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In MGS4 why does Akiba poop his pants?

Besides comical effect?

He and the rest of the Rat Patrol Team are either running covert operations or involved in one gun fight or the other. Under those sort of conditions, he probably holds it in too long to avoid discovery or to get through a battle.

Though it is most likely due to Kojima's sense of humor.

Note this is merely speculation. There is no in-game explanation of this phenomenon.
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