Further evidence for the capacity of mirror self-recognition in cleaner fish and the significance of ecologically relevant marks

Abstract

An animal that tries to remove a mark from its body that is only visible when looking into a mirror displays the capacity for mirror self-recognition (MSR), which has been interpreted as evidence for self-awareness. Conservative interpretations of existing data conclude that convincing evidence for MSR is currently restricted to great apes. Here, we address proposed shortcomings of a previous study on MSR in the cleaner wrasse Labroides dimidiatus, by varying preexposure to mirrors and by marking individuals with different colors. We found that (1) 14/14 new individuals scraped their throat when a brown mark had been provisioned, but only in the presence of a mirror; (2) blue and green color marks did not elicit scraping; (3) intentionally injecting the mark deeper beneath the skin reliably elicited spontaneous scraping in the absence of a mirror; (4) mirror-naive individuals injected with a brown mark scraped their throat with lower probability and/or lower frequency compared to mirror-experienced individuals; (5) in contrast to the mirror images, seeing another fish with the same marking did not induce throat scraping; and (6) moving the mirror to another location did not elicit renewed aggression in mirror-experienced individuals. Taken together, these results increase our confidence that cleaner fish indeed pass the mark test, although only if it is presented in ecologically relevant contexts. Therefore, we reiterate the conclusion of the previous study that either self-awareness in animals or the validity of the mirror test needs to be revised.

Fig 1

Ecologically relevant color mark (brown) on the throat of cleaner fish and ectoparasite (sea stag) (A). Bar is 1 mm. Meaningless color mark (green and blue) on the throat (B). These cleaner fish are just after mark injection and are still in the anesthetized condition.

Fig 2

Mean frequency (± SEM/h) of throat scraping on the substrate (A), and mean time (sec ± SEM/10 minutes) of position reflecting their throat on a mirror (B) of the 8 cleaner fish L. dimidiatus in Experiment 1: during the periods of no treatment, sham mark, brown mark without mirror, and brown mark with mirror presentation. Different letters denote statistically significant differences by exact Wilcoxon signed-rank tests with sequential Bonferroni correction. See S1 Data for raw data.

Fig 3

Mean frequency (± SEM/h) of throat scraping on the substrate (A) and mean time (sec ± SEM/10 minutes) of position reflecting their throat on a mirror (B) in no treatment, blue mark, green mark, and brown mark by cleaner fish L. dimidiatus in presence of mirror. No throat scrapings were observed in all cases in the absence of mirror, and the results are omitted. See S1 Data for raw data. SEM, standard error of the mean.

Fig 4. Changes in social responses of cleaner wrasse toward the mirror during the first week.

Mean ± SEM for the time spent mouth–mouth fighting (red), time of watching reflection within 5 cm of the mirror (blue), and frequency of mirror testing behavior/10 minutes (green) (see Table 2). Time spent mouth fighting: LMM, χ₆² = 31.07, p < 0.0001; Time < 5 cm from mirror: LMM, χ₆² = 63.38, p < 0.0001; and frequency of unusual behaviors: negative binomial GLMM, χ₆² = 59.42, p < 0.0001. Different letters of the same colors denote statistically significant differences by multiple comparisons using Tukey contrasts. See S1 Data for raw data. GLMM, generalized linear mixed model; LMM, linear mixed model.