When my wife and I heard this story.
The first thing that came to my mind; was how strange it seemed, for 17, 19, and 23 year old boys / men, to be at the zoo, on Christmas evening.
I know when I was that age, I wouldn't be at a zoo, unless my girlfriend worked there.
I wouldn't be surprised if the police find the boy's discarded video camera; and find they were doing a stunt for YouTube.