Biologists think that the giant bumphead parrotfish use their big, bumpy head to break corals into tiny chunks, but Roldan Munoz of the National Marine Fisheries Service has found another reason: headbutting rivals.
Ed Yong of Discover's Not Exactly Rocket Science wrote:
While watching the parrotfish at Wake Atoll, in the middle of the Pacific, Munoz’s team heard “loud jarring sounds”. They soon found that the males were smashing their heads together head-on, and then trying to bite each other in the flanks.