David, as Uncle Barry pointed out, the tanning process locks in the hairs on the hide - but even with tanned hides, you want to keep the dye bath temperature below 165 degrees F. (About 140 degrees F is where I try to keep it)

At 165 degrees and above, you are cooking the material you are trying to dye and changing it's physical properties. This can, for example, turn beautifully tanned rabbit zonkers into something that has the suppleness of a pretzel. Not much use for fly tying! (Yes, I know this first hand )

mAngler