Reproductive isolation from a cross between a horse and a donkey or between a mare and a donkey, respectively. These animals are nearly always sterile due to the difference in the number of chromosomes between the two parent species. Both horses and donkeys belong to the genus "Equus", but "Equus caballus" has 64 chromosomes, while "Equus asinus" only has 62. A cross will produce offspring (mule or hinny) with 63 chromosomes, that will not form pairs, which means that they do not divide in a balanced manner during meiosis. In the wild, the horses and donkeys ignore each other and do not
Equus (genus) each other both visually and vocally. Human activities have threatened wild equine populations. The word "equus" is Latin for "horse", and is cognate with the Greek ἵππος ("hippos"), "horse", and Mycenaean Greek "i-qo" /ikkʷos/, the earliest attested variant of the Greek word, written in Linear B syllabic script.; compare the alternative development of the Proto-Greek labiovelar in Ionic ἴκκος ("ikkos"). The genus "Equus" was first described by Carl Linnaeus in 1758. It is the only recognized extant genus in the family Equidae. The first equids were small, dog-sized mammals (e.g. "Eohippus") adapted for browsing on shrubs during the Eocene, around