Fishery biologists led by Dr Roldan Muñoz of NOAA’s Beaufort Laboratory have for the first time observed headbutting contests between males of the world’s largest parrotfish.

While observing large schools of the Green Humphead Parrotfish at Wake Atoll, the researchers heard loud jarring sounds. Surprisingly, they found that the sounds arose from violent impacts between males engaged in repeated, ritualized headbutting behavior.

“During headbutting bouts, males utilized their caudal fins to rapidly collide with their cephalic humps, immediately followed by fast swimming in a semicircle where each fish tried to bite the back and flank of its opponent,” the scientists explain in a paper in the open-access journal PLoS-ONE.

The Green Humphead Parrotfish, Bolbometopon muricatum, also known as Bumphead Parrotfish, Buffalo Parrotfish and Giant Parrotfish, is the largest species of parrotfish growing to lengths of 1.5 m and weighing up to 70 kg.

“How could this dramatic aspect of its social and reproductive behavior have gone unnoticed,” the researchers emphasize in the paper.

They propose two reasons: “i) low population densities resulting from overfishing dampen competition for resources (females or spawning territories) and/or disrupt the social system so that headbutting contests are uncommon and no longer advantageous; ii) headbutting contests are common, but negative responses to humans in exploited populations preclude observations of natural behavior.”

The team was also able to capture an entire headbutting bout on high definition video, consisting of four, successive charges between two males of B. muricatum.

The biologists note that the unusual aggressive headbutting behavior by the Green Humphead Parrotfish is the first of this type observed in marine fishes.

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Bibliographic information: Muñoz RC, Zgliczynski BJ, Laughlin JL, Teer BZ. 2012. Extraordinary Aggressive Behavior from the Giant Coral Reef Fish, Bolbometopon muricatum, in a Remote Marine Reserve. PLoS-ONE 7(6): e38120. doi:10.1371/journal.pone.0038120