Hinny - Fertility, Sterility, and Rarity

Fertility, Sterility, and Rarity

Hinnies are difficult to obtain because of the differences in the number of chromosomes of the horse and the donkey. A donkey has 62 chromosomes, whereas a horse has 64. Hinnies, being hybrids of those two species, have 63 chromosomes and are sterile. The uneven number of chromosomes results in an incomplete reproductive system. According to the ADMS, "The equine hybrid is easier to obtain when the lower chromosome count, the donkey, is in the male. Therefore breeding for hinnies is more hit-and-miss than breeding for mules."

Male hinnies and mules are usually castrated to help control their behavior by eliminating their interest in females. The male hinny or mule can and will mate, but the emission is not fertile. There are no recorded cases of fertile male hinnies or male mules.

Female hinnies and mules are not customarily spayed, and may or may not go through estrus. Female mules have been known to produce offspring when mated to a horse or donkey, though this is extremely uncommon. Since 1527 there have been more than sixty documented cases of foals born to female mules around the world. In contrast, according to the ADMS, there is only one known case of a female hinny doing so.

In China in 1981, a hinny mare proved fertile with a donkey stallion. When the Chinese hinny was bred to a donkey jack, she produced "Dragon Foal," who resembled a donkey with mule-like features.

In Morocco in 2003, a mule mare bred to a donkey stallion produced a male foal; DNA testing revealed the foal has a mixed karyotype hybrid like the Chinese hinny offspring, Dragon Foal.

There are other reasons for the rarity of hinnies. Female donkeys, jennies, and male horses, stallions, are choosier about their mates than horse mares and donkey jacks. Thus, the two parties involved may not care to mate. Even if they do cooperate, female donkeys are less likely to conceive when bred to a horse than horse mares are when bred to a donkey. Breeding large hinnies is an even bigger challenge, as it requires stock from a jenny of Mammoth donkey (Baudet de Poitou or American Mammoth Jack. Mammoth donkey stock is becoming increasingly rare and has been declared an endangered domestic breed. Fanciers are unlikely to devote a Mammoth jenny's valuable breeding time to producing sterile hinny hybrids when Mammoth females are in high demand to produce fertile pure-bred Mammoth foals.