A mule is a hybrid animal created by crossing two different species within the Equidae family. Specifically, it is the offspring of a male donkey, called a jack, and a female horse, known as a mare. Because the mule inherits sex chromosomes from both parents, mules are indeed biologically defined as either male or female. However, their unique genetic makeup prevents them from continuing the species.
The Hybrid Cross
Mules are produced by crossing a male donkey (jack) and a female horse (mare). The less common hybrid, known as a hinny, results from breeding a male horse (stallion) with a female donkey (jenny).
While both mules and hinnies share the same combination of parental genes, the maternal lineage significantly influences the size and general conformation of the offspring. The hinny tends to be smaller and more donkey-like in appearance due to the smaller size of the jenny mother. The physical constraints of the smaller uterine environment during gestation often result in a less imposing animal than the mule, which develops within the larger mare.
Male vs. Female Mules
A male mule is often referred to as a “John mule,” and a female mule is called a “Molly mule.” To manage their temperament and make them more suitable for work, nearly all John mules are subjected to gelding.
Gelding removes the male hormones, resulting in a calmer, more predictable disposition that is better for handling and labor. Molly mules are generally kept intact and exhibit typical female behaviors, occasionally even showing signs of estrus. Molly mules can sometimes be used as nurse mares for foals of other species.
Why Mules Cannot Reproduce
The fundamental reason for the mule’s sterility lies in the mismatch of its parental chromosomes. The domestic horse possesses 64 chromosomes in its somatic cells, organized into 32 pairs. In contrast, the donkey has 62 chromosomes, arranged in 31 pairs. When these two species mate, the resulting mule inherits a haploid set from each parent—32 from the mare and 31 from the jack—resulting in an odd total of 63 chromosomes.
The difficulty arises during meiosis, the specialized cell division process required to produce viable gametes, such as sperm and eggs. Meiosis necessitates that homologous chromosomes, which carry similar genetic information, pair up perfectly before the cell can divide into reproductive cells. Since the mule has 63 chromosomes, there is an unpaired chromosome that cannot properly align with a partner during the initial stages of division. This misalignment prevents the orderly segregation of genetic material into the daughter cells.
This failure in proper pairing means that the mule cannot produce functional gametes capable of fertilization. Although a male mule may produce small quantities of sperm, they are typically non-motile, malformed, and genetically unbalanced, rendering him infertile. Female mules are also sterile because their oocytes, or egg cells, do not develop properly due to this chromosomal pairing failure. The irregular chromosome number halts the reproductive process at a cellular level.
There have been fewer than 10 documented cases globally of a female mule giving birth to a live foal, but these instances are considered biological anomalies that do not change the species’ general classification as sterile. These rare exceptions involve unusual situations where the mule’s 63 chromosomes somehow manage a functional, though imperfect, segregation. However, the offspring of such an event are typically sterile themselves, confirming that the genetic barrier between the horse and donkey species remains robust.