in the zoo侧重于泛指在动物园里，at the zoo是指动物园里具体的一个地点。
例句
A small lion was cubbed in the zoo.
一只幼狮在动物园出生了。
Interest is abundant arrived in the zoo.
兴致盎然地来到了动物园。
We linked up with them at the gate of the zoo.
我们在动物园门口与他们会合。
I found when I tried to get a ticket at the zoo.
当我在动物园里想要买票时发现钱包没了。
