Fifty million years ago, whales were four-footed, tailed land mammals about the size of wolves. Over the next 12 million years, these terrestrial whales evolved into fully aquatic animals, complete with oceanic adaptations like flippers and flukes. Today, all of the Earth’s largest mammals live in the sea, including blue whales, considered the biggest animal to have ever existed. For a long time, scientists believed these ocean mammals grew to their modern sizes because the ocean provided them with immense space and the ability to float, but a new study presents an entirely different line of reasoning.

According to a study published Monday in the Proceedings of the National Academy of Sciences, mammal growth is actually more constrained in water than on land. Size in the ocean is bound by hard limits, the scientists from Stanford and Louisiana Universities Marine Consortium explain: Mammals that are too small struggle to retain heat in the cold water, and those that are too large must capture enough food to live.

“Many people have viewed going into the water as more freeing for mammals, but what we’re seeing is that it’s actually more constraining,” co-author and Stanford geological sciences professor Jonathan Payne, Ph.D., explained in a statement released Monday. “It’s not that water allows you to be a big mammal, it’s that you have to be a big mammal in water — you don’t have any other options.”

marine mammals, ocean life
A marine mammal, the blue whale, is the largest creature in the ocean.

Payne and his team compiled data sets of the body masses for 3,859 living species and 2,999 fossil mammal species, paying attention to when the mammals turned aquatic and when they became their modern size. They determined a pattern: Once the land mammals became aquatic, they evolved very quickly to their new size until they reached a plateau and stalled growth.

The researchers’ reason that being large means being in charge is because larger mammals are better at retaining heat in water that’s lower than their body temperature. Metabolism, however, increases with size more than the animal’s actual ability to gather food, which puts a limit on how big they can get.

Baleen whales, for example, can be so huge because they spend less energy on feeding. Their mouth contains a filter-feeder system, which means that when a baleen whale opens its mouth, it scoops up water, then pushes it out through its bristle-like baleen. While the water is filtered away, the krill captured in the water remains as a meal. This method allows them to grow larger than mammals that use teeth to munch and capture prey, like orcas.

“The sperm whale seems to be the largest you can get without a new adaptation,” lead author and Stanford Ph.D. candidate Will Gearty explained in a statement. “The only way to get as big as a baleen whale is to completely change how you’re eating.”

Sperm Whale - Kaikorua - New ZealandFJ0A3401
Sperm whales are thought to represent an upper size limit for swimming mammals; anything bigger requires additional adaptations.

What sizes comes down to, the researchers contend, are the basic principles of physics and chemistry. Living in water imposes selective pressures on metabolic rates and size — to be a big boy of the sea you need to live a pretty chill existence so that you don’t waste any precious energy not maintaining your largeness.

There’s one exception, though. The only aquatic mammals that didn’t rapidly evolve to be huge once they entered the sea are the much more manageable-sized sea otters. The scientists theorize that’s because these water-loving mammals took to the ocean relatively recently and, per the study’s accompanying statement, “many otter species still live much of their lives on land.”

sea otter
Sea otters don't really follow the marine mammal size pattern.