Scale and SkinSource Magic Tactics Toolbox pg. 5
When you are changed by magic, your skin also toughens.
Prerequisites: Con 13.
Benefit: Whenever a transmutation spell or spell-like ability affects you, your natural armor bonus increases by 1. If you have no natural armor bonus to Armor Class, treat your natural armor bonus as 0 for the purposes of this feat. The bonus to your natural armor bonus increases by 2 if the caster level of the effect is 10th or higher.