Whales are big. Really big. Enormously big. Tremendously big.
Fin whales can be 63 tonnes (63,000kg). Bowhead whales tip the scales at 90 tonnes. And the big mama of them all, the blue whale, can reach a whopping 170 tonnes, making it the largest animal to have ever lived. But for as long as whales have awed us with their great size, people have wondered how they became so colossal.
In a study published this week in the journal Proceedings of the Royal Society B, a team of researchers investigated gigantism in baleen whales, the filter-feeding leviathans that include blue whales, bowhead whales and fin whales. The marine mammals became jumbo-size relatively recently, they found, only within the past 4.5 million years. The cause? A climatic change that allowed the behemoths to binge-eat.
Whales have an interesting evolutionary history. They began as land-dwelling, hoofed mammals some 50 million years ago. Over several millions of years they developed fins and became marine creatures. Between about 20 million and 30 million years ago, some of these ancient whales developed the ability to filter-feed, which meant they could swallow swarms of tiny prey in a single gargantuan gulp. But even with this feeding ability, whales remained only moderately large for millions of years.
"But then all of a sudden – boom – we see them get very big, like blue whales," said Nick Pyenson, the curator of fossil marine mammals at the Smithsonian Institution's National Museum of Natural History and an author of the paper. "It's like going from whales the size of minivans to longer than two school buses."
Becoming giants
Pyenson and his colleagues measured more than 140 museum specimens of fossilised whales, and then plugged that data into a statistical model. It showed that several distinct lineages of baleen whales became giants around the same time, independently of one another. Starting about 4.5 million years ago, giant blue whales were popping up in oceans across the world alongside giant bowhead whales and giant fin whales.
The researchers suspected that an environmental change happened during that time that essentially caused the baleen whales to bulk up. After some investigation, they found that this time period coincided with the early beginnings of when ice sheets increasingly covered the Northern Hemisphere.
Runoff from the glaciers would have washed nutrients like iron into coastal waters and intense seasonal upwelling cycles would have caused cold water from deep below to rise, bringing organic material toward the surface. Together these ecological effects brought large amounts of nutrients into the water at specific times and places, which had a cascading effect on the ocean’s food web.
Throngs of zooplankton and krill would gather to feast on the nutrients. They would form dense patches that could stretch many miles long and wide and be more than 65 feet thick. The oceans became the whales' giant all-you-can-eat buffets. "Even though they had the anatomical machinery to filter-feed for a long, long time," said Jeremy Goldbogen, a comparative physiologist from Stanford University and author of the paper, "it wasn't until the ocean provided these patchy resources that it made bulk filter-feeding so efficient".
The baleen whales could now gulp down much larger amounts of prey, which allowed them to get bigger. But that was only part of the equation. "Plentiful food everywhere isn't going to get you giant whales," said Graham Slater, an evolutionary biologist at the University of Chicago and the study's lead author. "They have to be separated by big distances."
Migration
Because the ecological cycles that fuel the explosions of krill and zooplankton occur seasonally, Slater said the whales must migrate thousands of miles from food patch to food patch. Bigger whale ancestors that had bigger fuel tanks had a better chance of surviving the long seasonal migrations to feed, while smaller baleen whales became extinct.
If the food patches were not far apart, Slater said, the whales would have grown to a certain body size that was comfortable for that environment, but they would not be the giants we see today. “A blue whale is able to move so much further using so much less energy than a small-bodied whale,” Slater said. “It became really advantageous if you’re going to move long distances if you’re big.”
Ari Friedlaender, a behavioural ecologist at Oregon State University who was not involved in the study, said the research improved our understanding of how baleen whales became giants. "What this does is it allows us to be able to say that there are crucial processes in the ocean that allowed these animals to get this big," he said.
Richard Norris, a paleobiologist at the Scripps Institution of Oceanography, called the study a "nice piece of work" and said that it confirmed scientists' current understanding of changes to the oceans over time.
“When we think about what the planet has been like in its long history, a whale of 10 million years ago was a very different type of critter than we have now,” Mr Norris said. “So in a sense we live in a special time where we get to enjoy the majesty of really big animals out there in the ocean.”
New York Times