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 scotti Equus scotti Equus scotti (translated from Latin as Scott's horse, named after vertebrate paleontologist William Berryman Scott) is an extinct species of "Equus", the genus that includes the horse. "E. scotti" was native to North America and likely evolved from earlier, more zebra-like North American equids early in the Pleistocene Epoch. The species may have crossed from North America to Eurasia over the Bering land bridge during the Pleistocene. The species died out at the end of the last ice age in the large-scale Pleistocene extinction of megafauna. It was among the last of the native horse species in the