Tail-flicking, tail-flagging, and tail position in ungulates with special reference to black-tailed deer

Testing the functional significance of antipredator behaviors is exceedingly difficult given their potential dual functions (i.e., conspecific warning and pursuit deterrence), and this task is made even more difficult when the same term is used to describe different behavioral patterns by different authors. Tail movements and rump patch exposure in ungulates are well-documented phenomena, but the functional significance of each has remained elusive. I discuss the confusion in the literature regarding the definition of tail movements, redefine two behavioral patterns and three tail positions, and examine their performance in Columbian black-tailed deer (Odocoileus hemionus columbianus) in response to an approaching human. 'Tail-flicking', the brief swaying of the tail to one side or the other, was most commonly associated with feeding behavior, and 'tail-flagging', the lateral side-to-side movement of the tail in rapid succession, was common during both feeding and alert behaviors. Tail-flicking and tail-flagging: (i) were performed by both calm/foraging deer and alarmed deer; (ii) probably lack naturally selected antipredator function but may have the benefit of providing public information to conspecifics about potential danger, and (iii) likely lay at opposite ends of a continuum of tail movements that are the result of agitation and motor reflexes involved in locomotion, where flagging indicates greater agitation than flicking. Deer held their tail 'erect' during bouts of rapid flight, including stotting and trotting, and erected their tails sooner after flight during more threatening approaches. Tail erection as a behavioral pattern is an antigravity reflex performed when preparing for locomotion and probably lacks antipredator function. However, tail erection likely facilitated the evolution of bright rump patches in ungulates, which were probably shaped by natural selection to serve multiple antipredator or thermoregulatory functions.