Maybe someone has posted this or a similar solution, but I didn't see it. If there's a more efficient way, puh-leeze let me know!
These are ears with parallax from the Face behavior that also swap back and forth from behind to in front of the face with Head & Body Turner behavior triggered by face tracking. The idea started rolling around in my head a month or so ago when the guys at Digital Puppets commented on YT that they use a form of this to get some of their puppets' noses to seem to turn. I had success using okaysamurai's Stan puppet. This method also worked well on more photorealistic ears--but it takes a lot of tinkering with the parameters and nudging the art around so all the instances of ears interect well. You may not find it worth the effort.
1) Create two ear groups, something like Front Ears and Back Ears--both independent. One group goes above (in front of) the face or, in Stan's case, his whole Head, and one goes below (behind). In each parent group, create Frontal, Left Quarter, and Right Quarter sub-groups. You could use the two Profile views as well if you want of course.
2) Place your ear art according to both the angle view and whether that art should be above or below the face. [Note that no art goes in the front Frontal, it's just a placeholder in this group so CH doesn't get confused.] Keep in mind, the quarter turn groups only have one ear, because the other is in the same turn group on the other layer. Once you start, this will make sense, especially after you start seeing the results after step 3...
3) Tag the Parent groups (Front Ears & Back Ears) as a Head and add to these parent groups the behaviors Face and Head & Body Turner. The Face behavior will allow your ear elements to move with a parallax effect, as demonstrated in okaysamurai's Head Turns tutorial. Your head rotation should now turn the various ear layers on and off.
4) Tinker with the behavior parameters. Make sure the Head & Body Turner behaviors have the same sensitivity for both Front Ears and Back Ears or the ears won't switch instantly (this may be desireable in some cases, IDK).