Best to go in the mornings, when they usually feed. By afternoon, most are sleeping or hiding. Gorillas are always visible.
"Most of the zoo’s animals are crepuscular, active in the morning and at dusk, so the best times to see the animals are in the mornings when the zoo opens and the animals shift from their indoor sleeping enclosures to the outdoor exhibits,” So both morning and late afternoon are good, but the zoo closes at 6 in the summer and 4 in the winter. I have been mid-day and it wasn't as fun because all the animals were napping. Many out of sight.
Mornings definitely, that is also when they feed many of the animals. They are going to start having some nights open though, so keep an eye out for those if to want to see the evening active animals.
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