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Planet Earth

Swimming Trunks

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If elephant trunks look like snorkels, there's a good reason: The world's biggest land animals once lived in the water, says zoologist Anne Gaeth of the University of Melbourne. She came to this striking conclusion during an unrelated study of elephant fetuses. On the animals' kidneys, she saw hundreds of tiny ducts, called nephrostomes, that regulate the flow of oxygen and water in the body.

Most mammalian fetuses lose nephrostomes as they mature, but Gaeth found "heaps of them" in her late-stage embryo. She speculates that some unknown evolutionary pressures drove elephants into the water and then back onto dry land. Forty million years ago, ancestors of the elephant may have grazed on sea grass near the shore and used their trunks to breathe while submerged.

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