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,
One-male group offspring, compared to a noninbred offspring. Inbreeding depression resulted from the decreased genetic diversity within this population, meaning that the population as a whole experienced a decrease in fitness (i.e. reproductive success). Unlike infanticide, the high occurrence of inbreeding in one-male groups is a disadvantageous to both the females and males in the group. Experiments involving the hamadryas baboon species ("hamadryas hamadryas") provide evidence of feeding advantages for male and female members of one-male groups. However, the findings of feeding advantages were only evident when these one-male groups formed clans. It has been shown that males from single one-male groups