Intelligence, agility and stamina are all characteristics of mules. These qualities combine with a quirk of nature to make mules excellent pack animals. Mules are intelligent and possess a strong sense of self-preservation. You just can't make a mule do something it thinks will hurt it, no matter how much persuasion you use. So, some people confuse this trait with stubbornness.
Mules form close bonds with horses, especially mares.
The bond is so close that mules willingly follow a mare. A wrangler, or mule skinner, can usually control an entire pack string simply by controlling the bell mare. At night in the backcountry mule skinners can picket the bell mare and turn the mules loose. The mules will disperse and graze freely yet remain close to the mare. Environmental impacts are reduced and the mules are easy to gather in the morning.