There is an increasing emphasis on understanding individual variation in brain structure—such as overall brain size and the size of specific regions—and linking this variation to behaviour, cognition, and the driving social and environmental factors. However, logistical challenges arise when studying the brain, especially in research involving wild animals, such as dealing with small sample sizes and time-consuming methods. In this study, we used wild lizards, Podarcis bocagei, as our model. We developed an efficient and accurate method that combines X-ray micro-tomography and deep-learning segmentation to estimate the volume of six main brain areas: the olfactory bulbs, telencephalon, diencephalon, midbrain, cerebellum, and brain stem. Through quantitative comparisons, we show that a sufficiently trained deep-learning neural network can be developed with as few as five samples. Using this trained model, we obtained volume data for the six brain regions from 29 brain samples of Podarcis bocagei. This approach drastically reduced the time needed for manual segmentation from several months to just a few weeks. We present a comprehensive protocol detailing our methods, which includes sample preparation, X-ray tomography, and 3D volumetric segmentation. This work collectively provides valuable resources that can assist researchers not only in animal behaviour and physiology, but also in biomedical research and computer sciences.

Some cognitive abilities are suggested to be the result of a complex social life, allowing individuals to achieve higher fitness through advanced strategies. However, most evidence is correlative. Here, we provide an experimental investigation of how group size and composition affect brain and cognitive development in the guppy (Poecilia reticulata). For 6 months, we reared sexually mature females in one of 3 social treatments: a small conspecific group of 3 guppies, a large heterospecific group of 3 guppies and 3 splash tetras (Copella arnoldi)—a species that co-occurs with the guppy in the wild, and a large conspecific group of 6 guppies. We then tested the guppies’ performance in self-control (inhibitory control), operant conditioning (associative learning), and cognitive flexibility (reversal learning) tasks. Using X-ray imaging, we measured their brain size and major brain regions. Larger groups of 6 individuals, both conspecific and heterospecific groups, showed better cognitive flexibility than smaller groups but no difference in self-control and operant conditioning tests. Interestingly, while social manipulation had no significant effect on brain morphology, relatively larger telencephalons were associated with better cognitive flexibility. This suggests alternative mechanisms beyond brain region size enabled greater cognitive flexibility in individuals from larger groups. Although there is no clear evidence for the impact on brain morphology, our research shows that living in larger social groups can enhance cognitive flexibility. This indicates that the social environment plays a role in the cognitive development of guppies.

Executive functions are a set of cognitive control processes required for optimizing goal-directed behavior. Despite more than two centuries of research on executive functions, mostly in humans and nonhuman primates, there is still a knowledge gap in what constitutes the mechanistic basis of evolutionary variation in executive function abilities. Here, we show experimentally that size changes in a forebrain structure (i.e. telencephalon) underlie individual variation in executive function capacities in a fish. For this, we used male guppies (Poecilia reticulata) issued from artificial selection lines with substantial differences in telencephalon size relative to the rest of the brain. We tested fish from the up- and down-selected lines not only in three tasks for the main core executive functions: cognitive flexibility, inhibitory control, and working memory, but also in a basic conditioning test that does not require executive functions. Individuals with relatively larger telencephalons outperformed individuals with smaller telencephalons in all three executive function assays but not in the conditioning assay. Based on our findings, we propose that the telencephalon is the executive brain in teleost fish. Together, it suggests that selective enlargement of key brain structures with distinct functions, like the fish telencephalon, is a potent evolutionary pathway toward evolutionary enhancement of advanced cognitive abilities in vertebrates. 

In recent years, we witnessed an increasing number of funding agencies, scientific journals and scientists agreeing that society and science benefit from open access to research data. Benefits derive mainly from increased access to knowledge for all and improved transparency and credibility in academia. However, despite the advances in open science and open data, three significant aspects still need considerable policing: data quality, the accompanying summaries with basic information of the data files (i.e. metadata) and computational codes used to generate the research outcomes. Only by having these three components together, we can achieve efficient data sharing and reuse, and hence higher transparency. Here, we present two complementary approaches that potentially can help with shared data quality: (i) data file(s) sharing should be guided step-by-step in public archives with mandatory metadata, and (ii) journals creating assistant data editor positions at editorial boards with a leading role in data quality and computational reproducibility. Forty-four editors-in-chief in the field of behaviour, ecology and evolution shared their opinion with us regarding these two approaches. Although most of the views were divided, the majority estimated that their current editorial board members do not have the necessary skills to assess the quality of shared data. Since data are the core of research studies, we should consider not only data presence but also quality as a requirement for publication. 

Mosaic brain evolution, the change in the size of separate brain regions in response to selection on cognitive performance, is an important idea in the field of cognitive evolution. However, untill now, most of the data on how separate brain regions respond to selection and their cognitive consequences stem from comparative studies. To experimentally investigate the influence of mosaic brain evolution on cognitive ability, we used male guppies artificially selected for large and small telencephalons relative to the rest of the brain. Here, we tested an important aspect of executive cognitive ability using a detour task. We found that males with larger telencephalons outperformed males with smaller telencephalons. Fish with larger telencephalons showed faster improvement in performance during detour training and were more successful in reaching the food reward without touching the transparent barrier (i.e., through correct detouring) during the test phase. Together, our findings provide the first experimental evidence showing that evolutionary enlargement of relative telencephalon size confers cognitive benefits, supporting an important role for mosaic brain evolution during cognitive evolution.

Determining how variation in brain morphology affects cognitive abilities is important to understand inter-individual variation in cognition and, ultimately, cognitive evolution. Yet, despite many decades of research in this area, there is surprisingly little experimental data available from assays that quantify cognitive abilities and brain morphology in the same individuals. Here, we tested female guppies (Poecilia reticulata) in two tasks, colour discrimination and reversal learning, to evaluate their learning abilities and cognitive flexibility. We then estimated the size of five brain regions (telencephalon, optic tectum, hypothalamus, cerebellum and dorsal medulla), in addition to relative brain size. We found that optic tectum relative size, in relation to the rest of the brain, correlated positively with discrimination learning performance, while relative telencephalon size correlated positively with reversal learning performance. The other brain measures were not associated with performance in either task. By evaluating how fast learning occurs and how fast an animal adjusts its learning rules to changing conditions, we find support for that different brain regions have distinct functional correlations at the individual level. Importantly, telencephalon size emerges as an important neural correlate of higher executive functions such as cognitive flexibility. This is rare evidence supporting the theory that more neural tissue in key brain regions confers cognitive benefits. 

Over the last decades, we witnessed a growing interest in animal cognition, in general, and in fish cognition, in particular. Here, we provide various study examples that employ an ecological approach to study cognition through field observations, field manipulations and laboratory tests on wild teleost fishes. In this review, we focus on cases with implications for understanding endotherm vertebrate cognition, that is, cases that show fishes possess supposedly ‘complex’ cognitive processes originally thought to warrant a more complex brain. Furthermore, in contrast to the classic interpretation of high/low performance as high/low cognitive abilities, incorporating an individual-level ecological approach reveals that low performance in a cognitive task may be caused by a mismatch between the experimental paradigm and the individual’s experience. The future avenue for wild fish cognition is to grasp better how individual, population and species differences in performance stem from differences in their ecological conditions.

Although uniquely destructive and wasteful, intergroup conflict and warfare are not confined to humans. They are seen across a range of group-living species, from social insects, fishes and birds to mammals, including nonhuman primates. With its unique collection of theory, research and review contributions from biology, anthropology and economics, this theme issue provides novel insights into intergroup conflict across taxa. Here, we introduce and organize this theme issue on the origins and consequences of intergroup conflict. We provide a coherent framework by modelling intergroup conflicts as multi-level games of strategy in which individuals within groups cooperate to compete with (individuals in) other groups for scarce resources, such as territory, food, mating opportunities, power and influence. Within this framework, we identify cross-species mechanisms and consequences of (participating in) intergroup conflict. We conclude by highlighting crosscutting innovations in the study of intergroup conflict set forth by individual contributions. These include, among others, insights on how within-group heterogeneities and leadership relate to group conflict, how intergroup conflict shapes social organization and how climate change and environmental degradation transition intergroup relations from peaceful coexistence to violent conflict.

Supply and demand affect the values of goods exchanged in cooperative trades. Studies of humans and other species typically describe the standard scenario that an increase in demand leads to a higher price. Here, we challenge the generality of that logic with empirical data and a theoretical model. In our study system, “client” fishes visit cleaner wrasse (Labroides dimidiatus) to have ectoparasites removed, but cleaners prefer client mucus, which constitutes “cheating.” We removed 31 of 65 preselected cleaners from a large isolated reef patch. We compared cleaner-client interactions at the reef and a control reef before removal and 4 weeks after removal. Cleaner fish from the experimental treatment site interacted more frequently with large clients (typically visitors with access to alternative cleaning stations), but we did not observe any changes in service quality measures. A game-theoretic analysis revealed that interaction duration and service quality might increase, decrease, or remain unchanged depending on the precise relationships between key parameters, such as the marginal benefits of cheating as a function of satiation or the likelihood of clients responding to cheating as a function of market conditions. The analyses show that the principle of diminishing return may affect exchanges in ways not predicted by supply-to-demand ratios.

Across vertebrate species, intergroup conflict confronts individuals with a tension between group interests best served by participation in conflict and personal interest best served by not participating. Here, we identify the neurohormone oxytocin as pivotal to the neurobiological regulation of this tension in distinctly different group-living vertebrates, including fishes, birds, rodents, non-human primates and humans. In the context of intergroup conflict, a review of emerging work on pro-sociality suggests that oxytocin and its fish and birds homologues, isotocin and mesotocin, respectively, can elicit participation in group conflict and aggression. This is because it amplifies (i) concern for the interests of genetically related or culturally similar 'in-group' others and (ii) willingness to defend against outside intruders and enemy conspecifics. Across a range of social vertebrates, oxytocin can induce aggressive behaviour to 'tend-and-defend' the in-group during intergroup contests.This article is part of the theme issue 'Intergroup conflict across taxa'.