The offspring of a donkey and a horse is a hybrid animal, typically known as either a mule or a hinny, depending on which parent is which species. Both the horse (Equus caballus) and the donkey (Equus asinus) are members of the Equidae family. These hybrids combine desirable traits from both parents, resulting in animals prized for their strength and endurance. However, because they are the product of two different species, they are almost always unable to reproduce.
The Most Common Cross: The Mule
The most common hybrid is the mule, resulting from breeding a male donkey (jack) with a female horse (mare). This pairing is more frequently successful and produces a more robust animal than the reciprocal cross. Mules are renowned for hybrid vigor, exhibiting greater strength, resilience, and longevity than either parent species.
Physically, a mule displays a blend of characteristics. They often have the robust, muscular body and size of a horse, but with the longer ears, thin limbs, and narrow hooves of a donkey. Mules are known for their sure-footedness, a trait inherited from the donkey parent, making them excellent pack and draft animals, especially in mountainous or rugged terrain. Their temperament is patient and stoic, combining the horse’s agility with the donkey’s cautious nature and high intelligence. Mules vary widely in size, ranging from miniature types to large draft animals that can stand up to 70 inches tall.
The Less Common Cross: The Hinny
The less common hybrid is the hinny, produced by breeding a male horse (stallion) with a female donkey (jenny). Hinnies are significantly rarer than mules, partly because female donkeys are more difficult to breed with a horse and often have lower conception rates. The hinny often presents a different physical appearance, usually being smaller and less muscular than a mule.
A hinny tends to have a more horse-like body shape and shorter ears, unlike the mule’s distinctly long, donkey-like ears. Due to the smaller size of the jenny’s womb, hinnies lack the pronounced hybrid vigor seen in mules and are typically smaller than both parent species. While both hybrids are valued for work, the hinny is described as having a calmer, more horse-like temperament, making it suitable for lighter tasks and riding.
The Biological Reason for Sterility
Both mules and hinnies are functionally sterile, stemming from the difference in chromosome numbers between horses (64 chromosomes) and donkeys (62 chromosomes). When these two species interbreed, the resulting hybrid inherits a total of 63 chromosomes—32 from the horse and 31 from the donkey.
This odd number of chromosomes creates a problem during meiosis, the cell division process that produces reproductive cells. For successful meiosis to occur, chromosomes must precisely pair up with their homologous partner. The horse and donkey chromosomes are not perfectly matched and cannot align correctly. The single, unpaired chromosome further disrupts the division process. This mismatch prevents the formation of viable gametes, rendering both mules and hinnies unable to reproduce in almost all cases.