10,000 Years of Sterile Offspring
Humans have been breeding mules (horse × donkey) for over 10,000 years, despite knowing that nearly all mules are sterile and can't reproduce.
Mules are the quintessential hybrid — the offspring of a male donkey (jack) and female horse (mare). Humans have been breeding them for at least 10,000 years, possibly longer.
The key fact about mules: they're almost always sterile. A mule can't make more mules. Every single mule must be bred anew from a horse and donkey.
Why breed them anyway? Mules combine the best traits of both parents: - Strength of a horse - Endurance of a donkey - Surefootedness of a donkey - Speed (faster than donkeys) - Longevity (often outlive both parent species) - Intelligence (considered smarter than horses)
Why are they sterile? Horses have 64 chromosomes; donkeys have 62. Mules inherit 32 from mom and 31 from dad, giving them 63 — an odd number that can't pair up properly during reproduction.
Rare exceptions exist. About 60 cases of fertile female mules have been documented throughout history. These are so unusual that the Romans had a saying: *"cum mula peperit"* ("when a mule foals") — meaning "when pigs fly."
The reverse cross (male horse × female donkey) produces a hinny, which is similar but typically smaller and less common because the cross is harder to achieve.
Mules demonstrate that humans have long understood hybrid vigor and selectively bred for it, even without understanding genetics.