Fish recognise other fish faces

24 August 2017
Posted by Chris Smith.

Small fish, which are popular as pets in Japan and also favourites with genetics researchers, can tell each other apart, a new study has revealed. The research sheds light on a powerful mechanism that runs throughout the animal kingdom from penguins that can find their chick among thousands to humans that can distinguish a face in less than a quarter of a second.

Medaka, which are also called Japanese rice fish (Oryzias latipes) look like small goldfish. Common across Asia, they have been frequent inhabitants of paddy fields in the past, but now are frequently found swimming around in bowls in people's homes, or in fish tanks at research facilities. Scientists like working with them because they are easy to keep, grow and reproduce rapidly and also have a well-documented genetic fingerprint. 

But now we can add another attribute, which is that these fish have a discerning eye for mates, to the point where, University of Tokyo scientist Mu-Yun Wang has discovered, they can recognise the faces of other members of their shoals. And, just like humans, if presented with inverted images of their mates, they take much longer to recognise them than when the pictures are presented the right way up, suggesting that they have a dedicated system in their brains for processing faces.

Working with her colleague Hideaki Takeuchi, and writing in the journal eLife, Wang allowed medaka females sight of a potential mate over a 5 hour period. The female was then offered either the beau she had been staring at, or an alternative amour, and the researchers clocked the time it took her to accept the match. Females accept a familiar male much more rapidly than a newcomer, and in this instance the males that had been seen in advance were taken by the females significantly faster than the unrecognised animals.

In a second set of tests the researchers put fish into a tube with 2 arms and a different fish, clearly visible, in each arm. If the test fish swam towards one of the fish it received an electric shock, but the other fish was "safe". In multiple trials, in which the locations of the safe and "shock" fish were swapped at random, the test fish became adept at avoiding a shock and swimming into the arm where they could see the "safe" fish.

Faces were very important to the medaka fish, Wang found. In various tests, she covered either the head or tail of test fish and showed that the recognition only reliably took place when the face was visible. Because the researchers themselves couldn't see any discernible differences between the fish faces they also tried painting dots onto some of the test fish faces, but this did not appear to affect their accuracy.

"It might be down to how they see colour," says Wang. "Medaka have 8 colour-sensing pigments in the eye but humans have only 3, so they may be seeing patterns that we can't."

Incredibly, medaka also seem to succumb to a trick that trips up humans looking at faces: the researchers presented inverted images of fish that were either vertically or horizontally flipped. Left to right inversion made no difference, but a fish face seen upside down led to very poor recognition. The same happens in humans, with facial recognition speeds plummeting when viewing faces upside down. 

This, Wang and Takeuchi argue, suggests that, like us, these fish have a dedicated set of circuits in the brains that specifically process faces. And because these animals are so well understood and so well studied, there are many genetic tools available that can be used to unpick how the fish brain may be doing this, which may in turn show us where to go fishing inside our own brains for the mechanism of face recognition...

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