CAT PURRING:

(SFX: Cat purring)

PURRING IS A FUNDAMENTAL PART OF ANY CAT'S REPORTOIRE, FOR WHEN IT'S HAPPY,
WHEN IT WANTS FOOD, OR WHEN IT WANTS AFFECTION.  BUT JUST HOW DOES THE CAT
DO IT?  HUMAN'S CERTAINLY CAN'T PURR.  (TRY IT, ITS IMPOSSIBLE.)  CAT'S ON THE
OTHER HAND CAN USE THEIR VOICE THE WAY WE DO, SAYS BIOENGINEER DAVID RICE
AT TULANE UNIVERSITY.

RICE: "Cat's have a voice. they say meow and things like that." 

THE VOICE WORKS IN BOTH CATS AND HUMANS BY FORCING AIR PAST THE VOCAL
CHORDS, WHICH STRETCH BACK AND FORTH  TO PRODUCE DIFFERENT PITCHES. BUT
RICE DISCOVERED THAT PURRING HAPPENS DIFFERENTLY.
     
RICE: "it turns out that the purring seems to be caused by the vocal chords being drawn together and apart
. . . 25 times per second."

BY MOVING THEIR VOCAL CHORDS SO QUICKLY, A CAT CREATES A VIBRATING COLUMN
OF AIR IN THE THROAT THAT YOU HEAR AS A PURR.  OTHER MAMMALS INCLUDING
HUMANS JUST DON'T HAVE THE MUSCLES TO MOVE THE VOCAL CHORDS THAT
QUICKLY.  RICE POINTS OUT THAT THE PURRING OCCURS AT JUST THE RIGHT
FREQUENCY TO SET THE CAT'S ENTIRE BODY VIBRATING.

RICE: "This is one of the interesting characteristics of a purr because you can also feel by touching the cat
that its purring." 

 WHY WOULD THE CAT NEED THIS UNIQUE ABILITY TO PURR? NO ITS NOT JUST SO YOU
KNOW WHEN TO FEED IT.  SOME THEORIES SUGGEST THAT CATS USE THEIR PURR TO
COMMUNICATE WITH THEIR YOUNG BY FEEL INSTEAD OF SOUND, WHICH MIGHT ALERT
PREDATORS TO THE DEN.