A hinny is the hybrid offspring of a male horse and female donkey, whereas a mule is the product of a female horse and male donkey. Although mules and hinnies have the same genetics, they do exhibit some generally distinguishing characteristics. It is believed the differences may be the result of which species is the mother, the horse or donkey. Hinnies tend to be smaller than mules and have a more horse-like head and mane. The differences might be so slight that knowing the parents is the only sure way to identify the hybrid as a hinny.
Mules are much more common than hinnies. This is thought to be because of the genetics of the parents. Male donkey and female horse hybrid offspring are more easily conceived, producing a mule. It is also believed the combination required to produce hinnies is less likely to want to breed. For these reasons, hinnies are rare.
Having the donkey as the mother may be the cause of the generally smaller stature than a mule. The size of hinnies varies depending on the size of the parents. Large hinnies are very rare though, as the mother must be of the Mammoth donkey breed, which is considered an endangered domestic breed. The coat color of hinnies also varies widely depending on the appearance of the parents.
Although male and female hinnies are capable of mating, offspring almost never result. As with most hybrid animals, hinnies are almost always sterile. Males are usually gelded to prevent them from wanting to mate. This makes them easier to train and control. The female hinny may experience estrus and mate, but generally produces no offspring.
Both the hinny and mule have an odd number of chromosomes, which makes producing viable gametes, sex cells, almost impossible. The hinny has 63 chromosomes. This is the result of hybrid breeding, as the male horse has 64 and the female donkey has 62. Although there have been reports of female hinnies having offspring, only one documented case exists, while there are no reports of a male hinnies fathering offspring. In the one confirmed case, a female hinny mated with a male donkey, producing a female foal.
The majority of horse to donkey breeding is done to produce mules. These animals were considered superior work animals when compared to hinnies, although the hinny’s endurance is now considered the same as a mule’s of equal size. In the days of canal transportation, mules were regarded as a better choice than the frequently smaller hinny for pulling loaded barges along the canals. For centuries, the mule has been the preferred hybrid equine for all types of heavy work, but this may actually be the result of the rarity of hinnies.