Alan Yates: We really screwed ourselves this time, trying to, trying to stay there for the last shot. I don't even know where we are now, but I know, they... I know they, uh, followed us, and, and we lost everything trying to escape. We're screwed! We're trapped.
Chaco Losojos: In there, the more you carry, the quicker you get tired, the sooner you die.
Professor Harold Monroe: All I want to know, Mr. Chaco, is... do we have a chance of finding them alive?
Chaco Losojos: Who's to say? The only thing I know is we're gonna risk our lives to save those fuckers. Let's go see the Yacumo.
Chaco Losojos: You did it, goddamnit! They've just invited us to dinner.
Professor Harold Monroe: They what?
Professor Harold Monroe: Yes, yes, that is precisely what I want. I've seen the rest of the material, you haven't. You haven't seen the stuff that even your editors didn't have the stomach to put together, and if you had, you wouldn't hesitate but to agree with me.
Answer: He was bitten on the foot by a poisonous spider that was hiding in his boot. They cut his leg off to stop the poison from spreading.