When the ancestors of modern whales first dipped into the seas, it was the first step of a radical evolutionary journey: Arms morphed into flippers, and legs eventually vanished. But limbs weren't the only thing that needed to change. New whale fossils are revealing how the animal's ears were transformed in a process of compromise: Early whales had ears adapted for hearing both on land and underwater.
Land mammals perceive sound that enters the ear canal and hits the eardrum. The eardrum is connected to three small bones--ossicles called the malleus, incus, and stapes--that amplify and transmit the vibrations of the eardrum to the fluid-filled cochlea, where tiny hairs turn the sloshing into nerve signals.
When whales moved from land to water, they had to make two major changes to ensure that they kept hearing: They enlarged and rearranged the ossicles to transmit underwater sounds. And, in order to retain directional hearing, they had to prevent sounds from passing through the whole skull to the ear. (On land, airborne sounds bounce off the skull.) Whales accomplished this by isolating the cochlea in an air chamber. Instead of hearing through ear canals or the skull, whales channel sound to the ear via a fat pad in the lower jaw.
The earliest known whales, called pakicetids, lived 50 million years ago on land and had ears to match. By 35 million to 40 million years ago, the basilosauroid whales had ears that were essentially like those of modern toothed whales. To find out what happened in between, Sirpa Nummela and Hans Thewissen of the Northeastern Ohio Universities College of Medicine in Rootstown studied fossils that belong to two intermediate families.
Remingtonocetus and Indocetus--which had four limbs and probably were as aquatic as sea lions--both had heavy ossicles like modern whales do, and their jaws had space for a fat pad. But their cochleae were not completely surrounded by air, so their directional hearing would have been quite limited. These early whales could still detect sounds in the air via their ear canals, the team concludes in this week's issue of Nature. "This seems to be an early stage of experimentation," Thewissen says. The whales could hear underwater better than other land mammals did, but at the cost of not hearing as well on land.
"This is really documenting a very important evolutionary transition," says Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh. Although the ear of these transitional forms is what he expected, what's "amazing" is that Thewissen's team found the ossicles, which are rarely preserved.