If you’ve ever wondered whether mammalian evolution has a speed limit, here’s a number for you: 24 million.

That’s how many generations a new study estimates it would take to go from mouse- to elephant-sized while operating on land at the maximum velocity of change. The figure underscores just how special a trait sheer bigness can be.

“Big animals represent the accumulation of evolutionary change, and change takes time,” said evolutionary biologist Alistair Evans of Australia’s Monash University.

Evans and co-authors revisit a fossil record dataset of mammal body size during the last 70 million years, in a study published Jan. 31 in Proceedings of the National Academy of Sciences. The data was originally used to describe the evolutionary growth spurts experienced by mammals soon after dinosaurs ceased to be Earth’s dominant animals.

For the previous 140 million years, mammals had been rat-sized or smaller. With dinosaurs significantly reduced, mammals had a chance to fill newly vacant ecological niches, particularly that of the large-bodied plant-eater.

In this context, size isn’t simply a visible sign of change, but a proxy for modifications to diet, metabolism and body structure. To become big is to change, radically and fundamentally.

“How fast can all of these interconnected changes be made? This to me is the main question that drives why maximum evolutionary rates are fascinating,” said Evans.

In the new study, Evans’ team measures the time taken, in total years and likely number of generations, for 28 mammal lineages to become larger and smaller over the fossil record.

Odd-toed ungulates, including horses and rhinoceroses, had the highest maximum rates of growth. (The largest land mammal ever, the now-extinct Paraceratherium, was part of this group.) Rodents placed in the middle of the pack, while carnivores changed quite slowly, and primates even more slowly.

At the fastest observed terrestrial rates, going from rabbit- to elephant-sized takes roughly 10 million generations, while the aforementioned mouse- to elephant-sized jump takes 24 million generations. In the oceans, however, body size could change twice as fast, perhaps because water’s support of body weight lessened physiological constraints.

The researchers also found that mammals shrink more rapidly than they grow, with size lost 100 times faster than it’s gained. An implicit conservation message: Treasure bigness, because it’s difficult to achieve, and won’t likely happen again so long as humans remain Earth’s dominant species.

“Very large land mammals need a huge area to be able to source enough food,” said Evans, and there just isn’t enough remaining land. It’s likely that “animals will not get enough food or live long enough to grow as large as they have, even compared to 100 years ago,” he said.

Image: African elephant chasing a black rhinoceros. (Alistair Rae/Flickr)

Citation: “The maximum rate of mammal evolution.” By Alistair R. Evans, David Jones, Alison G. Boyer, James H. Brownd, Daniel P. Costa, S. K. Morgan Ernest, Erich M. G. Fitzgerald, Mikael Fortelius, John L. Gittleman, Marcus J. Hamilton, Larisa E. Harding, Kari Lintulaakso, S. Kathleen Lyons, Jordan G. Okie, Juha J. Saarinen, Richard M. Sibly, Felisa A. Smith, Patrick R. Stephens, Jessica M. Theodor, and Mark D. Uhen. Proceedings of the National Academy of Sciences, Vol. 109 No. 5, Jan. 31, 2012.