If a fish can pass the mark test, what are the implications for consciousness and self-awareness testing in animals?

Masanori Kohda,Alex L Jordan,Tomohiro Takeyama,Satoshi Awata,Hirokazu Tanaka,Jun-Ya Asai,Takashi Hotta,Frans B M De Waal

doi:10.1371/journal.pbio.3000021

Abstract

The ability to perceive and recognise a reflected mirror image as self (mirror self-recognition, MSR) is considered a hallmark of cognition across species. Although MSR has been reported in mammals and birds, it is not known to occur in any other major taxon. Potentially limiting our ability to test for MSR in other taxa is that the established assay, the mark test, requires that animals display contingency testing and self-directed behaviour. These behaviours may be difficult for humans to interpret in taxonomically divergent animals, especially those that lack the dexterity (or limbs) required to touch a mark. Here, we show that a fish, the cleaner wrasse Labroides dimidiatus, shows behaviour that may reasonably be interpreted as passing through all phases of the mark test: (i) social reactions towards the reflection, (ii) repeated idiosyncratic behaviours towards the mirror, and (iii) frequent observation of their reflection. When subsequently provided with a coloured tag in a modified mark test, fish attempt to remove the mark by scraping their body in the presence of a mirror but show no response towards transparent marks or to coloured marks in the absence of a mirror. This remarkable finding presents a challenge to our interpretation of the mark test—do we accept that these behavioural responses, which are taken as evidence of self-recognition in other species during the mark test, lead to the conclusion that fish are self-aware? Or do we rather decide that these behavioural patterns have a basis in a cognitive process other than self-recognition and that fish do not pass the mark test? If the former, what does this mean for our understanding of animal intelligence? If the latter, what does this mean for our application and interpretation of the mark test as a metric for animal cognitive abilities?Editor’s noteThis Short Report received both positive and negative reviews by experts. The Academic Editor has written an accompanying Primer that we are publishing alongside this article (https://doi.org/10.1371/journal.pbio.3000112). The linked Primer presents a complementary expert perspective; it discusses how the current study should be interpreted in the context of evidence for and against self-awareness in a wide range of animals.

Highlights

The ability to perceive and recognise a reflected mirror image as self is considered a hallmark of cognition across species
We show that a fish, the cleaner wrasse, shows behavioural responses that can be interpreted as passing the mark test, a classic test for self-awareness in animals
The mark test, in which a coloured mark is placed on a test subject in a location that can only be viewed in a mirror reflection, is held as the benchmark behavioural assay for assessing whether an individual has the capacity for self-recognition [1,2]

Summary

Introduction

The mark test, in which a coloured mark is placed on a test subject in a location that can only be viewed in a mirror reflection, is held as the benchmark behavioural assay for assessing whether an individual has the capacity for self-recognition [1,2]. The interpretation of these results is subject to wide debate, and the certainty with which behavioural responses during the mirror test can be taken as evidence of self-awareness in animals is questioned [8,12,13]. This problem is exacerbated when the taxonomic distance increases between the test species and the primate taxa for which the test was initially designed. Can the behavioural results recorded for chimpanzees during the mirror test be meaningfully compared with the responses of a bird? Can the behavioural results recorded for chimpanzees during the mirror test be meaningfully compared with the responses of a bird? If so, does this mean a bird that passes the mirror test is self-aware? More generally, if we are interested in understanding and comparing cognition and problem solving across taxa, can we assume that equivalent behaviours represent equivalent underlying cognitive processes? With particular reference to the mark test, here we explore what forms of behaviour in fish could be taken as evidence of self-awareness and whether the same conclusions that have been drawn in other taxa can be drawn for fish

Methods

Results

Conclusion