- Nearly all mammals have gotten smaller -- except for the blue whale.
- Blue whales, the largest mammal, may be gaining heft thanks to ocean currents boosting the amount of krill around Antarctica.
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.
For example, it took 10,000 million generations for a mammal to get 5000 times bigger, and over twice as long to evolve form the size of a mouse to the size of elephant.
Elephants have been the largest land animal for the past 10 million years, and before that the record was held by a now-extinct rhino-like animal, says Evans.
The researchers believe one reason for this faster evolution in size among marine animals is that it's easier to grow bigger in the sea.
With the water holding you up, fewer body modifications are required to handle the increase in weight.
Interestingly the new study found that almost all mammals are smaller today than they were in the last major ice ages -- a million or so years ago.
Evans says this may be because the biggest animals have been hunted to extinction, or because the weather is warmer and there is less advantage to being big.
But the blue whales are an exception, he says. "It's continued to get bigger," says Evans.
He says ocean currents boosting the amount of krill around the Antarctica are likely to be responsible for this growth.
Co-author zoologist Dr Erich Fitzgerald of Museum Victoria says whales in general have continued to get larger and could theoretically continue to grow assuming they could get the food they require.
But he says the future is uncertain given such things as overfishing, which threatens whales' food source.
"Their maximum size may be peaking during our lifetime," says Fitzgerald.
Evans says there are certain advantages to increasing in size.
For example, your relative metabolic rate decreases with size which means you don't have to eat as much food per gram of your own tissue. "It's more efficient to be big," he says.
This means you can eat more abundant low energy-dense foods, like trees, leaves and grass.
And you can avoid being eaten by other animals, can store more energy and can travel further distances.
Small animals by contrast have a high metabolic rate and need to eat insects, seeds and fruit, which are less available.
But not all mammals got larger after the demise of the dinosaurs. Some, especially those isolated on islands, got smaller -- including now extinct dwarf mammoths off the coast of California and dwarf elephants in the Mediterranean.
Getting smaller can have advantages too, adds Evans, including helping animals to adapt to a smaller food resource.
Interestingly, these animals evolved smaller size much faster than those evolving larger size, the researchers found.