Researchers have discovered that a tiny tropical fish sizes itself up in a mirror before attacking another fish, suggesting they have sophisticated self-awareness.
The bluestreak cleaner wrasse – a finger-sized, territorial coral reef fish – was the first fish to pass the ‘mirror test.’
Scientists use this assessment to determine whether an animal recognizes its reflection as its own body and not as another animal.
Now, a new study has found that this fish not only knows when it’s looking at itself, but actually uses its reflection to compare its own body size to the size of a threat.
A new study has found that bluestreak cleaner wrasse fish size themselves up in a mirror before attacking another fish
‘Our study established cleaner fish as the first non-human animal to be demonstrated to possess private self-awareness,’ the researchers said in their report.
The research team, led by Taiga Kobayashi at Osaka Metropolitan University in Japan, performed two types of tests on 15 wrasse fish.
Each fish was tested individually, and the team published their findings in Scientific Reports.
The first test assessed the fish’s willingness to go on the attack. The researchers showed a wrasse inside a fish tank an image of another wrasse that was either 10 percent larger or smaller than the live fish.
No matter the size of the fish in the photo, the real ones attacked.
Next, the researchers tested whether the presence of a mirror would influence the fish’s decision to fight.
They repeated the first test while also showing the live fish a mirror.
The tank was partitioned to prevent the fish from seeing their own reflection and the photo at the same time.
After looking at themselves in the mirror, the fish picked a fight with the photo of a smaller fish, but not the photo of a larger one
These findings should help scientists better understand self-awareness not just in other animals, but in humans too
This time, the researchers found that the live fish checked their reflection before deciding whether or not to fight, and this seemed to influence their choice.
After looking at themselves in the mirror, the live fish only attacked the image of the smaller fish – not the larger one.
This suggests that the fish sized themselves up against the threat, weighing the risk of attacking before making a decision.
‘This was unexpected because we had an image that this fish always shows aggression against rivals, regardless of size,’ Kobayashi told New Scientist.
This is the first time that a non-human animal has demonstrated some mental states – such as mental body image, standards, intentions and goals – that are elements of ‘private awareness,’ or awareness of one’s own internal state.
‘These findings suggest that cleaner fish would likely possess two mental states: a mental image of their body and internal standards for comparing body size,’ the researchers stated in their report.
But there are no mirrors in the ocean – which raises an important question: how did the bluestreak cleaner wrasse evolve to have this unique skill?
In both nature and in the lab, it benefits fish to be able to compare their own size to that of a threat before attacking, Kobayashi told New Scientist. The results of this study suggest that the fish are smart enough to use the mirror as a decision-making tool, he said.
These findings have important implications for our understanding of self-awareness not just in other animals, but in humans too.
‘The results that fish can use the mirror as a tool can help clarify the similarities between human and non-human animal self-awareness and provide important clues to elucidate how self-awareness has evolved,’ Kobayashi said in a press release.