Measuring close to 5 feet in length and weighing more than 165 pounds, the giant bumphead parrotfish (Bolbometopon muricatum) is the largest parrotfish in the world and is the largest herbivorous fish living on coral reefs. Scientists have studied their behaviors on Wake Atoll, a pristine reef ecosystem in the North Pacific Ocean that was listed as a U. S. Marine National Monument by President George W. Bush. The scientists have recorded these fish showing extraordinary displays of headbutting among males. According to a paper detailing this behavior in the PLoS One journal, this marks the first documentation of ritualized head butting seen among marine fish. According to the paper, the fish engage in headbutting often, with males propelling toward each other using their caudal fins and then ramming each other with their cephalic hump on their forehead. This is followed by the fish swimming around and trying to bite its opponent on the back or backside.
The ecological role of this species has been studied on Australia’s Great Barrier Reef, where no commercial fishing of this species takes place. That study says that individual fish are capable of bioeroding up to 5 tons of reef carbonate each year during feeding, presumably using its forehead to crush corals. Without this species, the coral reef communities on which they live would be disrupted, including the reduction of reef structural stability caused by invasive erosion by sea urchins and the reduction of sediment transport.
The giant bumphead parrotfish was listed as vulnerable by IUCN in 2007 and in 2010 became an Endangered Species list candidate. Bolbometopon muricatum has been overfished throughout most of its range, and, according to the study in PloS One, is one of the cautious and wary of the parrotfish species toward humans. Check out the video below provided by the authors of the study showing the headbutting.