The answer to this can be found in Chapter 8, on pages 171 and 172 in the paperback edition of the book. In this chapter, Diamond argues that the reason that animals like zebras were not domesticated is that they were not really suited for it. He rejects the idea that Africans were not smart enough or not motivated enough to domesticate zebras. Instead, he says that zebras were simply not domesticable (which he supports by noting that they still have not been domesticated).
Diamond says that there are six factors that can make an animal impossible to domesticate. They are:
- Diet. You can only really feed herbivores so carnivores can’t be domesticated very well.
- Growth rate. An animal has to grow fast to make it worth keeping around.
- Problems of captive breeding. Some animals will not breed very well in captivity.
- Disposition. Some animals are simply too mean.
- Tendency to panic. If animals are easily panicked, it will be hard to keep them confined.
- Social structure. Herd animals are the easiest to domesticate.
Of these factors, the only one that is really a problem with zebras is disposition. Diamond says that zebras tend to bite and not let go. He says that they injure more American zookeepers than tigers do. Zebras also cannot be lassoed because they can watch the rope and dodge it.
For these reasons, zebras are not good for domestication.