… and perhaps more importantly: when?
Five years ago, on a boat off the southern coast of Sri Lanka, I met the largest animal that exists or has ever existed.
The blue whale grows up to 110 feet in length. Its heart is the size of a small car. Its major artery is big enough that you could wedge a small child into it (although you probably shouldn’t). It’s an avatar of hugeness. And its size is evident if you ever get to see one up close. From the surface, I couldn’t make out the entire animal—just the top of its head as it exposed its blowhole and took a breath. But then, it dove. As its head tilted downwards, its arching back broke the surface of the water in a graceful roll. And it just kept going, and going, and going. By the time the huge tail finally broke the surface, an unreasonable amount of time had elapsed.
For scientists who study whales—and especially the sieve-mouthed baleen whales like the blue, fin, and humpback—it’s hard to escape the question of size. “They are big!” says Nick Pyenson at the Smithsonian Institution. “As an evolutionary biologist, you always have to wonder why.”
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 for Pyenson, the secret to really understanding why the baleen whales got so big is to really nail down when they got so big. And more importantly, when did they get really big? The sheer size of the baleen whales can distract us from the fact that some are much bigger than others—there’s a considerable difference between a 20-foot minke and a 100-foot blue. When did the latter titans emerge?
To find out, Pyenson teamed up with Graham Slater and Jeremy Goldbogen to collect data on the size of baleen whales, both past and present. For extinct species, they measured fossils. For living ones, they turned to museum specimens, records of beached whales, and even data from aboriginal harvests. They then mapped these measurements onto a family tree that unites all of these species, showing how they’re related and when each group evolved.
The trio found
that the really big baleen whales—those measuring 33 feet (10 meters) or more—only show up in the final act of the whale drama. When this group first appeared, around 35 million years ago, they were already big, but they only became Big with a capital B within the last 4.5 million years. What’s more, gigantic sizes evolved independently in at least three different lineages of baleen whales. One gave rise to the fins and humpbacks, a second led to the blues and seis, and a third produced the rights and bowheads.
The result of these repeated forays into gigantism is that “modern whales are dramatically larger than any whales in their evolutionary history,” says Pyenson. He has noticed this pattern for a long time, but his data show it plainly. Even the smallest living baleen whale—the amusingly named pygmy right whale—is 20 feet long, making it about the same size as average prehistoric whales. And it’s not just that the biggest members of the family have become bigger. Almost all the smallest species disappeared. The family as a whole ballooned in size.
Cope’s rule can’t explain this late-breaking increase. It’s also unlikely that factors like competition within the group or threats from huge predators or the advent of filter-feeding were responsible, since these were already part of whale life before they became giants. Instead, Pyenson says, “We think the ice ages had something to do with it.”
