Eh. How far is the zoo from the photograher's home? What kind of planning goes on for the photo shoot? If they are shooting for an hour, I can see how it could get to several hours of work easily.
I wouldn't tip the photographer, but it wouldn't be based on this. It would be based on them running their own business and setting the price they need to make it worthwhile. If it was a wedding, and I felt they had gone above and beyond in some way, maybe. But not for this.