Abstract
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.
Highlights
Lake Tanganyika cichlids show a great diversity of colour patterns among populations and species, and the divergence in visual signals can function in species recognition (e.g. [1, 2])
Focal fish spent less time watching familiar neighbour models (FfFb) than stranger models (SfSb) (Wilcoxon signed-rank test, Z = 2.266, p < 0.02, n = 8, Fig 2), and they did not differentiate among models with the same face but different bodies (FfFb vs. familiar neighbour’s face on stranger’s body (FfSb), Z = 0.421, p = 0.73; SfSb vs. s face on familiar neighbour’s body (SfFb), Z = 0.141, p = 0.96, n = 8)
When the digital model emerged from the black square first time, there was no difference in the distance between the focal fish and the digital model of familiar neighbour-face models (FfFb and FfSb, 8.9 cm ± 3.1SD, n = 16) and stranger-face models (SfSb and SfFb, 10.7 ± 3.1, n = 16, Mann-Whitney U-test U = 85, p = 0.10)
Summary
Lake Tanganyika cichlids show a great diversity of colour patterns among populations and species, and the divergence in visual signals can function in species recognition (e.g. [1, 2]). The cues for individual recognition such as chemically mediated cues are unlikely used in social communication [27] The visual signals they use for individual identification are unknown, but the variations in facial colour patterns among individuals of N. pulcher (Fig 1A and 1B) suggest a role for facial colour patterns in recognition. We hypothesize that this variability in facial colour pattern may play a role in individual recognition independently of other more conserved visual cues like body shape and overall colouration in this fish. We tested the role of facial recognition by generating familiar and unfamiliar pairs of individuals and using digital images modified to separate the effects of facial colouration and body characteristics on individual recognition behaviour
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