How blue whales became giants of the sea

Blue whales can grow up to 30m in length and weigh 100 tonnes.

Blue whales are the most massive animals to exist in the history of animals. Dreadnoughtus and those other thundering, 60-tonne dinosaurs? Bantamweights next to one of today's 100-tonne Balaenoptera musculus.

"We truly live in an age of giants," said Nicholas D. Pyenson, an expert in the paleobiology of marine mammals at the Smithsonian's National Museum of Natural History. Blue whales, he said, can grow as long as three city buses parked end to end. Living blue whales would be even bigger, too, if it weren't for the sailors who killed most of the 33m, 110-tonne specimens 100 years ago.

Yet evolutionarily speaking, whales are recent leviathans. After the largest dinosaurs died off, land mammals bulked up, leading to elephant-size rhinoceroses, sloths and armadillos about 35 million years ago. The ancestors of today's giant whales, meanwhile, stayed curiously small.

"It is only since around the beginning of the so-called ice ages that whales have not just evolved to be huge, but titanic in size," Erich M.G. Fitzgerald, a vertebrate paleontologist at Museum Victoria in Australia, said in an email. "Most baleen whales that ever lived were little fellows compared to their modern descendants."

To Pyenson and other paleontologists, what exactly jump-started the age of aquatic giants remained a mystery.

"It's such an obvious question," he said. "If you're an evolutionary biologist or a paleobiologist, you want to know how it came to be that way."

Relying on the extensive collection of fossil whale skulls at the Smithsonian, Pyenson and his colleagues tracked the evolution of baleen whale size. (Baleen whales lack teeth, instead using the moustache-like bristles that hang in their mouths to scoop up krill, fish and other tiny sea creatures.) From the skull sizes, the scientists could estimate the body lengths of about 60 species of modern and extinct whales.

Fossils of the first baleen whales appeared about 25 to 20 million years ago. For millions of years afterward, most baleen whales remained about 4.5m long. But 3 million years in the past, the scientists reported this week in the Proceedings of the Royal Society B, baleen whales underwent a dramatic size shift. The smallest baleen whales vanished. The others grew to double or triple the size.

Paleobiologist Jorge Velez-Juarbe, a marine mammal curator at the Natural History Museum of Los Angeles County who was not involved with the new study, said that experts previously proposed baleen whales grew large as a way to avoid being eaten. But baleen whales didn't outgrow such carnivores until millions of years after predatory sperm whales appeared in the fossil record.

In the new study, Pyenson and his co-authors note that shoreline ecosystems changed about 3 million years ago, around the same time that baleen whales got huge. First, seasonal windy upwellings began to kick up nutrients along the coast; later, glacial runoff added to the richness of these pockets. The net results were souped-up marine ecosystems where whales could feast.

It was not that the ocean necessarily had more krill or other whale food. But the food was more densely packed.

Pyenson likened the seasonal upwellings to a bag of marbles dumped in a corner of a room, rather than the balls careening across the floor. When the marbles are clustered, it's easier to hoover them up in a single go instead of plucking the marbles one by one.

For suspension feeders such as baleen whales, "the bigger you get, the more efficient you are", Pyenson said. "It maximises the return on density of prey."

"A large size allows them to take a giant bite," Velez-Juarbe said in an email, "as well as makes it easier to migrate between areas where food is concentrated."

Because the increased marine productivity changed with the seasons, the giants would have to swim vast distances to find new pockets. Gigantism may have allowed large baleen whales to outcompete their smaller cousins.