Improved Toughness( Monster Manual III, p. 207)
A creature with this feat is signifi cantly tougher than normal.
Base Fortitude save bonus +2,
The creature gains a number of hit points equal to its current Hit Dice. Each time it gains a Hit Die (such as by gaining a level), it gains 1 additional hit point. If it loses a Hit Die (such as by losing a level), it loses 1 hit point permanently.
A fighter can select Improved Toughness as one of his fighter bonus feats.