When you hear this phrase, a picture probably pops into your mind -- someone trying to drag a mule forward by a rope while the mule resists, digging its hooves into the dirt and refusing to budge. Or maybe you're picturing a donkey. If so, are both animals stubborn? For starters, let's discuss their differences. A mule is not an animal species, like a horse or donkey. It's a hybrid, or the product of two other species -- in this case, the pairing of a male donkey with a female horse. Donkeys have 62 chromosomes and horses have 64; mules are born with 63. This odd number of chromosomes means they can't reproduce [source: Lucky Three Ranch].
Donkeys and mules both have reputations as animals with, um, mulish personalities. They're widely seen as stubborn. Willful. Obstinate, even. Guess what? They aren't. A study done by Canterbury Christ Church University and Devon's The Donkey Sanctuary showed that when it came to showing flexibility toward solving a problem (learning to learn), mules came out on top, followed by donkeys, with horses and dogs bringing up the rear. So why the common misperception? Mules -- and donkeys -- are smart. Really smart. They also have a deep-seated tendency toward self-preservation. So they won't let owners overwork them, nor will they typically put themselves in danger. These characteristics led to the "stubborn" label [source: Canterbury Christ Church University].