Depends on time of the year, which you didn't specify. Right now, it's not getting dark until close to 9pm. We were there a month ago, and the animals were really active beginning an hour to an hour and a half before it got dark. There was a baby giraffe on the Arusha Savanna (which isn't visible from Sanaa) that would begin running around the savanna every evening before dusk.
Just saw where you listed when the sun sets....you may be a little late, but not necessarily.