Hearing clue to whale evolution

ROOTSTOWN, Ohio, 12 August, 2004 - The evolution of whales from
four-legged land dwellers into streamlined swimmers has been traced in
fossilised ears, the journal Nature reports.

The ancestors of cetaceans (whales, dolphins and porpoises) slowly
lost their ability to move around on land to become efficient swimmers.

This shift is recorded in whale ears, as they evolve a sound
transmission better able to hear underwater.

Whales went through a stage where their hearing was crude in both air
and water.

"The change from living on land to living in water caused great
problems for hearing", said Dr Hans Thewissen, Northeastern Ohio
Universities College of Medicine

Directional hearing - the ability to determine which direction a sound
is coming from underwater - is vital for modern toothed whales.

They pinpoint their prey through echolocation, in which some of the
high-pitched sound whales send out bounces off an object and returns
to them.

Toothed whales then interpret this returning echo to determine the
object's distance, shape and other characteristics.

Almost like a whale

"In less than 15 million years, bodies of cetaceans evolved to adapt
to life in the water. However, the change from living on land to
living in water caused great problems for hearing," said co-author
Hans Thewissen, associate professor of anatomy at Northeastern Ohio
Universities College of Medicine.

"As every swimmer knows, mammals are unable to determine the direction
from which sound originates under water."

The earliest cetaceans, the pakicetids - which lived about 50 million
years ago - used the same sound transmission system as land mammals
and had poor underwater hearing.

By 43 million years ago, their relatives the remingtonocetids and
protocetids (which lived between 43 and 46 million years ago) had
developed a new sound transmission system.

These whales could hear better in water than pakicetids, while
retaining their ability to hear in air. But this halfway house meant
hearing in both air and water was crude.

With the advent of the basilosaurids about 40 million years ago, this
compromise was abandoned in favour of a hearing system more akin to
that of modern whales, giving them refined underwater sound reception.

However, the basilosaurids lacked the ability to echolocate like
modern toothed whales.

http://news.bbc.co.uk/1/hi/sci/tech/3558350.stm

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