When a horse and a donkey have an offspring, it is called a mule--a hybrid that cannot reproduce itself. Why are horses and donkeys considered separate species?

Horses and donkeys are two different species but belong to the same family Equidae and the genus Equus. They both have different numbers of chromosomes.

Although they look quite similar in appearance, there are many dissimilarities.

The physical differences between horses and donkeys include:

Donkeys backs are flatter than horses.

Donkeys have smaller hoofs than horses.

Donkeys have longer and thicker ears than horses.

Horses tend to have a long face than Donkeys.

Horses have six vertebrates, while donkeys have only five.

Tails of horses and donkey differ. Horses have long tails, whereas donkey’s tail looks like a cow’s tail.

On the genetic level, the total number of chromosomes in the horse is 64 and donkey have 62 number of chromosomes. Mule is a hybrid of a female horse and a male donkey. Hinny is a hybrid of a female donkey and a male horse. Both the offspring (hybrid offspring) produced by donkeys and horses are infertile, sterile, cannot have its own offspring and both have 31 pairs of chromosomes each.

If the produced offspring is fertile then the animals are of the same species and if the produced offspring is sterile then they are of different species. Because of this, both horses and donkeys are considered as a separate species. A species is generally defined as a group of organisms, which are capable of interbreeding and creating viable offspring. In this case, mule and the hinny are not really a new species and are produced by two different species.