Our boy Hogan does this exact same thing, and so we took him to the vet a
when this behavior first began.  He doesn't have adrenal disease, and the
vet attributes this odd habit of his as just a behavioral problem.
 
Hogan was used as a breeder before we brought him home, and so we thought
his ear sucking was in some way related to his past history of having his
way with the ladies.  It also appears to be a behavior Hogan only exhibits
when he's bored.
 
An interesting twist to Hogan's dirty little secret is that he only sucks
the ears of female albino ferrets -- he shares a cage with three to four
sable girls, whose ears he leaves alone, but poor Snickers constantly
suffers from his oral aural fixation (and before Annabelle died he sucked
her ears too).  Bitter apple didn't help -- he just made faces when he
sucked on the white girls' bitter apple soaked ears -- and so we added a
"girls only" hutch into their cage, which is reachable by a 3" diameter
tube, through which Hogan refuses to pass.
 
Leonard & Elayne                http://cascade-ferret.org
[Posted in FML issue 2979]