Good job. It really shows the dynamics of such a herd. Shadows in dust really does it too. If you want more light on the zebra's the only thing I can think of is either comping them in later (and do 2 renders), but that would be an enormous task, or just light up the texture. You can go above 1 if needed in the default shader, I tend to do that if I want ground colors to seep into grass objects.
Or give them a little (more) soft reflection, which would lighten up their back mostly.