do you have to purchase tickets in advance?

Yes

Yes if you can purchase them for as early in the day as possible . Saturday it took us more then 5 hours to go through the Zoo and we did skip a few things . It's a great place to visit

Yes you need to purchase tickets on line and it has your start time

I went with my sons school and they wanted us to purchase before going because they'll let so many in at a time

Reservations are not generally required after 2pm, but patrons attending before 2pm are required to make a reservation.

We had to make reservations to be able to get in.