What happens to extra captured aliens?

Possible Duplicate:
Capturing multiple of the same alien

You can capture aliens with the Arc Thrower, allowing you to recover them and their equipment intact; however, I cannot seem to sell them on the Grey Market, and you only need 1 of each to unlock interrogation bonuses (Barring any renovation "accidents"). Do they stay alive in your facility indefinitely? Do the extras expire and turn into useful corpses? If they do stay alive, can I turn them into usable corpses?