I would suggest picking up a 300 f/2.8 tamron, with the matched teleconvertors, they are very reasonable now a days, they go on ebay for $400-700, and even with the 2X still gives you a 600 f/5.6.

My main three lenses I used when I went to Africa were a 80-200 f/2.8 a 300 f/2.8 with 1.4 convertor and a 600 f/4, the 300 got the most use, and for many situations the 80-200 worked very well.

Have a good trip, Africa is a wonderous place to visit.