We have two litter mates, a male and female, 3 years old.  The male
has always been more aggressive, or less afraid of things. They have
aways gotten along wonderfully, cuddling, and sleeping together.

Today, Things have changed.  Some time during the day, something
happend.  Tonight, the male is cowering in the basement, while the
female is behaving normally.  When I came home, it was odd that the
male wasn't there to greet me,,with the female as normal. later, we
found him hiding in the basement.  They are both indoor cats, and
don't go outdoors.

Since earlier in the evening, the cats have tried to approach each
other, but the male reteats to the basement, and the female hisses.

I would appreciate any information from anyone who has gone through
this...cats who have been loving compansions, who are now adversaries.