Some mammals need roughly 24 million generations to go from mouse-size to elephant-size, a new study says.

Using both fossil and living specimens, scientists calculated growth rates for 28 different mammalian groups during the past 65 million years—and found that, for mammals, getting big takes longer than shrinking.

It takes a minimum of 1.6 million generations for mammals to achieve a hundredfold increase in body size, about 5 million generations for a thousandfold increase, and about 10 million generations for a 5,000-fold increase, the team discovered.

For land mammals, odd-toed ungulates—such as horses and rhinos—displayed the fastest maximum rates. Curiously, primates showed the slowest rates among the mammals examined.

"It's a bit of a mystery," said study leader Alistair Evans, an evolutionary biologist at Australia's Monash University.

"It's a lot harder to make a big primate than it is to make a big rhino or elephant ... There could be many reasons for this, but staying a primate and getting big seems to be very difficult."

Among all mammals, cetaceans—the group that includes whales and dolphins—experienced the highest rate of body inflation, requiring only about three million generations for a thousandfold size increase.

Evans and his team speculate that difference is likely because their body weight is supported by water, which makes growing larger less challenging than on land.

That's because there are fewer constraints on a marine mammal evolving bigger. For instance, without the buoyancy of water, a whale's internal organs would be crushed by its own weight.

Incredible Shrinking Mammals

The study also found that mammals shrink up to 30 times faster than they increase.

"That's quite amazing that there'd be such a difference in the rate of decreasing in size compared to increasing," said Evans, whose study appears this week in the journal Proceedings of the National Academy of Sciences.

There could two main reasons for the fast shrinkage, the team says. First, increasing body size requires a lot of skeletal and muscular changes to support and move the increased weight, said study co-author James Brown of the University of New Mexico.

"As an organism gets bigger, they run up against different design constraints," Brown said.

"Dealing with those problems requires innovation. It requires new genes and new ways of reading the developmental program."

Secondly, Evans said, all mammals, regardless of their sizes, must grow from a single cell and go through a developmental phase when they are small. So, to shrink, all a mammal species has to do is cease development earlier.

"You're stopping the developmental program earlier, instead of trying to extend it," Evans said.

Mammal-Evolution Study Most Thorough Yet?

David Polly, a paleontologist at Indiana University, said the new study is one of the most comprehensive investigations to date of mammal body-size evolution.

"Measuring rates of evolution using fossils is a laborious task," said Polly, who was not involved in the study. "So while people have studied rates of evolution, it's usually on a pretty specific group of animals."

Polly added that the new findings also seem to confirm what many scientists suspected: Evolution of very large body sizes requires much more time than past studies had indicated. Previous research had focused on body-size changes of individual species over short geological time periods.

Evans and colleagues suspect their findings might also apply to other animal groups, such as dinosaurs.