I'd say that the monkeys weren't actually introduced to the concept of money it the first place, as they've already had other means of transferring value between themselves. Every living organism is involved in trade in some kind of way. What the discs have brought to the equation was the ability to preserve the resources, so they can be used later. I think the reason that the monkeys eventually went corrupt is that the discs were fake money in a sense that it was given to them in exchange for nothing (no work had to be performed) and there was no fixed cap on it, so there was no way to put an objective price on it. I'd say that the experiment outcome was inevitable, rather than surprising :)