Ghoul Ecology

Ghoul Ecology is a perk added to Fallout 3 in the Point Lookout add-on. All your weapons do an additional 5 damage against ghouls.

Obtaining
The perk is gained by reading Plik's journal inside the Coastal Grotto. The book itself is located on a table inside Plik's bedroom next to a Mini Nuke and a Chessboard.

Bugs
Due to a misplaced condition, damage is applied to all enemies. The condition is placed into the conditions list used for perks that are selectable at level-up, instead of in the Perk Entry's Target condition where it belongs. This means that the damage bonus is not just restricted to ghouls like the Entomologist perk is against insects, but instead the damage is added against EVERY enemy you encounter.

The bonus is applied on each shot fired in V.A.T.S. Therefore, hunting rifles (which fire once) get +5 and Chinese assault rifles (which shoot 3 times at once) get a bonus of +15. This can be useful for players who use weapons that fire many shots in V.A.T.S, such as the Minigun, which fires 8 times providing a bonus of +40 damage.

There might also be a bug where upon saving, the damage of all weapons gets decreased by 3. (confirmed Xbox 360)

It is unknown when this happens, but the first time it was spotted was during the quest, "Big Trouble In Big Town". (needs confirmation)

With the Superior Defender perk, upon saving and loading, the DAM of all weapons is increased or decreased between a maximum and minimum DAM. Atleast it's listed in the Pip-boy, but unconfirmed if that is the actual damage. (confirmed Xbox 360)