Written by Alexander Fordyce
It’s not often that individual scientific studies make international headlines. Around two weeks ago, a group of scientists in Japan demonstrated for the first time that a common reef fish was able to pass the mark test for self-recognition, calling into question how we perceive the intelligence of other animals on the planet and how we go about measuring this. Having worked in fish behavioural ecology before delving into my current field, I thought I’d take a look and help communicate this wonderful discovery. After all, we should celebrate any piece of work that challenges our understanding of the natural world.
The Critter
The colourful critter in question is the bluestreak cleaner wrasse, an almost constant denizen of coral reefs. These cleaner fish set up ‘salons’ on the reef, known as cleaning stations, where other fish of all shapes and sizes come to be groomed. The wrasse grazes on parasites and dead skin in every nook and cranny, while the client gets a bit of rest and relaxation. This is epitomised in the mesmerising dance that wrasses perform to chill out particularly hungry clients that might otherwise consider them a small morsel. From a behavioural perspective, this makes them a complex little fish in their own right. Add in a bit of self-recognition and suddenly, they appear to be rather clever.
The Mark Test
The mark test is a behavioural classic which we’ve all probably come across one way or another. The principle is simple – can an animal see a coloured dot on its body and recognise it as being on itself? The test involves passing three hurdles. Phase I involves recording their initial behaviour in order to establish baseline behaviour. It is expected that you initially see either social behaviour or perhaps aggression, as if the animal were acting towards an unknown conspecific. Phase II is achieved if these expected behaviours is gradually replaced by those that are abnormal or idiosyncratic. Actions that fall outside the framework of ‘normal’ behaviours as seen in the wild or in the absence of a mirror. Effectively, is the mirror causing it to behave significantly differently compared to a lack of mirror? Importantly, these odd behaviours have to be repeatable. This underpins all behavioural ecology – the repeatability of behaviour. Perhaps this seems obvious. But when there is so much potential for individual interpretation to skew results through anthropomorphising animal behaviour, you really have to be quite ruthless in classifying the actions of your test subjects. It’s a challenge to objective when you’re classifying animal behaviour.
The most well-known component of the test, Phase III, is where the coloured dot comes in. At this point, we have observed the animal recognising that the reflection is not another animal, followed by some unique behaviours not seen in the absence of a mirror. We then need to paint a coloured dot onto the animal, in a place it can’t see without the mirror like the forehead, and then give it a mirror. Self-recognition is confirmed when the animal sees the reflection and then attempts to remove the dot from its own body. Simple, repeatable and effective.
The Caveat
This test was originally designed for primates for two reasons: they have recognisable facial expressions and hands. Fish obviously don’t have hands (not even the handfish) and honestly, not very expressive faces. Of course this hasn’t stopped people applying the test to elephants, dolphins and birds, all of whom certainly don’t have hands. So emerges the caveat to the international headlines and this study. That a fish passed this test suggests, when viewed through the framework of human and primate behaviours, that the wrasse is self-aware. Alternatively, it might actually mean the test isn’t really appropriate for all these different taxonomic groups. To their credit, the authors are very honest about this; conservative scientific conclusions, however, rarely make for punchy headlines.
The Results
Even in light of the alterations required to apply this test to fish, the apparent changes in the wrasses’ behaviour really are quite astonishing. First it displayed territorial aggression (Phase I) which was gradually replaced by repeated posturing in front of the mirror, upside down (Phase II) – an almost unheard of behaviour never seen in the wild. Atypical to say the least. Meanwhile, mirrorless fish seemed as bored as you might expect a cleaner fish to be without something to clean. Finally, the authors injected an elastomer under the skin of the fishes’ throat (Phase III). Far from comfortable and your first thought is probably that this is likely to screw with their behaviour a bit. Don’t worry, fish get an acclimation period to get used to the injection and by injecting controls with transparent dots to ensure that the injection itself was not altering behavior
Lo and behold, the test fish appear to consistently posture in a way that allowed the best view of the coloured dot i.e. exposing their ventral side. After a bit of inspection, they would then move away from the mirror and scrap their front against artificial substrate. Then head back to the mirror to see the result. Back and forth.
Updating our perception
To quote the authors of this study, “The results we present here will by their nature lead to controversy and dispute, and we welcome this discussion”. An elegant and honest study, but can we now say that fish are self-aware? This Quanta article really delves into the controversy behind the work. The further we get, taxonomically, from ourselves then the more we should question whether studying behaviour through the lens of the human experience is enough to draw conclusions about the nature of intelligence. Insects in their hundreds of thousands show remarkable self-organisation, social behaviour and the ability to learn from others; bees can even count! Tusk fish use corals as anvils to break open hardy molluscs. How do these behaviours fit into our framework? Does self-awareness really indicate intelligence or is it just one evolutionary adaptation to a social lifestyle? As well as making us question our perceptions of animal cognition, this study should make us reconsider our definition of intelligence.
Further reading
Kohda M, Hotta T, Takeyama T, Awata S, Tanaka H, Asai JY, Jordan AL. If a fish can pass the mark test, what are the implications for consciousness and self-awareness testing in animals?. PLoS Biology. 2019 Feb 7;17(2):e3000021.
Grutter AS. Cleaner fish use tactile dancing behavior as a pre conflict management strategy. Current Biology. 2004 Jun 22;14(12):1080-3.
Howard SR, Avarguès-Weber A, Garcia JE, Greentree AD, Dyer AG. Numerical cognition in honeybees enables addition and subtraction. Science Advances. 2019 Feb 1;5(2):eaav0961.
Harborne AR, Tholan BA. Tool use by Choerodon cyanodus when handling vertebrate prey. Coral Reefs. 2016 Sep 1;35(3):1069-.
Anderson JR, Gallup GG. Mirror self-recognition: a review and critique of attempts to promote and engineer self-recognition in primates. Primates. 2015 Oct 1;56(4):317-26.
Resource: Featured Image taken from https://www.biolib.cz/en/image/id168984/
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