It's easier to grow bigger in the sea with the water holding up your weight, say researchers.
Blue whales are the largest animal to have ever lived on Earth and -- for now -- are continuing to get bigger, say researchers.
The findings come out of a study by evolutionary biologist Alistair Evans, of Monash University in Melbourne, and colleagues.
"The biggest animal ever is potentially still getting bigger," says Evans, whose study is reported this week in Proceedings of the Royal Academy of Sciences.
When the dinosaurs and their marine cousins went extinct 65 million years ago, mammals took the opportunity to take advantage of the space these creatures had previously occupied, says Evans.
He and colleagues investigated the increase in the size of mammals since this time.
The study estimated the body size of hundreds of species in 28 different orders of animals in 20 time periods over the past 70 million years.
The researchers used teeth, skulls and limb bones to work out the size of the animal, based on comparisons with current day species.
The researchers found it took whales 5 million generations, or 30 million years, to go from 25 kilograms to 190 tons -- the weight of a blue whale.
By contrast land mammals got bigger half as quickly as marine mammals.
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