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Mammal Evolution has a Speed Limit, says USU Biologist

Thursday, Feb. 02, 2012

USU biologist Morgan Ernest holding a kangaroo rat

USU biologist Morgan Ernest holds a kangaroo rat at the NSF-funded Portal, New Mexico Long-Term Ecological Research Project, where she is principal investigator. She has long studied the role of body size in structuring mammalian communities.

Alistair Evans of Monash University

Ernest's colleague Alistair Evans of Monash University displays the skulls of a mouse and an elephant. The two scientists and colleagues published findings on mammal evolution in the Jan. 31 issue of PNAS. Photo courtesy of Monash University.

For the first time, scientists have measured how fast large-scale evolution can occur in mammals. The answer? Twenty-four million generations for a mouse-sized animal to evolve to the size of an elephant.

Utah State University biologist Morgan Ernest is among the authors of the study published Jan. 31, 2012, in the Proceedings of the National Academy of Sciences. She and her colleagues describe increases and decreases in mammal size following the extinction of the dinosaurs 65 million years ago.

“The demise of dinosaurs provided vast evolutionary opportunities for mammals,” says Ernest, associate professor and co-director of graduate programs in USU’s Department of Biology. “It is well known in biology that size profoundly influences everything from how quickly a species reproduces to its vulnerability to extinction.”  

The research team of 20 biologists and paleontologists, led by Alistair Evans of Australia’s Monash University's School of Biological Sciences, discovered size rates decrease much faster than growth rates. It takes only 100,000 generations for very large decreases, leading to dwarfism, to occur.

Evans, an evolutionary biologist and Australian Research Fellow, says the study is unique because most previous work focused on microevolution, the small changes that occur within a species.

“Instead, we concentrated on large-scale changes in body size,” he says. “We can now show it took at least 24 million generations to make the proverbial mouse-to-elephant size change — a massive change, but also a very long time. A less dramatic change, such as rabbit-sized to elephant-sized, takes 10 million generations."

The study looked at 28 different groups of mammals, including elephants, primates and whales, from various continents and ocean basins over the past 70 million years. Size change was tracked in generations rather than years to allow meaningful comparison between species with differing life spans. 

Co-author Erich Fitzgerald, senior curator of vertebrate paleontology at Museum Victoria, says changes in whale size occurred at twice the rate of land mammals.

“This is probably because it’s easier to be big in the water — it helps support your weight,” he says.

Evans says he was surprised to find decreases in body size occurred more than ten times faster than the increases.

“The huge difference in rates for getting smaller and getting bigger is really astounding — we certainly never expected it could happen so fast,” he says.

Many miniature animals, such as the pygmy mammoth, dwarf hippo and ‘hobbit’ hominids lived on islands, helping to explain the size reduction.

“When you get smaller, you need less food and can reproduce faster, which are real advantages on small islands,” Evans says.

The research furthers understanding of conditions that allow certain mammals to thrive and grow bigger and circumstances that slow the pace of increase and potentially contribute to extinction.

Related links:

Contact: S.K. Morgan Ernest, 435-797-8751,

Source: Monash University

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