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The blue whale is the largest animal that’s ever existed, heavier than even the biggest of dinosaurs. But scientists have debated why whales are so huge for a long time.

The prevailing idea is that if you live in water, buoyancy allows you to get bigger than if you lived on land. But it might not be quite that simple. Marine mammals may actually have more constraints on their body size than land mammals, and scientists now think they have to be big to conserve energy.

Land mammals have slipped back into the water four separate times. Whales evolved from ancient hippo-like mammals; seals from a dog-like ancestor. Manatees and dugongs are a branch of the elephant family tree, and sea otters are basically just really big weasels.

And until 2018, most scientists thought that their buoyancy in water gave marine mammals the freedom to grow larger, because it kind of literally lifted the weight from their skeletons. But when researchers looked for patterns of change in body size in almost 4,000 living and 3,000 fossil mammals, they found something more complicated. In 3 out of the 4 marine mammal groups (whales, seals, and manatees) species converged on the same size: an average of around 500 kg or 1100 pounds.

And when they tested various hypotheses to explain that, they found while buoyancy might allow marine mammals to get big it’s not the reason they grow. That probably has more to do with keeping warm. Ocean waters are colder than most mammals’ ideal body temperature, and water is better than air at conducting heat away from you.

Since larger and rounder bodies have less surface area in relation to their volume, they lose heat less rapidly even in chilly seas. You’d think that means marine mammals could just keep growing bigger and bigger, but the researchers found that if they get too big, there’s no way they could eat enough to feed all those cells. 500 kg seems to be just the right size to stay warm but still be able to eat enough to maintain a huge body. But there are outliers.

Baleen whales like blue whales managed to hack the equation a bit when they developed those fibrous baleen bristles, because filter feeding is a very efficient way to get lots of calories while spending very little energy. And sea otters were the oddball that didn’t converge on that 500kg size. They’re bigger than their weasel ancestors, but not as big as other marine mammals.

That might be because there’s something special about weasels and their relatives that limits their size, or allows them to stay trim. Or, it’s possible sea otters are still growing, and that milennia from now, they’ll be huge. Wait, would sea otters the size of polar bears still be ridiculously cute, or utterly terrifying?

I’m going with terrifying. Anyhow, the findings have got scientists rethinking what they thought they knew about the life in the seas. And they just go to show that if you keep asking good questions, you might find the answers aren’t as simple as you thought—but they’re also much cooler.

Thanks for watching this episode of SciShow, a Complexly Production. If you find marine mammals fascinating, you might like the episode of our sister channel, Eons, where they delve into how whales made the transition from land to sea. [♪OUTRO].