It’s often said that the ocean releases them from the constraints of life on land. It’s actually the opposite.
The first time I came face to face with a sea lion, I nearly screamed. I was snorkeling, and after a long time spent staring down at colorful corals, I looked up to see a gigantic bull, a couple of feet in front of my mask. Its eyes were opalescent. Its long canines hinted at its close evolutionary ties to land-based predators like bears and dogs. And most unnervingly of all, it was huge.
Mammals tend to get that way when they invade the ocean. The pinnipeds—seals, sea lions, and walruses—tend to be immense blobs of muscle and blubber. The same could be said for manatees and dugongs. And whales are almost synonymous with bigness. Time and again, lineages of furry mammals have gone for a swim and over evolutionary time, they’ve ballooned in size. Why?
Most of the explanations for this trend treat the ocean as a kind of release. The water partly frees mammalian bodies from the yoke of gravity, allowing them to evolve heavy bodies that they couldn’t possibly support on land. The water unshackles them from the constraints of territory, giving them massive areas over which to forage. The water liberates them from the slim pickings of a land-based diet and offer them vast swarms of plankton, crustaceans, and fish to gorge upon.
But William Gearty from Stanford University has a very different explanation. To him, the ocean makes mammals big not because it relieves them of limits, but because it imposes new ones.
“As you enter the water, you start to lose heat from your body that you aren’t losing on land or air,” he explains. To counteract that constant loss of heat, humans use wet suits, whales have blubber, and otters have thick fur. “But really the easiest way to counteract it is to get bigger,” Gearty says. As bodies balloon, volume increases faster than surface area does, so you produce more heat in your body but lose comparatively less of it from your skin. But animals can’t become infinitely big because larger bodies also demand more fuel.