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Abstract Research on several social fishes has revealed that shoals constituted by familiar individuals behave remarkably differently compared to shoals formed by unfamiliar individuals. However, whether these behavioural changes may arise also in shoals composed by a mixture of familiar and unfamiliar individuals, a situation that may commonly occur in nature, is not clear. Here, we observed the behaviour of Mediterranean killifish (Aphanius fasciatus) shoals that were composed by both familiar and unfamiliar individuals (i.e. individuals were familiar to each other in pairs) and compared it with shoals entirely made by either unfamiliar or familiar individuals. Shoals formed by familiar individuals took longer to emerge from a refuge and swam more cohesively compared to shoals formed by unfamiliar fish. Shoals formed by a mixture of familiar and unfamiliar individuals behaved as shoals formed by unfamiliar individuals. Moreover, mixed shoals did not segregate in pairs according to their familiarity. This study suggests that mixed shoals do not show the behavioural effects of familiarity. Significance statement Laboratory studies have compared the behaviour of shoals formed by familiar fish versus shoals formed by unfamiliar fish, finding notable advantages in the former ones, such as improved antipredator and foraging behaviour. However, comparing these two opposite shoal types may not provide information on the natural situation, because in nature, shoals often change composition. We investigated how shoals formed by a mixture of familiar and unfamiliar fish behaved. We analysed shoals’ preference for open environment versus covers and shoals’ swimming cohesion. Results showed that shoals formed by both familiar and unfamiliar individuals mostly behave like shoals entirely formed by unfamiliar individuals. This suggests that the advantages of social groups formed by familiar fish might be hardly seen in nature for species in which shoal composition changes frequently.

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.

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.

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.