Elephants don't grow so large overnight. And a new study published in PNAS shows that it takes at least 24 million generations for a mammal the size of a mouse to evolve into something as big as an elephant, reports Veronique Greenwood at Discover magazine's 80beats blog. "To get that number, researchers looked at the evolution of body mass over the last 70 million years, after the dinosaurs went extinct and surviving animals expanded into the ecological niches they left behind," Greenwood says. "That estimate is far longer than earlier estimates, which, extrapolating from bursts of super-fast evolution in mice, range from just 200,000 to 2 million generations."
The researchers also found that the opposite is true when the process is reversed. A large mammal grows smaller about 30 times faster than a small mammal getting larger does. This could reflect the fact that smaller organisms have an easier time finding resources and places to live than larger organisms, the researchers posit. "Certain physical constraints — the pull of gravity, for instance — might also make great size difficult to attain without unusual, physiologically expensive adaptations," Greenwood adds. "Tellingly, the rate of growth changes when you look at marine mammals, which are supported by the water around them. Body size can grow twice as fast in the water."