Mule Deer have large ears that move constantly and independently, hence they get their name, "Mule" or "Burro Deer."
The deer ears are critical to its survival, and they can easily detect a faint sound, and in a split second determine if danger is present. The ears contain many complex muscles, which allow free movement of the ears with incredible control. The deer's ears can rotate in any direction, allowing the animal to determine the direction of the sound. Hearing is so acute that the deer can detect the time it takes for a sound to reach one ear relative to the other; thus allowing the deer to establish how far away a sound is. If several deer are together, the ears can be a form of communication, and a simple flick of the ears can warn the others of danger.
Here's a good example of how the ears rotate behind the deer: (Compare to photo above)
(http://www.wrightwoodcalif.com/nature/6_27_deerEar1.JPG)
(http://www.wrightwoodcalif.com/nature/6_27_deerEar2.JPG)