Elephants, dolphins, orcas, great apes and magpies (a type of bird) have all demonstrated the ability to recognize themselves in mirrors. Except maybe the magpie, these are the species we would expect to be able to recognize themselves.
But the exclusive club of animals that has passed the mirror test just got an unexpected new member: a little fish that's only about as long as your pinky finger.
First things first: What is the mirror test?
You may have heard of the mirror test as a way of measuring animals' intelligence. Researchers will place a mark somewhere on an animal's body, and then give them the opportunity to look in the mirror. If the animal looks in the mirror and then interacts with the mark on its own body, it has passed the test.
Until magpies passed the test, many biologists assumed that only mammals were capable of self-recognition. Now that a fish has proven its ability to recognize itself, we again must reconsider the prevalent idea that only animals with large, complex brains are capable of self-awareness.
Meet the bluestreak cleaner wrasse.
Wikimedia/Matthias Kleine (selbst fotographiert)
The bluestreak cleaner wrasse is a little reef fish that cleans parasites and dead skin from bigger fish. In the past, researchers have observed these fish exhibiting behaviors that suggests a high level of intelligence.
For example, according to the Atlantic, cleaner wrasse will sometimes sneak a bite of the protective mucus layer on a fish they are cleaning — but they will only do so if the client is not watching. Cleaner wrasse also prioritize serving large predators like sharks or grouper before smaller fish.
Now, a new experiment has shown that when a cleaner wrasse is marked and then placed in front of a mirror, the fish will try to remove the mark.
The experimenters also placed an invisible mark on the fish, to make sure they were not reacting to the sensation of being touched. But the fish did not attempt to remove the invisible mark — only the one they could see in the mirror.
What does this say about animal intelligence?
This experiment has not yet been peer-reviewed and published in a journal, but the preliminary findings suggest that self-recognition may be more widespread throughout the animal kingdom than we originally thought. Many mammals have failed to recognize themselves in the mirror, while magpies, cleaner wrasse and even ants seem to have succeeded.
Does this mean cleaner wrasse are more intelligent than dogs, for example, who have failed the mirror test? Not necessarily. A study published last year showed that dogs actually can recognize themselves by scent. They don't fail the mirror test because they have no self-recognition ability, but rather because sight is not their prominent sense.
What we are learning is that making generalizations about animal intelligence is very tricky, because each species expresses intelligence in unique ways. As we invent more creative ways to test animal intelligence, we will probably find that many species are much smarter than we previously gave them credit for.