The abandonment
The abandonment of Jennings by his mother represents the archetypal maximum for abandonment. She could have done rehab and gotten better for her family, but instead, Jennings's mother takes the easy way out. She leaves him at the orphanage and tells him "I'll be right back," and he doesn't see her for years. This abandonment is so thorough that Jennings must work through the damage the rest of his life.
The motif of alcohol
Alcohol is consistently responsible for the failures of Jennings's family in the book. First, his alcoholic, abusive father abandoned the family, and then the alcoholic mother did the very same thing, sending Jennings off in orphanage. What about Jennings's older siblings? Well, they're as alcoholic as their parents, and they are as contemptuous as they are as well. The motif signifies that alcohol is often a very horrifying addiction.
The motif of false rescue
The Carpenters sound like nice people, and it's hard for Jennings not to be at least a little excited by the prospect of a home. But, the Carpenters are money hungry people who only foster for the tax benefits, and then they treat the child with contempt. They rescue him, but to an abusive fate. The other foster parents and orphanages for a motif of false expectation and disappointment.
The symbolism of the Zoo
What do the animals have in common with Jennings? They are universally contained. They are caged at night, which the title aptly notices, like Jennings feels caged at night. In his body, his instincts tell him that he should be safe at night, sleeping in a family that loves him, in a home that someone cares for, but instead he ends up like a lion in a small cage.
Sal as a symbol for family
Sal ends up being the best family to Jennings of all. There is that police officer who is friendly, but Sal invites Jennings into his own home. Sal is a bus driver who has watched Jennings suffer. When suddenly, he is thrust back into orphanhood, Sal puts his foot down. If his own mother won't do it, Sal will do it. He accepts him and loves him, and he is a family to Jennings.
