smaller species of whales also began to evaporate. Pyenson notes that larger whales appeared in several different lineages around the same time, indicating that immense size was somehow advantageous during that timeframe.
“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 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.”
This evolutionary shift, which took place at the beginning of the Ice Ages, corresponds to climatic changes that would have reshaped whales’ food supply on the planet ‘s oceans. Before ice sheets started to cover the Northern Hemisphere, food resources would have been pretty equally dispersed through the oceans, Pyenson said. However, while glaciation started, run off from the new ice caps would have washed nutrients into coastal waters at particular times of the year, seasonally boosting food supplies.
At that time of this transition, baleen whales, which filter small quarry, like krill, out of seawater, were well equipped to take advantage of these dense patches of food. Goldbogen, whose studies of contemporary whale foraging behavior have attested that filter-feeding is particularly efficient when whales have access to very compact aggregations of quarry, said the foraging strategy becomes much more efficient as body size increases.
What is more, big whales can migrate tens of thousands of miles to take good advantage of seasonally abundant food supplies. Thus, the scientists said, baleen whales’ filter-feeding systems, which evolved about 30 million years ago, seem to have set the stage for important size rises once rich sources of quarry became focused specifically places and times of the year.
“An animal’s size determines so much about its ecological role,” Pyenson said. “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.”
Funds for this particular study was provided by the Remington Kellogg Fund of the Smithsonian and with support from the Basis Foundation.