Humans aren’t the only species with left-handed and right-handed dominance. Many animals, including chimpanzees, rats, mice, toads and even insects share this trait. But fish?

Researchers at Toyama University’s Graduate School of Medicine and Pharmaceutical Sciences have been studying the predatory, scale-eating cichlid (Perissodus microlepic) from Africa’s Lake Tanganyika. They have found the cichlid to be the model organism for demonstrating the development of behavioral laterality.

Though scientists have been aware of laterality and documented its advantages in many animals, the means by which left or right preference is acquired during development (brain and organ development) has been unclear until now.


The researchers discovered that cichlids gradually acquire the trait during development as they learn which is the more effective side of their mouth for tearing off scales to eat. They were able to determine the age at which fish transition from eating scales from both sides of their prey to preferentially eating one side by taking measurements of the lower jawbone, revealing a gradual increase in mouth asymmetry as the fish age.

The researchers' fieldwork concludes that mouth asymmetry precedes behavioral acquisition and that behavioral laterality in this species is a learned strategy that develops in association with morphology.

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Cichlid, showing the pronounced right-morphing (a) and pronounced left-morphing (b)

Adult left-morphing fish open its mouth toward the left and preferentially feed on the scales of the right side of its victim. Adult right-morphing fish bite from the victim’s left side. This facilitates efficient scale eating.

Yuichi Takeuchi, lead author of the study writes,

This is a truly important study because it allows us to observe mouth direction development with age and the relationship between behaviors laterality and mouth asymmetry in these fish. We can now also address the question of which came first, scale eating or mouth asymmetry.

The genetic basis of mouth asymmetry, however, has yet to be determined.

Now we know that fish can be southpaws … or would it be southmouths?

Nancy Loyan Schuemann

Nancy Loyan Schuemann is a writer specializing in architecture, safes, profiles, histories and a multi-published fiction and non-fiction author and is Nailah, Middle Eastern dancer.

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