Before diving into a brawl, some fish take a moment to check themselves out in the mirror. This intriguing behavior has been observed in the bluestreak cleaner wrasse (Labroides dimidiatus), which appears to assess its size in the mirror before deciding whether to confront another fish, whether larger or smaller. This discovery comes from a recent study published in Scientific Reports.
The researchers suggest that this might be the first instance of non-human animals demonstrating a mental body image.
Osaka Metropolitan University
“Cleaner fish will be the first non-human animals to be shown to possess private self-awareness,” the researchers noted in their paper.
Self-awareness in animals is often evaluated through a method called the mirror self-recognition (MSR) test, which determines if an animal can recognize its own reflection. Successfully passing this test indicates a level of self-awareness, as the animal understands that the reflection represents itself, not another creature.
“Self-awareness refers to the ability to focus on oneself and recognize one’s own existence. The MSR test is widely regarded as a reliable indicator of self-awareness,” the authors explained.
Previously, animals like great apes, elephants, dolphins, and Eurasian magpies have been documented as passing the MSR test, and now the bluestreak cleaner wrasse joins this list.
What’s fascinating is that the study shows the cleaner wrasse not only recognizes itself in a mirror but actually uses its reflection to inform its decision-making.
This small, vibrant fish is known for its cleaning habits, removing parasites and dead skin from larger fish. It’s a mutually beneficial relationship: the wrasse gets food, while the larger fish get a clean slate.
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Bluestreak cleaner wrasses are protective of their cleaning stations, which can lead to confrontations when another cleaner enters their territory. Like many fish species, they can also exhibit aggressive behavior in mate competition.
In the study, researchers showed the fish photographs of other cleaner wrasses that were either 10% smaller, the same size, or 10% larger than themselves. After looking into the mirror, they tracked the fish’s behavior toward the images.
The results revealed that while the fish displayed aggression towards all sizes, they were notably less aggressive towards the larger and same-sized fish than towards those that were smaller.
“The findings suggest that fish trained with mirrors might reduce their aggression not simply due to repeated exposure but because they are better able to gauge size differences by understanding their own reflection,” the researchers stated.
This indicates that these fish are capable of building a mental image of their body and have standards for assessing their size. Additionally, when confronted with a larger fish’s image, the cleaner wrasse swam back and forth between the mirror and photo, suggesting they were anticipating the potential outcome of a confrontation, hinting at their ability to form intentions and goals.
“This study significantly adds to the evidence of various mental states, providing stronger support for the idea that cleaner fish possess private self-awareness,” concluded the researchers.
References
Kobayashi, T., Kohda, M., Awata, S., Bshary, R., & Sogawa, S. (2024). Cleaner fish with mirror self-recognition capacity precisely realize their body size based on their mental image. Scientific Reports, 14(1). https://doi.org/10.1038/s41598-024-70138-7