Line (queue) to get in

I arrived at 9am with a few cars in the lot and no line to get in. When I left at 12:30pm, this is what the entrance line looked like!

I wonder if the zoo is busier in the winter months as I've been to ASDM twice (both times in August) and I almost melted due to the intense Arizona heat. Some folks in your photo are wearing jackets and in the summer they'd be sopping wet with sweat standing in a line with zero shade.

It is definitely busier in winter - much busier. Tucson in general is busier in winter as there are a large population of retired folks called snowbirds who have a winter house here and a summer house back east or up north. This week the museum is especially busy due to out of state (and out of country) visitors seeing family over the Christmas holiday.