It might not seem like a big deal, but the ability to recognize that the face in the mirror is your own makes us part of a pretty exclusive club in the animal kingdom. Humans obviously can do it (from about 15 months of age), and so can apes, monkeys, dolphins, elephants, and some birds. Now a fish species has passed the mirror test for the first time, which may suggest that the animals are smarter than we give them credit for.

Researchers from the Max Planck Institute, the University of Konstanz and Osaka City University conducted the experiment with a species called the cleaner wrasse. The team put a colored mark on each fish in a place that can only be seen in reflection, then held up a mirror. The idea was to check whether the fish could tell they were looking at themselves and would try to clean the mark off. Since these fish are naturally in the habit of cleaning parasites off other fish, they're fairly well-adapted to spotting marks like these.

Sure enough, after seeing their reflection the fish rubbed themselves on hard surfaces, trying to clean the marks away. That suggests that the creature understood that the reflected image was itself and not a neighboring fish – an ability that was long thought to be beyond their capabilities.

The team investigated other scenarios to rule out any false positives. To make sure the fish weren't just reacting to the act of being marked, the scientists gave the fish transparent marks, and they didn't try to wipe them off after looking in a mirror. They also didn't react when they were marked but not shown their reflection at all.

The researchers also tried to outsmart the fish by marking the mirror itself, or placing a marked fish on the other side of a clear divide in the tank. In neither case did the fish try to clean itself, which adds evidence that the creatures understood when they were seeing the mark on their own bodies.

While these results suggest that fish are self-aware, the team isn't jumping straight to that conclusion – perhaps they're using other mental processes to figure it out. The fact that the fish have aced the mirror test is so surprising that the researchers are now questioning the validity of the test itself.

"Personally, I find the most parsimonious interpretation to be that these fish do pass the test as given, but this doesn't mean they are self-aware," says Alex Jordan, corresponding author of the study. "Rather they come to recognize the reflection as a representation of their own bodies without the involvement of self-consciousness. Given this, we should critically evaluate whether the mark test remains the gold-standard for awareness testing in animals."

Either way, it seems like we're constantly discovering that animals like fish, and even bees, are smarter than we thought.

The research was published in the journal PLOS Biology.

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