If you're after more zoom for going to Africa, consider one of the higher power zooms. I have an old Sigma 170-500mm APO which I like on my 5D because it doesn't weigh so much that it annoys me - I comfortably walk around with it hanging off my camera, I have a hand strap on the camera with which to easily hold it by its grip.
There's a newer Sigma 150-500mm APO out I think.
Others here have preferred the Tamron 200-500mm.
Still others here have been fans of the old Minolta 100-400mm APO, but I can't say I'm that partial to it.
The ultimate zoom is the Sigma 50-500mm APO, which is very good, but heavy enough to annoy me.
For non zoom, the 500mm Reflex lens is very light to carry around and works well - only problem is the doughnut shape in objects when you focus on something close.
For more power, you can add 1.4X or 2X teleconverters to the above lenses, but you lose autofocus.
Renato