"Male European red deer have a distinctive "roar"-like-sound (not to be confused with actual roars made by lions, panthers and the like) during the rut, which is an adaptation to forested environments, as opposed to male wapiti (or American elk) which "bugle" during the rut in adaptation to open environments. The male deer roars to keep his harem of females together. The females are initially attracted to those males that both roar most often and have the loudest roar call. Males also use the roar call when competing with other males for females during the rut, and along with other forms of posturing and antler fights, is a method used by the males to establish dominance. Roaring is most common during the early dawn and late evening, which is also when the crepuscular deer are most active in general."
(This is a composite of two photos taken at Strathglass)