The monkeys, at the zoo, of course, are in literal cages. But nearly every character is in a metaphorical cage. John feels trapped by his father's expectation that John will become a businessman like himself; John's father is trapped in his narrow world at the Coffee Exchange; John's mother is trapped by her obsession with cleaning; Lorraine is trapped by her mother's suspicions; Lorraine's mother is trapped in her awful job; Mr. Pignati is trapped in his grief over his wife's death.
One moment at the zoo (at the bat exhibit), which Lorraine thinks is an omen, explores the cage as metaphor:
[…] a little kid about ten years old who was sitting right up on the railing and leaning against the glass of the bat cage. Only he wasn't looking at the bats. He was looking at you when you came to look at the bats. And when I came up to the cage to see these ugly blood-sucking creatures, I had to look right into this little kid's face that had a smirk on it. He made me feel as though I was a bat in a cage and he was on the outside looking in at me. It all made me very nervous. (6)
Wow! That's quite a passage! We knew Lorraine was slightly paranoid, but this is weird.