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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).

Modern whales have many adaptations that allow them to overcome these obstacles. How these features developed, however, remained a mystery until the new fossil discoveries.

The finds enabled Thewissen—together with his graduate student, Sirpa Nummela, and a team of biologists and paleontologists—to document the evolutionary transition of whale ears.

From Air to Water

The team found fossil specimens of four groups of early whales from Pakistan, which allowed them to interpret how the ear evolved as whales took to the water. (These detailed fossils include ear bones as well as marks on the skull that hint at how those ears worked).

The earliest ancestors of whales, found in 50 million-year-old sediments, bore little resemblance to modern whales. These animals, known as pakicetids, were wolf-size terrestrial carnivores. They were most closely related to cows, pigs, antelopes, and other hoofed animals.

Skull features give strong evidence that pakicetids were the ancestors of whales. New fossils reveal that pakicetids had ears similar to those of terrestrial mammals, however, and would have had very poor hearing underwater, according to Thewissen.

Researchers have also recently found new fossils for later intermediate species of whales associated with 43- to 46-million-year-old deposits. Fossils from these species, known as remingtonocetids and protocetids, revealed that they had developed some of the features of the modern whale ear.

Remingtonocetids had short legs, long tails, and long snouts. Some were the size of river otters. Protocetids had powerful hind limbs for swimming and were the first whales to disperse to tropical oceans across the world.

Both of these ancient whale ancestors had developed a system whereby sound vibrations were detected from the water and transmitted to the ear via a large fat pad in the lower jaw. (The animals still retained the air-filled outer ear canal for hearing on land, however.)

Analysis of specimens of one more group of whales—basilosauroids, which lived around 40 million years ago—reavealed that they had developed one of the final defining features of modern whale ears.

Unlike their semi-aquatic predecessors, the massive, snakelike basilosauroids had tiny residual hind limbs and would not have been able to leave the water.

The mammals, which grew up to 60 feet (18 meters) in length, possessed air-filled sacs that insulated each ear from sound on all sides—except the left or right side of the head. This adaptation allowed high-definition, directional hearing underwater and is used by whales today, Thewissen said.

"Remarkable Transition"

Annalisa Berta, an expert on the evolution of marine mammals at San Diego State University in California, said the new study on whale-ear evolution "elucidates how a land-mammal ear capable of hearing airborne sounds … was transformed into the precise, directional underwater ear exemplified by modern toothed whales."

"The evolution of the ear and hearing in whales adds to an impressive suite of [physical] transformations, such as teeth and limbs, that document the remarkable transition of whales from land to sea," she added.

Zhe-Xi Luo, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, described the new study as "elegant" and said it "demonstrates for the first time that [intermediate species such as remingtonocetids] had a fascinating mosaic of primitive characters for hearing sound in air, and advanced characters for hearing sound underwater."

Thewissen, meanwhile, noted that his study of how the whale ear developed only recently become possible as "fossils have slowly accumulated"—and because the mechanism of hearing in modern whales has been explained only in the last decade.

For more whale news, scroll down.
 

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