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.