Recent social conditions affect boldness repeatability in individual sticklebacks

Bolder together: conformity drives behavioral plasticity in eastern gartersnakes

Personality traits drive individual differences in behaviour that are consistent across time and context. Personality limits behavioural plasticity, which could lead to maladaptive choices if animals cannot adapt their behavior to changing conditions. Here, we assessed consistency and flexibility in one personality trait, boldness, across non-social and social contexts in eastern gartersnakes (Thamnophis sirtalis sirtalis). Snakes explored a novel open arena either alone or in a pair. Pairs were assigned based on the data from the solo trials, such that each snake was paired once with a bolder and once with a less bold partner. We predicted that snakes would conform when in a social context, displaying plasticity in their personality, and causing boldness scores to converge. We found that snakes were consistent within contexts (solo or paired), but changed their behavior across contexts (from solo to paired). Plasticity in boldness resulted from an interaction between conformity and repeatable individual differences in plasticity. In line with some data on other species, snakes conformed more when they were the less bold partner. Personality reflects a consistent bias in decision-making, but our results highlight that the cognitive processes that drive the expression of personality traits in behavior are flexible and sensitive to social context. We show that both consistency and plasticity combine to shape snake social behavior in ways that are responsive to competition. This pattern of behavior may be particularly beneficial for species in which group-living is seasonal.

Are bold-shy personalities of European perch (Perca fluviatilis) linked to stress tolerance and immunity? A scope of harnessing fish behavior in aquaculture

The sensitivity to stress and its impact on immunity are supposedly related to a fish's personality. In the present study, European perch (Perca fluviatilis) were exposed to an open-field and a novel-object test to identify distinctive shy and bold individuals. This series of cognitive tests revealed clear differences between proactive individuals with pronounced exploration behavior (bold personality) and reactive individuals that took a freeze-hide position (shy personality). A cohort of shy and bold perch was then exposed to elevated stocking density. Frozen activity and lower explorative behavior were related to higher basal and stocking-induced cortisol levels compared to proactive individuals. Since cortisol is a well-known modulator of immune-gene expression, we used multiplex real-time PCR to profile the differential immune responses to the intraperitoneal injection of Aeromonas hydrophila in the head kidney and peritoneal cells of bold and shy perch individuals. These expression differences between stimulated bold and shy perch were generally modest, except for the genes encoding the complement component c3 and the matrix metallopeptidase mmp9. The strong differential expression of these two bactericidal and inflammatory genes in the context of the modestly regulated features suggests that a fish's personality is linked to a particular immune-defense strategy. In conclusion, our approach, based on behavioral video observations, phagocytosis and enzyme assays, immunogene-expression profiling, and quantification of stress-relevant metabolites, revealed indications for divergent coping styles in cohorts of bold or shy European perch. This divergence could be exploited in future selective breeding programs.

Maze design: size and number of choices impact fish performance in cognitive assays

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.

Changes in group behaviour in response to a preferred environment reflect positive affect

When observed in their preferred environments, animals display behavioural changes, such as an increase in resting or a reduction in agonism, suggestive of positive affect and improved welfare. However, most studies focus on the behaviour of individuals or, at most, pairs of animals; even though in group-living animals beneficial environmental changes may impact on how the group behaves as a whole. In this study, we investigated whether experiencing a preferred visual environment affected the shoaling behaviour of zebrafish (Danio rerio) groups. We first confirmed a group preference for an image of gravel placed underneath the base of a tank compared to a plain white image. Second, we observed replicated groups either with or without the preferred (gravel) image present to determine if a visually enriched and preferred environment could elicit changes in shoaling behaviour. We found a significant interaction between the observation time and test condition, with differences in shoaling behaviour reflective of increased relaxation emerging gradually over time in the gravel condition. The findings of this study reveal that experiencing a preferred environment can alter group behaviour, making such holistic changes valuable as potential indicators of positive welfare.

The effect of heterospecific and conspecific competition on inter-individual differences in tungara frog tadpole (Engystomops pustulosus) behavior

Repeated social interactions with conspecifics and/or heterospecifics during early development may drive the differentiation of behavior among individuals. Competition is a major form of social interaction and its impacts can depend on whether interactions occur between conspecifics or heterospecifics and the directionality of a response could be specific to the ecological context that they are measured in. To test this, we reared tungara frog tadpoles (Engystomops pustulosus) either in isolation, with a conspecific tadpole or with an aggressive heterospecific tadpole, the whistling frog tadpole (Leptodactylus fuscus). In each treatment, we measured the body size and distance focal E. pustulosus tadpoles swam in familiar, novel and predator risk contexts six times during development. We used univariate and multivariate hierarchical mixed effect models to investigate the effect of treatment on mean behavior, variance among and within individuals, behavioral repeatability and covariance among individuals in their behavior between contexts. There was a strong effect of competition on behavior, with different population and individual level responses across social treatments. Within a familiar context, the variance in the distance swam within individuals decreased under conspecific competition but heterospecific competition caused more variance in the average distance swam among individuals. Behavioral responses were also context specific as conspecific competition caused an increase in the distance swam within individuals in novel and predator risk contexts. The results highlight that the impact of competition on among and within individual variance in behavior is dependent on both competitor species identity and context.

Western Hognose Snakes (Heterodon nasicus) Prefer Environmental Enrichment

The environmental enrichment needs of snakes are often disregarded. Using preference testing, we aimed to shed light on the enrichment preferences of a popular pet species, the western hognose snake (Heterodon nasicus). Snakes' enclosures were divided into enriched and standard sides. The enriched half had substrate for burrowing, interactive stimuli, and a large water dish. The standard half had paper towel substrate and a small water dish. Each side also contained a single shelter. We provided belly heat to create a thermal gradient on one side of the cage. Snakes were observed for 6 days, four times daily. We predicted a preference for enriched conditions and, as snakes are ectothermic, a preference for the warmer side. Snakes were additionally given an exploration assay, to explore whether differences in preference for environmental enrichment interact with boldness levels. We found that hognose snakes preferred enrichment, and the strength of this preference increased over time. Preference for enrichment was stronger when the enriched side was cooler. This may be due to the burrowing tendencies of these snakes. We found no relationship between preference and boldness. These findings emphasise the importance of preference testing in establishing research-informed enrichment opportunities for reptiles.

Social context mediates the expression of a personality trait in a gregarious lizard

The social environment is a key factor that influences behavioural traits across a wide array of species. Yet, when investigating individual differences in behaviour, studies tend to measure animals in isolation from other conspecifics—even in social species. Surprisingly, whether behavioural traits measured in isolation are predictive of individual-level behaviour when in social groups is still poorly understood. Here, we repeatedly measured risk-taking behaviour (i.e. boldness; 741 total trials) in both the presence and absence of conspecifics in a social lizard, the delicate skink (Lampropholis delicata). Further, we manipulated food availability during group trials to test whether the effect of the social environment on risk-taking behaviour was mediated by competition over resources. Using 105 lizards collected from three independent populations, we found that individual risk-taking behaviour was repeatable when measured in either social isolation or within groups both with and without food resources available. However, lizards that were bolder during individual trials were not also bolder when in groups, regardless of resource availability. This was largely driven by individual differences in social behavioural plasticity, whereby individual skinks responded differently to the presence of conspecifics. Together, this resulted in a rank order change of individual behavioural types across the social conditions. Our results highlight the importance of the social environment in mediating animal personality traits across varying levels of resource availability. Further, these findings suggest that behavioural traits when measured in isolation, may not reflect individual variation in behaviour when measured in more ecologically realistic social groups.

Cascading indirect genetic effects in a clonal vertebrate

Understanding how individual differences arise and how their effects propagate through groups are fundamental issues in biology. Individual differences can arise from indirect genetic effects (IGE): genetically based variation in the conspecifics with which an individual interacts. Using a clonal species, the Amazon molly (Poecilia formosa), we test the hypothesis that IGE can propagate to influence phenotypes of the individuals that do not experience them firsthand. We tested this by exposing genetically identical Amazon mollies to conspecific social partners of different clonal lineages, and then moving these focal individuals to new social groups in which they were the only member to have experienced the IGE. We found that genetically different social environments resulted in the focal animals experiencing different levels of aggression, and that these IGE carried over into new social groups to influence the behaviour of naive individuals. These data reveal that IGE can cascade beyond the individuals that experience them. Opportunity for cascading IGE is ubiquitous, especially in species with long-distance dispersal or fission-fusion group dynamics. Cascades could amplify (or mitigate) the effects of IGE on trait variation and on evolutionary trajectories. Expansion of the IGE framework to include cascading and other types of carry-over effects will therefore improve understanding of individual variation and social evolution and allow more accurate prediction of population response to changing environments.

How feedback and feed-forward mechanisms link determinants of social dominance

In many animal societies, individuals differ consistently in their ability to win agonistic interactions, resulting in dominance hierarchies. These differences arise due to a range of factors that can influence individuals' abilities to win agonistic interactions, spanning from genetically driven traits through to individuals' recent interaction history. Yet, despite a century of study since Schjelderup-Ebbe's seminal paper on social dominance, we still lack a general understanding of how these different factors work together to determine individuals' positions in hierarchies. Here, we first outline five widely studied factors that can influence interaction outcomes: intrinsic attributes, resource value asymmetry, winner-loser effects, dyadic interaction-outcome history and third-party support. A review of the evidence shows that a variety of factors are likely important to interaction outcomes, and thereby individuals' positions in dominance hierarchies, in diverse species. We propose that such factors are unlikely to determine dominance outcomes independently, but rather form part of feedback loops whereby the outcomes of previous agonistic interactions (e.g. access to food) impact factors that might be important in subsequent interactions (e.g. body condition). We provide a conceptual framework that illustrates the multitude potential routes through which such feedbacks can occur, and how the factors that determine the outcomes of dominance interactions are highly intertwined and thus rarely act independently of one another. Further, we generalise our framework to include multi-generational feed-forward mechanisms: how interaction outcomes in one generation can influence the factors determining interaction outcomes in the next generation via a range of parental effects. This general framework describes how interaction outcomes and the factors determining them are linked within generations via feedback loops, and between generations via feed-forward mechanisms. We then highlight methodological approaches that will facilitate the study of feedback loops and dominance dynamics. Lastly, we discuss how our framework could shape future research, including: how feedbacks generate variation in the factors discussed, and how this might be studied experimentally; how the relative importance of different feedback mechanisms varies across timescales; the role of social structure in modulating the effect of feedbacks on hierarchy structure and stability; and the routes of parental influence on the dominance status of offspring. Ultimately, by considering dominance interactions as part of a dynamic feedback system that also feeds forward into subsequent generations, we will understand better the factors that structure dominance hierarchies in animal groups.

Social Group Size and Shelter Availability Influence Individual Metabolic Traits in a Social Fish

Group living is widespread among animal species and yields both costs and benefits. Presence of conspecifics can restrict or enhance the expression of individual behavior, and the recent social environment is thought to affect behavioral responses in later contexts, even when individuals are alone. However, little is known about how social group size influences the expression of individual physiological traits, including metabolic rates. There is some evidence that shoaling can reduce fish metabolic rates but this variable may be affected by habitat conditions such as shelter availability via density-dependent processes. We investigated how social group size and shelter availability influence Eurasian minnow (Phoxinus phoxinus) metabolic rates estimated by respirometry. Respirometry trials were conducted on fish in isolation before and after they were housed for 3 weeks in a social treatment consisting in a specific group size (n = 4 or 8) and shelter availability (presence or absence of plant shelter in the experimental tank). Plant shelter was placed over respirometers for half of the duration of the respirometry trials, allowing estimation of minimum daytime and nighttime metabolic rates in both conditions (in the presence or absence of plant shelter). Standard metabolic rate (SMR), maximum metabolic rate (MMR), and aerobic scope were also estimated over the entire trial. Minimum daytime and nighttime metabolic rates estimated while in presence of plant shelter were lower than when estimated in absence of plant shelter, both before and after individuals were housed in their social treatment. After the social treatment, SMRs were higher for fish that were held in groups of 4 as compared with those of fish held in groups of 8, while MMR showed no difference. Plant shelter availability during the social treatments did not influence SMR or MMR. Our results suggest that social group size may directly influence energy demands of individuals, highlighting the importance of understanding the role of group size on variations in physiological traits associated with energy expenditure.

Trap versus net: Behavioural sampling bias caused by capture method in three-spined sticklebacks

Wild-caught animals are often used in behavioural or other biological studies. However, different capture methods may target individuals that differ in behaviour, life history and morphology, thereby giving rise to sampling biases. Here, we investigated whether juvenile three-spined sticklebacks caught in a natural population by passive and active sampling methods using frequently used tools (i.e. trap and hand net) differ in behaviours related to cognition and personality. The fish caught by traps were more prone to take risks and shoal (i.e. bolder and more sociable), but smaller in size and mass than the fish caught by hand nets. Individual variation in boldness was greater in the fish caught by hand nets, suggesting that this active sampling method may capture more representative samples of the natural population. Our results show the importance of capture method to avoid sampling bias in behavioural studies using wild-caught animals.

Population differences in the effect of context on personality in an invasive lizard

Within populations, individuals often differ consistently in their average level of behavior (i.e., animal personality), as well as their response to environmental change (i.e., behavioral plasticity). Thus, changes in environmental conditions might be expected to mediate the structure of animal personality traits. However, it is currently not well understood how personality traits change in response to environmental conditions, and whether this effect is consistent across multiple populations within the same species. Accordingly, we investigated variation in personality traits across two ecological contexts in the invasive delicate skink (Lampropholis delicata). Specifically, lizards from three different populations were repeatedly measured for individual activity in group behavioral assays under differing levels of food availability. We found that environmental context had a clear effect on the structure of lizard personality, where activity rates were not repeatable in the absence of food, but were repeatable in the presence of food resources. The difference in repeatability of activity rates across contexts appeared to be largely driven by an increase in among-individual variance when tested in the presence of food resources. However, this was only true for one of the populations tested, with food context having no effect on the expression of personality traits in the other two populations. Our results highlight the important role of environmental context in mediating the structure of animal personality traits and suggest that this effect may vary among populations.

Does shelter influence the metabolic traits of a teleost fish?

Availability of shelter is an important component of habitat selection for animals as it can influence survival (protection against harsh physical conditions and predation) and growth (energy acquisition and expenditure). Few studies address the effect of shelter on metabolic expenditures associated with non-mechanical tasks (excluding station holding or movement). The main goal of this study was to investigate the influence of shelter use on metabolic traits of smallmouth bass (Micropterus dolomieu) from two populations (Kiamika River and Lake Long). Respirometry experiments on smallmouth bass were conducted to measure standard metabolic rate (SMR), resting metabolic rate (RMR), aerobic scope (AS), recovery time (RT) and excess post-exercise oxygen consumption (EPOC) in the presence or absence of shelter. The presence of shelter did not affect most metabolic traits, except for RMR, which was reduced in the presence of shelter for Lake Long fish. The results of this study also show that larger fish had lower SMR in the presence of shelter than when it was absent. When accounting for social hierarchy, there were no differences in most metabolic traits in dominant or subordinate fish in the presence or absence of shelter, except for RT, which was significantly lower in the presence of shelter for dominant fish. These results do not support the existence of an unequivocal relationship between individual metabolic traits and the presence of shelter. If physiological motives may influence the use of shelter, sheltering in itself might not have important consequences on energy expenditures required for non-mechanical tasks.

Animal Personality and Conservation: Basics for Inspiring New Research

Simple Summary

The study of animal personality is important to conserve animals because it can help in selecting the most appropriate individuals to be released into the wild. Individuals not so bold or aggressive, less stressed, who explore their new environment with greater caution are often more likely to survive after release into the wild. In contrast, bolder and more aggressive animals reproduce more successfully and, therefore, can be released with the aim of rapid repopulation of an area. These and other aspects of how animal personality can help in conservation programs, as well as how to collect personality data are covered in this paper.

Abstract

The number of animal species threatened with extinction are increasing every year, and biologists are conducting animal translocations, as one strategy, to try to mitigate this situation. Furthermore, researchers are evaluating methods to increase translocation success, and one area that shows promise is the study of animal personality. Animal personality can be defined as behavioral and physiological differences between individuals of the same species, which are stable in time and across different contexts. In the present paper, we discuss how animal personality can increase the success of translocation, as well as in the management of animals intended for translocation by evaluating personality characteristics of the individuals. Studies of the influence of birthplace, parental behavior, stress resilience, and risk assessment can be important to select the most appropriate individuals to be released. Finally, we explain the two methods used to gather personality data.

"Micropersonality" traits and their implications for behavioral and movement ecology research

Many animal personality traits have implicit movement-based definitions and can directly or indirectly influence ecological and evolutionary processes. It has therefore been proposed that animal movement studies could benefit from acknowledging and studying consistent interindividual differences (personality), and, conversely, animal personality studies could adopt a more quantitative representation of movement patterns.Using high-resolution tracking data of three-spined stickleback fish (Gasterosteus aculeatus), we examined the repeatability of four movement parameters commonly used in the analysis of discrete time series movement data (time stationary, step length, turning angle, burst frequency) and four behavioral parameters commonly used in animal personality studies (distance travelled, space use, time in free water, and time near objects).Fish showed repeatable interindividual differences in both movement and behavioral parameters when observed in a simple environment with two, three, or five shelters present. Moreover, individuals that spent less time stationary, took more direct paths, and less commonly burst travelled (movement parameters), were found to travel farther, explored more of the tank, and spent more time in open water (behavioral parameters).Our case study indicates that the two approaches-quantifying movement and behavioral parameters-are broadly equivalent, and we suggest that movement parameters can be viewed as "micropersonality" traits that give rise to broad-scale consistent interindividual differences in behavior. This finding has implications for both personality and movement ecology research areas. For example, the study of movement parameters may provide a robust way to analyze individual personalities in species that are difficult or impossible to study using standardized behavioral assays.

Individual behavioural traits not social context affects learning about novel objects in archerfish

Abstract

Learning can enable rapid behavioural responses to changing conditions but can depend on the social context and behavioural phenotype of the individual. Learning rates have been linked to consistent individual differences in behavioural traits, especially in situations which require engaging with novelty, but the social environment can also play an important role. The presence of others can modulate the effects of individual behavioural traits and afford access to social information that can reduce the need for ‘risky’ asocial learning. Most studies of social effects on learning are focused on more social species; however, such factors can be important even for less-social animals, including non-grouping or facultatively social species which may still derive benefit from social conditions. Using archerfish, Toxotes chatareus, which exhibit high levels of intra-specific competition and do not show a strong preference for grouping, we explored the effect of social contexts on learning. Individually housed fish were assayed in an ‘open-field’ test and then trained to criterion in a task where fish learnt to shoot a novel cue for a food reward—with a conspecific neighbour visible either during training, outside of training or never (full, partial or no visible presence). Time to learn to shoot the novel cue differed across individuals but not across social context. This suggests that social context does not have a strong effect on learning in this non-obligatory social species; instead, it further highlights the importance that inter-individual variation in behavioural traits can have on learning.

Significance statement

Some individuals learn faster than others. Many factors can affect an animal’s learning rate—for example, its behavioural phenotype may make it more or less likely to engage with novel objects. The social environment can play a big role too—affecting learning directly and modifying the effects of an individual’s traits. Effects of social context on learning mostly come from highly social species, but recent research has focused on less-social animals. Archerfish display high intra-specific competition, and our study suggests that social context has no strong effect on their learning to shoot novel objects for rewards. Our results may have some relevance for social enrichment and welfare of this increasingly studied species, suggesting there are no negative effects of short- to medium-term isolation of this species—at least with regards to behavioural performance and learning tasks.

The importance of individual variation for the interpretation of behavioural studies: ethanol effects vary with basal activity level in zebrafish larvae

Standardization and reduction of variation is key to behavioural screening of animal models in toxicological and pharmacological studies. However, individual variation in behavioural and physiological phenotypes remains in each laboratory population and can undermine the understanding of toxicological and pharmaceutical effects and their underlying mechanisms. Here, we used zebrafish (ABTL-strain) larvae to explore individual consistency in activity level and emergence time, across subsequent days of early development (6-8 dpf). We also explored the correlation between these two behavioural parameters. We found inter-individual consistency over time in activity level and emergence time, but we did not find a consistent correlation between these parameters. Subsequently, we investigated the impact of variation in activity level on the effect of a 1% ethanol treatment, suitable for our proof-of-concept case study about whether impact from pharmacological treatments might be affected by inter-individual variation in basal locomotion. The inter-individual consistency over time in activity level did not persist in this test. This was due to the velocity change from before to after exposure, which turned out to be a dynamic individual trait related to basal activity level: low-activity individuals raised their swimming velocity, while high-activity individuals slowed down, yielding diametrically opposite response patterns to ethanol exposure. We therefore argue that inter-individual consistency in basal activity level, already from 6 dpf, is an important factor to take into account and provides a practical measure to improve the power of statistical analyses and the scope for data interpretation from behavioural screening studies.

Repeatable individual variation in migration timing in two anadromous salmonids and ecological consequences

Consistent individual differences in behavior have been demonstrated for many animals, but there are few studies of consequences of such repeated behavior in the wild. We tested consistency in migration timing to and from the sea among anadromous Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta), using data from a study period of about 25 years, including more than 27,000 uniquely Carlin-tagged individuals that migrated to sea for feeding in the spring and returned to the river in late summer for up to 13 successive years. Consistency was found between individuals across time in timing of the seaward migration. Individuals migrating early during their first migration tended to migrate early the following years, and late migrants tended to migrate late. The same pattern was found also at ascent to freshwater. Hence, this study demonstrated that individual fish in nature can differ in behavior related to migration timing and that these differences can be consistent during their lifetime. Early migrants increased their mass more than late migrants and had a higher specific growth rate. Early migrating Arctic char, but not brown trout, experienced a longer life after the first migration to sea than late migrants. In both species, maturity occurred earlier in individuals that migrated early. For brown trout, but not for Arctic char, fecundity was significantly correlated to the timing of smolt migration. Hence, the repeatable individual variation in migration timing seemed to have ecological and fitness consequences in terms of growth, longevity, timing of maturity, and lifetime fecundity.

Becoming creatures of habit: Among- and within-individual variation in nesting behaviour shift with age

The quantification of repeatability has enabled behavioural and evolutionary ecologists to assess the heritable potential of traits. For behavioural traits that vary across life, age-related variation should be accounted for to prevent biasing the microevolutionary estimate of interest. Moreover, to gain a mechanistic understanding of ontogenetic variation in behaviour, among- and within-individual variance should be quantified across life. We leveraged a 30-year study of painted turtles (Chrysemys picta) to assess how age contributes to variation in the repeatability of nesting behaviours. We found that four components of nesting behaviour were repeatable and that accounting for age increased the repeatability estimate for maternal choice of canopy cover over nests. We detected canalization (diminished within-individual variance with age) of canopy cover choice in a reduced data set despite no shift in repeatability. Additionally, random regression analysis revealed that females became more divergent from each other in their choice of canopy cover with age. Thus, properly modelling age-related variance should more precisely estimate heritable potential, and assessing among- and within-individual variance components in addition to repeatability will offer a more mechanistic understanding of behavioural variation across age.

Schistocephalus parasite infection alters sticklebacks' movement ability and thereby shapes social interactions

Parasitism is ubiquitous in the animal kingdom. Although many fundamental aspects of host-parasite relationships have been unravelled, few studies have systematically investigated how parasites affect organismal movement. Here we combine behavioural experiments of Schistocephalus solidus infected sticklebacks with individual-based simulations to understand how parasitism affects individual movement ability and thereby shapes social interaction patterns. High-resolution tracking revealed that infected fish swam, accelerated, and turned more slowly than did non-infected fish, and tended to be more predictable in their movements. Importantly, the strength of these effects increased with increasing parasite load (proportion of body weight), with more heavily infected fish showing larger changes and impairments in behaviour. When grouped, pairs of infected fish moved more slowly, were less cohesive, less aligned, and less temporally coordinated than non-infected pairs, and mixed pairs were primarily led by the non-infected fish. These social patterns also emerged in simulations of self-organised groups composed of individuals differing similarly in speed and turning tendency, suggesting infection-induced changes in mobility and manoeuvrability may drive collective outcomes. Together, our results demonstrate how infection with a complex life-cycle parasite affects the movement ability of individuals and how this in turn shapes social interaction patterns, providing important mechanistic insights into the effects of parasites on host movement dynamics.

Is repeatability of metabolic rate influenced by social separation? A test with a teleost fish

Metabolic rates are typically thought to have important influences on fitness and more broadly be relevant to the ecology and evolution of animals. Previous studies demonstrate that metabolic rates are repeatable to a certain extent under constant conditions, but how social conditions influence the repeatability of metabolic rate remains largely unknown. In this study, we investigated the repeatability of resting metabolic rate (RMR) in the highly social crucian carp (Carassius auratus) after being socially separated for different time periods relative to control fish that were not socially separated. We found that RMR was repeatable in fish in the control group, while the repeatability of RMR disappeared quickly (even within 7 days) when fish were exposed to social separation. This study is the first to our knowledge to examine the role of social separation for different time periods on the repeatability of intra-individual physiological variation in fish. We highlight that the inter-individual repeatability of metabolic rate can be substantial over time but was eliminated by social separation. The findings indicate that the repeatability of metabolic rate in fish is condition dependent, and that the change in repeatability of metabolic rate should not be overlooked when considering the ecological and evolutionary effects of environmental change.

Social experiences shape behavioral individuality and within-individual stability

Individual repeatability characterizes many behaviors. Repeatable behavior may result from repeated social interactions among familiar group members, owing to adaptive social niche specialization. In the context of aggression, in species like field crickets, social niche specialization should also occur when individuals repeatedly interact with unfamiliar individuals. This would require the outcome of social interactions to have carry-over effects on fighting ability and aggressiveness in subsequent interactions, leading to long-term among-individual differentiation. To test this hypothesis, we randomly assigned freshly emerged adult males of the southern field cricket Gryllus bimaculatus to either a solitary or social treatment. In social treatment, males interacted with a same-sex partner but experienced a new partner every 3 days. After 3 weeks of treatment, we repeatedly subjected treated males to dyadic interactions to measure aggression. During this time, we also continuously measured the 3-daily rate of carbohydrate and protein consumption. Individual differentiation was considerably higher among males reared in the social versus solitary environment for aggressiveness but not for nutrient intake. Simultaneously, social experience led to lower within-individual stability (i.e., increased within-individual variance) in carbohydrate intake. Past social experiences, thus, shaped both behavioral individuality and stability. While previous research has emphasized behavioral individuality resulting from repeated interactions among familiar individuals, our study implies that behavioral individuality, in the context of aggression, may generally result from social interactions, whether with familiar or unfamiliar individuals. Our findings thus imply that social interactions may have a stronger effect on individual differentiation than previously appreciated.

Social cues affect quantitative genetic variation and covariation in animal personality traits

The social environment is expected to have substantial effects on behavior, and as a consequence, its heritability and evolvability. We investigated these effects by exposing Australian field crickets (Teleogryllus oceanicus) to either silence or recordings of male acoustic sexual signals. We used a combined pedigree and full-sib/half-sib breeding design to estimate the repeatability, heritability, and evolvability of behaviors related to boldness, exploration, and activity. All behaviors measured were significantly repeatable in both social environments. Additionally, most behaviors showed significant heritabilities in the two environments. We found no difference in repeatabilities between the silent and the acoustic environment but did find significant differences in the heritabilities and evolvabilities between these environments. There was a high degree of similarity between the phenotypic covariance matrices across the two environments, while the genotypic covariance matrices were highly dissimilar. Reflecting this, we found significant genotype-by-environment interactions for most of the behaviors. Lastly, we found that the repeatable aspect of behavior ("personality") was significantly heritable for most behaviors, but that these heritabilities were higher in the acoustic than in the silent environment. We conclude that the social environment can have a significant impact on the heritability and evolvability of behavior, and argue that evolutionary inferences from phenotypic studies should be made with caution.

Repeatable group differences in the collective behaviour of stickleback shoals across ecological contexts

Establishing how collective behaviour emerges is central to our understanding of animal societies. Previous research has highlighted how universal interaction rules shape collective behaviour, and that individual differences can drive group functioning. Groups themselves may also differ considerably in their collective behaviour, but little is known about the consistency of such group variation, especially across different ecological contexts that may alter individuals' behavioural responses. Here, we test if randomly composed groups of sticklebacks differ consistently from one another in both their structure and movement dynamics across an open environment, an environment with food, and an environment with food and shelter. Based on high-resolution tracking data of the free-swimming shoals, we found large context-associated changes in the average behaviour of the groups. But despite these changes and limited social familiarity among group members, substantial and predictable behavioural differences between the groups persisted both within and across the different contexts (group-level repeatability): some groups moved consistently faster, more cohesively, showed stronger alignment and/or clearer leadership than other groups. These results suggest that among-group heterogeneity could be a widespread feature in animal societies. Future work that considers group-level variation in collective behaviour may help understand the selective pressures that shape how animal collectives form and function.

The role of personality variation, plasticity and social facilitation in cockroach aggregation

Personality variation has been proven to affect ecology, evolution and group behaviour in many ways. Nevertheless, how social context influences behavioural strategies and individual personality variation has rarely been addressed. This study sheds light on the relationship between social interactions, personality variation and plasticity in a collective context. For this purpose, we used a binary setup (i.e. an arena with two identical shelters) to study the aggregation process of cockroaches. We tested the same individuals in isolated and social (groups of 16 individuals) conditions. We show that even if social interactions reduce the observation of personality variation, the behaviour in a group is correlated to individual preferences displayed in isolation. Furthermore, our results suggest that individuals show different levels of plasticity according to their shelter occupancy; individuals with high occupancy rates show low levels of plasticity and are less affected by social amplification in social conditions.

Individual behavioral variation reflects personality divergence in the upcoming model organism Nothobranchius furzeri

In the animal kingdom, behavioral variation among individuals has often been reported. However, stable among-individual differences along a behavioral continuum-reflective of personality variation-have only recently become a key target of research. While a vast body of descriptive literature exists on animal personality, hypothesis-driven quantitative studies are largely deficient. One of the main constraints to advance the field is the lack of suitable model organisms. Here, we explore whether N. furzeri could be a valuable model to bridge descriptive and hypothesis-driven research to further unravel the causes, function and evolution of animal personality. As a first step toward this end, we perform a common garden laboratory experiment to examine if behavioral variation in the turquoise killifish Nothobranchius furzeri reflects personality divergence. Furthermore, we explore if multiple behavioral traits are correlated. We deliver "proof of principle" of personality variation among N. furzeri individuals in multiple behavioral traits. Because of the vast body of available genomic and physiological information, the well-characterized ecological background and an exceptionally short life cycle, N. furzeri is an excellent model organism to further elucidate the causes and implications of behavioral variation in an eco-evolutionary context.

The origins and evolution of sleep

Sleep is nearly ubiquitous throughout the animal kingdom, yet little is known about how ecological factors or perturbations to the environment shape the duration and timing of sleep. In diverse animal taxa, poor sleep negatively impacts development, cognitive abilities and longevity. In addition to mammals, sleep has been characterized in genetic model organisms, ranging from the nematode worm to zebrafish, and, more recently, in emergent models with simplified nervous systems such as Aplysia and jellyfish. In addition, evolutionary models ranging from fruit flies to cavefish have leveraged natural genetic variation to investigate the relationship between ecology and sleep. Here, we describe the contributions of classical and emergent genetic model systems to investigate mechanisms underlying sleep regulation. These studies highlight fundamental interactions between sleep and sensory processing, as well as a remarkable plasticity of sleep in response to environmental changes. Understanding how sleep varies throughout the animal kingdom will provide critical insight into fundamental functions and conserved genetic mechanisms underlying sleep regulation. Furthermore, identification of naturally occurring genetic variation regulating sleep may provide novel drug targets and approaches to treat sleep-related diseases.

Boldness traits, not dominance, predict exploratory flight range and homing behaviour in homing pigeons

Group living has been proposed to yield benefits that enhance fitness above the level that would be achieved through living as solitary individuals. Dominance hierarchies occur commonly in these social assemblages, and result, by definition, in resources not being evenly distributed between group members. Determinants of rank within a dominance hierarchy can be associated with morphological characteristics, previous experience of the individual, or personality traits such as exploration tendencies. The purpose of this study was to investigate whether greater exploration and positive responses to novel objects in homing pigeons (Columba livia) measured under laboratory conditions were associated with (i) greater initial exploration of the local area around the home loft during spontaneous exploration flights (SEF), (ii) faster and more efficient homing flights when released from further afield, and (iii) whether the traits of greater exploration and more positive responses to novel objects were more likely to be exhibited by the more dominant individuals within the group. There was no relationship between laboratory-based novel object exploration and position within the dominance hierarchy. Pigeons that were neophobic under laboratory conditions did not explore the local area during SEF opportunities. When released from sites further from home, neophobic pigeons took longer routes to home compared to those birds that had not exhibited neophobic traits under laboratory conditions, and had spontaneously explored to a greater extent. The lack of exploration in the neophobic birds is likely to have resulted in the increased costs of homing following release: unfamiliarity with the landscape likely led to the greater distances travelled and less efficient routes taken. Birds that demonstrated a lack of neophobia were not the dominant individuals inside the loft, and thus would have less access to resources such as food and potentially mates. However, a lack of neophobia makes the subordinate position possible, because subordinate birds that incur high travel costs would become calorie restricted and lose condition. Our results address emerging questions linking individual variation in behaviour with energetics and fitness consequences.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Aggression supersedes individual oxygen demand to drive group air-breathing in a social catfish

Group-living is widespread among animals and comes with numerous costs and benefits. To date, research examining group-living has focused on trade-offs surrounding foraging, while other forms of resource acquisition have been largely overlooked. Air-breathing has evolved in many fish lineages, allowing animals to obtain oxygen in hypoxic aquatic environments. Breathing air increases the threat of predation, so some species perform group air-breathing, to reduce individual risk. Within species, individual air-breathing can be influenced by metabolic rate as well as personality, but the mechanisms of group air-breathing remain unexplored. It is conceivable that keystone individuals with high metabolic demand or intrinsic tendency to breathe air may drive social breathing, especially in hypoxia. We examined social air-breathing in African sharptooth catfish Clarias gariepinus, to determine whether individual physiological traits and spontaneous tendency to breathe air influence the behaviour of entire groups, and whether such influences vary in relation to aquatic oxygen availability. We studied 11 groups of four catfish in a laboratory arena and recorded air-breathing behaviour, activity and agonistic interactions at varying levels of hypoxia. Bimodal respirometry was used to estimate individual standard metabolic rate (SMR) and the tendency to utilize aerial oxygen when alone. Fish took more air breaths in groups as compared to when they were alone, regardless of water oxygen content, and displayed temporally clustered air-breathing behaviour, consistent with existing definitions of synchronous air-breathing. However, groups displayed tremendous variability in surfacing behaviour. Aggression by dominant individuals within groups was the main factor influencing air-breathing of the entire group. There was no association between individual SMR, or the tendency to obtain oxygen from air when in isolation, and group air-breathing. For C. gariepinus, synchronous air-breathing is strongly influenced by agonistic interactions, which may expose subordinate individuals to risk of predation. Influential individuals exerted an overriding effect on risk-taking by the entire group, for reasons independent of their physiological oxygen requirements. Overall, this illustrates that social context can obscure interactions between an individual's physiological and behavioural traits and their tendency to take risks to obtain resources.

Social conformity affects experimental measurement of boldness in male but not female monk parakeets (Myiopsitta monachus)

The standard approach for characterizing boldness rarely considers the influence of social environment on the expression of boldness in group-living animals. We studied a wild-caught, captive population of monk parakeets (Myiopsitta monachus) to investigate the impact of conspecific presence on boldness — a personality trait in monk parakeets — in a controlled environment. We quantified seven boldness metrics across three types of behavioural assay: novel object, emergence, and predator exposure tests in the presence of a companion pair of flock mates. Because of the high degree of sociality in this species, we hypothesized that the presence of companion birds would facilitate the focal individuals’ behavioural responses (i.e., increase the average boldness level). We found that behavioural response in a risky foraging context was inversely correlated between solitary and social condition in males, but not in females. Our results have implications for characterizing sex-specific differences of risk-taking behaviour in social animals.

Developmental social isolation affects adult behavior, social interaction, and dopamine metabolite levels in zebrafish

The zebrafish is a social vertebrate and an excellent translational model for a variety of human disorders. Abnormal social behavior is a hallmark of several human brain disorders. Social behavioral problems can arise as a result of adverse early social environment. Little is known about the effects of early social isolation in adult zebrafish. We compared zebrafish that were isolated for either short (7 days) or long duration (180 days) to socially housed zebrafish, testing their behavior across ontogenesis (ages 10, 30, 60, 90, 120, 180 days), and shoal cohesion and whole-brain monoamines and their metabolites in adulthood. Long social isolation increased locomotion and decreased shoal cohesion and anxiety in the open-field in adult. Additionally, both short and long social isolation reduced dopamine metabolite levels in response to social stimuli. Thus, early social isolation has lasting effects in zebrafish, and may be employed to generate zebrafish models of human neuropsychiatric conditions.

Consistent Individual Differences Drive Collective Behavior and Group Functioning of Schooling Fish

The ubiquity of consistent inter-individual differences in behavior ("animal personalities") [1, 2] suggests that they might play a fundamental role in driving the movements and functioning of animal groups [3, 4], including their collective decision-making, foraging performance, and predator avoidance. Despite increasing evidence that highlights their importance [5-16], we still lack a unified mechanistic framework to explain and to predict how consistent inter-individual differences may drive collective behavior. Here we investigate how the structure, leadership, movement dynamics, and foraging performance of groups can emerge from inter-individual differences by high-resolution tracking of known behavioral types in free-swimming stickleback (Gasterosteus aculeatus) shoals. We show that individual's propensity to stay near others, measured by a classic "sociability" assay, was negatively linked to swim speed across a range of contexts, and predicted spatial positioning and leadership within groups as well as differences in structure and movement dynamics between groups. In turn, this trait, together with individual's exploratory tendency, measured by a classic "boldness" assay, explained individual and group foraging performance. These effects of consistent individual differences on group-level states emerged naturally from a generic model of self-organizing groups composed of individuals differing in speed and goal-orientedness. Our study provides experimental and theoretical evidence for a simple mechanism to explain the emergence of collective behavior from consistent individual differences, including variation in the structure, leadership, movement dynamics, and functional capabilities of groups, across social and ecological scales. In addition, we demonstrate individual performance is conditional on group composition, indicating how social selection may drive behavioral differentiation between individuals.

To be so bold: boldness is repeatable and related to within individual behavioural variability in North Island robins

Behavioural research traditionally focusses on the mean responses of a group of individuals rather than variation in behaviour around the mean or among individuals. However, examining the variation in behaviour among and within individuals may also yield important insights into the evolution and maintenance of behaviour. Repeatability is the most commonly used measure of variability among individuals in behavioural research. However, there are other forms of variation within populations that have received less attention. One such measure is intraindividual variation in behaviour (IIV), which is a short-term fluctuation of within-individual behaviour. Such variation in behaviour might be important during interactions because it could decrease the ability of conspecific and heterospecific individuals to predict the behaviour of the subject, thus increasing the cost of the interaction. In this experiment, we made repeated measures of the latency of North Island robins to attack a prey in a novel situation (a form of boldness) and examined (i) repeatability of boldness (the propensity to take a risk), (ii) IIV of boldness, and (iii) whether there was a significant relationship between these two traits (a behavioural syndrome). We found that boldness was highly repeatable, that there were high levels of IIV in boldness, and that there was a negative relationship between boldness and IIV in boldness. This suggests that despite high levels of repeatability for this behaviour, there were also still significant differences in IIV among different individuals within the population. Moreover, bolder individuals had significantly less IIV in their boldness, which suggests that they were forming routines (which reduces behavioural variability) compared to shyer individuals. Our results definitively demonstrate that IIV itself varies across individuals and is linked with key behavioural traits, and we argue for the importance of future studies aimed at understanding its causes and consequences for behavioural interactions.

A standardized method for quantifying consistent individual differences in schooling behaviour

A method for quantifying consistent individual differences in schooling behaviour is presented. This method, which utilizes a school of models, improves on previous methods by removing the unwanted variation that is introduced by live stimulus fish while still providing the physiological experience of schooling to the focal fish. Three-spined stickleback Gasterosteus aculeatus observed in the model school assay exhibited consistent individual differences in schooling behaviour.

Behavioural consistency and group conformity in humbug damselfish

Humbug damselfish, Dascyllus aruanus, are a common coral reef fish that form stable social groups with size-based social hierarchies. Here we caught whole wild groups of damselfish and tested whether social groups tended to be comprised of animals that are more similar to one another in terms of their behavioural type, than expected by chance. First we found that individuals were repeatable in their level of activity and exploration, and that this was independent of both absolute size and within-group dominance rank, indicating that animals were behaviourally consistent. Secondly, despite the fact that individuals were tested independently, the behaviour of members of the same groups was significantly more similar than expected under a null model, suggesting that individual behaviour develops and is shaped by conformity to the behaviour of other group members. This is one of the first studies to demonstrate this group-level behavioural conformity in wild-caught groups.

Seasonal changes in neophobia and its consistency in rooks: the effect of novelty type and dominance position

Neophobia, or the fear of novelty, may offer benefits to animals by limiting their exposure to unknown danger, but can also impose costs by preventing the exploration of potential resources. The costs and benefits of neophobia may vary throughout the year if predation pressure, resource distribution or conspecific competition changes seasonally. Despite such variation, neophobia levels are often assumed to be temporally and individually stable. Whether or not neophobia expression changes seasonally and fluctuates equally for all individuals is crucial to understanding the drivers, consequences and plasticity of novelty avoidance. We investigated seasonal differences and individual consistency in the motivation and novelty responses of a captive group of rooks, Corvus frugilegus, a seasonally breeding, colonial species of corvid that is known for being neophobic. We tested the group around novel objects and novel people to determine whether responses generalized across novelty types, and considered whether differences in dominance could influence the social risk of approaching unknown stimuli. We found that the group's level of object neophobia was stable year-round, but individuals were not consistent between seasons, despite being consistent within seasons. In contrast, the group's avoidance of novel people decreased during the breeding season, and individuals were consistent year-round. Additionally, although subordinate birds were more likely to challenge dominants during the breeding season, this social risk taking did not translate to greater novelty approach. Since seasonal variation and individual consistency varied differently towards each novelty type, responses towards novel objects and people seem to be governed by different mechanisms. Such a degree of fluctuation has consequences for other individually consistent behaviours often measured within the nonhuman personality literature.

Behavioural plasticity across social contexts is regulated by the directionality of inter-individual differences

An individual's behavioural phenotype is a combination of its unique behavioural propensities and its responsiveness to environmental variation, also known as behavioural plasticity. In social species, we must not only explore how individuals respond to variations in the physical environment but also how they react to changes in their social environment. A growing body of work has demonstrated that the behavioural heterogeneity of a group can alter its responsiveness, decision making, and fitness. Whether an individual is more or less extreme than a partner - what we term its 'relative personality' - may also alter individual behavioural responses. We determined exploratory tendencies of individual zebrafish (Danio rerio) and then constructed pairs with varying differences in 'relative personality' to determine the effect of differences between partners on behavioural plasticity. We find that relative personality, but not the magnitude of the difference between partners, is the most important determinant of behavioural plasticity across social treatments. Despite this overall effect, pairs of fish exhibited no predictable leader-follower interactions, suggesting that details of the experimental paradigm may be important in shaping social dynamics.