A Tiny Reef Fish Can Recognize Itself in a Mirror


It’s something most of us do every morning without a second thought. We wake up, stumble to the bathroom and glance at ourselves in the mirror as we wipe the sleep from our eyes. It may not seem like much, but the simple act of looking at that mirror—and understanding that the eye-rubbing person staring back is really one’s own reflection—demonstrates a remarkably sophisticated level of understanding.

Only a handful of the world’s other brainiest species have proved capable of this: chimpanzees, bonobos, gorillas, orangutans, bottlenose dolphins, killer whales, magpies and at least one Asian elephant. But now a group of researchers based in Japan and Germany has discovered the bluestreak cleaner wrasse(Labroides dimidiatus) a coral reef fish found throughout the Indo-Pacific, may have the same ability. What remains to be seen is what this says about the “mirror self-recognition” test, a longtime scientific standby for assessing self-awareness.

Mirror, Mirror on the Wall

In the classic test devised by psychologist Gordon Gallup, Jr., in the early 1970s, an animal is first positioned near a mirror to become comfortable with it. Then a small mark is painted onto the animal’s body in a spot that cannot be seen without the aid of the mirror. If the animal spends time examining the spot in the mirror, this suggests some understanding of its reflection. But to truly “pass” the test, the animal must try to remove the mark from its own body. “We wanted to look at the tests that are held up as being the hallmarks of advanced cognition, so the mirror test came up,” says Alex Jordan, a biologist at Max Planck Institute for Ornithology in Germany. Jordan specializes in studying the evolution of social behavior, and the biology that underlies it. Because complex social behaviors have been linked to the evolution of braininess, he decided to put some highly social fish to the test.

Most of Jordan’s research had focused on a diverse group of African freshwater fishes called cichlids—which failed the test. “They showed suggestive evidence that they were exploring their reflections,” he says. “But they never tried to remove the mark.” Of course that does not necessarily mean the fish were unintelligent or lacked self-awareness. It could be they simply do not care about unusual marks on their bodies, and the test just is not relevant for the species. And because fish lack limbs or an elephantine trunk, it would be hard to interpret potential mark-removal behaviors anyway—suggesting the test itself might be biased toward animals that more closely resemble humans.

Still, the mirror test is widely considered a benchmark for sophisticated cognition. So Jordan and his team turned to 10 wild-caught cleaner wrasse, a type of fish known to care quite a bit about unusual spots on others; it gets its name from eating parasites and dead tissue from larger fishes’ skin. This practice has led to the evolution of a suite of impressive social-cognitive capacities, including the ability to recognize and remember individuals, cooperate with others, reconcile after a fight, predict others’ actions and intentionally deceive others.

Because fish skin is coated in mucus, Jordan could not mark them with paint. So after anesthetizing the fish the researchers injected a small amount of dye under the wrasses’ skin. To control for the potential effects of the jab itself, some fish received an injection without the pigment. And based on their behaviors alone, the fish indeed seem to have passed the mirror test.

Swimming Upside Down

When first shown a mirror, the wrasses generally reacted aggressively, as if they perceived their reflection as another individual. They quickly changed tactics however, and began performing odd or atypical behaviors in front of the mirror, including swimming upside down—something the researchers had never observed in any other context. Some fish repeated such strange actions many as 400 times per day. After the wrasses were injected with dye on their throats and returned to a mirrorless tank, they went about their lives as if nothing had happened. But when placed in a tank with a mirror, several of them scraped their throats against the tank bottom, apparently trying to remove the marks. The findings were published Thursday in PLOS Biology.

Emory University primatologist Frans de Waal, who was not involved in the wrasse study, is not quite convinced these fish passed the test as originally devised, however. “When I read the paper,” he says, “I became a bit doubtful about the behavior that they report, and the fact that they cannot do a purely visual mark.” In the classic test, a mark is simply painted onto the animal. But the injection necessary for fish pairs a tactile stimulation—the physical irritation of the jab—with the visual cue. Jordan notes this makes it harder to directly compare the wrasses’ responses with those of superficially marked apes, elephants and other animals.

Shades of Gray

“These fish were doing unusual things,” Jordan says. And although he maintains they convincingly passed the mirror test, he does not claim this necessarily means they possess humanlike self-awareness.

Both he and de Waal say the way cognitive scientists have historically thought of self-awareness—as an all-or-nothing capacity—needs an overhaul. “We're not really comfortable with that kind of black-and-white thinking,” de Waal says. Perhaps self-awareness exists along a continuum, with humans and other apes at one extreme end. “We need to think broader about self-awareness, and that’s maybe what this fish study is going to stimulate,” he adds.

De Waal compares the cleaner wrasse with some types of monkeys, which eventually demonstrate an understanding the animal in the mirror is not an unfamiliar stranger, and can even use mirrors as tools to find hidden food. But the monkeys do not quite pass the test. “That’s my conclusion for the moment, that [the wrasses] operate at the level of a macaque” he says. But still, “it's truly remarkable for a fish.”