Hybrid (biology) are the mule, a cross between a female horse and a male donkey, and the hinny, a cross between a female donkey and a male horse. Pairs of complementary types like the mule and hinny are called reciprocal hybrids. Among many other mammal crosses are hybrid camels, crosses between a bactrian camel and a dromedary. The first known instance of hybrid speciation in marine mammals was discovered in 2014. The clymene dolphin ("Stenella clymene") is a hybrid of two Atlantic species, the spinner and striped dolphins. Cagebird breeders sometimes breed bird hybrids known as mules between species of finch, such
Domestication of the horse less than expected based on the overall genetic variation in the remaining genetic material. This indicates that a relatively few stallions were domesticated and that it is unlikely that many male offspring originating from unions between wild stallions and domestic mares were included in early domesticated breeding stock. Genes located in the mitochondrial DNA are passed on along the maternal line from the mother to her offspring. Multiple analyses of the mitochondrial DNA obtained from modern horses as well as from horse bones and teeth from archaeological and palaeological finds consistently shows an increased genetic diversity in the mitochondrial DNA