Help! My Bum is Balding

Alice has a problem.  Every winter this ewe loses all the wool off her rear.  Her bum always starts out evenly covered in wool.  But starting around this time of year, the wool gets thinner and thinner and thinner.

Aw.  Her bum must be cold.

I am not sure why this happens.  I have checked her repeatedly for lice/scab and for itchy hay stuck in her bum wool - these do not appear to be the problem.  Maybe she is getting unwanted advances from Abbott that are thinning the wool on her bottom?  Alice is not scratching excessively and seems otherwise perfectly healthy.  It really is an unusual condition, though she does not seem to be bothered by it in the least.  Very strange.