The answer to whether donkeys and horses can breed is yes, they can, producing hybrid offspring. This interspecies breeding is possible because both animals belong to the same genus, Equus, which signifies a close evolutionary relationship. The horse (Equus caballus) and the donkey (Equus asinus) are genetically similar enough to overcome the reproductive barriers that separate different genera of animals.
The Basic Science of Cross-Breeding
The biological reason this cross is possible lies in the slight difference in the parents’ chromosome counts. A horse possesses 64 chromosomes, while a donkey has 62 chromosomes. When these two species mate, each parent contributes half of its genetic material to the resulting embryo. The horse contributes 32 chromosomes and the donkey contributes 31 chromosomes, resulting in a hybrid with an odd number of 63 chromosomes. This odd number is present in every cell of the offspring’s body and directly influences its ability to reproduce. The specific name given to the hybrid animal is determined by which species is the mother and which is the father.
Understanding Mules and Hinnies
The resulting offspring are known as either mules or hinnies, depending on the maternal and paternal lineage. A mule is the hybrid produced from a male donkey, called a jack, and a female horse, known as a mare. Mules are characterized by a horse-like body structure, but they typically inherit the donkey’s long ears, smaller, tougher hooves, and often a coarser mane and tail.
Conversely, a hinny is the offspring of a male horse, or stallion, and a female donkey, called a jenny. Hinnies generally present with a more horse-like head and shorter ears, yet they often retain the sturdy, heavy build and temperament of their donkey mother. Both hybrids benefit from a phenomenon known as “hybrid vigor,” which grants them greater endurance, intelligence, and longevity than either parent species.
Mules are significantly more common in breeding programs than hinnies. The larger size of the mare compared to the jenny provides a more accommodating environment for a hybrid fetus, leading to higher gestation success rates. Additionally, jennies have a lower success rate in carrying the hybrid fetus to term, possibly due to a genetic incompatibility that can cause the mother’s body to reject the horse’s DNA during early pregnancy.
Why Hybrids Cannot Reproduce
The inability of mules and hinnies to reproduce stems directly from their odd number of 63 chromosomes. Sexual reproduction requires a specialized cell division process called meiosis, which creates gametes, or sex cells, with exactly half the number of chromosomes. This process necessitates that chromosomes pair up precisely with their homologous partner.
In a hybrid with 63 chromosomes, the odd number means that one chromosome is left without a pair, preventing the successful pairing of the entire set. Furthermore, the structural differences between the horse and donkey chromosomes contribute to this failure. The chromosomes are too dissimilar to align correctly for the exchange of genetic material and subsequent division.
This failure of homologous chromosomes to pair during meiosis, known as a meiotic block, makes it impossible to produce viable sperm or eggs. Consequently, male mules, called johns, are universally sterile, and the vast majority of female mules, known as mollies, are also sterile. While rare instances of a female mule producing an offspring have been documented, these events do not alter the classification of these hybrids as reproductively sterile.