Big or Small: Scientists Discover How Mammals Evolve Their Body Sizes

Mammals vary enormously in size--from the tiny mouse to the enormous blue whale. Yet researchers haven't been able to pinpoint exactly why some animals evolve to be so large. Now, scientists have come up with a theory that may explain how certain groups are able to become gigantic while others remain tiny.

Mammals can broadly be placed into two categories. There are animals like rabbits that breed quickly and die young. Then there are animals like humans that mature slowly, live longer and are relatively older when they first have children. Building off of the observations of these two different "types" of mammals, the scientists developed a theory that proposed that "fast" species can actually evolve a larger size more quickly than slow species--and that their maximum size will be greater.

In order to test their theory, the scientists used the fossil records of mammals over the last 70 million years. More specifically, they looked at the maximum size of each mammal group throughout that time, including whales, elephants, rodents, seals and primates. They found that, in fact, it seemed that their theory was correct.

"Primates have evolved very slowly, and never got bigger than 1,000 pounds," said Jordan Okie, one of the researchers, in a news release. "The opposite was true of whales, which evolved their large size at the fastest rates recorded."

Baleen whales are actually part of the more productive clades, which represent the fast end of the slow-fast lifestyle continuum. In other words, whales have a high reproduction rate for their body size. Primates, in contrast, represent part of the less productive clades, which means that they have a relatively slow reproduction rate for their body size.

They didn't only examine the rate of size evolution, though. The new theory also makes predictions about the relative risks of extinction for large animals compared to small. The maximum size of an animal is limited by the rate of mortality in the population. Because larger animals tend to breed less frequently than smaller animals, if the mortality rate doubles, then the maximum size is predicted to be about 16 times smaller.

"This is a really surprising finding," said Alistair Evans, co-author of the new study, in a news release. "It points to another reason why many of the large animals went extinct after the last Ice Age, and their high risk of extinction in modern environments."

The findings reveal a little bit more about the differences among the main groups of mammals and make further predictions about how the change in body size affect evolutionary potential.

The findings are published in Proceedings of the Royal Society B: Biological Sciences.