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We tested the hypothesis that part of the gap in numerical competence between fish and warm-blooded vertebrates might be related to the more efficient procedures (e.g. automated conditioning chambers) used to investigate the former and could be filled by adopting an adapted version of the Skinner box in fish. We trained guppies in a visual numerosity discrimination task, featuring two difficulty levels (3 vs. 5 and 3 vs. 4) and three conditions of congruency between numerical and non-numerical cues. Unexpectedly, guppies trained with the automated device showed a much worse performance compared to previous investigations employing more “ecological” procedures. Statistical analysis indicated that the guppies overall chose the correct stimulus more often than chance; however, their average accuracy did not exceed 60% correct responses. Learning measured as performance improvement over training was significant only for the stimuli with larger numerical difference. Additionally, the target numerosity was selected more often than chance level only for the set of stimuli in which area and number were fully congruent. Re-analysis of prior studies indicate that the gap between training with the Skinner box and with a naturalistic setting was present only for numerical discriminations, but not for colour and shape discriminations. We suggest that applying automated conditioning chambers to fish might increase cognitive load and therefore interfere with achievement of numerosity discriminations.

Lateralization of cognitive functions impacts many behaviours related to fitness and, in most species, varies greatly among individuals. Laboratory and field studies have suggested that within-species variation in lateralization is partly due to phenotypic plasticity. For example, in fish, prey that have experienced predation risk during early ontogeny develop highly lateralized phenotypes, and this lateralization often favours prey in evading predators. In contexts other than predation, plasticity of lateralization has also been reported for adult fish. Therefore, we asked whether adult fathead minnows, Pimephales promelas, exposed to high predation risk would also show plasticity linked to increase lateralization. We exposed minnows to conspecific alarm cues for up to 8 days to simulate predation risk and tested their lateralization with a standard detour test. The treatment affected lateralization but in an unexpected direction: Individuals exposed to high predation risk showed lower lateralization scores compared to control fish. In addition, fish within groups exposed to risk reduced the variability in their directionality of lateralization; that is, they showed a similar turning preference in the detour task. Our study suggests that lateralization can vary in response to predation risk in adult fish. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

An optimal foraging strategy often requires identifying and choosing the larger amount of food in the presence of multiple options, in order to maximize food intake. Food quantity estimation frequently depends on the perceptual ability to segregate food from the surrounding background. In human and nonhuman animals, it has recently been shown that the perception of food size is affected by the size of the background on which food is presented, with a tendency to overestimate food items encircled by a small background. This perceptual bias resembles an illusory effect known as the Delboeuf illusion. We investigated whether this bias occurs in five fish species: zebrafish, Danio rerio, redtail splitfin, Xenotoca eiseni, angelfish, Pterophyllum scalare, Siamese fighting fish, Betta splendens, and three-spot gourami, Trichopodus trichopterus. In control trials, we observed their spontaneous preference for choosing the larger of two amounts of food. In test trials, two same-sized food amounts were presented on a small and a large plate, a pattern that causes the Delboeuf illusion in humans. Similarities in subjects’ choices between the five species were greater than differences, with a significant bias towards selecting the food amount on the large plate. This suggests the existence of a perceptual bias, although one operating in the opposite direction from that of humans, which might interfere with the accurate food size estimation necessary for an optimal foraging strategy.

In a number of species, males and females have different ecological roles and therefore might be required to solve different problems. Studies on humans have suggested that the 2 sexes often show different efficiencies in problem-solving tasks; similarly, evidence of sex differences has been found in 2 other mammalian species. Here, we assessed whether a teleost fish species, the guppy, Poecilia reticulata, displays sex differences in the ability to solve problems. In Experiment 1, guppies had to learn to dislodge a disc that occluded a feeder from which they had been previously accustomed to feed. In Experiment 2, guppies had to solve a version of the detour task that required them to learn to enter a transparent cylinder from the open sides to reach a food reward previously freely available. We found evidence of sex differences in both problem-solving tasks. In Experiment 1, females clearly outperformed males, and in Experiment 2, guppies showed a reversed but smaller sex difference. This study indicates that sex differences may play an important role in fish’s problem-solving similar to what has previously been observed in some mammalian species.

Behavioural innovations allow an individual to solve new problems or find new solutions to an existing problem. Despite being considered an important source of phenotypic plasticity and evolutionary changes, innovative problem-solving remains poorly understood, except in a few species of mammals and birds. We investigated innovative problem-solving performance and its underlying psychological mechanisms in a teleost fish, the guppy Poecilia reticulata. We assayed guppies in Thorndike’s puzzle-box problem: we placed them in a small chamber, where they had to learn to dislodge an object to access a tunnel leading to their home tank. Guppies showed heightened performance with most individuals (23 out of 24) solving the problem, within, on average, three trials. After a fish solved the task for the first time, improvement was still visible in the form of an increased likelihood to solve the problem over trials. An individual’s sex and willingness to solve the task were unimportant, but behavioural traits related to neophilia significantly predicted problem-solving performance. High exploration in a new environment and high attraction towards novel objects favoured the guppies in learning the task solution. Our finding suggested that this fish species shows remarkable performance and individuality in innovative problem-solving. As observed in warm-blooded vertebrates, these cognitive features may have important consequences for individual fitness and the species’ invasiveness in nature. Behavioural innovation is an important mechanism that allows animals to adapt to their environment. Among the others, it permits solving new problems and obtaining new resources. However, innovative problem-solving remains poorly understood except for a few species of mammals and birds. Our study suggests that fish can learn a novel problem-solving task and improve over time to find the solution. Fish learned to dislodge an object to access a tunnel leading to their home tank. Problem-solving performance was linked to the propensity to explore novel objects and novel environments. Individuals with greater neophilia may have higher propensities for innovative problem-solving.

In humans, individual and sex differences have been long reported for several cognitive tasks and are at least in part due to variability in the function that inhibits behaviour (i.e. inhibitory control). Similar evidence of individual and sex differences in inhibitory abilities is also present in other vertebrates, but is scarce outside primates. Experiments on reversal learning, which requires inhibiting behaviours, suggest that this variability may exist in a teleost fish, the guppy, Poecilia reticulata. We tested this hypothesis by observing guppies in an inhibitory task. Guppies were exposed to unreachable prey inside a transparent tube for six trials. Guppies showed a marked reduction in the number of attempts to catch the prey within the first trial and also over repeated trials. We found a striking sex difference in the capacity to inhibit foraging behaviour. Males attempted to attack the prey twice as often as females and showed negligible improvement over trials. Irrespective of sex, individuals remarkably differed in their performance, with some guppies being systematically more skilled than others across the repeated trials. These results confirm that individual and sex differences in the ability to inhibit behaviour are not restricted to humans and other primates, suggesting that they might be widespread among vertebrates. Variability in inhibitory ability provides an explanation for emerging records of variability in other cognitive tasks in fish.

In a number of animal species, individuals differ in their ability to solve cognitive tasks. However, the mechanisms underlying this variability remain unclear. It has been proposed that individual differences in cognition may be related to individual differences in behavior (i.e., personality); a hypothesis that has received mixed support. In this study, we investigated whether personality correlates with the cognitive ability that allows inhibiting behavior in 2 teleost fish species, the zebrafish Danio rerio and the guppy Poecilia reticulata. In both species, individuals that were bolder in a standard personality assay, the open-field test, showed greater inhibitory abilities in the tube task, which required them to inhibit foraging behavior toward live prey sealed into a transparent tube. This finding reveals a relationship between boldness and inhibitory abilities in fish and lends support to the hypothesis of a link between personality and cognition. Moreover, this study suggests that species separated by a relatively large phylogenetic distance may show the same link between personality and cognition, when tested on the same tasks.

Once considered a human characteristic, the presence of correlations between individuals' performance in cognitive tasks has now been reported in a range of vertebrates. In humans, an important source of cognitive variability is inhibitory control: some individuals are consistently more efficient in inhibitory tasks and this affects individual differences in other cognitive tasks, including measures of general intelligence. We looked for these two types of individual differences in a teleost fish, the guppy Poecilia reticulata. First, we observed guppies in two inhibitory control tasks. In the tube task, guppies had to inhibit the tendency to attack live prey sealed into a transparent tube. In the cylinder task, guppies had to inhibit the tendency to swim directly toward a food item placed inside a transparent cylinder and rather detour and enter the cylinder from the open sides. Individual rank performance was maintained between the two inhibitory tasks, suggesting individual differences in inhibitory control across tasks in this species. Then, we tested the same set of guppies in a problem-solving task, whereby they had to learn to dislodge an object that prevented the access to a food reward. Neither the tube task nor the cylinder task score predicted guppies' problem-solving performance. Our study demonstrates that fish exhibit consistent individual differences in inhibitory control, as expected if this trait has a common evolutionary origin in vertebrates. Yet, in fish, these individual differences appear not to be related to other cognitive processes such as those required for problem solving.

Due to their unique characteristics, the zebrafish plays a key role in the comprehension of neurobiology of cognition and its pathologies, such as neurodegenerative diseases. More and more molecular tools for this aim are being developed, but our knowledge about the cognitive abilities of zebrafish remains extremely scarce compared to other teleost fish. We aimed to investigate the complex cognitive abilities of zebrafish using a tracking-based automated conditioning chamber that allowed precise experimental control, avoided potential cueing provided by the observer (Clever Hans phenomenon), and was shown to considerably improve learning in other teleosts. A computer presented two visual stimuli in two sectors of the chamber, and zebrafish had to enter the correct sector to obtain a food reward. Zebrafish quickly learned to use the conditioning device and easily performed up to 80 trials per day. In Experiment 1, zebrafish efficiently discriminated between two differently coloured sides, reaching a 75 % accuracy in only 10 training sessions. Surprisingly, zebrafish failed to choose the correct chamber when the stimuli were two shapes, a small circle and a small triangle, even when, in Experiment 2, training on shape discrimination was prolonged for up to 30 sessions. In Experiment 3, we tested the hypothesis that simultaneously learning to use the conditioning chamber and learning discrimination imposes a too-high cognitive load. However, zebrafish that first successfully learned how the conditioning chamber functioned (in the colour discrimination) subsequently failed in the shape discrimination. Conversely, zebrafish that firstly failed the shape discrimination subsequently learned colour discrimination. In Experiment 4, zebrafish showed some evidence of learning when the stimuli were two large shapes, suggesting that zebrafish did not discriminate between the shapes of the previous experiments because they were not salient enough. Altogether, results suggest constraints in the discrimination learning abilities of zebrafish, which should be taken into account when developing cognitive tasks for this species.

As a consequence of sexual selection, males and females may exhibit wide behavioural differences, for example, spatial behaviour. In fish, the two sexes often show different exploratory tendencies. This sex difference has been usually studied by testing individual fish. As many fish species live in social groups with different sex composition, the aforementioned approach might not picture the natural variation of the exploratory behaviour expressed by males and females. Here, we observed shoals of four Mediterranean killifish, Aphanius fasciatus, with three different sex compositions (4 females, 4 males, or 2 females and 2 males) during the exploration of a novel environment. Sex composition of the shoals did not predict the latency to emerge from a shelter into the novel environment. However, once emerged, shoals composed by four males displayed reduced exploratory behaviour compared to 4-female and mixed-sex shoals. These results indicated that sex differences in exploration subsist also at group level and highlighted the importance of sex composition in determining the behaviour of the entire shoal.

Many fish species are social, and individuals spend most of their lives in shoals, but sociability can vary greatly between species, populations and individuals. Sociability has been largely studied by measuring the time spent by a focal fish in proximity to one or more conspecifics. To control for the behaviour of the stimulus fish, the conspecifics have often been substituted by a mirror, on the assumption that the subject perceives its mirror image as a conspecific. The validity of the mirror test has recently been questioned, at both the behavioural and the molecular level, because of the discrepancy in fish responses when exposed to a mirror image and to a live conspecific. In this study, we compared the sociability scores of a social fish, the guppy, Poecilia reticulata, obtained using live fish or a mirror as a stimulus, to assess the validity of the mirror test. We found that the sociability score assessed using the standard mirror test was not significantly correlated with the sociability assessed using live stimuli. Nevertheless, we observed a positive correlation between the scores of the two tests when the mirror test was performed in a more naturalistic context, controlling for the minimum distance between the stimulus fish and the mirror. Our findings provide evidence of the reliability of the mirror test as a measure of sociability compared to the test using live conspecifics when certain requirements are satisfied.

Living in a group offers the chance to follow the choices and the behaviours of other individuals. Following a group mate might confer fitness advantages if the group mate knows about resources such as food or shelters. Shoaling fish often follow larger group mates which, in most species, are generally older and therefore more experienced. Yet, the effect of individuals' characteristics other than size on following behaviour remains to be understood. For example, familiar fish and female shoals have been reported as more cohesive, which might be due to a differential tendency to follow in relation to familiarity and sex. Here, we investigated whether size, familiarity, sex, and the interaction between these factors affect fish following behaviour. We observed pairs of differently sized Mediterranean killifish, Aphanius fasciatus, exploring a new environment, and we recorded whether the rear fish followed the front fish when the latter changed swimming direction. In female and male pairs, and in unfamiliar pairs, smaller fish were more likely to follow the directional change of the larger fish than vice versa. In mixed-sex pairs and in familiar pairs, however, size did not affect following behaviour and larger fish followed as much as smaller fish did. Our results revealed that killifish's following decisions are determined by the size of the individuals, their level of familiarity, and their sex. These characteristics may have a notable impact on the behaviour of fish groups in nature.

Animal species differ considerably in their ability to detour around a see-through obstacle to reach a goal positioned behind it. This variation is commonly assumed to derive from interspecific differences in the cognitive functions involved in the execution of the task, such as spatial abilities and inhibitory motor control. A teleost fish, the guppy, Poecilia reticulata, has recently been found to outperform many mammals and birds in this type of task. To determine whether this is a typical condition in teleost fish or whether detour abilities vary among fish species as observed in mammals and birds, we compared four distantly related teleosts in a transparent barrier task using a group of conspecifics as the goal. The scores of three species (Poecilia reticulata, Xenotoca eiseni, Oryzias sarasinorum) were similar to those previously reported for fish. The remaining species, the zebrafish, Danio rerio, showed a much higher performance, close to that of warm-blooded animals with the highest scores (e.g. corvids and monkeys). In comparative cognition studies, contextual variables rather than differences in cognitive ability may be responsible for observed differences between species. In a second experiment, we found that the four species were similarly gregarious, excluding a different motivation to reach the target as an explanation for the different performance. In another experiment, however, we found evidence that the zebrafish's higher detour performance might be due to a sensory advantage. Zebrafish used olfactory cues (towards which the barrier was opaque) to navigate to the social stimuli, whereas the guppy, which we used as a control species, preferentially relied on visual information. This study highlights the importance of sensory differences as a source of potential experimental confound in comparative cognition research.

Behavioural responses to the environment often require the suppression of strong internal predispositions or the overriding of external lures, tasks performed by a cognitive function called inhibitory control. Inhibitory control of nonhuman animals is generally measured with the cylinder task: subjects are presented with food inside a transparent cylinder and must inhibit their tendency to reach the food directly and instead detour round the cylinder to solve the task. However, several studies have raised concerns about the validity of this test to compare different species. Recently, a tiny teleost fish, the guppy, Poecilia reticulata, was tested with the cylinder task and scored higher than many mammals and birds. Before considering that guppies display unpredictably high inhibitory control, it is important to exclude that they were advantaged by some noncognitive factors. In particular, guppies could enjoy three advantages: experience with transparent surfaces (i.e. the walls of maintenance aquaria), experience with detouring round see-through obstacles (vegetation in the maintenance aquaria) and the spread of food odour in the water from the sides of the cylinder. We tested whether these factors affect guppies' performance in the cylinder task by manipulating both their experience with transparent surfaces before the task and the diffusion of food odour cues from the cylinder. Guppies raised in transparent aquaria or with transparent panels placed inside the tank did not show advantages over guppies with no experience with transparent surfaces. Furthermore, the guppies’ performance was not reduced when the cylinder was pierced in the middle, so that both visual and olfactory cues lured them in the same direction. These results seem to exclude methodological explanations for the high inhibitory control score of guppies, and they indicate that even teleost fish can display efficient inhibitory control.

Many animals, from humans to invertebrates, exhibit sexual preference for unfamiliar partners. This may reduce the risk of inbreeding and increase offspring heterozygosity. An alternative less tested hypothesis is that selection for neophilia in other contexts (e.g. exploration of unfamiliar environments) may promote mate preference for unfamiliar partners. We tested whether exploratory tendency covaries with female preference for unfamiliar mates in the guppy, Poecilia reticulata, a species in which exploratory behaviour show significant phenotypic and genetic variation. We measured the exploratory behaviour of females using an unfamiliar environment test. The same females were also tested for preference for an unfamiliar male over a male with which they had spent the previous 24 h. We found that a female's tendency to explore an unfamiliar environment was positively correlated with the strength of her preference for unfamiliar males. Our results support the hypothesis that exploration might play a role in the evolution and maintenance of preference for unfamiliar males via functional pleiotropy.

To cope with the variable environment, animals are continuously required to learn novel behaviours or, in certain cases, to inhibit automatic and previously learned behaviours. Traditionally, inhibition has been regarded as cognitively demanding and studied mostly in primates, other mammals and birds, using laboratory tasks, such as the cylinder task. Recent studies have also revealed that fish show high levels of inhibition in the cylinder task. However, conclusions on such results are undermined by evidence that the cylinder task may be inappropriate to compare such phylogenetically distant species. Here, we studied whether a fish, the guppy, Poecilia reticulata, could learn to inhibit behaviour using a different paradigm, which exploited spontaneous foraging behaviour and overcame some drawbacks that characterised the cylinder task. We exposed guppies to live brine shrimp nauplii, Artemia salina, enclosed within a transparent tube. Initially, the guppies attempted to attack the prey but over time showed a rapid decrease of the attacks. Control tests seemed to exclude the possibility that this behavioural trend was due to response to novelty or habituation, and suggested that the guppies were learning to inhibit the foraging behaviour. Memory tests indicated that guppies retained the inhibition of foraging behaviour for at least 24 h. Our study seems to indicate that teleost fish display rapid and durable inhibition of spontaneous foraging behaviour; this may be related to previous evidence, from the cylinder task, supporting efficient behavioural inhibition in this taxon.

The shoal-choice test is a popular method to investigate quantity discrimination in social fish based on their spontaneous preference for the larger of two shoals. The shoal-choice test usually requires a long observation time (20-30 min), mainly because fish switch between the two shoals with low frequency, thus reducing the possibilities of comparison. This duration limits the use of the shoal-choice test for large-scale screenings. Here, we developed a new version of the shoal-choice test in which the subject was confined in a large transparent cylinder in the middle of the tank throughout the experiment to bound the minimum distance from the stimulus shoals and favour switching. We tested the new method by observing guppies (Poecilia reticulata) in a 4 versus 6 fish discrimination (experiment 1). The new method allowed for a faster assessment of the preference for the larger shoal (

A recent study found that guppies (Poecilia reticulata) can be trained to discriminate 4 versus 5 objects, a numerical discrimination typically achieved only by some mammals and birds. In that study, guppies were required to discriminate between two patches of small objects on the bottom of the tank that they could remove to find a food reward. It is not clear whether this species possesses exceptional numerical accuracy compared with the other ectothermic vertebrates or whether its remarkable performance was due to a specific predisposition to discriminate between differences in the quality of patches while foraging. To disentangle these possibilities, we trained guppies to the same numerical discriminations with a more conventional two-choice discrimination task. Stimuli were sets of dots presented on a computer screen, and the subjects received a food reward upon approaching the set with the larger numerosity. Though the cognitive problem was identical in the two experiments, the change in the experimental setting led to a much poorer performance as most fish failed even the 2 versus 3 discrimination. In four additional experiments, we varied the duration of the decision time, the type of stimuli, the length of training, and whether correction was allowed in order to identify the factors responsible for the difference. None of these parameters succeeded in increasing the performance to the level of the previous study, although the group trained with three-dimensional stimuli learned the easiest numerical task. We suggest that the different results with the two experimental settings might be due to constraints on learning and that guppies might be prepared to accurately estimate patch quality during foraging but not to learn an abstract stimulus-reward association.

Recognising predators accurately is key to making fine-scale adjustments to behaviour that enhance survival and maximise overall fitness for prey. Prey incorporate information from specific predator features in order to recognise predators and the risk they pose. For olfactory cues, prey can use both predator odour and diet cues to recognise predators. The role of diet cues in predator recognition has only been tested when they provide information about risk and act as an unconditioned stimulus. Thus, it is unclear whether prey use diet cues in the development of more general predator recognition templates. Here, we tested whether diet cues that contain no apparent information about the prey’s vulnerability to the predator are used by prey when they learn to recognise predators. We trained predator-naive wood frog tadpoles (Lithobates sylvaticus) to recognise the odour of a novel crayfish (Orconectes virilis) as risky by pairing tadpole alarm cues with the odour of crayfish fed one of two diets: alfalfa pellets or earthworms (Lumbricus sp.). We tested tadpoles from each group for their response to one of the two crayfish diet odour combinations or a water control. Tadpoles displayed antipredator responses to crayfish odour, irrespective of diet. However, their responses were stronger when tadpoles were exposed to crayfish fed the same diet as during training. Such results demonstrate that diet cues play a previously unrecognised but subtle role in predator recognition and suggest that flexibility in prey choice can lead to an advantage for the predator. Recognising predators and the threat they pose is critical for prey to adjust their behaviour in response to fluctuations in predation risk. There is therefore a need to understand how prey use different cues to develop effective recognition templates that allow for threat-sensitive adjustments to behaviour. Here, we demonstrate that diet cues of predators contribute to the development of predator recognition templates by prey. These results provide new information about how prey develop recognition templates for predators and that, by incorporating diet cues, they are able to adjust their responses to variable risk posed by different predators within a population. Additionally, we suggest that generalist diets may provide unrecognised benefits to predators when switching between prey types.

To make foraging, reproductive and antipredator decisions, animals often have to discriminate discrete and continuous quantities (numbers and sizes of objects, respectively). Few studies have investigated discrete quantity discrimination in amphibians, but this has been done only in the context of prey selection. Using a species with arboreal habits, the Italian treefrog, Hyla intermedia, we investigated whether amphibians discriminate both discrete and continuous quantities when choosing between microhabitats. In field experiments, we showed that newly metamorphosed treefrogs exhibit a preference for microhabitats with abundant and tall grass. In the laboratory, treefrogs presented with the dichotomous choice between two sets comprising different numbers of vertical green bars (simulating grass clumps) showed a preference for the larger set and discriminated between 1 and 2 bars and between 2 and 4 bars, but not between 2 and 3 bars and between 3 and 4 bars. When presented with two bars of different size (i.e. one bar was taller and wider), treefrogs preferred the larger bar up to a 0.25 surface area ratio. Control experiments suggested that treefrogs represent numbers rather than continuous variables to discriminate between sets of bars and that they use the height but not the width of the bars to discriminate sizes. We also found evidence of a possible trade-off between speed and accuracy: individuals that chose more quickly did not display a significant preference for the larger bar/set of bars. These findings suggest that for amphibians, as for other vertebrates, a variety of decision-making processes can rely on quantitative abilities.

Many aquatic animals learn to recognize novel predators when they simultaneously perceive the odours of these novel threats paired with alarm cues released by injured conspecifics. Since the odours of several organisms may be present simultaneously in the environment during this process, selection is expected to favour learning mechanisms that allow prey to respond independently to the odour of each species in a mixture of odours. We tested this hypothesis by exposing tadpoles of the edible frog, Pelophylax esculentus, to injured conspecific cues paired with either the odour of two fish species (experiment 1) or one fish and one crayfish species (experiment 2). We subsequently tested the ability of tadpoles to respond to each odour separately. We found clear evidence that tadpoles learned to recognize the odour of individual species in the mixture and that the response to each odour of a mixture was equally strong. However, the learned response was weaker overall in tadpoles conditioned with the mixture of fish and crayfish compared to those with the two fish species. Our study reveals that tadpoles can adaptively handle the presence of multiple predator odours in their environment during conditioned learning, but highlights some constraints that may due to the diversity of predators in the mix.

In several mammalian and avian species, females show a higher performance than males in tasks requiring cognitive flexibility such as the discrimination reversal learning. A recent study showed that female guppies are twice as efficient as males in a reversal learning task involving yellow-red discrimination, suggesting a higher cognitive flexibility in female guppies. However, the possibility exists that the superior performance exhibited by females does not reflect a general sex difference in cognitive abilities, but instead, is confined to colour discrimination tasks. To address this issue, we compared male and female guppies in two different discrimination reversal learning tasks and we performed a meta-analysis of these experiments and the previous one involving colour discrimination. In the first experiment of this study, guppies were tested in a task requiring them to learn to select the correct arm of a T-maze in order to rejoin a group of conspecifics. In experiment 2, guppies were observed in a numerical task requiring them to discriminate between 5 and 10 dots in order to obtain a food reward. Although females outperformed males in one condition of the T-maze, we did not find any clear evidence of females' greater reversal learning performance in either experiment. However, the meta-analysis of the three experiments supported the hypothesis of females' greater reversal learning ability. Our data do not completely exclude the idea that female guppies have a generally higher cognitive flexibility than males; however, they suggest that the size of this sex difference might depend on the task.

Inhibitory control is an executive function that positively predicts performance in several cognitive tasks and has been considered typical of vertebrates with large and complex nervous systems such as primates. However, evidence is growing that some fish species have evolved complex cognitive abilities in spite of their relatively small brain size. We tested whether fish might also show enhanced inhibitory control by subjecting guppies, Poecilia reticulata, to the motor task used to test warm-blooded vertebrates. Guppies were trained to enter a horizontal opaque cylinder to reach a food reward; then, the cylinder was replaced by a transparent one, and subjects needed to inhibit the response to pass thought the transparency to reach the food. Guppies performed correctly in 58% trials, a performance fully comparable to that observed in most birds and mammals. In experiment 2, we tested guppies in a task with a different type of reward, a group of conspecifics. Guppies rapidly learned to detour a transparent barrier to reach the social reward with a performance close to that of experiment 1. Our study suggests that efficient inhibitory control is shown also by fish, and that its variation between-species is only partially explained by variation in brain size.

Evidence from a growing number of organisms suggests that individuals show consistent performance differences in cognitive tasks. According to empirical and theoretical studies, these cognitive differences might be at least partially related to personality. We tested this hypothesis in the guppy, Poecilia reticulata, by comparing individuals with different degree of sociability in the discrimination of shoals formed by a different number of conspecifics. We found that individual guppies show repeatability of sociability as expected for personality traits. Furthermore, individuals with higher sociability showed poorer shoal size discrimination performance and were less efficient in choosing the larger shoal compared to individuals with low sociability. As choosing the larger shoal is an important strategy of defense against predators for guppies, we discuss this relationship between personality and cognition in the light of its fitness consequences.

Individual differences in cognitive abilities have been thoroughly investigated in humans and to a lesser extent in other mammals. Despite the growing interest in studying cognition in other taxonomic groups, data on individual differences are scarce for non-mammalian species. Here, we review the literature on individual differences in cognitive abilities in teleost fishes. Relatively few studies have directly addressed this topic and have provided evidence of consistent and heritable individual variation in cognitive abilities in fish. We found much more evidence of individual cognitive differences in other research areas, namely sex differences, personality differences, cerebral lateralisation and comparison between populations. Altogether, these studies suggest that individual differences in cognition are as common in fish as in warm–blooded vertebrates. Based on the example of research on mammals, we suggest directions for future investigation in fish.

In many social species, when an individual is associated with familiar conspecifics, it displays an array of behaviours that may confer benefits (e.g., increased boldness and faster habituation to novel environments). In fish, these effects of familiarity have been studied using individuals of only one sex or juveniles. Since shoals often vary regarding sex composition and males and females show different social behaviours, we hypothesised that social familiarity's effects vary with group sex composition. We tested this hypothesis by comparing the exploratory behaviour of groups of two females, two males, and one male and one female Mediterranean killifish, Aphanius fasciatus, which were either familiar or unfamiliar. Pairs of familiar females were bolder than pairs of unfamiliar females, whereas males showed the opposite trend. Pairs of familiar females also showed faster habituation to the novel environment and, at the beginning of the test, were more cohesive compared to pairs of unfamiliar females. Pairs of familiar mixed-sex fish habituated faster to the novel environment than unfamiliar pairs. Pairs of familiar males did not show any beneficial effect of familiarity relative to pairs of unfamiliar males. Hence, the effects of social familiarity on exploratory behaviour, and likely the associated benefits, appear to depend on the sex composition of the pair in the Mediterranean killifish.

When males and females differ in their spatial ecology, selection is expected to promote sex differences in spatial abilities. Although this prediction applies to many species, few studies have looked at sex differences in spatial abilities outside mammals. Here, we addressed this hypothesis in the guppy, Poecilia reticulata, a polygynous fish in which males disperse more than females and inhabit more spatially complex environments. We compared the performance of male and female guppies in two spatial tasks to test whether males have been selected for enhanced spatial abilities. In a detour task (experiment 1), the two sexes showed similar ability to navigate around an obstacle to reach a target. However, males were more persistent in trying to pass through the transparent obstacle, an effect that is likely to be related to sex differences in cognitive flexibility rather than to spatial abilities. In the second experiment, with a more complex maze in which guppies had to choose between alternative routes to reach the target, males learned the task after only one presentation, whereas females did not show any evidence of learning after five trials. The direction of these differences is the same as that observed in most polygynous species investigated, suggesting a common pattern of cognitive sex differences across vertebrates.

Rats, mice and other rodents are well-known for their ability to solve complex spatial tasks, such as learning to negotiate complicated mazes. This ability might be an adaptation for the fossorial habit that characterizes most rodents, but the scarcity of data from other taxa prevents us from confirming this hypothesis. We tested guppies, Poecilia reticulata, for their ability to navigate a maze consisting of six consecutive T-junctions. Guppies learned to solve the complex maze, and both the number of errors and the time to exit significantly decreased during the training period, which consisted of 30 trials over 5 test days. Guppies reached 68% correct responses on the first day of training and they exceeded 80% correct responses by the last day of training. We found no difference between a condition in which colour cues made each T-junction distinct from the others and a condition with no such cues. In contrast with the male advantage in spatial tasks previously observed in guppies, we found a small but significant female advantage in complex maze learning. Our work suggests that the ability to learn complex mazes is not a prerogative of those species that inhabit burrow systems such as mice and rats, but it might be common in vertebrates.

The ability to discriminate between different quantities is useful to an animal since it allows adaptive decisions in several contexts. In humans, some individuals are generally better at solving different numerical and mathematical problems, but we currently ignore whether other species also show individual differences in quantitative abilities across different contexts. Aiming to investigate this possibility, we observed the performances of individual guppies (Poecilia reticulata) in an antipredator task that required the discrimination between shoals made of a different number of conspecifics (4 versus 6 fish) and, successively, in two foraging tasks that required the discrimination between different numbers of food items (4 versus 6 items) and food items with different sizes (0.67 ratio between the two surface areas). Overall, guppies achieved comparable performances in the discrimination of shoal size and food item size, but showed reduced accuracy in the discrimination between different numbers of food items. Performances of guppies were correlated between the discrimination of shoal size and food item size, suggesting the existence of individual differences in quantity discrimination abilities in different contexts in this species. Unexpectedly, this relationship was negative, and some guppies were better in the antipredator task, while others were better in the foraging task. Our results agree with the general trend of the literature on animals that suggests the existence of multiple cognitive functions whose activation are task specific, and also introduce the possibility that some of these functions are subject to constraints. In this work, we provided the first clear evidence of individual differences in the ability to solve quantity discriminations in a fish. Guppies were observed in three quantity discrimination tasks: recognising and joining the larger shoal of conspecifics among two options, a behaviour that confers protection from predators; selecting the larger number of food items between two sets; and selecting the food item with larger size. Guppies that were more accurate in the shoal size discrimination achieved lower scores in the food item size discrimination. Some individuals gain advantage from cognitive abilities that confer defence against predators, while other individuals from those abilities that improve foraging success.

Due to the costs of antipredator behaviour, prey have the ability to finely modulate their response according to the risk they have experienced, and adjust it over different scales of ecological time. Information on which to base their responses can be obtained from direct experience, but also indirectly from nearby conspecifics. In aquatic environments, alarm cues from injured conspecifics are an important and reliable source of information about current predation risk. We used wood frog tadpoles, Lithobates sylvaticus , to investigate whether prey responses to alarm cues match the level of background predation risk experienced by injured conspecifics. We found that tadpoles exposed to alarm cues from conspecifics raised in a high-risk environment showed a stronger antipredator response and an enhanced learned response to novel predators, when compared with tadpoles exposed to alarm cues from conspecifics raised in a low-risk environment. Alarm cues not only allow prey to cope with an ongoing predation event, but also to adjust their behaviour to match background risk in the environment.

For a diversity of species, differences in sexual and parental roles, along with differences in body morphology, often result in males and females having different diets, distinct predators and even different patterns of habitat use. As a consequence, the two sexes often face different environmental challenges and selection may favour the evolution of sex differences in cognition. We tested this prediction in the guppy (Poecilia reticulata). Under perceived hazard, individual guppies join the larger available social group, a behaviour that is thought to minimise predation risk. In this species, females are more frequently exposed to predation and more averse to predation risk; we therefore expected greater accuracy in shoal size discrimination in females. We compared the accuracy of male and female guppies in discriminating shoals of 4 and 6 conspecifics, which represents the upper limit of discrimination for this species. Overall, we found no sex differences in the accuracy of discriminating the two shoals. However, while females showed this ability at the beginning of the test, males began to select the larger group only after several minutes. In three control experiments, we found indications that this sex difference cannot be accounted for by differences in motivation or antipredator strategies between the two sexes, suggesting female guppies are more efficient at rapidly estimating shoal size.

In many species, males and females have different reproductive roles and/or differ in their ecological niche. Since in these cases the two sexes often face different cognitive challenges, selection may promote some degree of cognitive differentiation, an issue that has received relatively little attention so far. We investigated the existence of sex differences in visual discrimination learning in the guppy, Poecilia reticulata, a fish species in which females show complex mate choice based on male colour pattern. We tested males and females for their ability to learn a discrimination between two different shapes (experiment 1) and between two identical figures with a different orientation (experiment 2). In experiment 3, guppies were required to select an object of the odd colour in a group of five objects. Colours changed daily, and therefore, the solution for this task was facilitated by concept learning. We found males' and females' accuracy practically overlapped in the three experiments, suggesting that the two sexes have similar discrimination learning abilities. Yet, males showed faster decision time than females without any evident speed-accuracy trade-off. This result indicates the existence of consistent between-sex differences in decision speed perhaps due to impulsivity rather than speed in information processing. Our results align with previous literature, indicating that sex differences in cognitive abilities are the exception rather than the rule, while sex differences in cognitive style, i.e. the way in which an individual faces a cognitive task, are much more common.