Why Did Whales Get So Massive?
The answer is a tale of massive proportions
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Whales are curious creatures: Why did the animals get so big? When did they balloon to such massive proportions? And as Elizabeth Pennisi reports for Science, a new study has some surprising answers.
Researchers have long guessed at why baleen whales—a group of cetaceans that includes humpback, minke, right whales and others—grew so big. As Ed Yong writes for the Atlantic:
"We’re not short of possible answers. Some scientists have suggested that giant bodies were adaptations to the recent Ice Age: At a time of uncertain climate and unstable food supplies, bigger whales could store more fat, and their large bodies allowed them to more efficiently migrate in search of the best feeding grounds. Some pointed their fingers at competition between early baleen whales, forcing some members to become giant filter-feeders. Others said that whales became big to escape from titanic killers, like the megalodon shark, or the sperm whale Livyatan. Yet others have pointed to Cope’s rule—the tendency for groups of creatures to get bigger over evolutionary time."
But Nicholas Pyenson, a paleobiologist at Smithsonian's National Museum of Natural History, wanted to know more. Pyenson previously showed that the width of a whale’s skull is a good overall indicator of its length. So he and his team measured the skulls of 63 extinct whale species, including some of the earliest known baleen whales, which swam through the oceans some 30 million years ago. They also examined 13 specimen of modern whales. Using that data, the researchers were able to estimate changes in the sizes of the fossil whales over time. They published their results in the journal Proceedings of the Royal Society B.
It turns out, whales got fairly large and stayed that way for a while, reports Pennisi. They didn’t hit that massive growth spurt (growing to over 33 feet or longer) until 4.5 million years ago. Today, the creatures can measure up to some 100 feet long—at least two school busses in length.
“We think of [baleen whales] as being giants but if you consider this in the context of their 36m-year evolutionary history, they have only been giants for a 10th [of it],” Graham Slater, an author of the study and evolutionary biologist at the University of Chicago, tells Nicola Davis at The Guardian.
So why did whales swell up in the relatively recent past? As Davis reports, the recent advent of gigantism rules out the idea that megapredators, which were already on the way out at the time, caused the change. Instead, the scientists suggest that the whales morphed in response to an emerging global ice age.
Before glaciers covered the northern hemisphere, food resources were spread somewhat equally around the oceans. But when the ice appeared, seasonality emerged on the planet. During the warmer spring and summer, nutrient-rich melt water streamed from the ice caps of the coasts into the oceans, causing tiny sea creatures to blossom. And as the climate changed, Yong reports, new wind patterns kicked up spurring nutrient-rich waters to upwell from the ocean depths—a phenomena that often takes place near the coasts.
Baleen whales, which filter feed on tiny crustaceans like krill, were able to take advantage of these resources. And by growing large, the whales could migrate thousands of miles to munch on krill in other parts of the globe.
“They can travel from one feeding zone to the next very efficiently because their big size means their 'miles per gallon', their MPG, is very high,” Slater tells Jonathan Amos at the BBC. “And they seem to know precisely the right time to turn up at these feeding grounds.”
In fact, Yong reports, modern blue whales can filter 120 tons of water and slurp up half a million calories worth of krill in a single mouthful. Feats like that allowed the giant baleen whales to thrive as the oceans changed while smaller baleen whales went extinct.
But the results also suggest that the massive cetaceans might not fare well in the future as the climate changes once again, Annalisa Berta, a cetacean researcher from San Diego State University, tells Yong. “So what will happen to the baleen whales if there is less food available?” she says. “Will they adapt fast enough? It took millions of years for them to reach large size. Can they shrink in 100 years?”
Though the latest study has no answers for this whale of a question, the new data is a step toward figuring out these creatures' evolutionary past. But, Pyenson tells Yong, more fossils are needed to complete this tale of massive proportions.