Horse outward similarity is an unreliable measure of relatedness. Besides the horse, there are six other species of genus "Equus" in the Equidae family. These are the ass or donkey, "Equus asinus"; the mountain zebra, "Equus zebra"; plains zebra, "Equus quagga"; Grévy's zebra, "Equus grevyi"; the kiang, "Equus kiang"; and the onager, "Equus hemionus". Horses can crossbreed with other members of their genus. The most common hybrid is the mule, a cross between a "jack" (male donkey) and a mare. A related hybrid, a hinny, is a cross between a stallion and a jenny (female donkey). Other hybrids include the zorse,
Cryptic female choice in which a male's interaction with a female is of detriment to her personal well-being and that of her offspring, one such situation occurs when a male harasses a female prior to copulation. In such circumstances where the female's lifespan and fertility are compromised, it benefits females to develop evasive mechanisms. In addition, in some species where multiple males inseminate a female, a female is able to select the most desirable sperm for her offspring by rejecting that which she desires less. In both of the aforementioned situations it is a female's reproductive actions that affect male reproductive success following