How long did it take mammals the size of mice to become as large as elephants? Is it easier to evolve larger or smaller body sizes? Research published today in the Proceedings of the National Academy of Sciences USA (PNAS) describes increases and decreases in mammal size following the extinction of the dinosaurs 65 million years ago.

The international team of scientists, including several at the University of New Mexico, found that it took about 24 million generations for terrestrial mammals to evolve from the size of a mouse to that of an elephant after the dinosaurs went extinct.

Getting smaller is easier: once large size was achieved, it only took about one hundred thousand generations for very large decreases, such as extreme dwarfism, to occur.

"Our work demonstrates, for the first time, how quickly the major changes in body size have happened in the history of mammals," says Alistair Evans, an evolutionary biologist at Monash University in Melbourne, Australia who was the lead author on the paper.

The research team looked at 28 different groups of mammals from the four largest continents (Africa, Eurasia, and North and South America) and all ocean basins during the last 70 million years. These groups included elephants, rhinos, hippos, carnivores and whales.

"In earlier work we looked at the overall body size trajectory and constraints operating on body size," said Felisa Smith, a co-author and paleoecologist at the University of New Mexico. "Here instead of chronological time, we use generations. This allowed us to compare the rates of change among very small and large mammals and ask whether it matters if you are getting bigger or smaller. It turns out it does."

Researchers were surprised to learn how quickly body size decreased: the rate is more than 10 times faster than the increases.

"The apparently different rates of dwarfing and gigantism are relevant to understanding the evolution of body size of domestic animals under artificial selection," said Jim Brown, a co-author also at the University of New Mexico. "Both dogs and horses exhibit much greater decreases than increases in size, and problems associated with large size in dogs (such as hip displasia, reduced life expectancy. etc.) are not seen - or at least nowhere near so severe - in small dogs."

The work was funded by a research coordination grant from the National Science Foundation to Smith and two colleagues: Kate Lyons at the Smithsonian and Morgan Ernest at Utah State University. "Without support from NSF, this work could never have happened" said Smith. "It took us years and a core group of 10 scientists to assemble the data we needed to address this question."

This research will help scientists to better understand mammal evolution: what conditions allow mammals to grow to bigger sizes and when smaller size is favored.

To read the complete paper, visit: The maximum rate of mammal evolution.

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