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 lambei of the modern wild horse, including the domestic horse, "Equus caballus". Controversy still surrounds "E. lambei" and the divergence of other similar extinct horse species. Through examinations of the bones and teeth of "Equus lambei", many similarities can be seen with the modern horse "Equus caballus". There is also a strong resemblance to "E. lambei" in the metapodials of "E. caballus przewalskii" and the phalanges of "E. caballus lenensis" "." Metrical and morphological studies of horse teeth from the Bluefish Caves confirm the close similarity between "Equus lambei" with wild and domestic horses alive today. "E. lambei" is a "caballus"