Blue whales are famously the largest animals to have ever lived, exceeding even the great beasts of the Cretaceous. Giant aquatic mammals make it easy to conclude that living in water allows animals to grow larger, since there is less fighting gravity. However, a new study finds the limits in size on aquatic mammals are actually tighter than for their relatives on land, but the lower limit is more restrictive.

"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," Stanford Professor Jonathan Payne said in a statement. "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."

At the bottom end of the size scale, the problem is heat retention. Water is a much better conductor of heat than air. Small mammals and birds face the problem that their surface area is relatively large compared to their volume, so they lose heat more quickly than larger equivalents. To replace this heat, warm-blooded small creatures need more food, relative to their size, than larger ones, and the problem is magnified in water. Fur or blubber can only do so much.

On the other hand, whether on water or land, above a certain point an animal just can't find enough food to meet a really large body's demands. Sperm whales, the largest toothed whales, weigh a little less than the biggest titanosaurs did. "The range of viable sizes for mammals in the ocean is actually smaller than the range of viable sizes on land," Payne said.

Only baleen whales have broken this apparent maximum, likely because filter feeding requires less energy than chasing prey. For others, the upper limit is similar to on land.

Payne reached these conclusions by comparing the evolution of marine mammals with their nearest terrestrial relatives. Fortunately, there are plenty of study subjects, as seals are more closely related to dogs, manatees to elephants, and whales to hippos than any of them are to each other.

In the Proceedings of the National Academy of Sciences, Payne reports on the body masses of almost 4,000 living and 3,000 fossil mammal species. He found that when mammals start living in water, they evolve to meet the new conditions fairly quickly, most often growing or shrinking to reach a weight of around 500 kilograms (1,100 pounds). Otters represent a curious exception, perhaps because many still spend so much of their time on land.

It seems if you are a mammalian species entering the water, the message is; Get big or get out... but not too big.