A tiny fish has shown signs of a remarkable level of intelligence in mirror test experiments, not only recognizing itself in the reflection but also using a piece of food to explore how the mirror works.
Mirror mark tests are a standard scientific experiment used to explore animal self-recognition, gauging how similar other species' intelligence might be to our own.
Fixing your smudged eyeliner on seeing your reflection is a clear sign of self-recognition. You know the smudge on your face is out-of-place, and you can use the reflection as a proxy for pinpointing exactly where to wipe the smudge away.
In theory, a reaction to an out-of-place mark in a reflection is assumed to be evidence of self-recognition for other animals as well. Chimpanzees, elephants, and dolphins are just some that have passed this mark test, which many regard as a sign of intelligence similar to our own.
The cleaner wrasse (Labroides dimidiatus) is a finger-sized marine fish that earned its name by eating the parasites and dead tissue it finds on other, larger fish. It was first reported to pass the mirror test in 2018.
It's not hard to see why the cleaner wrasse would make for a good candidate. Its modus operandi is cleaning stuff off fish, so perhaps if it saw a parasite-like mark on its own body, it would similarly be compelled to use the mirror to 'freshen up'.
Though the 2018 study reported that the fish had passed, the mirror mark test's founder, evolutionary psychologist Gordon Gallup, told New Scientist he believed the fish in these experiments were mistaking the marks on their own bodies for parasites on other fish.
Now, a team from Osaka Metropolitan University in Japan and the University of Neuchâtel in Switzerland has tweaked the experiment to test the wrasse's self-awareness even further.
"In earlier cleaner wrasse mirror studies, the procedure was typically the fish see a mirror for several days, they habituate to it and stop reacting socially, and a mark is added," animal behavioralist Shumpei Sogawa from Osaka Metropolitan University in Japan explains.
"In this study, the order was reversed; the fish were marked first, then the mirror was introduced for the first time. The fish were likely aware of something unusual on their body, but they couldn't see it. When the mirror appeared, it immediately provided visual information that matched an existing bodily expectation, hence scraping occurred much faster."
While it's far from a perfect control, this goes some way to improving the initial experimental design, giving the fish time to identify the 'parasite' on their own body before encountering their reflection.
The scientists were surprised by how fast the fish reacted in these new experiments: on average, they tried to rub off the 'parasite' within about 82 minutes. This, they say, implies self-awareness before being exposed to the mirror.
After the fish had several days to get used to the mirror, the scientists noticed some performing an unusual behavior. They would pick up a little piece of shrimp from the bottom of the tank, carry it over to the mirror, and drop it.
As the mirror-shrimp fell in sync with the real thing, the fish followed the reflection closely, touching the mirror surface with their mouths.
Sogawa and team believe this is the fish's way of exploring the mirror's properties, using an object separate from themselves to better understand how the reflected images work.
This kind of 'contingency testing' and mirror tool use has been observed in several other species that failed the mark-based mirror test, including pigs, rhesus monkeys, manta rays, and corvids.
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"The findings from this research will likely influence not only academic issues, such as revising evolutionary theory and constructing concepts of self, but also directly impact matters relevant to our lives, including animal welfare, medical research, and even AI studies," says biologist Masanori Kohda, who was involved in both this new study and the original paper on cleaner wrasse self-recognition.
The team says these findings suggest self-awareness, once thought to be unique to great apes, is actually a skill that may have arisen in a much wider range of animals, including fish.
"Our results suggest that self-awareness evolved at a minimum with the bony fishes (450 million years ago) and is likely widespread across vertebrates," they write.
The research was published in Scientific Reports.