I've had horn get really brittle under 30% rh, lost two good pieces just by flexing them.  I suppose it could have been those pieces, but now I keep all my horn around 55% rh and haven't had one snap in a flex test since.  Never thought about moths, that's a good point kiwijim.  My workshop has a lot of critters in it around at any given point, so I decided to give everything a once over.  Luckily no moths munching on horns, but I found a lot of tiny bedbug-looking insects all over my fish bladders and sinew, with a pile of very powder at the bottom of the bags, presumably their droppings.  Froze them, then sealed them up better.