There are lots of different ones you can find just by Googling animal care volunteering or animal care internships, but I'd do some research into voluntourism and what it is or what it means to the cultures affected. There are plenty of unique experiences that aren't so much a pay to do it event. Plenty of people will go ahead and do it any way, it's really a personal choice. The rat one sounds super interesting and as a research project, is far less likely to be disadvantageous to the local populations. I was also looking at a neat one that would cost you only airfare and you'd assist with Galápagos turtle conservation and research. Not in Africa obviously but still neat.
I'm not trying to sound super negative or judgemental, just trying to encourage more research. Africa is neat and if your heart is set on Africa, do it! I know better than anyone that when your heart is set on going somewhere that nothing will change your mind. But there are plenty of opportunities outside of Africa with less ugly programs.