Rates of growth

In the story of “Alice in Wonderland,” the little girl first shrinks down to a fraction of her size when she drinks an unidentified liquid, and then grows large enough to fill a house after eating a small cake. These experiences don’t stop her from experimenting even further later on in the book when she comes across a mushroom. As Lewis Carroll wrote, “she set to work very carefully, nibbling first at one and then at the other, and growing sometimes taller and sometimes shorter, until she had succeeded in bringing herself down to her usual height.”

Unlike Alice, people and other mammals can’t change their size as quickly by snacking. A recent study by an international team of scientists suggests that going from something the size of a 20-gram mouse—roughly equivalent to five teaspoons of sugar—to a 2,000-kilogram elephant takes millions of years and multiple generations.

“We concentrated on large-scale changes in body size,” said the study’s first author Alistair Evans of Australia’s Monash University in a statement. “We can now show that 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.”

To find out just “how big was it and how fast did it happen,” the researchers tracked the rates of evolution for 28 mammal orders found in Africa, Eurasia, North America and South America to calculate how many years and how many generations it took for the size of a mammal to increase. The mammals in this selected group varied widely in size and ranged from mice to foxes to gazelles to humans to elephants to whales.

Rates of evolution

Based on their calculations, the researchers reported, it took at least 1.6 million generations for land-based mammals to grow to 100 times their original size and at least 10 million generations for the mammals to grow to 5,000 times their original size.

“This substantial length of time illustrates just how challenging this great transformation is,” the researchers wrote in their article.

For mammals in aquatic regions however, their calculations suggest that it took only five million generations for them to grow 5,000-fold. The feeling of weightlessness when in water and the ability to move more easily might be contributing factors as to why mammals such as whales thrive in marine environments but not on land.

While the focus of the study was about maximum growth rates, the team also looked at the time it took for animals to shrink. They found that the regions where mammals were known to be much smaller compared to their relatives found elsewhere shared a common trait: isolation. For example, until 10,000 years ago there were pygmy elephants on islands in the Mediterranean that were one-fourth the size of their relatives in Africa. Today the Borneo pygmy elephant of Indonesia stands at least a foot shorter than the Asian elephants that are more common throughout Southeast Asia.

“What caused their dwarfism? They may have needed to be small to survive in their environment or perhaps food was scarce and a small stature would require less nutrients,” said study coauthor Jessica Theodor of Canada’s University of Calgary.

The study was published ahead online Jan. 30 in the journal Proceedings of the National Academy of Sciences USA.

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