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.

Cooperative breeding has been studied intensively in many species of birds and mammals but remain less well studied in fish. We report a remarkable new example of a cooperatively breeding cichlid from Lake Tanganyika, Neolamprologus obscurus. Using field observations and microsatellite DNA analyses, we studied group structure, helping behavior, relatedness, and dispersal of this species. We present four major observations. First, large territorial breeding males mated with one to eight breeding females, each of which was territorial and unrelated to another. Second, one to ten smaller fish ("subordinates") of both sexes were allowed to stay inside the breeding females' territories. Subordinates were often highly related to both the respective breeding male and female and performed territory defense and shelter maintenance, which is regarded as helping behaviors. Third, one to three subordinate males, similar in size to breeding females, were allowed to stay inside a breeding male's territory but were not tolerated in the breeding females' territories. Pairwise relatedness suggests these individuals are usually sons of the respective breeding male. Fourth, pairwise relatedness estimates suggest that juveniles delay dispersal and assist their mothers in raising offspring. As female subordinates grow up, they leave the father's territory and disperse into other groups. In contrast, male subordinates leave their mother's territory but remain within the territory of their father. The described social system makes N. obscurus a promising new model species to study the evolution of cooperative breeding.

The theoretical underpinnings of the mechanisms of sociality, e.g. territoriality, hierarchy, and reciprocity, are based on assumptions of individual recognition. While behavioural evidence suggests individual recognition is widespread, the cues that animals use to recognise individuals are established in only a handful of systems. Here, we use digital models to demonstrate that facial features are the visual cue used for individual recognition in the social fish Neolamprologus pulcher. Focal fish were exposed to digital images showing four different combinations of familiar and unfamiliar face and body colorations. Focal fish attended to digital models with unfamiliar faces longer and from a further distance to the model than to models with familiar faces. These results strongly suggest that fish can distinguish individuals accurately using facial colour patterns. Our observations also suggest that fish are able to rapidly (≤ 0.5 sec) discriminate between familiar and unfamiliar individuals, a speed of recognition comparable to primates including humans.

Summary 1.Communities of different species are often structured according to niche differentiation associated with competitive interactions. We show that similar principles may apply on an ecological time-scale when individuals of a species having a wide size variation compete for resources, using the Lake Tanganyika cichlid Lobochilotes labiatus (5-30 cm). This species has a mouth especially adapted to suck up invertebrates from rock crevices. 2. Individuals defended feeding territories against similar-sized conspecifics, but not against differentsized ones. Thus, territories of similar-sized fish rarely overlapped, but up to a total of seven individuals (of seven size-ranks) had broadly overlapping territories with dissimilar-sized individuals. Comparison with expectation from the null model demonstrated clearly that observed size ratios between adjacent size rank were determined non-randomly regardless of sexual combinations. 3. Larger individuals took larger prey types of larger average size, but more importantly used wider rock crevices from which to suck food than smaller individuals. We calculated pairwise values of Schoener's index of diet overlap C d and the values of Levin's index of diet breadth B d (prey type and prey size) and the same for the width of the rock crevices used for foraging ( C r and B r ). C d remained high among all combinations of the seven ranks. In contrast, C r declined strongly in combinations of adjacent ranks (to 0·27), and was low or zero among further different size ranks. This shows that fish with overlapping territories divided the food resources largely through foraging site partitioning. Accordingly, B d did not depend on the size difference to the nearest two coinhabiting fish, whereas B r did. 4. We conclude that this L. labiatus community is structured non-randomly: body size-dependent effects on foraging site usage result in competition with, and territorial exclusion of, similar-sized individuals, but not of dissimilar-sized individuals that were accepted as coinhabitants. Accordingly, mean body size ratios (large/small) between two adjacent ranks were consistently approximately 1·28 [standard deviation (SD) = 0·07, n = 104], while approximately 1·34 from the null model (SD = 0·34, n = 10 400 simulations). We discuss our results as an example of Hutchinson's rule, applied originally to size ratios of different species.