>I however cannot find the answer to this question:
>My two make this sound when they are playing either with me or with each
>other.  I hope I can describe it.  They run around jumping, then they
>look at you and sort of laugh.  It is like they are breathing in and out
>really fast with their mouth open.  Like a HEE HEE HEE HEE.
 
Yes, it is laughter.  Laughter is a pretty old noise -- a number of
critters do it.  It's not as old as the purr (which does not equate to a
strict feline purr), the growl, or the hiss each of which is found in
reptiles, birds, and mammals (and I think the hiss may go back further,
but not sure).  There are some neat studies of these.
[Posted in FML issue 3764]