Fossils Show How Whales Evolved to Hear Underwater

John Pickrell
for National Geographic News
August 11, 2004

Starting 50 million years ago, modern-looking whales began to evolve from terrestrial wolflike ancestors. Their transition to fully fledged aquatic behemoths took 15 million years. It is one of the best-recorded examples of an evolutionary transition in the fossil record.

A well-preserved series of fossils from India and Pakistan have already helped scientists understand how whales rapidly evolved limbs, teeth, kidneys, and other organs to cope with the pressures of the marine environment.

Now newly described fossils, with tiny ear bones intact, reveal for the first time how the ancestors of whales and dolphins developed their finely tuned underwater hearing.

A study detailing this evolutionary change will appear tomorrow in the science journal Nature.

"Whales had to rebuild their ears to regain the ability to hear clearly underwater and pinpoint the direction of sounds," said Hans Thewissen, a study co-author and evolutionary biologist at the Northeastern Ohio Universities College of Medicine in Rootstown.

Vital Sense

Hearing is the most important sense in modern whales, Thewissen said. Toothed whales, such as dolphins, rely on this auditory sense when hunting prey by echolocation. "A blind dolphin can find food easily, but a deaf dolphin will starve," he said.

While sounds travel farther and faster underwater, hearing in the marine environment presents a different set of challenges to hearing on land.

On land, sound vibrations strike the mammalian eardrum through an air-filled, outer ear canal. But when a typical mammal is submerged, however, water fills that ear canal, diminishing the ability of the eardrum to transmit sound.

An additional challenge to underwater hearing is distortion: Sound easily transfers from water through an animal's body, arriving at the ears via the bone and tissue of the head.

As a result, sound strikes both ears simultaneously, making it impossible to detect the sound's direction of origin. (We humans detect the direction of sounds by the volume and speed with which they arrive at each of our ears).

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