Ancient whales: thirsty at sea.

Water, water, everywhere but what's a whale to drink? That's the question J.G.M. Thewissen posed while studying the earliest chapter in the story of cetaceans.

Although modern whales can ingest seawater, they evolved from four-legged land mammals that needed freshwater to survive. Indeed, the first whales must have lived in freshwater-their fossils are found in deposits from ancient rivers and lakes in Pakistan. Thewissen, a paleontologist at Northeastern Ohio Universities College of Medicine in Rootstown, and his colleagues wondered when whales developed the ability to drink saltwater.

To answer the question, they turned their attention to teeth. As mammals grow, oxygen atoms in the tooth enamel record information about the type of water the animal drinks. Oxygen typically comes in two isotopes, and saltwater contains a significantly higher ratio of heavy oxygen than freshwater does. The researchers measured the isotopic ratio of oxygen in four ancient whale fossils. The three oldest species showed isotopic signatures similar to those of modern river dolphins, which drink freshwater. The fourth fossil whale, called Indocetus, had a greater proportion of heavy oxygen, like modern whales.

Indocetus lived around 48 million years ago, only 4 million years after the earliest known whales. Within that geologically short period, whales must have evolved the specialized kidneys that enable them to drink saltwater, the researchers conclude in the May 30 Nature.

"The exciting thing is that this tells you when whales became independent from freshwater," says Thewissen. The earliest whales could not have strayed far from coastlines because they had to return to a river to drink, as manatees do today. One of the fossil whales, Ambulocetus, apparently led such a lifestyle: Its bones come from saltwater deposits, but its teeth show a freshwater isotopic signature.

Once whales could survive on seawater, though, they could migrate across oceans and spread around the world, says Thewissen. The fossil record supports this theory. Whales had reached several other continents by the time of Indocetus.