Fishing for clues: fish recognise fish faces
Fish are social creatures capable of learning from one another and some species also have a hierarchy; so it figures that they should be able to recognise one another’s faces… Chris Smith hears how from the University of Tokyo's Mu-Yun Wang...
Mu-Yun: So medaka fish is only about 2 or 3 centimetres long and they live in the rice field in East Asia and they are very popular pets in Japan; so almost all the Japanese kids will have them as pets. Recently, they’ve become very popular molecular models. So we have a good number of molecular toolsets to edit their genome and they're also very good for studying neuroscience. That’s why we are interested in this small fish.
Chris: In particular, what were you interested in studying this time?
Mu-Yun: So, we observe like the female medaka, they prefer the familiar male. During mating, the male will swim in a circle in front of the female and if the female had seen this male before, she will accept him quickly. But if it is an unfamiliar male, then the female will take a longer time to accept this male. It seems they can actually identify different individuals. So even after we exchange the male after familiarisation - so we put one pair of male and female together and then exchange the male. But the female was able to notice this exchange. So, “this is not my boy anymore” so she would take a longer time to accept him. So, it seems they can perform very strict individual recognition.
Chris: Do you think they're doing that by visual processing? They're looking at each other? or might they be using other cues; for instance, smell or perhaps a combination of those things?
Mu-Yun: Yes. So the using of odour cues is very common in fish because an odour cue can spread more widely in the water. But in this case, we try to use visual cue, odour cue, and both cues in the fish and we found, with only visual cue, they are able to accept the familiar male very quickly. So probably, they use only visual cues for individual recognition, and odour cues maybe are not so important in this case.
Chris: How did you physically do the experiment?
Mu-Yun: Well, we would put the male in the transparent cup and would familiarise them for more than 5 hours and then put them together for a mating test. Also, we try to condition them with electric shock so we use a Y maze. When the females stay at the safe males’ side then she will not get punishment. But when she swims to the wrong male’s side, she will receive an electric shock. We found after 40 trials of training, the females were able to stay at the safe males’ side. So with both the mating test and electric shock test, we think the female medaka fish were able to discriminate between different males.
Chris: In other words, in order not to get an electric shock, the female has to recognise which of the males she’s seen before which is the safe male, so she swims the right way. So they're clearly visually recognising the males that they like.
Mu-Yun: Yes. And also, we are wondering which body parts they use for discriminating different individuals. So we try to cover their face and cover their tails. We found that the signal around the face is very important for individual recognition. So, when you cover the heads then the female cannot recognise the male anymore. But when we cover only the tail then they are still able to recognise this male. We also tried the fish inversion effect. So, in humans, faces are very special. So we recognise the faces as a whole so not only with the eyes or nose, or mouth, so we look at the face as a whole. So, if the face is upside down, we cannot recognise the face so fast. This is because we have a special brain part for processing faces and when the face is upside down, the process becomes different. So, we also tested this in medaka fish. We used a prism to invert the medaka faces and we found after they were inverted, the female also cannot recognise the same male. So, I think it’s comparable to the human face inversion effect. Maybe faces are also processed specifically in their brain.