Snarl Bars are a little tricky to do but when done can add a lot to the realism and expressiveness of your costume.
One person who done a wonderful job of this is Wolftail http://www.wolftronix.com/head/index.html
How do Snarl bar's work. basically you make your head as normal, except when you are furring the muzzle you put a piece of bicyle break cable around the edge of the upper jaw line.
Then within the muzzle you can use servo's to pull up on these (simple pulley type) and this will raise the edges of the muzzle flaps.. And hence a snarl.
Another way if to make flaps that can be raised into the sides of your muzzle so that when they are extended the push the fur outwards and up giving a little more wrinkle.
A great exercise of anyone wanting to develop this level of realism is to look at nature, and then try and reverse engineer what you see.
Have Fun… and happy tinkering
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