Your physical durability allows you to shrug off attacks that would cripple a lesser person. Rather than depend on agility or willpower, you use your raw toughness to survive.
Prerequisites: Endurance.
Benefit: You can use your Constitution modifier in place of your Wisdom modifier on Will saves.
You do not automatically fail Fortitude saves on a roll of natural 1.
Source: Player’s Handbook II, page 83.