Shareen Finucan, age 10, of Montreal, Quebec, Canada, for her question:

How many horse species are there?

You may think that there are dozens of different horse species. But the scientists who classify the animal kingdom say that there are only six. To scientists a species is a group of animals that breed with each other but not with members of other species. All our domestic horses descended from the same two or three wild species. Eventually, our tame pintos and palaminos, shire horses and Shetland ponies became different breeds of the same species.

All the world's horses belong in the family Equidae and the genus group Equus. Five of the six species are divided into smaller groups called subspecies. Most of the wild asses belong to one species, with several subspecies. Most of the zebras belong to the same species, with several subspecies. The only horsy type with a genus of his own is a stripy fellow called Grevy's zebra.