Environmental factors such as temperature and oxygen are well-established modulators of animal physiology, but the influence of social context remains under-integrated into comparative and environmental physiology. Although numerous studies across behavioural, ecological and biomedical fields show that social interactions alter metabolic, hormonal, immune and stress-related traits, these insights are not routinely incorporated into physiological study design or interpretation. Social effects arise through mechanisms such as isolation, dominance hierarchies, altered energy use and social buffering, and can amplify or dampen responses to abiotic stressors. Because metabolic and hormonal pathways regulate multiple physiological systems, socially induced shifts can cascade to affect cardiovascular, immune, neural, digestive, osmoregulatory and reproductive function over both acute and evolutionary time scales. Thus, overlooking social context places researchers at risk of taking two critical missteps in comparative and environmental physiology: (1) measuring animals under socially unrealistic or uncontrolled conditions, which can yield unrepresentative physiological estimates; and (2) extrapolating these findings to natural populations where trait expression is influenced by social dynamics that are absent from the experimental context. Together, these issues might bias estimates of physiological trait values, plasticity and heritability, and limit the ecological relevance and predictive power of physiological research. Here, we outline general strategies to incorporate social context into experimental design, including the use of emerging tools that allow physiological measurements in naturalistic social settings. Integration of social context, alongside abiotic drivers, will improve our capacity to predict organismal responses to environmental change through comparative physiological research.

Temperature and oxygen concentration can alter the behavior of animals. While often studied in isolation, these factors frequently co-occur. Together they may have nonadditive effects. The effect of abiotic stressors may also change in the presence of biotic stressors, like predators, due to altered costs and benefits of behaviors. Additionally, species may balance the demands of co-occurring stressors differently depending on their unique needs. Here, we investigated the effects of acute exposures to elevated temperature (16 °C versus 12 °C) and decreased dissolved oxygen (30% versus 100% DO) on the social behavior and activity of two co-occurring freshwater fish (common minnow, Phoxinus phoxinus, and three-spined stickleback, Gasterosteus aculeatus) before and after a simulated predator attack. Prior to the release of the model predator, the effect of one stressor was dependent on the other stressor in minnows. Evidence suggested that minnows were more social in warmer waters, but only in hypoxic conditions, and were less active under hypoxia, but only in warm water. Sticklebacks were more active in warm water at both oxygen concentrations. Following predator release, there were no differences in social behavior, likely because of overall reductions in activity. Our study highlights the need to consider stressor combinations. Minnows appeared to compensate for one abiotic stressor such that effects were only apparent when the stressors were experienced together. However, the arrival of the "predator" masked differences between the treatment groups. Different responses between the species suggest that studies in species interactions are needed to understand community level impacts of multiple stressors.

Basal and standard metabolic rate (BMR and SMR) are cornerstones of physiological ecology and are assumed to be relatively fixed intrinsic properties of organisms that represent the minimum energy required to sustain life. However, this assumption is conceptually flawed. Many core maintenance processes underlying SMR are temporally partitioned across sleep and wakefulness and are not continuously active. We argue that instead of representing a singular metabolic state, SMR is better defined as a shifting metabolic mosaic where maintenance functions are distributed unevenly across different sleep-wake states, including metabolically and functionally distinct phases such as non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. SMR measured during wakefulness will mainly represent ion regulation, thermoregulation, sensory processing, and substrate cycling. Meanwhile, SMR measured during sleep primarily includes processes upregulated during sleep, including protein synthesis, cellular repair, immunity, and synaptic plasticity. Our models demonstrate that SMR values measured exclusively during wake or sleep consistently over- or underestimate daily maintenance costs depending on the time spent in specific sleep states and when SMR was measured. In addition, treatment or environmental effects on the costs of specific processes may be entirely missed if metabolic measures occur during an inappropriate sleep-wake state. The temporal partitioning of maintenance processes suggests that traditional and current approaches to SMR measurement may confound true metabolic variation with individual and species-specific differences in sleep architecture, with implications for the estimation of energy budgets, trait heritability, environmental effects on metabolic rate, and metabolic scaling relationships. We propose redefining organismal maintenance costs as a time-integrated profile of metabolic demands, but also suggest that state-specific SMR measurements are appropriate if the sleep-wake measurement period aligns with that of the behavioural, physiological, or ecological context of interest. Moving beyond the fiction of a constant maintenance baseline would provide more refined insights into the bioenergetic foundations of ecological performance and evolutionary constraints.

Social animals may reduce their overall metabolic demand through group living due to a “calming effect”. However, it remains unclear whether the metabolic response of individuals to group members varies depending on the social system, and how individual sociability influences this response. We measured the metabolic rates of the territorial three-striped dwarf cichlid (Apistogramma trifasciata) and the shoaling cardinal tetra (Paracheirodon axelrodi) in the presence or absence of three conspecifics, examining the relationship between individual sociability and metabolic rates, with a focus on how sociability influences the metabolic response to conspecific presence. Territorial cichlids exhibited increased standard metabolic rate (SMR) and routine metabolic rate (RMR) in response to the presence of conspecifics, while shoaling tetras showed unaltered RMR or SMR. In tetras only, sociability was negatively correlated with both SMR and RMR, but this relationship weakened when conspecifics were present—indicating that metabolic responses to conspecific presence differ among individuals of varying sociability. The contrasting metabolic responses to grouping in these species demonstrate that the energetic consequences of social proximity depend on the species’ evolved social structure. Given the lack of correlation between individual sociability and metabolic rates in cichlids, we suggest that sociability and maintenance metabolism may evolve independently, without underlying genetic covariation. This study highlights the intricate relationship between group living and individual energy expenditure, indicating that a species’ social system plays a significant role in determining the extent to which the metabolic response to social partners serves as an adaptive element of group living. In social species, the presence of conspecifics may reduce individual metabolic rates through a “calming effect”. However, whether these effects differ among species with varying social systems is unclear. In this study, we compared the metabolic responses to conspecific presence in two species: (1) the dwarf cichlid (Apistogramma trifasciata), a territorial species with social dominance, and (2) the cardinal tetra (Paracheirodon axelrodi), a shoaling species. We measured metabolic rates under two conditions: alone and with three conspecifics. Territorial cichlids showed increased metabolic rates in the presence of conspecifics, while shoaling tetras exhibited unaltered metabolic rates. This study highlights the crucial role of social systems in shaping individuals’ physiological responses to grouping, providing new insights into the benefits and costs of group living.

Ecosystems around the globe are under unprecedented pressure from human activities. Chemical pollution and biological invasions are two leading drivers of environmental change, each of which causes substantial harm to wildlife and the ecosystems they inhabit. However, despite their individual impacts being well-documented, the combined effects of these pervasive environmental pressures are seldom studied. Here, we address this critical gap by first examining the potential interactions between chemical pollution and biological invasions in animals. We then discuss possible impacts of chemical pollution on animals—both invasive and native—across the distinct stages of the invasion process. Further, we examine gaps in our current understanding of the potential interactions between chemical pollution and biological invasions, including the role of pollutants in mediating interactions between native and non-native species, how pollutants may influence the potential for the invasion process to act as a selective filter, and the relevance of phenotypic plasticity and behavioural syndromes in this context. By synthesizing current knowledge and identifying key research gaps, this review underscores the importance of considering chemical pollution and biological invasions in combination in ecological research. Understanding the combined impacts of these widespread and frequently co-occurring phenomena is essential for developing effective conservation and restoration measures in an increasingly human-modified world.

Within species, individuals vary in their preference to interact with conspecifics, shaping ecological and evolutionary processes. Measures of individual sociability are essential in behavioural and physiological ecology, yet the impact of experimental design, particularly arena size, on sociability estimates has been largely overlooked. Arena size is known to influence other fish behaviours, but its effect on accurately measuring sociability remains understudied. Here, measuring individuals twice at each of four different arena sizes in a binary choice assay, we show that the size of the experimental arena strongly affects absolute and relative measures of sociability in wild-caught threespine sticklebacks Gasterosteus aculeatus. Individual fish were less social with increasing arena size (arena size range: 10–40 cm length x 30 cm width), and sociability also decreased as the ratio of arena size to fish body length decreased. In addition, smaller individuals were more social than larger conspecifics at all arena sizes. Importantly, while absolute sociability estimates varied with arena size, the effect of body size remained relatively consistent. However, individual repeatability of sociability varied across arenas sizes, with the highest observed in the 30 cm arena, suggesting this may represent a practical minimum size for detecting consistent behavioural differences in G. aculeatus or similarly sized species. Together, these results provide the first empirical evidence that experimental arena size can bias both absolute estimates of individual sociability and their repeatability in fishes. This effect may have implications for the comparability and transferability of behavioural data, possibly limiting comparisons among studies and species. However, the results here represent a step toward understanding how methodological details can bias measurements in standardised assays in behavioural ecology. This study investigates a crucial but understudied aspect of experimental design in behavioural assays: the influence of arena size on measures of individual sociability in fish and its repeatability. Our research reveals that arena size impacts sociability estimates in Gasterosteus aculeatus, with larger arenas resulting in lower absolute sociability measures. We further demonstrate that the repeatability of sociability estimates is highest in intermediate-sized arenas. These findings reveal that physical constraints of experimental design may bias sociability estimates, potentially limiting cross-study comparability and ecological conclusions. Our results highlight the importance of standardizing behavioural assays to improve data quality, transferability, and ecological inference.

Microplastic (MP) ingestion acts as an evolutionary trap with various ecological consequences. Cues that lead animals to respond differently to MPs are key factors driving MP ingestion, yet they remain poorly understood. Here, we quantified the susceptibility of three fish species to different types of MPs across different social contexts. Our results showed that bass were more attracted to MPs that resembled food visually, whereas carp tended to select MPs that shared olfactory cues with food. Goldfish relied more on oral processing to make foraging decisions on MPs. Structural differences in the oropharynx supported these discriminated oral processes. Enlarged group size and fasting time altered the foraging behaviors of MPs of goldfish and bass, both of which were suction-feeding species. Such behavioral changes, regardless of whether fish ultimately ingested or rejected MPs, could pose indirect costs to fish. However, changed group sizes and fasting times did not affect the intake of MPs by the filter-feeding carp. We also proposed four pathways causing the MP-induced evolutionary trap and discussed the potential of fish to escape this trap. Our results contribute to experimental and theoretical understanding of the ecological risks posed by MPs to aquatic species.

The use of different mazes to assess spatial learning has become more common in fish behavior studies in recent decades. This increase in fish cognition research has opened the door to numerous possibilities for exciting and diverse questions, such as identifying ecological drivers of spatial cognition and understanding the role individual variation plays in navigational abilities. There are many different types of mazes, each with its own specific considerations, making it challenging to determine exactly which spatial test is the most relevant and appropriate for a particular experiment. Many spatial mazes, such as the T-maze and Y-maze, have been successfully adapted from rodent studies, particularly with respect to zebrafish, a widely accepted non-mammalian model in biomedical studies. Standardization across studies is increasing with these easily accessible maze designs, validating them for use in fish; however, variations in design (e.g., length of arms and scale) and procedure still exist, and the impact of these variations on results is largely unknown. The efforts to standardize mazes outside zebrafish work are also more limited. Other mazes have been developed specifically for use on fish, with design modifications varying widely, making it difficult to draw comparisons. In this review, we have highlighted the many design and procedural elements that should be considered for the acquisition of reliable behavioral data, with the goal of drawing readers' attention to aspects of experimentation that are often not given the careful consideration that they deserve. We then argue that additional focused research and reporting is needed to produce more reliable methods in spatial learning research across a broader range of subjects.

Anthropogenic pollutants are near-ubiquitous in aquatic systems. Aquatic animals such as fishes are subject to physiological stress induced by pollution present in aquatic systems, which can translate to changes in behaviour. Key adaptive behaviours such as shoaling and schooling may be subject to change as a result of physiological or metabolic stress or neurosensory impacts of pollution. This can result in fitness and ecological impacts such as increased predation risk and reduced foraging success. Here, we conducted a systematic metanalysis of the existing literature, comprising 165 studies, on the effects of anthropogenic pollution on sociability and group cohesion in fish species. Both organic (number of studies = 92, posterior mean (PM) = -0.483, p < 0.01) and inorganic (n = 24, PM = -1.453, p < 0.001) chemical pollutants, as well as light exposure (n = 21, PM = -3.038, p < 0.01) were found to reduce sociability. These pollutants did not reduce group cohesion, indicating that effects may be masked in group settings, though fewer studies were carried out on group cohesion and this is a key area for future research. Mixtures of chemical pollutants (n = 16) were found to reduce cohesion (PM = -43.71, p < 0.01), but increase sociability (PM = 44.27, p < 0.01). Evidence was found that fish may behaviourally acclimate to two forms of pollutant, namely mixed chemical pollutants (PM = -0.668, p < 0.01) and noise exposure (n = 22, PM = -4.043, p < 0.01). While aquatic systems are often subject to pollution from multiple sources and of multiple types, very few studies investigated the effects of multiple stressors concurrently. This review identifies trends in the existing literature, and highlights areas where further research is required in order to understand the behavioural and ecological impacts of anthropogenic pollutants in aquatic systems.

ABSTRACT Fishing causes direct removal of individuals from wild populations but can also cause a physiological disturbance in fish that are released or discarded after capture. While sublethal physiological effects of fish capture have been well studied in commercial and recreational fisheries, this issue has been overlooked for the ornamental fish trade, where it is common to capture fish from the wild and discard non-target species. We examined metabolic responses to capture and discard procedures in the three-striped dwarf cichlid Apistogramma trifasciata, a popular Amazonian aquarium species that nonetheless may be discarded when not a target species. Individuals (n = 34) were tagged and exposed to each of four treatments designed to simulate procedures during the capture and discard process: 1) a non-handling control; 2) netting; 3) netting +30 seconds of air exposure; and 4) netting +60 seconds of air exposure. Metabolic rates were estimated using intermittent-flow respirometry, immediately following each treatment then throughout recovery overnight. Increasing amounts of netting and air exposure caused an acute increase in oxygen uptake and decrease in available aerobic scope. In general, recovery occurred quickly, with rapid decreases in oxygen uptake within the first 30 minutes post-handling. Notably, however, male fish exposed to netting +60 seconds of air exposure showed a delayed response whereby available aerobic scope was constrained <75% of maximum until ~4–6 hours post-stress. Larger fish showed a greater initial increase in oxygen uptake post-stress and slower rates of recovery. The results suggest that in the period following discard, this species may experience a reduced aerobic capacity for additional behavioural/physiological responses including feeding, territory defence and predator avoidance. These results are among the first to examine impacts of discard practises in the ornamental fishery and suggest ecophysiological research can provide valuable insight towards increasing sustainable practises in this global trade.

Group-living in animals comes with a number of benefits associated with predator avoidance, foraging, and reproduction. A large proportion of fish species display grouping behaviour. Fish may also be particularly vulnerable to climate-related stressors including thermal variation, hypoxia, and acidification. As climate-related stressors are expected to increase in magnitude and frequency, any effects on fish behaviour may be increased and affect the ability of fish species to cope with changing conditions. Here we conduct a systematic review of the effects of temperature, hypoxia, and acidification on individual sociability and group cohesion in shoaling and schooling fishes. Searches of the published and grey literature were carried out, and studies were included or excluded based on selection criteria. Data from studies were then included in a meta-analysis to examine broad patterns of effects of climate-related stressors in the literature. Evidence was found for a reduction in group cohesion at low oxygen levels, which was stronger in smaller groups. While several studies reported effects of temperature and acidification, there was no consistent effect of either stressor on sociability or cohesion. There was some evidence that marine fishes are more strongly negatively affected by acidification compared with freshwater species, but results are similarly inconsistent and more studies are required. Additional studies of two or more stressors in combination are also needed, although one study found reduced sociability following exposure to acidification and high temperatures. Overall, there is some evidence that hypoxia, and potentially other climate-related environmental changes, impact sociability and group cohesion in fishes. This may reduce survival and adaptability in shoaling and schooling species and have further ecological implications for aquatic systems. However, this synthesis mainly highlights the need for more empirical studies examining the effects of climate-related factors on social behaviour in fishes.

Foraging is the behavioral process of searching for, capturing, and consuming food. It is inextricably linked with individual metabolism in animals and is influenced by numerous physiological systems and environmental factors. This article describes the progression of a typical foraging bout in fish and describes how internal and external variables can modulate decision-making during foraging and potential outcomes throughout this sequence. Although foraging is a fundamental, ubiquitous facet of fish behavior with numerous links to physiology, there are many aspects of foraging we do not fully understand, particularly related to the effects of ongoing environmental change.

Schooling is a form of group behavior in fishes, in which individuals move in the same direction, at a similar speed, with synchrony and some consistency in the spatial separation among individuals. It is critical for foraging and predator avoidance in many fish species and interacts with numerous physiological traits and environmental factors. This article describes links among schooling behavior and swimming performance, energy balance, stress physiology, and sensory systems, and the ecological relevance of these effects. While schooling behavior is widespread across fish species and is influenced by numerous environmental variables including temperature and hypoxia, there is a surprising dearth of knowledge regarding how various forms of anthropogenic change are affecting schooling behavior and especially via effects on individual physiology.

Cognitive abilities are crucial for survival and adaptation, enabling animals to navigate their environment, recognize predators and remember the location of food resources. However, underlying factors related to learning and memory can be energetically demanding and thus may vary depending on an individual's metabolism or aerobic capacity, potentially affecting individuals' cognitive performance. In this study, we explored the link between cognitive performance and whole-body metabolic traits, including aerobic scope (AS), maximum metabolic rate (MMR) and standard metabolic rate (SMR). European minnows, Phoxinus phoxinus, were trained over 20 days to locate a food reward in a maze. Individuals were trained in either a simple (two-door) or a complex (four-door) maze. Fish in the simpler maze had consistently higher success and a lower latency to reach the reward, suggesting the two-door maze was less cognitively challenging. We found a correlation between metabolic traits and cognitive performance traits (i.e. success and latency to reach the reward) at the end of training. However, this relationship varied depending on maze complexity. In the two-door maze, individuals with higher MMR and SMR had higher success and a lower latency to reach the reward. However, in the more complex maze, fish with lower metabolic rates (MMR and SMR) had higher success and lower latency to reach the reward. AS followed similar patterns but mostly affected the success to reach the reward. In simpler environments, having a higher metabolism may be more beneficial for cognitive performance, whereas in complex environments, having a lower metabolism may be more beneficial as it could be associated with a slower but more thorough exploration and learning process.

The "cognitive styles" hypothesis suggests that individual differences in behavior are associated with variation in cognitive performance via underlying speed-accuracy trade-offs. While this is supported, in part, by a growing body of evidence, some studies did not find the expected relationships between behavioral type and cognitive performance. In some cases, this may reflect methodological limitations rather than the absence of a true relationship. The physical design of the testing arena and the number of choices offered in an assay can hinder our ability to detect inter-individual differences in cognitive performance. Here, we re-investigated the cognitive styles hypothesis in threespine stickleback (Gasterosteus aculeatus), adapting the maze design of a previous study which found no cost to decision success by faster (bolder) individuals. We used a similar design but increased the size of the maze and incorporated an additional choice in the form of a third maze arm. We found, in accordance with cognitive style expectations, that individuals who were consistently slower to emerge from the start chamber made fewer errors than fish that emerged faster. Activity in an open field test, however, did not show evidence of a relationship with decision success, possibly due to the low number of repeated observations per fish in this separate assay. Our results provide further empirical support for the cognitive styles hypothesis and highlight important methodological aspects to consider in studies of inter-individual differences in cognition. Inter-individual variation in cognitive ability is hypothesized to be associated with behavioral traits through trade-offs between speed and performance. Previous research testing a common model species-the stickleback-did not find evidence to support the expected relationship between decision success and behavioral type. Using a more challenging test, with an additional choice and larger maze, we found that individuals that emerge faster (i.e. "bolder") were also more error-prone, as predicted by the cognitive style hypothesis.

Sailfin catfish, Pterygoplichthys, is a genus of fish common in the ornamental aquarium trade.Originally from South America, they are now invasive in numerous locations around the globe.We report the first records of Pterygoplichthys in the United Kingdom.We captured one Pterygoplichthys joselimaianus and one Pterygoplichthys pardalis in an artificial side channel of the River Kelvin in Glasgow, Scotland on 30 August 2021 and 6 September 2021, respectively.Further monitoring and public education will be important as river temperatures increase to prevent establishment of these species, which have become invasive in other parts of the world.

Evidence of behavioural sleep has been observed in every animal species studied to date, but current knowledge of the behaviour, neurophysiology and ecophysiology associated with sleep is concentrated on mammals and birds. Fish are a hugely diverse group that can offer novel insights into a variety of sleep-related behaviours across environments, but the ecophysiological relevance of sleep in fish has been largely overlooked. Here, we systematically reviewed the literature to assess the current breadth of knowledge on fish sleep, and surveyed the diverse physiological effects and behaviours associated with sleep. We also discuss possible ways in which unstudied external factors may alter sleep behaviours. For example, predation risk may alter sleep patterns, as has been shown in mammalian, avian and reptilian species. Other environmental factors - such as water temperature and oxygen availability - have the potential to alter sleep patterns in fish differently than for terrestrial endotherms. Understanding the ecological influences on sleep in fish is vital, as sleep deprivation has the potential to affect waking behaviour and fitness owing to cognitive and physiological impairments, possibly affecting ecological phenomena and sensitivity to environmental stressors in ways that have not been considered.

Microplastics (< 5 mm) are widely found in organisms and have the potential harm to ecosystems. Despite their widespread prevalence in environments, there is high individual varation in the abundance of microplastics found in individuals of the same species. In the present study, juvenile cichlid fish (Chindongo demasoni) were chosen to determine the ingestion personality for microplastics in the laboratory. The visible implant fluorescent tags were used for individual recognition. The fish were fed with microplastic fiber, pellet, and food for comparison. Our results showed that the observation of the behaviors of fish could be successfully matched with subsequent measurements for each individual through the tag method in microplastic research. The difference in the abundance of fiber (0-27 items/ind.) among fish individuals was also observed in our study. Meanwhile, the abundance of fiber showed a positive correlation with the average speed and covered area of fish, which indicates the degree of activity of fish. Moreover, fish with higher speed or a front position had higher capturing times for pellet. Our results suggest that the swimming behaviors of fish affect their ingestion of microplastics, and active fish had a higher likelihood of ingesting microplastics, which might be one of the reasons for the common phenomena, i.e., great individual differences observed in microplastic studies.

Although studies on fish cognition are increasing, consideration of how methodological details influence the ability to detect and measure performance is lagging. Here, in two separate experiments the authors compared latency to leave the start position, latency to make a decision, levels of participation and success rates (whether fish entered the rewarded chamber as first choice) across different physical designs. Experiments compared fish performance across (a) two sizes of T-mazes, large and standard, and a plus-maze, and (b) open choice arenas with either two or four doors. Fish in T-mazes with longer arms took longer to leave the start chamber and were less likely to participate in a trial than fish in T-mazes with shorter arms. The number of options, or complexity, in a maze significantly impacted success but did not necessarily impact behavioural measures, and did not impact the number of fish that reached a chamber. Fish in the plus-maze had similar latencies to leave the start box and time to reach any chamber as fish in the same-sized T-maze but exhibited lower overall success. Similarly, in an open choice arena, increasing the number of options - doors to potential reward chambers - resulted in lower probability of success. There was an influence of reward position in the choice arena, with rewarded chambers closest to the sides of the arena resulting in lower latencies to enter and higher probability of decision success. Together the results allow the authors to offer practical suggestions towards optimal maze design for studies of fish cognition.