Howdy,
 
Why aren't ferret yawns contagious?  The answer is really quite simple.  A
yawn is triggered when there is an air pressure differential between the
lungs and the outside air.  So a person yawns to balance the differential.
Yawns seem to be contagious because when one person yawns, he draws in air
from the outside, thereby lowering the air pressure outside and causing a
pressure differential in any person that happens to be standing too close.
That person then has to yawn to alleviate their own pressure differential.
This can go back and forth for quite a few yawns, until equilibrium is
reached.  This rarely happens between humans and ferrets because a ferret's
lung capacity is much too small to create a significant air pressure
differential in a human when they yawn.  Therefore, Humans don't usually
catch yawns from ferrets.  However, highly erratic behavior is known to be
highly contagious between ferrets and humans.  Just look at how many ferret
owners have gone nuts.
 
May your yawns be forever in equilibrium,
Limejello & The Weezils of Doom
[Posted in FML issue 2578]