Whales are big so they can stay warm in cold water
Retaining body heat in the chilly ocean is the main reason why whales are so big, say scientists.
But there is a limit to how large marine mammals can grow, imposed by the need for energy and the challenge of finding enough food.
The study which, looked at 3,859 living and 2,999 extinct species, found that once land mammals take to the water they evolve very quickly to a larger size. Canine ancestors of seals grew faster than the hippo-like forbears of whales and dolphins.
Generally, bigger seemed to be better for aquatic life, but only up to a point.
Professor Jonathan Payne, from Stanford University in the US, said: "It's not that water allows you to be a big mammal, it's that you have to be a big mammal in water."
Larger size helped aquatic mammals retain heat in water colder than their own body temperature. "When you're very small, you lose heat back into the water so fast, there's no way to eat enough food to keep up," said Prof Payne.