You might also consider a Canon 50mm f/1.8. It is small, light & inexpensive at just over $100, especially considering the quality, would give you some low light capability, and act as a general purpose backup should something unforeseen happen to your 24-105 while on international travel, where access to repair or replacement services may not be easy or speedy.
As an alternative to the 100-400, if you don't already have it, you might also consider Canon's relatively new 70-300L lens. It has the latest generation image stabilization & its image quality is supposed to be great. You could also couple that 70-300L with a Kenko Teleplus Pro 300 DGX 1.4x teleconverter. Combining a 1.4x teleconverter with a 1 stop decrease in light with the 70-300L which has a f/5.6 max aperture at the long end will yield 98-420 lens with a net f/8 max aperture at the long end, which Canon states will not autofocus on non-high end DSLRs; however there are anecdotal reports that many Canon DSLRs will in fact AF with this combo in reasonable light, albeit a little slow at the long end. Also, yet to be determined, it might be possible that Canon will include f/8 AF capability, at least for select AF points, using the 5D MkIII.
Lastly, if you don't have to sell your 40D, you might consider taking it with you on your trip to South Africa, for several reasons:
1. as a backup camera (while rare, camera failures do occasionally occur)
2. with its 1.6x crop factor, the 40D will gain you some extra range which may be useful for distant wildlife shots
3. you could keep the 24-105 on your 5D MkIII, and a telephoto (e.g. 70-300 or 100-400) on your 40D, minimizing the need to change lenses in a dusty environment or the possibility of missing a shot due to not having the correct lens mounted
4. if in dangerous areas or wanting to travel light, you could just carry a 50mm f/1.8, if you get one, on your 40D. Yes, it's a little long for a general purpose lens on a 1.6x crop body, but would still enable you to get some decent shots.