The capacity to breathe air is rare in fish, but it does exist. Why, then, do only two percent of fish species develop a function that has proven so useful for mammals, birds and reptiles? According to one study, it's about a lack of personality.
Air-breathing can only be done at the water's surface, which also tends to be where predators can be found. “Air is a rich source of oxygen but breathing it is risky, so it made me wonder if it might be linked to personality,” said Dr. David McKenzie of CNRS Montpellier in a statement.
Studies of animal personality have concentrated on the shy-bold axis, as this appears to explain the most about animal behavior, although it is far from the whole story. McKenzie reasoned that if fish were deterred from coming to the surface by predators, the boldest in species that can breathe air would make most use of their ability.
African sharptooth catfish (Clarias gariepinus) can breathe directly from the air or use gills to extract oxygen dissolved in water. C. gariepinus are an invasive species in Brazil, so McKenzie decided to do his bit by collecting 29 from São Paulo rivers.
McKenzie reported his findings in a paper published in the Journal of Experimental Biology, fabulously titled “To Boldly Gulp.” He allowed the catfish to rest for 24 hours after being collected and then tested the ratio of oxygen obtained from air and water.
Researchers then tested the personalities of their captives by tapping the tanks and measuring the delay before they went back to breathing the air. “We assumed that how quickly they returned to air breathing was a measure of how bold they were,” McKenzie said.
The study tested the fish in water with different levels of dissolved oxygen, including some so rich the catfish didn't need to come to the surface at all. Even in fully oxygenated water, some catfish chose to come to the surface, but unsurprisingly they did so much more in stagnant waters where oxygen levels were low.
There was a strong correlation between a high metabolism and extra air-breathing, but personality mattered as well. Catfish that returned to the surface within 75 minutes of being frightened were likely to breathe more often during daylight than the shy ones who took more than 115 minutes before resurfacing.
High-metabolism fish breathed more air when placed in oxygen-poor water, but were not more likely to surface than their low-metabolism counterparts when dissolved oxygen was available. On other hand, shy fish breathed less air during the day, when predators would be a threat, but were as likely to surface at night as their bold cousins.
While high-metabolism fish were bolder on average, the correlation was not strong enough to make this a simple case of high metabolisms forcing personality changes. “Some animals take risks because they have to,” McKenzie said. “Others seem to take risks just because they want to.”
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