Wolves have incredibly good hearing and can hear sounds up to six miles away, including some high-pitched sounds that even a human can't hear. (http://www.wolfsource.org/?page_id=70)

A wolf needs to be alert at all times in case of danger. When a wolf is sleeping, its ears will be in an upright position, so it can catch any sounds made by other animals. A wolf can use each ear on its own, turning the ear to locate where a noise is coming from.