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Inner ears. From left to right: a land mammal (bushbaby), a land-living early whale, a marine early whale, and a modern dolphin, adjusted for body size. Each inner ear would easily fit on a penny.

Dizzy Dolphins

About 40 million or 50 million years ago the ancestors of today's dolphins and whales gave up their landlubbing ways and plunged into the sea. A team of researchers now says that the balance-sensing organs in the inner ear may have been the first thing to change--preceding the more dramatic sleeking of body shape. The modified ear threw the animals off balance on land, they argue, and set their fate as creatures of the sea.

Cetaceans--the group that includes dolphins and whales--evolved from a hippopotamus-like ancestor (ScienceNOW, 19 September 2001). These creatures bore little resemblance to modern cetaceans, and paleontologists have little idea what drove them into the sea and transformed them into obligate swimmers. The new study suggests the answer may lie deep in the inner ear.

The inner ear contains three fluid-filled tubes called semicircular canals that detect head and body movements. These structures are about three times smaller in cetaceans than in similarly sized land mammals. Previous studies with land mammals suggested that larger semicircular canals might be a feature of animals that are especially agile on land. Intrigued, paleobiologist Fred Spoor of University College London and colleagues used x-ray tomography to reconstruct the inner ears in the skulls of four early cetaceans.

Only 5 million years after the origin of the first whalelike creature, the semicircular canals had shrunk to the exceptionally small size found in modern cetaceans, the team reports in the 9 May issue of Nature. That's about 10 million years before early cetaceans evolved fluke tails, short necks, and fins. The team argues that smaller semicircular canals would diminish the sense of balance, and the creatures would have had a harder time evading predators on land. They speculate that the change might have been an advantage in the sea--allowing the animals to execute acrobatic flips and turns without getting dizzy.

Zhe-Xi Luo, paleontologist at the Carnegie Museum of Natural History in Pittsburgh, is fascinated that the balance organ of early cetaceans adapted to life in the sea before the skeleton changed. "It is a great surprise that they still have their limbs capable of crawling or running, but lost their sensory system to be active on land."

Related sites
Spoor's site
Co-author Hans Thewissen's site on whale evolution
Primer on cetacean evolution