Looking in the mirror and understanding that the person staring back at you is indeed yourself and not another person entirely is something that humans do without effort. But this understanding has long been used as a hallmark of advanced thinking and self-awareness for other species in the animal kingdom. Very few animals are considered self-aware, and this exclusive club is currently limited to mammals including chimpanzees, elephants and dolphins, as well as one bird species. However, researchers from universities in Japan, Switzerland and Germany have suggested that a small tropical fish – the cleaner wrasse – might also be a part of this elite group of thinkers, or perhaps the mirror test is somewhat flawed…
The mirror test typically works like this. First, an animal is shown a mirror. Next, a researcher removes the mirror and places a distinct mark somewhere on the animal's body that could only be seen with the help of a mirror. The mirror is then returned, and the researcher observes whether the animal attempts to remove the mark. Seems simple enough. Passing this test tells us that the animal understands that the reflection is itself, and no fish has ever passed it. But Masanori Kohda and colleagues thought that the cleaner wrasse might be able to. The fish are called cleaner wrasse because they munch parasites from the skin of other fish, or clients, and they are known for having the social intelligence to navigate their cleaner–client relationships. Kohda wondered whether this social intelligence might extend to self-recognition.
The researchers tested the wrasse in several phases. First, they left the fish unmarked and recorded how the individuals responded to a mirror. Having never seen a mirror before, the fish attacked – how did another wrasse get in my tank?! But this anger quickly subsided and shifted to something stranger. The fish began moving in ways that had never before been recorded for this species. For instance, they would swim upside-down towards the mirror or rush at it, only to stop right before crashing into it. The team of scientists interpreted these oddities as ‘contingency behaviours’, meaning the fish were exploring their bodies and may be testing whether their reflection would follow suit if they behaved weirdly.
Finally, the researchers marked the fish. They started by adding a small injection of clear paint under the scales (a common method for marking fish) to make sure the marking procedure itself did not indicate to the fish they had been marked; it didn't. The fish only inspected the mark in the mirror when the coloured paint was added. Amazingly, the fish would then leave the mirror and try to scrape off the mark by brushing themselves against the sand in their tanks. According to the rules of the mirror test, the wrasses passed.
However, the team has not been quick to declare that cleaner wrasses are self-aware. Instead, they conclude that these fish must have some level of self-recognition; whether this extends to a fully realized ‘sense of self’ remains to be seen. Perhaps self-awareness exists on a continuum, instead of being either present or absent, and the mirror test may not capture these nuances. The researchers know that their findings are controversial; the prevailing view among biologists has long been that fish have few advanced cognitive abilities. Kohda's team hopes that their work will encourage others to think carefully about the methods we use to compare intelligence across species. They also point out that we need to re-evaluate our preconceived notions about fish: they may be much smarter than we give them credit for.