Are these two all that different? In species that practice "male combat", the males don't force the females to mate with them. The females "choose" to mate with the winners of the combat ritual.
In fact, the peacock's tail is just another form of "male combat", except that the males strut around displaying their tails rather than butt heads.
My guess is that if we think real hard, we could probably come up with a fairly smooth continuum of behaviors from rams butting heads to peacocks displaying tails. (Song birds come to mind, for example.)
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