Parrotfish are more aggressive with strangers than neighbors

Neighbors can often be annoying, intruding on our personal space or being too loud. But is it worthwhile to fight them? Parrotfish, for instance, choose not to battle it out with their neighbors. 

In a new study led by the University of Colorado Boulder, researchers revealed that the stoplight parrotfish – a brightly colored species living in the shallow waters off Florida and in the Caribbean Sea – tends to behave more tolerantly toward neighboring parrotfish, but rather aggressively toward strangers.

Ecological role of parrotfish 

The experts spent days underwater observing parrotfish, to gather new insights into a species which plays a major role in maintaining healthy coral reefs, while also contributing to the white sand beaches of the Caribbean.

“Parrotfish are an important part of the coral reef ecosystem and the ecological functions it provides,” said lead author Joshua Manning, a postdoctoral researcher in the Department of Ecology and Evolutionary Biology at CU Boulder. “Understanding their behavior will help us evaluate whether and how they can buffer the effects of climate change on coral reefs.”

Parrotfish with beak-like teeth 

Stoplight parrotfish are some of the largest parrotfish species in the Caribbean reefs, measuring up to 1.5 feet long. They sport strong, beak-like teeth which allow them to spend nearly 90% of the day eating microscopic organisms that grow on and within the calcium carbonate structures produced by corals. 

As the parrotfish remove coral skeletons, they make space for new corals to grow and produce white sand as their digestive waste. 

Claiming their territory 

Since they are highly territorial animals, male stoplight parrotfish defend territories as large as two tennis courts, where they spend their time foraging and mating with a small group of female followers. 

However, not every parrotfish has a specific territory. Some of them are “floaters,” as Manning calls them, constantly scouting the reefs and claiming space that becomes available. 

Parrotfish respect their neighbors

As an experienced diver, Manning spent over 400 hours underwater during his doctoral studies trying to understand how the stoplight parrotfish behave and interact with one another. His observations revealed that the fish might be smarter than previously thought. 

By following ten spotlight parrotfish off the coast of Bonaire, a Caribbean island, he discovered that each time a floater swam by an occupied territory, the territory holders would puff up, displaying their fins and aggressively chasing the floater away. 

Surprisingly though, when parrotfish from neighboring territories approached the boundary lines, territory holders appeared to be much less aggressive. 

The “dear enemy” effect

When parrotfish did behave aggressively toward their neighbors, it was most frequently because they had strayed too far into other parrotfish’s territory while chasing another parrotfish.

This “dear enemy” effect, as scientists call it – when territory holders show less aggression toward neighbors than strangers – has also been observed in frogs, squirrels, sparrows, and several other animal species. However, this was the first time this phenomenon was discovered in parrotfish. 

According to Manning, parrotfish might be more aggressive toward floaters because they are more likely to try to take over their territories. 

Moreover, since territory-holding males have better mating opportunities, their continuous patrolling and defense efforts take a toll on their physical condition and, as a consequence, they have to focus their energy on fighting off the most critical threats.  

The capacity to learn and use information 

To the scientists’ surprise, floaters appeared to recognize territorial boundaries. For instance, floaters often swam across the reefs using the buffer zones between established territories in order to avoid aggression.

“These fish may be smarter than what we give them credit for. They seem to recognize neighbors, find the boundaries of territories, and have the capacity to learn and use information,” Manning said. 

Parrotfish accelerate reef recovery 

Due to global warming, coral reefs worldwide are declining rapidly. Between 2023 and mid-May 2024, experts have found mass coral bleaching – which happens when corals expel the algae living in their tissues due to stress caused by high ocean temperatures and other climatic factors – in 62 countries and territories worldwide. 

Since parrotfish depend on coral reefs for shelter and food, coral loss due to ocean acidification and warming can have major impacts on their habitat and populations. Yet, parrotfish are able to accelerate reef recovery from bleaching events by creating space for coral larvae to settle and grow.

“Reefs are a vital source of food for us and support immense biodiversity, including species with significant medical potential,” said Manning. 

“By studying how parrotfishes use space and how their grazing influences coral recruitment patterns, we can better understand how reefs can recover from disturbances and adapt to climate change.”

The study is published in the journal Ecology.

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