By KER THAN - NATIONAL GEOGRAPHIC NEWS
Added: Sun, 05 Feb 2012 05:42:29 UTC
Large mammals such as the black rhino (pictured) take longer to evolve than do small mammals.
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.