Tuesday, 23 August 2011

The Echolocation Chambers In Whale Skulls Originally Evolved To Allow Directional Hearing

A computer model of the skull of Basilosaurus. The swirl moves
clockwise to show the direction of asymmetrical warping
The whale family is divided into two groups; the baleen whales which use bristles in the mouth to filter krill and plankton from sea water and toothed whales which are active predators. A well known and highly unusual characteristic in toothed whales is an asymmetric skull. It was previously thought that the asymmetric skull was a recent evolutionary step that was solely to enable echolocation in toothed whales, properly known as the Odontocetes.

However studies of ancient whale fossils shows that a skewed skull evolved much earlier. Scientists now believe that the distinctive shape evolved to allow sensitive, directional hearing. A team of scientists led by Dr Julia Fahlke made a study of prehistoric whale skulls to try and unpick the details of this puzzle. Fahlke originally looked at whales from the perspective of their feeding habits and tooth shape. The team made studies of an early predatory whale called Basilosaurus isis.

Basilosaurus was a top predator of its time at over 15 metres long with a short but powerful skull and long, sharp teeth designed to grip struggling prey. Fahlke worked with a well preserved, but warped skull. She believed that its twisted nature came from warping during fossilisation. To study feeding in Basilosaurus, Fahlke created 'corrected' models. However when it came to fitting the jaws together, there was a problem. The morphological arrangement just did not fit.

This made the team believe that the skulls were not warped and would have actually been asymmetrical in life. They made more rigorous tests using six more well preserved skulls. Not only did the jaw models and feeding data all fit together, it also revealed that the distinctive body shape of whales might have evolved along with the jaws to channel sound waves and vibrations for echolocation and sensitive hearing.