A fish that scratches a mark only a mirror reveals
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In an aquarium, a small reef fish swims up to a mirror, then starts scratching a coloured spot on its body, in a place it can only see through its reflection. Until now, this kind of behaviour was known in only a few animals.
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The fish is the bluestreak cleaner wrasse (Labroides dimidiatus), which cleans the skin of other fish by eating their parasites. In one experiment, researchers placed a coloured mark on its body, invisible without a reflection. The fish tried to scratch it only when the mirror was present, and never when the mark was transparent.
The mark test is used to measure self-awareness. So far, it has been passed by a small number of vertebrates, including dolphins, Asian elephants and magpies. Most species tested fail it, including large parrots.
The authors themselves remain cautious: the test observes behaviour and does not measure self-awareness directly. A follow-up study reported similar results, but it was carried out by the same teams. The debate is therefore still open.
