Since many of us no longer live in small agricultural societies, while you might be familiar with the idea of a mule, you might not know about the specifics of the actual animal. Beyond its use as a label for a very stubborn person, you might be aware the mule is a hybrid animal, related to horses and donkeys but properly belonging to neither species. What Makes a Mule? A mule is a cross between a male donkey (also known as a jack) and a female horse (called a mare). The result is a creature that inherits the best traits from both its parents. Humans have used mules for thousands of years because of their strength, stamina and resilience. But what's really cool about mules is their unique status as a sterile hybrid. Because they're a combination of two different species, they can't reproduce, which adds another layer of intrigue to their biology. The first recorded mules appeared in ancient civilizations, with evidence of their use dating back to at least 1000 B.C.E.

Middle East. The modern word "mule" comes from Middle English, which, following the Anglo-French languages, modified the word "mulus" from the Latin. Mules are sterile hybrids, meaning they inherit characteristics from both parents but are unable to reproduce. While horses have 64 chromosomes and donkeys have 62, mules end up with 63, which disrupts their reproductive capabilities. There are also specific terms for different types of mules. But mules can only be born when a male donkey and a female horse mate