Listen, I’m not going to sit here and tell you that baleen whales are big. You know that. In fact, I bet you know that at 300,000 pounds, the blue whale is the heaviest creature to ever exist. But I bet you don’t know why such filter-feeding whales are so big—because not even scientists know that.
Today, though, one group of researchers offers a fascinating theory in Proceedings of the Royal Society B: Fusing the fossil record and phylogenetic work (that is, determining the relatedness of species to one another), they found that baleen whales probably got colossal just 3 million years ago—a sliver of time in the grand evolutionary scheme of things—and climate change probably triggered the transformation. That, of course, carries troubling implications for how the giants might fare as Earth’s oceans warm and acidify.
Baleen whales—which gulp massive amounts of water and filter out tiny critters like krill—have been around for about 30 million years. But for the vast majority of that time, they were modestly proportioned, topping out at 30 feet or so, or about one-third the length of the modern blue whale. Now, being small offered some perks. For one, it’s easier to open your maw when it’s smaller.
But at the beginning of the ice ages some 3 million years ago, the oceanic ecosystem transformed on a grand scale. Ice sheets expanded in the north, and runoff dragged nutrients into the sea. An increase in wind-driven upwelling brought even more nutrients up from the depths—as wind blows away the top bit of the water, deeper waters rush up to replace it. This resulted in blooms of plant-like phytoplankton, leading to blooms of zooplankton like krill that feed on the phytoplankton. Whale food began concentrating in certain places at certain times of year.
Being big would have been an advantage for whales in the midst of this climatic transformation in a number of ways (the trait evolved independently in different baleen whale lineages, strongly suggesting it gave them an edge). For one, bigger whales reoriented within the food chain, growing so large that they put themselves out of reach of predators. And a bigger mouth more efficiently gulps zooplankton. If you’ve got more fat reserves, you can make longer migrations to bounce around intercepting these seasonal events, while smaller whales are stuck with more modest concentrations of plankton in their usual foraging waters.
“We think that kind of ecological explanation underpins why we see this shift in body size,” says paleobiologist and study co-author Nick Pyenson of the Smithsonian. “Not just the origin of the very big, but also the extinction of very small baleen whales, which today are only represented in that lineage by the pygmy right whale.” Smaller whales just couldn’t compete in this new world order, but why the pygmy right whale escaped extinction remains a mystery.
But couldn’t whales have grown so large because, well, they just could? Floating in water is a whole lot different than struggling with a big body on land. “But that doesn’t hold up when you look at the data,” says Pyenson. “It’s not true that as soon as whales got into the water they got very, very big—as big as a blue whale.” After all, baleen whales lived in the water for tens of millions of years before growing into giants.
What this research makes clear is that this is an extraordinary moment in the natural history of Earth. Think about what it takes to sustain not just blue whales, but other baleen giants like gray whales. They fuel themselves for journeys of thousands upon thousands of miles. “There has not been another time when there has been so much large predator biomass reliant on such productive oceans,” says vertebrate paleontologist Erich Fitzgerald, who wasn’t involved in the study. “There is no analog in history. We can go back right to the end of the age of dinosaurs, if you like, 66 million years ago, and these last 3 million years are unique.”
Humanity may be threatening to bring it all crashing down. Minute changes in ocean temperature can have massive cascading effects on ecosystems. If currents change or cold upwelling waters stop surfacing nutrients, phytoplankton don’t bloom and zooplankton don’t feed. When zooplankton don’t feed, neither do baleen whales. If blooms shift to other parts of the sea and the whales can’t find them, that size advantage suddenly becomes a burden.
Climate change made the giants, and while it’s still too early to say exactly how it’ll affect them in the coming decades, they’re certainly in a precarious situation. “One thing we can say though from the fossil record,” says Fitzgerald, “is where you tend to have specialists, and they can be small or they can be titanic in proportion like the blue whale, they are really—and this is a phrase used in the paper—they are really on an evolutionary knife-edge.”
So sure, science knows that climate change made big whales big. Now the challenge is making sure it isn’t climate change that does them in.