Scientists reveal secret of humongous mammals

The largest primate — the group to which humans belong — was Gigantopithecus blacki, an extinct ape that weighed about 1,100 lbs. (500 kg). As impressive as that might look, primates showed the slowest rate of size increase of any group; Evans is not sure what's behind the slow rate.

"There seems to be some intrinsic maximum rate that each order evolves at, which may have something to do with the basic construction or physiology of each group," he wrote. "So it may be really hard to be built like a primate and get very big."

Things can get smaller much faster than they can get big, they also found. Mammals can shrink at more than 10 times the rate at which they get bigger, and among animals living in isolated environments, primarily on islands, the decrease in size can be even more rapid.

For example, dwarf elephants that once inhabited islands in the Mediterranean Sea weighed about 220 lbs. (100 kg). They are believed to be descended from larger European elephants, weighing 100 times as much, which lived on mainland Europe. This decrease happened in less than 800,000 years, much faster than any rate of increase over the last 70 million years, Evans said.

The research was published Monday (Jan. 30) in the journal Proceedings of the National Academy of Sciences.

You can follow LiveScience senior writer Wynne Parry on Twitter @Wynne_Parry. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

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