Mules exist as both males and females. They are hybrid animals, resulting from crossing two different species within the Equus family. The common misconception that all mules are male likely stems from their inability to reproduce, a biological fact affecting both sexes. Because of this sterility, most male mules are castrated, meaning the working animal commonly encountered is a gelded male or a female.
The Unique Hybrid Parentage
The mule is the specific offspring of a male donkey (jack) and a female horse (mare). This pairing combines the donkey’s endurance and hardiness with the horse’s size and speed. The reciprocal cross—a male horse (stallion) and a female donkey (jenny)—results in a hinny, a less common hybrid.
Creating a mule introduces a genetic hurdle due to the parents’ differing chromosome counts. Horses have 64 chromosomes, while donkeys have 62. The mule inherits 32 chromosomes from the horse mother and 31 from the donkey father, resulting in a total of 63 chromosomes.
The mule’s 63-chromosome count is an odd number, which is a genetic anomaly. Producing a mule is more successful than producing a hinny, partly because a mare’s uterus is generally larger than a jenny’s, providing a more accommodating environment for the hybrid foal. The willingness of a jack to mate with a mare also contributes to the greater prevalence of mules.
Defining Male and Female Mules
Both sexes of mules are common and have specific terminology. A female mule is known as a molly or a mare mule. A male mule is typically called a John mule or a horse mule. The term “jack” is sometimes used for an intact male mule, borrowing the name from its donkey sire.
The general working mule is overwhelmingly likely to be a gelded male mule (a castrated John mule). This practice is common because uncastrated male mules, also called stud mules, are difficult to handle. An intact male mule retains a high hormonal drive, making its temperament unpredictable and potentially dangerous.
Male mules are routinely castrated to regulate their behavior, making them calmer and more manageable for work. This widespread practice of gelding is the likely source of the inaccurate idea that all mules are male. The physical appearance of the mule is also a blend, inheriting the donkey’s head shape, long ears, and thin legs, but often possessing the horse’s larger size and shape of the rump and tail.
The Biological Basis for Sterility
The mule’s 63-chromosome count is the definitive biological basis for its infertility, regardless of whether the animal is male or female. Reproduction requires the process of meiosis, a specialized cell division that halves the number of chromosomes to create viable reproductive cells, or gametes. During meiosis, chromosomes must pair up precisely with their homologous partner before dividing.
The mule’s set of 63 chromosomes consists of mismatched sets from two different species. The 32 horse chromosomes and 31 donkey chromosomes cannot align into 31 complete, homologous pairs. This chromosomal mismatch prevents the successful pairing required at the beginning of meiosis, a failure known as meiotic arrest.
Because the chromosomes cannot separate in a balanced manner, the mule is unable to produce functional sperm or egg cells. All male mules are considered completely sterile because this meiotic block prevents the full development of viable sperm.
While female mules, or mollies, are also overwhelmingly sterile, there have been a handful of extremely rare, documented cases of a molly carrying a foetus to term. For all practical purposes, however, the mule exists as a sterile hybrid, which means the species must be recreated anew with every generation.