Giant reef fish found to be marine head-butters

Jun 11, 2012 by Bob Yirka report
Sexual dimorphism in Bolbometopon muricatum. Image from doi:10.1371/journal.pone.0038120.g002

(Phys.org) -- Because humans have a history of disrupting marine environments when they move into an area, mainly due to overfishing, few places in the oceans of the Earth remain as they were before people arrived. And because of that, it’s been difficult for biologists to observe some species of fish in their so-called natural environment. The aptly named giant bumphead parrotfish is just one such example. It’s the largest species of parrotfish in the world and one of the largest of all reef dwellers. At one time it was also very common. But like many other species of fish, its numbers have dwindled to the point where it’s now on the endangered list and over time has learned to fear the presence of human beings. And it’s likely for these two reasons that until now, no one had seen, or at least documented, an important ritual performed by the males of the species; head butting one another. Researchers discovered this behavior recently when studying the fish in a protected part of Wake Atoll. They’ve filmed the fish in action, and have published a paper in PLoS One describing what they found.

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Ritualized headbutting of Bolbometopon muricatum at Wake Atoll. Video: doi:10.1371/journal.pone.0038120

The nearly human sized fish are not what most would call comely, both genders sport a large bumpy ridge on their foreheads and they have a beak sort of like a parrot, hence their name; or at least that was thought to be how they got their name. It might have instead come about because the males of the species, like bighorn sheep and some of deer, ram one another head-on as an apparent mating ritual. Afterwards, they circle around one another like dogs trying to bite each other. Because the behavior has only now been discovered, researchers will have to look a little deeper to discover if the fish are actually doing it for the same reasons.

Such aggressive behavior comes as a surprise to researchers both because it’s never been observed before, but also because the bumphead appears to be otherwise very docile. Prior to this discovery in fact, most ocean biologists had assumed the ridge on its head was used to knock pieces of reef loose, its main source of food; this despite the fact that no one had ever seen the actually engage in such a practice.

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More 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

Abstract
Human impacts to terrestrial and marine communities are widespread and typically begin with the local extirpation of large-bodied animals. In the marine environment, few pristine areas relatively free of human impact remain to provide baselines of ecosystem function and goals for restoration efforts. Recent comparisons of remote and/or protected coral reefs versus impacted sites suggest remote systems are dominated by apex predators, yet in these systems the ecological role of non-predatory, large-bodied, highly vulnerable species such as the giant bumphead parrotfish (Bolbometopon muricatum) has received less attention. Overfishing of Bolbometopon has lead to precipitous declines in population density and avoidance of humans throughout its range, contributing to its status as a candidate species under the U. S. Endangered Species Act and limiting opportunities to study unexploited populations. Here we show that extraordinary ecological processes, such as violent headbutting contests by the world’s largest parrotfish, can be revealed by studying unexploited ecosystems, such as the coral reefs of Wake Atoll where we studied an abundant population of Bolbometopon. Bolbometopon is among the largest of coral reef fishes and is a well known, charismatic species, yet to our knowledge, no scientific documentation of ritualized headbutting exists for marine fishes. Our observations of aggressive headbutting by Bolbometopon underscore that remote locations and marine reserves, by inhibiting negative responses to human observers and by allowing the persistence of historical conditions, can provide valuable opportunities to study ecosystems in their natural state, thereby facilitating the discovery, conservation, and interpretation of a range of sometimes remarkable behavioral and ecological processes.

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