Individual recognition and the 'face inversion effect' in medaka fish (Oryzias latipes)

The Medaka approach to evolutionary social neuroscience

Previously, the integration of comparative biological and neuroscientific approaches has led to significant advancements in social neuroscience. This review highlights the potential and future directions of evolutionary social neuroscience research utilizing medaka fishes (the family Adrianichthyidae) including Japanese medaka (Oryzias latipes). We focus on medaka social cognitive capabilities and mate choice behavior, particularly emphasizing mate preference using visual cues. Medaka fishes are also advantageous due to their abundant genetic resources, extensive genomic information, and the relative ease of laboratory breeding and genetic manipulation. Here we present some research examples of both the conventional neuroscience approach and evolutionary approach involving medaka fishes and other species. We also discuss the prospects of uncovering the molecular and cellular mechanisms underlying the diversity of visual mate preference among species. Especially, we introduce that the single-cell transcriptome technology, particularly in conjunction with 'Adaptive Circuitry Census', is an innovative tool that bridges comparative biological methods and neuroscientific approaches. Evolutionary social neuroscience research using medaka has the potential to unveil fundamental principles in neuroscience and elucidate the mechanisms responsible for generating diversity in mating strategies.

Male medaka continue to mate with females despite sperm depletion

In animals where males engage in multiple matings, sperm depletion can substantially reduce the reproductive success of both sexes. However, little is known about how successive matings affect sperm depletion, fertilization rates and mating behaviour. Here, we investigated this phenomenon under laboratory conditions. Medaka (Oryzias latipes), an externally fertilizing fish, is an ideal model to test predictions of sperm depletion because there are established methods to observe its mating and count sperm. Medaka males mated with multiple females (19 per day, on average; range, 4-27), experiencing significant sperm depletion, with sperm release declining markedly after the first few matings, reaching only 0.5-6.3% by the last mating of the day. Fertilization rates decreased, particularly after approximately 10 consecutive matings, although there was some recovery in the next-day's matings. The decline in courtship effort and mating duration probably resulted from the males becoming increasingly fatigued. Despite the reduced sperm availability, females did not adjust their clutch size as a counterstrategy. These results suggest substantial reproductive costs for males and the potential for sexual conflict owing to limited sperm availability. For species with frequent successive mating, these findings highlight the need to reconsider reproductive strategies and their impact on sexual selection.

Innate face-selectivity in the brain of young domestic chicks

Shortly after birth, both naïve animals and newborn babies exhibit a spontaneous attraction to faces and face-like stimuli. While neurons selectively responding to faces have been found in the inferotemporal cortex of adult primates, face-selective domains in the brains of young monkeys seem to develop only later in life after exposure to faces. This has fueled a debate on the role of experience in the development of face-detector mechanisms, since face preferences are well documented in naïve animals, such as domestic chicks reared without exposure to faces. Here, we demonstrate that neurons in a higher-order processing brain area of one-week-old face-naïve domestic chicks selectively respond to a face-like configuration. Our single-cell recordings show that these neurons do not respond to alternative configurations or isolated facial features. Moreover, the population activity of face-selective neurons accurately encoded the face-like stimulus as a unique category. Thus, our findings show that face selectivity is present in the brains of very young animals without preexisting experience.

The Evolution of Simplifying Heuristics in Visual Cognition: Categorization, Specialization, and Visual Illusions

Animals live in visually complex environments. As a result, visual systems have evolved mechanisms that simplify visual processing and allow animals to focus on the information that is most relevant to adaptive decision making. This review explores two key mechanisms that animals use to efficiently process visual information: categorization and specialization. Categorization occurs when an animal's perceptual system sorts continuously varying stimuli into a set of discrete categories. Specialization occurs when particular classes of stimuli are processed using distinct cognitive operations that are not used for other classes of stimuli. We also describe a nonadaptive consequence of simplifying heuristics: visual illusions, where visual perception consistently misleads the viewer about the state of the external world or objects within it. We take an explicitly comparative approach by exploring similarities and differences in visual cognition across human and nonhuman taxa. Considering areas of convergence and divergence across taxa provides insight into the evolution and function of visual systems and associated perceptual strategies.

An evolutionarily distinct Hmgn2 variant influences shape recognition in Medaka Fish

Protein sequence diversification significantly impacts physiological traits. In this study, using medaka fish (Oryzias latipes), we identify a novel protein variant affecting shape preference behavior. Re-analysis of sequencing data reveals that LOC101156433 encodes a unique Hmgn2 variant with unusual subnuclear localization, clustered separately from the Hmgn2 clades of other species. Medaka mutants with this variant showed reduce telencephalic regions and altered shape preference, suggesting a link between protein sequence variation and behavioral changes. Additionally, this Hmgn2 variant is common in Acanthopterygii fishes, which are adapted to a variety of environments, indicating its potential evolutionary significance. Our findings highlight the relationship between amino acid sequence variation and the development of new molecular and behavioral adaptations, providing insights into the visual shape perception system in fish.

Face processing in animal models: implications for autism spectrum disorder

Processing facial features is crucial to identify social partners (prey, predators, or conspecifics) and recognize and accurately interpret emotional expressions. Numerous studies in both human and non-human primates provided evidence promoting the notion of inherent mechanisms for detecting facial features. These mechanisms support a representation of faces independent of prior experiences and are vital for subsequent development in social and language domains. Moreover, deficits in processing faces are a reliable biomarker of autism spectrum disorder, appearing early and correlating with symptom severity. Face processing, however, is not only a prerogative of humans: other species also show remarkable face detection abilities. In this review, we present an overview of the current literature on face detection in vertebrate models that could be relevant to the study of autism.

Do toddlers reason about other people's experiences of objects? A limit to early mental state reasoning

Human social life requires an understanding of the mental states of one's social partners. Two people who look at the same objects often experience them differently, as a twinkling light or a planet, a 6 or a 9, and a random cat or Cleo, their pet. Indeed, a primary purpose of communication is to share distinctive experiences of objects or events. Here, we test whether toddlers (14-15 months) are sensitive to another agent's distinctive experiences of pictures when determining the goal underlying the agent's actions in a minimally social context. We conducted nine experiments. Across seven of these experiments (n = 206), toddlers viewed either videotaped or live events in which an actor, whose perspective differed from their own, reached (i) for pictures of human faces that were upright or inverted or (ii) for pictures that depicted a rabbit or a duck at different orientations. Then either the actor or the toddler moved to a new location that aligned their perspectives, and the actor alternately reached to each of the two pictures. By comparing toddlers' looking to the latter reaches, we tested whether their goal attributions accorded with the actor's experience of the pictured objects, with their own experience of the pictured objects, or with no consistency. In no experiment did toddlers encode the actor's goal in accord with his experiences of the pictures. In contrast, in a similar experiment that manipulated the visibility of a picture rather than the experience that it elicited, toddlers (n = 32) correctly expected the actor's action to depend on what was visible and occluded to him, rather than to themselves. In a verbal version of the tasks, older children (n = 35) correctly inferred the actor's goal in both cases. These findings provide further evidence for a dissociation between two kinds of mental state reasoning: When toddlers view an actor's object-directed action under minimally social conditions, they take account of the actor's visual access to the object but not the actor's distinctive experience of the object.

Chemical and Visual Cues as Modulators of the Stress Response to Social Isolation in the Marine Medaka, Oryzias melastigma

The marine medaka is emerging as a potential behavioral model organism for ocean studies, namely on marine ecotoxicology. However, not much is known on the behavior of the species and behavioral assays lack standardization. This study assesses the marine medaka as a potential model for chemical communication. We investigated how short exposure to visual and chemical cues mediated the stress response to social isolation with the light/dark preference test (LDPT) and the open field test (OFT). After a 5-day isolation period, and 1 h before testing, isolated fish were randomly assigned to one of four groups: (1) placed in visual contact with conspecifics; (2) exposed to a flow of holding water from a group of conspecifics; (3) exposed to both visual and chemical cues from conspecifics; or (4) not exposed to any stimuli (controls). During the LDPT, the distance traveled and transitions between zones were more pronounced in animals exposed to the conspecific's chemical stimuli. The time spent in each area did not differ between the groups, but a clear preference for the bright area in all animals indicates robust phototaxis. During the OFT, animals exposed only to chemical cues initially traveled more than those exposed to visual or both stimuli, and displayed lower thigmotaxis. Taken together, results show that chemical cues play a significant role in exploratory behavior in this species and confirm the LDPT and OFT as suitable tests for investigating chemical communication in this species.

Visual Perception of Density and Density-Dependent Growth in Medaka (Oryzias latipes): A Suitable Model for Studying Density Effects in Fish

High stocking densities have negative effects on fish. However, the mechanism mediating density perception and growth inhibition is still unknown. This study was conducted to confirm the occurrence of growth inhibition and evaluate changes in growth-related factors in fish reared under high-stocking-density conditions and to determine the role of vision in density perception of medaka. In the graduated-stocking experiment, growth inhibition was clearly observed in fish reared at higher densities, although environmental factors, such as water quality, dissolved oxygen, and feeding conditions, were the same in each experimental group. Differences in growth were observed between the 6-fish and 8-fish groups, indicating that medaka have a superior sense that allows them to accurately perceive the number of individuals in their surroundings. In the pseudo-high stocking experiment, the inner 2-L tank in both groups contained six fish; however, the outer 3-L tank in the pseudo group contained several fish, while that of the control group contained only water. Growth inhibition was observed among the fish in the inner tank of the pseudo group despite having similar spatial density with the control group. These findings suggest that vision is important for density perception. The gene expression of growth-related and metabolic-regulatory hormones decreased in the high-density group. Furthermore, neuropeptide Y expression increased, while pro-opiomelanocortin expression decreased in the high-density group. This study is the first to report that fish can visually perceive density and the resulting growth inhibition, and concluded that medaka is a suitable model for studying density effects and perception in fish.

Optogenetic control of medaka behavior with channelrhodopsin

Optogenetics enables the manipulation of neural activity with high spatiotemporal resolution in genetically defined neurons. The method is widely used in various model animals in the neuroscience and physiology fields. Channelrhodopsins are robust tools for optogenetic manipulation, but they have not yet been used for studies in medaka. In the present study, we used the clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-mediated knock-in approach to establish a transgenic medaka strain expressing the Chloromonas oogama channelrhodopsin (CoChR) in the ISL LIM homeobox 1 (isl1) locus. We demonstrated that light stimuli elicited specific behavioral responses, such as bending or turning locomotion in the embryos and pectoral fin movements in the larvae and adults. The response probabilities and intensities of these movements could be controlled by adjusting the intensity, duration, or wavelength of each light stimulus. Furthermore, we demonstrated that the pectoral fin movements in the adult stage could be elicited using a laser pointer to irradiate region including the caudal hind brain and the rostral spinal cord. Our results indicate that CoChR allows for manipulation of medaka behaviors by activating targeted neurons, which will further our understanding of the detailed neural mechanisms of motor control or social behaviors in medaka.

Male Guppies Recognize Familiar Conspecific Males by Their Face

Individual recognition is a necessary cognitive ability for the maintenance of stable social relationships. Recent studies have shown that like primates, some fish species can distinguish familiar fish from unfamiliar strangers via face-recognition. However, the taxa of the studied fish species are restricted (within Perciformes) and the visual signal used for the recognition of fish remains unclear. Here, we investigated the visual signal for individual-recognition in males of a sexually dichromatic guppy (Poecilia reticulata, Cyprinodontiformes). Using guppy males, we examined the hypothesis that fish distinguish between familiar individuals and unknown strangers by their faces rather than by body coloration. We randomly presented focal fish with four types of composite photo-models: familiar (familiar-face and familiar-body = F/F), stranger (stranger-face and stranger-body = S/S), familiar face combined with stranger body (F/S) and stranger face combined with familiar body (S/F). Focal males infrequently attacked familiar-face models but frequently attacked stranger-face models, regardless of body types. These behavioral reactions indicate that guppy males discriminate between familiar and stranger males by their face, not body coloration with wide variation. Importantly, male faces contain clear individual-variation in white/metallic colored patches on the operculum visible for humans. Considering the photo-model, our results suggest that these patches might be an important visual stimulus for face-recognition in guppy males, like some cichlids. Comparative examination among males of different guppy variants, including wild type phenotype, suggests that the face color-patch is stable regardless of variation in body color, with a different genetic mechanism potentially underlying face and body colors.

Cleaner fish recognize self in a mirror via self-face recognition like humans

Some animals have the remarkable capacity for mirror self-recognition (MSR), yet any implications for self-awareness remain uncertain and controversial. This is largely because explicit tests of the two potential mechanisms underlying MSR are still lacking: mental image of the self and kinesthetic visual matching. Here, we test the hypothesis that MSR ability in cleaner fish, Labroides dimidiatus, is associated with a mental image of the self, in particular the self-face, like in humans. Mirror-naive fish initially attacked photograph models of both themselves and unfamiliar strangers. In contrast, after all fish had passed the mirror mark test, fish did not attack their own (motionless) images, but still frequently attacked those of unfamiliar individuals. When fish were exposed to composite photographs, the self-face/unfamiliar body were not attacked, but photographs of unfamiliar face/self-body were attacked, demonstrating that cleaner fish with MSR capacity recognize their own facial characteristics in photographs. Additionally, when presented with self-photographs with a mark placed on the throat, unmarked mirror-experienced cleaner fish demonstrated throat-scraping behaviors. When combined, our results provide clear evidence that cleaner fish recognize themselves in photographs and that the likely mechanism for MSR is associated with a mental image of the self-face, not a kinesthetic visual-matching model. Humans are also capable of having a mental image of the self-face, which is considered an example of private self-awareness. We demonstrate that combining mirror test experiments with photographs has enormous potential to further our understanding of the evolution of cognitive processes and private self-awareness across nonhuman animals.

Mating experiences with the same partner enhanced mating activities of naïve male medaka fish

Mating experience shapes male mating behavior across species, from insects, fish, and birds, to rodents. Here, we investigated the effect of multiple mating experiences on male mating behavior in "naïve" (defined as sexually inexperienced) male medaka fish. The latency to mate with the same female partner significantly decreased after the second encounter, whereas when the partner was changed, the latency to mate was not decreased. These findings suggest that mating experiences enhanced the mating activity of naïve males for the familiar female, but not for an unfamiliar female. In contrast, the mating experiences of "experienced" (defined as those having mated > 7 times) males with the same partner did not influence their latency to mate. Furthermore, we identified 10 highly and differentially expressed genes in the brains of the naïve males after the mating experience and revealed 3 genes that are required for a functional cascade of the thyroid hormone system. Together, these findings suggest that the mating experience of naïve male medaka fish influences their mating behaviors, with neural changes triggered by thyroid hormone activation in the brain.

Visual Perception of Photographs of Rotated 3D Objects in Goldfish (Carassius auratus)

This study examined goldfishes' ability to recognize photographs of rotated 3D objects. Six goldfish were presented with color photographs of a plastic model turtle and frog at 0° in a two-alternative forced-choice task. Fish were tested with stimuli at 0°, 90°, 180°, and 270° rotated in the picture plane and two depth planes. All six fish performed significantly above chance at all orientations in the three rotation planes tested. There was no significant difference in performance as a function of aspect angle, which supported viewpoint independence. However, fish were significantly faster at 180° than at +/-90°, so there is also evidence for viewpoint-dependent representations. These fish subjects performed worse overall in the current study with 2D color photographs (M = 88.0%) than they did in our previous study with 3D versions of the same turtle and frog stimuli (M = 92.6%), although they performed significantly better than goldfish in our two past studies presented with black and white 2D stimuli (M = 67.6% and 69.0%). The fish may have relied on color as a salient cue. This study was a first attempt at examining picture-object recognition in fish. More work is needed to determine the conditions under which fish succeed at object constancy tasks, as well as whether they are capable of perceiving photographs as representations of real-world objects.

Comparison of anxiety-like and social behaviour in medaka and zebrafish

The medaka, Oryzias latipes, is rapidly growing in importance as a model in behavioural research. However, our knowledge of its behaviour is still incomplete. In this study, we analysed the performance of medaka in 3 tests for anxiety-like behaviour (open-field test, scototaxis test, and diving test) and in 3 sociability tests (shoaling test with live stimuli, octagonal mirror test, and a modified shoaling test with mirror stimulus). The behavioural response of medaka was qualitatively similar to that observed in other teleosts in the open-field test (thigmotaxis), and in 2 sociability tests, the shoaling test and in the octagonal mirror test (attraction towards the social stimulus). In the remaining tests, medaka did not show typical anxiety (i.e., avoidance of light environments and preference for swimming at the bottom of the aquarium) and social responses (attraction towards the social stimulus). As a reference, we compared the behaviour of the medaka to that of a teleost species with well-studied behaviour, the zebrafish, tested under the same conditions. This interspecies comparison indicates several quantitative and qualitative differences across all tests, providing further evidence that the medaka responds differently to the experimental settings compared to other fish models.

Face perception: computational insights from phylogeny

Studies of face perception in primates elucidate the psychological and neural mechanisms that support this critical and complex ability. Recent progress in characterizing face perception across species, for example in insects and reptiles, has highlighted the ubiquity over phylogeny of this key ability for social interactions and survival. Here, we review the competence in face perception across species and the types of computation that support this behavior. We conclude that the computational complexity of face perception evinced by a species is not related to phylogenetic status and is, instead, largely a product of environmental context and social and adaptive pressures. Integrating findings across evolutionary data permits the derivation of computational principles that shed further light on primate face perception.

Validation of the three-chamber strategy for studying mate choice in medaka

The three-chamber experiment, in which one test animal can choose between two animals placed in physically inaccessible compartments, is a widely adopted strategy for studying sexual preference in animals. Medaka, a small freshwater teleost, is an emerging model for dissecting the neurological/physiological mechanisms underlying mate choice for which intriguing findings have been accumulating. The three-chamber strategy has rarely been adopted in this species; therefore, here we investigated its validity using medaka colour variants that mate assortatively. First, a total of 551 movies, in which a test male and two choice females interacted for 30 min under a free-swimming condition, were manually analysed. The sexual preference of the males, calculated as a courtship ratio, was highly consistent between human observers (r > 0.96), supporting the objectivity of this manual-counting strategy. Second, we tested two types of three-chamber apparatuses, in which choice fish were presented in either a face-to-face or side-by-side location. Test fish (regardless of sex) spent most of the time associating with choice fish in the compartments. However, their sexual preference, calculated as an association ratio, was poorly reproduced when the locations of the choice fish were swapped. Third, the sexual preferences of males quantified using the manual-counting and either of the three-chamber strategies did not correlate (r = 0.147 or 0.297). Hence, we concluded that, even for individuals of a species like medaka, which spawn every day, sexual preference could not be reliably evaluated using the three-chamber strategy. Optimization of the protocol may solve this problem; however, the explanation for the observation that animals that are ready for spawning persist with never-accessible mating partners must be reconsidered.

A modified Tet-ON system minimizing leaky expression for cell-type specific gene induction in medaka fish

The Tet-ON system is an important molecular tool for temporally and spatially-controlled inducible gene expression. Here, we developed a Tet-ON system to induce transgene expression specifically in the rod photoreceptors of medaka fish. Our modified reverse tetracycline-controlled transcriptional transactivator (rtTAm) with 5 amino acid substitutions dramatically improved the leakiness of the transgene in medaka fish. We generated a transgenic line carrying a self-reporting vector with the rtTAm gene driven by the Xenopus rhodopsin promoter and a tetracycline response element (TRE) followed by the green fluorescent protein (GFP) gene. We demonstrated that GFP fluorescence was restricted to the rod photoreceptors in the presence of doxycycline in larval fish (9 days post-fertilization). The GFP fluorescence intensity was enhanced with longer durations of doxycycline treatment up to 72 h and in a dose-dependent manner (5-45 μg/ml). These findings demonstrate that the Tet-ON system using rtTAm allows for spatiotemporal control of transgene expression, at least in the rod photoreceptors, in medaka fish.

Long photoperiod impairs learning in male but not female medaka

Day length in conjunction with seasonal cycles affects many aspects of animal biology. We have studied photoperiod-dependent alterations of complex behavior in the teleost, medaka (Oryzias latipes), a photoperiodic breeder, in a learning paradigm whereby fish have to activate a sensor to obtain a food reward. Medaka were tested under a long (14:10 LD) and short (10:14 LD) photoperiod in three different groups: mixed-sex, all-males, and all-females. Under long photoperiod, medaka mixed-sex groups learned rapidly with a stable response. Unexpectedly, males-only groups showed a strong learning deficit, whereas females-only groups performed efficiently. In mixed-sex groups, female individuals drove group learning, whereas males apparently prioritized mating over feeding behavior resulting in strongly reduced learning performance. Under short photoperiod, where medaka do not mate, male performance improved to a level similar to that of females. Thus, photoperiod has sex-specific effects on the learning performance of a seasonal vertebrate.

Different measures of holistic face processing tap into distinct but partially overlapping mechanisms

Holistic processing, which includes the integration of facial features and analysis of their relations to one another, is a hallmark of what makes faces 'special'. Various experimental paradigms purport to measure holistic processing but these have often produced inconsistent results. This has led researchers to question the nature and structure of the mechanism(s) underlying holistic processing. Using an individual differences approach, researchers have examined relations between various measures of holistic processing in an attempt to resolve these questions. In keeping with this, we examined relationships between four commonly used measures of holistic face processing in a large group of participants (N = 223): (1) The Face Inversion Effect, (2) the Part Whole Effect (PWE), (3) the Composite Face Effect, and (4) the Configural Featural Detection Task (CFDT). Several novel methodological and analytical elements were introduced, including the use of factor analysis and the inclusion of control conditions to confirm the face specificity of all of the effects measured. The four indexes of holistic processing derived from each measure loaded onto two factors, one encompassing the PWE and the CFDT, and one encompassing the CE. The 16 conditions tested across the four tasks loaded onto four factors, each factor corresponding to a different measure. These results, together with those of other studies, suggest that holistic processing is a multifaceted construct and that different measures tap into distinct but partially overlapping elements of it.

Fish as a model in social neuroscience: conservation and diversity in the social brain network

Mechanisms for fish social behaviours involve a social brain network (SBN) which is evolutionarily conserved among vertebrates. However, considerable diversity is observed in the actual behaviour patterns amongst nearly 30000 fish species. The huge variation found in socio-sexual behaviours and strategies is likely generated by a morphologically and genetically well-conserved small forebrain system. Hence, teleost fish provide a useful model to study the fundamental mechanisms underlying social brain functions. Herein we review the foundations underlying fish social behaviours including sensory, hormonal, molecular and neuroanatomical features. Gonadotropin-releasing hormone neurons clearly play important roles, but the participation of vasotocin and isotocin is also highlighted. Genetic investigations of developing fish brain have revealed the molecular complexity of neural development of the SBN. In addition to straightforward social behaviours such as sex and aggression, new experiments have revealed higher order and unique phenomena such as social eavesdropping and social buffering in fish. Finally, observations interpreted as 'collective cognition' in fish can likely be explained by careful observation of sensory determinants and analyses using the dynamics of quantitative scaling. Understanding of the functions of the SBN in fish provide clues for understanding the origin and evolution of higher social functions in vertebrates.

Individual recognition is associated with holistic face processing in Polistes paper wasps in a species-specific way

Most recognition is based on identifying features, but specialization for face recognition in primates relies on a different mechanism, termed 'holistic processing' where facial features are bound together into a gestalt which is more than the sum of its parts. Here, we test whether individual face recognition in paper wasps also involved holistic processing using a modification of the classic part-whole test in two related paper wasp species: Polistes fuscatus, which use facial patterns to individually identify conspecifics, and Polistes dominula, which lacks individual recognition. We show that P. fuscatus use holistic processing to discriminate between P. fuscatus face images but not P. dominula face images. By contrast, P. dominula do not rely on holistic processing to discriminate between conspecific or heterospecific face images. Therefore, P. fuscatus wasps have evolved holistic face processing, but this ability is highly specific and shaped by species-specific and stimulus-specific selective pressures. Convergence towards holistic face processing in distant taxa (primates, wasps) as well as divergence among closely related taxa with different recognition behaviour (P. dominula, P. fuscatus) suggests that holistic processing may be a universal adaptive strategy to facilitate expertise in face recognition.

Sperm Allocation Strategies Depending on Female Quality in Medaka (Oryzias latipes)

Sperm production is costly, and males are expected to strategically allocate this potentially limiting resource to maximize their fitness. Sperm allocation theory predicts that males should adjust their sperm expenditure in relation to female quality. However, the available empirical evidence is limited. In this study, we assessed whether wild male medaka (Oryzias latipes) would allocate their sperm depending on female quality under controlled conditions. Behavioral observations revealed that spawning behavior could be classified into four stages: male quivering, female quivering, sperm release, and stay. Of these behaviors, only the duration of sperm release was positively correlated with the number of sperm ejaculated in the aquarium, suggesting that males may adjust the sperm number expended for each mating by controlling the duration of sperm release. The estimated amount of sperm released per spawning was positively and significantly correlated with the body depth and weight of paired females. This result is consistent with the prediction arising from the sperm allocation hypothesis, and suggests that male medaka may allocate their sperm depending on paired female body depth and weight. However, the number of released sperm did not correlate with the number of spawned eggs, which was positively correlated with body depth and weight of females. These results imply that medaka females may have counter-strategies against sperm allocation by males, and consequently, males might not be able to adjust the number of sperm efficiently. We disscuss our results from the perspective of sexual conflict over sperm as a limited resource.

No evidence for individual recognition in threespine or ninespine sticklebacks (Gasterosteus aculeatus or Pungitius pungitius)

Recognition plays an important role in the formation and organization of animal groups. Many animals are capable of class-level recognition, discriminating, for example, on the basis of species, kinship or familiarity. Individual recognition requires that animals recognize distinct cues, and learn to associate these with the specific individual from which they are derived. In this study, we asked whether sticklebacks (Gasterosteus aculeatus and Pungitius pungitius) were capable of learning to recognize individual conspecifics. We have used these fish as model organisms for studying selective social learning, and demonstrating a capacity for individual recognition in these species would provide an exciting opportunity for studying how biases for copying specific individuals shape the dynamics of information transmission. To test for individual recognition, we trained subjects to associate green illumination with the provision of a food reward close to one of two conspecifics, and, for comparison, one of two physical landmarks. Both species were capable of recognizing the rewarded landmark, but neither showed a preference for associating with the rewarded conspecific. Our study provides no evidence for individual recognition in either species. We speculate that the fission-fusion structure of their social groups may not favour a capacity for individual recognition.

Sexually dimorphic role of oxytocin in medaka mate choice

Oxytocin is a central neuromodulator required for facilitating mate preferences for familiar individuals in a monogamous rodent (prairie vole), irrespective of sex. While the role of oxytocin in mate choice is only understood in a few monogamous species, its function in nonmonogamous species, comprising the vast majority of vertebrate species, remains unclear. To address this issue, we evaluated the involvement of an oxytocin homolog (isotocin, referred herein as oxt) in mate choice in medaka fish (Oryzias latipes). Female medaka prefer to choose familiar mates, whereas male medaka court indiscriminately, irrespective of familiarity. We generated mutants of the oxt ligand (oxt) and receptor genes (oxtr1 and oxtr2) and revealed that the oxt-oxtr1 signaling pathway was essential for eliciting female mate preference for familiar males. This pathway was also required for unrestricted and indiscriminate mating strategy in males. That is, either oxt or oxtr1 mutation in males decreased the number of courtship displays toward novel females, but not toward familiar females. Further, males with these mutations exhibited enhanced mate-guarding behaviors toward familiar females, but not toward novel females. In addition, RNA-sequencing (seq) analysis revealed that the transcription of genes involved in gamma-amino butyric acid metabolism as well as those encoding ion-transport ATPase are up-regulated in both oxt and oxtr1 mutants only in female medaka, potentially explaining the sex difference of the mutant phenotype. Our findings provide genetic evidence that oxt-oxtr1 signaling plays a role in the mate choice for familiar individuals in a sex-specific manner in medaka fish.

Fish focus primarily on the faces of other fish

“Face” is a special stimulus in humans and, nonhuman primates, and some other social mammals; that is, they perceive the face differently from the other body parts and other stimuli. In these species, the face conveys much information, so individuals examine the face at first sight rather than other body parts. Similar to mammals, the faces of fish also convey much information, but little is known about whether fish pay attention to the face or face-viewing patterns. Here we document the face-viewing patterns of the cichlid fish Neolamprologus brichardi, which can distinguish between conspecifics based on facial colouration. First, we established a method to identify the point at which subject fish inspected. Fish often fixated in direction to their heads toward the object of attention, suggesting that the extended body axis indicated the attention point. Using this attribute, we examined the point of attention of subject fish presented with photographs of conspecifics and heterospecifics. The results revealed that the fish inspected initially and repeatedly at the face and the duration was longer for the face than other body parts.

Visual perception of planar-rotated 2D objects in goldfish (Carassius auratus)

This study examined the ability of goldfish to visually identify 2D objects rotated in the picture plane. This ability would be adaptive for fish since they move in three dimensions and frequently view objects from different orientations. Goldfish performed a two-alternative forced choice task in which they were trained to discriminate between two objects at 0°, then tested with novel aspect angles (+/- 45°, +/- 90°, +/- 135°, 180°). Stimuli consisted of an arrow and half circle (Experiment 1) and line drawings of a turtle and frog (Experiments 2 and 3). In the first two experiments, the S+ and S- were presented at the same aspect angle. Performance in these experiments exceeded chance on four of seven novel aspect angles. Overall accuracy was not significantly different with complex stimuli (animal drawings) vs. simple stimuli (geometric shapes). In Experiment 3, when fish were tested with the S+ at varying aspect angles and the S- always presented at 0°, the fish failed to discriminate among the stimuli at all but one aspect angle. These goldfish viewing planar-rotated 2D objects did not display viewpoint-invariant performance, nor did they show a systematic decrement in performance as a function of aspect angle.

Does Holistic Processing Require a Large Brain? Insights From Honeybees and Wasps in Fine Visual Recognition Tasks

The expertise of humans for recognizing faces is largely based on holistic processing mechanism, a sophisticated cognitive process that develops with visual experience. The various visual features of a face are thus glued together and treated by the brain as a unique stimulus, facilitating robust recognition. Holistic processing is known to facilitate fine discrimination of highly similar visual stimuli, and involves specialized brain areas in humans and other primates. Although holistic processing is most typically employed with face stimuli, subjects can also learn to apply similar image analysis mechanisms when gaining expertise in discriminating novel visual objects, like becoming experts in recognizing birds or cars. Here, we ask if holistic processing with expertise might be a mechanism employed by the comparatively miniature brains of insects. We thus test whether honeybees (Apis mellifera) and/or wasps (Vespula vulgaris) can use holistic-like processing with experience to recognize images of human faces, or Navon-like parameterized-stimuli. These insect species are excellent visual learners and have previously shown ability to discriminate human face stimuli using configural type processing. Freely flying bees and wasps were consequently confronted with classical tests for holistic processing, the part-whole effect and the composite-face effect. Both species could learn similar faces from a standard face recognition test used for humans, and their performance in transfer tests was consistent with holistic processing as defined for studies on humans. Tests with parameterized stimuli also revealed a capacity of honeybees, but not wasps, to process complex visual information in a holistic way, suggesting that such sophisticated visual processing may be far more spread within the animal kingdom than previously thought, although may depend on ecological constraints.

Attraction of posture and motion-trajectory elements of conspecific biological motion in medaka fish

Visual recognition of conspecifics is necessary for a wide range of social behaviours in many animals. Medaka (Japanese rice fish), a commonly used model organism, are known to be attracted by the biological motion of conspecifics. However, biological motion is a composite of both body-shape motion and entire-field motion trajectory (i.e., posture or motion-trajectory elements, respectively), and it has not been revealed which element mediates the attractiveness. Here, we show that either posture or motion-trajectory elements alone can attract medaka. We decomposed biological motion of the medaka into the two elements and synthesized visual stimuli that contain both, either, or none of the two elements. We found that medaka were attracted by visual stimuli that contain at least one of the two elements. In the context of other known static visual information regarding the medaka, the potential multiplicity of information regarding conspecific recognition has further accumulated. Our strategy of decomposing biological motion into these partial elements is applicable to other animals, and further studies using this technique will enhance the basic understanding of visual recognition of conspecifics.

Territorial fish distinguish familiar neighbours individually

True individual recognition (TIR), the ability to distinguish multiple familiar members individually, is more elaborate than class-level recognition, and evidence for the ability to perform TIR is reported from primates, some other social mammals, birds and lizard in vertebrates. These animals exhibit a highly social structure, wherein TIR is essential for their social interactions. Such high sociality has been documented in fish, but clear evidence of TIR has been limited. The cichlid, Neolamprologus pulcher, a cooperative breeder that guards a territory, exhibits the dear enemy relationship. Here, we show that this fish distinguishes two familiar neighbours individually, i.e., TIR ability, using one-way mirrors in experimental tanks. Focal fish established the dear enemy relationship with two neighbours, NA and NB, and displayed limited aggression towards these familiar neighbours. However, their aggressiveness towards neighbour NB increased when they were shifted from the original side of the tank after NA was removed, suggesting that they distinguished NB from NA or regarded NB as a stranger. Interestingly, this aggression level against the shifted neighbour NB largely decreased within 1 min. This decrease contrasted with the longer and more frequent aggressiveness towards unfamiliar strangers. These results suggest that focal fish recognised neighbour NB as a familiar stranger but probably punished NB that moved beyond its territory, that is, betrayed the dear enemy relationship. We prevented the effects of the behavioural reactions of exposed individuals using a one-way mirror. Thus, we conclude that this fish species displays TIR and discuss that TIR is prevalent in territorial animals in which the dear enemy effect is common.