On the Validity of a Single (Boldness) Assay in Personality Research

Multi-assay approach shows species-associated personality patterns in two socially distinct gerbil species

We aimed to investigate whether two closely related but socially distinct species of gerbils differ in personality patterns. Using a suit of multivariate repeated assays (docility test, dark-light emergence test, startle test, novel object test, elevated platform test, and stranger test), we assessed contextual and temporal consistency of docility, boldness, exploration, anxiety, and sociability in the solitary midday gerbil, Meriones meridianus, and social Mongolian gerbil, M. unguiculatus. We revealed contextually consistent and highly repeatable sex-independent but species-specific personality traits. Species differed in temporal repeatability of different behaviours, and contextual consistency was more pronounced in solitary M. meridianus than in social M. unguiculatus. This finding contradicts the social niche specialization hypothesis, which suggests that personality traits should be more consistent in more social species. Instead, we hypothesize that social complexity should favour more flexible and less consistent behavioural traits. The habituation effect indicative of learning abilities was weak in both species yet stronger in social M. unguiculatus, supporting the relationship between the sociality level and cognitive skills. In both species, only a few different behavioural traits covaried, and the sets of correlated behaviours were species-specific such that the two species did not share any pair of correlated traits. Between-species differences in personality traits, habituation, and behavioural syndromes may be linked to differences in sociality. The lack of prominent behavioural syndromes is consistent with the idea that context-specific individual behavioural traits might be favoured to allow more flexible and adequate responses to changing environments than syndromes of correlated functionally different behaviours.

Personality and cognition: shoal size discrimination performance is related to boldness and sociability among ten freshwater fish species

Several studies have reported that animals' personalities are often correlated with individual differences in cognition. Here, we tested whether personality is related to cognition across species, focusing on 10 freshwater fishes and a task relevant for fitness, the ability to discriminate shoal size. Bolder species exhibited more 'shuttle' behavior for information sampling during shoal selection and showed high performance (HP) in the numerical discrimination than shyer species, i.e., low performance (LP) species. Species at both the high and low ends of sociability showed LP, possibly due to loosened selection pressure because of either no need to perform shoal size discrimination tasks frequently in nature for very high sociability species or decreased willingness and motivation to join and stay within shoals for very low sociability species. Notably, the numerical discrimination was sensitive to the numerical contrast ratio in LP species but not in HP species, suggesting that the numerical system used for size discrimination also varied between species. Overall, we demonstrated the interspecies relationship between personality and shoal size discrimination across fish species, suggesting an evolutionary link between numerical abilities and behavior.

Time-specific convergence and divergence in individual differences in behavior: Theory, protocols and analyzes

Over the years, theoreticians and empiricists working in a wide range of disciplines, including physiology, ethology, psychology, and behavioral ecology, have suggested a variety of reasons why individual differences in behavior might change over time, such that different individuals become more similar (convergence) or less similar (divergence) to one another. Virtually none of these investigators have suggested that convergence or divergence will continue forever, instead proposing that these patterns will be restricted to particular periods over the course of a longer study. However, to date, few empiricists have documented time-specific convergence or divergence, in part because the experimental designs and statistical methods suitable for describing these patterns are not widely known. Here, we begin by reviewing an array of influential hypotheses that predict convergence or divergence in individual differences over timescales ranging from minutes to years, and that suggest how and why such patterns are likely to change over time (e.g., divergence followed by maintenance). Then, we describe experimental designs and statistical methods that can be used to determine if (and when) individual differences converged, diverged, or were maintained at the same level at specific periods during a longitudinal study. Finally, we describe why the concepts described herein help explain the discrepancy between what theoreticians and empiricists mean when they describe the "emergence" of individual differences or personality, how they might be used to study situations in which convergence and divergence patterns alternate over time, and how they might be used to study time-specific changes in other attributes of behavior, including individual differences in intraindividual variability (predictability), or genotypic differences in behavior.

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.

Statistical Analysis and Machine Learning Used in the Case of Two Behavioral Tests Applied in Zebrafish Exposed to Mycotoxins

Machine learning is a branch of artificial intelligence that allows computer systems to learn directly from examples, data, and experience. Statistical modeling is more about finding connections between variables and consequently the impact of these relationships, while also catering for prediction. It should be clear that these two methodologies are different in terms of their purpose, despite the fact that they use similar means to get there. The evaluation of the machine learning algorithm uses a set of tests to validate its accuracy. Although, for a statistical model, the analysis of regression parameters by confidence intervals, significance tests and other tests can be used to assess the legitimacy of the model. To demonstrate the applications and usefulness of this theory, an experimental study was conducted on zebrafish exposed to mycotoxin. Methods: Patulin (70 µg/L) and kojic acid (100 mg/L, 204 mg/L, and 284 mg/L) were administered by immersion to zebrafish once daily for a period of 7 days before the behavior testing. The following behavioral tests were performed: a novel tank test (NTT) (to assess the explorative behavior and anxiety); and a Y-maze test (which measures the spontaneous explorative behavior). Behavioral tests were performed on separate days. For the behavior tests, the statistical analysis was performed using ANOVA variation analysis (two-way ANOVA). All results are expressed as the mean ± standard error of the mean. The values of the general index F for which p < 0.05 were considered statistically significant. Results: Y-maze—patulin exposure led to an intensification of the locomotor activity and an increased traveled distance and number of arm entries. By increasing the spontaneous alternation between the aquarium’s arms, patulin has shown a stimulating effect on spatial memory. In the case of zebrafish exposed to 100 mg/L kojic acid, the traveled distance was shorter by 27% than the distance attained by those in the control group. The higher doses of kojic acid (204 mg/L and 284 mg/L) led to an increased locomotor activity, distance traveled, number of arm entries, and the spontaneous alternation. The increase in spontaneous alternation demonstrates that 204 mg/L and 284 mg/L kojic acid doses had a stimulating effect on spatial memory. Novel tank test—compared to the control group, the traveled distance of the patulin-exposed fish is slightly reduced. Compared to the control group, the traveled distance of the kojic acid-exposed fish is reduced, due to a shorter mobile time (by 25–27% in the case of fish exposed to 204 mg/L and 284 mg/L kojic acid). Patulin and kojic acid exhibit toxic effects on zebrafish liver, kidney, and myocardium and leads to severe alteration. We continued the analysis by trying some machine learning algorithms on the classification problems in the case of the two behavioral tests MAZE and NTT, after which we concluded that the results were better in the case of the NTT test relative to the MAZE test and that the use of decision tree algorithms leads to amazing results, knowing that their hierarchical structure allows them to learn signals from both classes. Conclusions: The groups exposed to patulin and kojic acid show histological changes in the liver, kidneys, and myocardial muscle tissue. The novel tank test, which assesses exploratory behavior, has been shown to be conclusive in the behavioral analysis of fish that have been given toxins, demonstrating that the intoxicated fish had a decreased explorative behavior and increased anxiety. We were able to detect a machine learning algorithm in the category of decision trees, which can be trained to classify the behavior of fish that were given a toxin in the category of those used in the experiment, only by analyzing the characteristic features of the NTT Behavior Test.

Behavioural effects of early-life exposure to parabens in zebrafish larvae

Parabens are classified as endocrine disrupting chemicals due to their ability to activate several nuclear receptors causing changes in hormones-dependent signalling pathways. Central nervous system of developing organisms is particularly vulnerable to changes in hormonal pathways, which could lead to altered brain function, abnormal behaviour and even diseases later in life. The aim of the present study was to investigate the effects of exposure to butylparaben (BuP), ethylparaben (EtP) and methylparaben (MeP) during early development on nervous system using zebrafish larvae's behavioural models. Zebrafish were exposed until 4 days post fertilization (dpf) to three concentrations of each paraben chosen considering the environmentally realistic concentrations of human exposure and the benchmark-dose lower bound calculated for zebrafish larvae (BuP: 5, 50 and 500 μg/L; EtP: 50, 500 and 5000 μg/L; MeP: 100, 1000 and 10,000 μg/L). Activity in novel and in familiar environment, thigmotaxis, visual startle response and photic synchronization of the behavioural circadian rhythms were analysed at 4, 5 and 6 dpf. Zebrafish larvae exposed to BuP 500 μg/L and EtP 5000 μg/L revealed increased anxiety-like behaviour in novel environment. Larvae treated with 500 μg/L of BuP showed reduced activity in familiar and marginally in unfamiliar environment, and larvae exposed to 5000 μg/L of EtP exhibited hyperactivity in familiar environment. Parabens exposure did not influence the visual startle response and the photic synchronization of circadian rhythms in zebrafish larvae. This research highlighted as the exposure to parabens has the potential to interfere with behavioural development of zebrafish.

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.

Ain’t going down without a fight: state-and environment-dependence of antipredator defensive aggressive personalities in Carpetan rock lizard

Abstract

Aggression is one of the most frequently studied behavioural traits across a wide range of taxa; however, most studies evaluate aggressive behaviour in a social context, in which aggressive interactions between conspecifics are motivated by resource control (offensive or social aggression). However, in an antipredator context, the primary role of aggression is defence (defensive or antipredator aggression). Although the neuroendocrinology of antipredator aggression is often studied in domesticated and laboratory animals, how environment and individual state affect this behavioural trait in the wild is largely unknown. Here, by conducting a manipulative experiment, we tested whether (i) consistent between-individual differences (i.e. animal personality) are present in antipredator aggression in adult male Carpetan rock lizards (Iberolacerta cyreni) and (ii) short-term environmental changes (presence vs. absence of predator cues) and differences in individual state (body length, head size, hind limb length) affect individual mean behaviour (i.e. behavioural type). We found moderate-high repeatability in antipredator aggression (willingness to bite a human), indicating the presence of animal personality in this behavioural trait. Lizards were on average more defensive in the presence of predator cues; furthermore, short-legged males showed higher antipredator aggression than long-legged males in the presence of predator cues, probably as an attempt to balance their decreased escape speed. Larger (~ older) males were more defensive than smaller ones, probably due to their increased fighting ability. We conclude that antipredator aggression is an important part of an individual’s behavioural repertoire and its expression is driven by both environmental situation and individual state.

Significance statement

Antipredator/defensive aggression is not the primary antipredatory response; however, when other ways of escape are not possible, actually hurting the predator could be the only way of survival. While this behaviour obviously has substantial effects on fitness, it is severely understudied compared to social/offensive aggression. In a manipulative experiment, we found that there are consistent between-individual differences in antipredator aggression (i.e. willingness to bite during handling) of adult male Carpetan rock lizards (Iberolacerta cyreni), supporting the presence of animal personality and suggesting that this behavioural trait might respond to natural selection. Furthermore, short-term environmental variation (i.e. presence vs. absence of predator cues) in interaction with individual state affected antipredator aggression of individuals, emphasising the ecological and evolutionary relevance of this behaviour.

The impact of personality, morphotype and shore height on temperature-mediated behavioural responses in the beadlet anemone Actinia equina

Between-individual variation in behavioural phenotype, termed personality, is an important determinant of how populations cope with acute environmental fluctuation related to climate change. Personality in the beadlet sea anemone Actinia equina is linked to genetically distinct morphotypes, which are associated with different heights on the shore. In the intertidal zone, high-shore environments experience more environmental fluctuation due to longer periods of exposure, and animals adapted to live in these environments are predicted to deal more effectively with environmental perturbation than their low-shore counterparts. We collected beadlet anemones of two different morphotypes from three different shore heights. We investigated variation in two behaviours at three different temperatures and in a temporal control treatment where the temperature was not changed: startle response time, the time it took an anemone to re-extend its tentacles after a threatening stimulus, and immersion response time, the time to re-extend tentacles after simulated tidal immersion. These behaviours reflect risk-taking and allow individuals to be categorized as bold, shy or intermediate based upon response times. Both behaviours showed significant changes as the temperature increased. For immersion response, the morphotype associated with the low-shore-lengthened response times at high temperatures. For startle response, all animals lengthened their response times at high temperatures but animals collected from the low-shore lengthened theirs to the greatest degree. At the individual level, although control individuals exhibited temporal changes in their response times, a clear effect of temperature was present in both behaviours. Shy and bold individuals became more intermediate at higher temperatures in immersion response (this effect was present to a lesser degree in control individuals), while intermediate individuals raised their response times at higher temperatures for startle response. Given that prolonged tentacle retraction reduces foraging opportunities and can negatively impact respiratory efficiency, our data suggest that some individuals within a single population of A. equina, particularly those associated with the lower shore, may exhibit less effective behavioural responses to temperature shifts than others. These findings demonstrate that acute temperature changes influence risk-taking, and could have profound short and long-term implications for survival in the face of climate change.

Does Divergence in Habitat Breadth Associate with Species Differences in Decision Making in Drosophila Sechellia and Drosophila Simulans?

Decision making is involved in many behaviors contributing to fitness, such as habitat choice, mate selection, and foraging. Because of this, high decision-making accuracy (i.e., selecting the option most beneficial for fitness) should be under strong selection. However, decision making is energetically costly, often involving substantial time and energy to survey the environment to obtain high-quality information. Thus, for high decision making accuracy to evolve, its benefits should outweigh its costs. Inconsistency in the net benefits of decision making across environments is hypothesized to be an important means for maintaining variation in this trait. However, very little is known about how environmental factors influence the evolution of decision making to produce variation among individuals, genotypes, and species. Here, we compared two recently diverged species of Drosophila differing substantially in habitat breadth and degree of environmental predictability and variability: Drosophilasechellia and Drosophilasimulans. We found that the species evolving under higher environmental unpredictability and variability showed higher decision-making accuracy, but not higher environmental sampling.

The role of personality, cognition, and affective state in same-sex contests in the red junglefowl

Abstract

Intra-species contests are common in the animal kingdom and can have fitness consequences. Most research on what predicts contest outcome focuses on morphology, although differences in personality and cognition may also be involved. Supporting this, more proactive individuals often have dominant status, although the causality of this relationship is rarely investigated. Contest initiators often win; thus, individuals that are more proactive in their personality (e.g., more aggressive, risk-taking) or cognition (e.g., more optimistic, impulsive) may initiate contests more often. To investigate this, we assayed the behavior and cognition of sexually mature male and female red junglefowl (Gallus gallus), a species in which both sexes contest over social status, before staging intra-sexual contests. We confirm that contest initiators were more likely to win. In males, individuals that behaved more boldly in a novel arena test were more likely to initiate and win contests. Female initiators tended to be less active in novel object test, more aggressive in a restrained opponent test, and respond less optimistically in a cognitive judgement bias test, whereas the main predictor of whether a female would win a contest was whether she initiated it. These results suggest that behaviors attributed to proactive and reactive personalities, and—at least for female red junglefowl—optimism, can affect contest initiation and outcome. Therefore, within species, and depending on sex, different aspects of behavior and cognition may independently affect contest initiation and outcome. The generality of these findings, and their fitness consequences, requires further investigation.

Significance statement

In red junglefowl, we explored how behavior previously shown to describe personality, cognition, and affective state affected initiation and outcome of intra-sexual contests, by staging contests between sexually mature individuals previously assayed in behavioral and cognitive tests. In both sexes, contest initiators usually won. Bolder males were more likely to initiate and win contests. Female contests initiators were less active, more aggressive, and less optimistic. Our results suggest that personality and cognition could affect the initiation and outcome of contests and that how this occurs may differ between sexes.

Roll with the fear: environment and state dependence of pill bug (Armadillidium vulgare) personalities

Most studies on animal personality evaluate individual mean behaviour to describe individual behavioural strategy, while often neglecting behavioural variability on the within-individual level. However, within-individual behavioural plasticity (variation induced by environment) and within-individual residual variation (regulatory behavioural precision) are recognized as biologically valid components of individual behaviour, but the evolutionary ecology of these components is still less understood. Here, we tested whether behaviour of common pill bugs (Armadillidium vulgare) differs on the among- and within-individual level and whether it is affected by various individual specific state-related traits (sex, size and Wolbachia infection). To this aim, we assayed risk-taking in familiar vs. unfamiliar environments 30 times along 38 days and applied double modelling statistical technique to handle the complex hierarchical structure for both individual-specific trait means and variances. We found that there are significant among-individual differences not only in mean risk-taking behaviour but also in environment- and time-induced behavioural plasticity and residual variation. Wolbachia-infected individuals took less risk than healthy conspecifics; in addition, individuals became more risk-averse with time. Residual variation decreased with time, and individuals expressed higher residual variation in the unfamiliar environment. Further, sensitization was stronger in females and in larger individuals in general. Our results suggest that among-individual variation, behavioural plasticity and residual variation are all (i) biologically relevant components of an individual's behavioural strategy and (ii) responsive to changes in environment or labile state variables. We propose pill bugs as promising models for personality research due to the relative ease of getting repeated behavioural measurements.

Behavioral and Perceptual Differences between Sexes in Dogs: An Overview

In this paper, we review the scientific reports of sex-related differences in dogs as compared to the outcomes described for wild animals. Our aim was to explore whether the differences in male and female dogs were affected by the domestication process, in which artificial selection is the main driver. For this purpose, we used information regarding personality traits, cognitive processes, and perception, for which there is a wide theoretical framework in behavioral ecology. Aggressiveness and boldness, described as a behavioral syndrome, were reported as being higher in males than females. Females also seemed more inclined to interspecific social interactions with humans in tasks that require cooperative skills, whereas males appeared more inclined to social play, thus implying different levels of social engagement between the sexes, depending on the context. Studies on cognitive processes underlined a greater flexibility in resorting to a particular navigation strategy in males. Most lateralization studies seem to support the view that males are preferentially left-handed and females are preferentially right-handed. Reports on visual focusing coherently rank females as superior in focusing on single social and physical stimuli. Only male dogs are able to discriminate kin; however, the timing of the olfactory recording in sexes is related to the stimulus relevance. Dogs are largely in line with life-history theories, which indicate that sex differences in dogs are mainly rooted in their biological and evolutionary heritage, remaining unchanged despite artificial selection. In contrast, the higher intraspecific sociability in wild male animals was not replicated in dogs.

Island tameness and the repeatability of flight initiation distance in a large herbivore

Antipredator behaviours can be lost relatively quickly in populations that are relieved of predation, as is known for several species inhabiting islands. Flight initiation distance (FID) is often studied in the context of island tameness; however, little is known about the factors that influence and maintain FID variation in predation-free populations. Here, we studied FID in foals of an isolated predator-free population of feral horses (Equus caballus L., 1758) on Sable Island, Canada, to determine if FID could be used for research on consistent individual differences in risk aversion and island tameness. In addition to testing for temporal, spatial, and sex effects on FID, we compared repeatability estimates at two temporal scales (within and among days). Similar FID for measurements obtained on the same day and for males and females indicated an absence of short-term desensitization and sex effects. In contrast, FID decreased for measurements made on subsequent days and from east to west, which could reflect habituation to human presence and (or) other temporal and spatial processes. Repeatability was high (0.42 ± 0.06), but tended to decrease with increasing time intervals. This study highlights the potential of FID for individual-based research on the ecology and evolutionary dynamics of risk aversion in predation-free populations.

Rapid divergence of animal personality and syndrome structure across an arid-aquatic habitat matrix

Intraspecific trait variation, including animal personalities and behavioural syndromes, affects how individual animals and populations interact with their environment. Within-species behavioural variation is widespread across animal taxa, which has substantial and unexplored implications for the ecological and evolutionary processes of animals. Accordingly, we sought to investigate individual behavioural characteristics in several populations of a desert-dwelling fish, the Australian desert goby (Chlamydogobius eremius). We reared first generation offspring in a common garden to compare non-ontogenic divergence in behavioural phenotypes between genetically interconnected populations from contrasting habitats (isolated groundwater springs versus hydrologically variable river waterholes). Despite the genetic connectedness of populations, fish had divergent bold-exploratory traits associated with their source habitat. This demonstrates divergence in risk-taking traits as a rapid phenotypic response to ecological pressures in arid aquatic habitats: neophilia may be suppressed by increased predation pressure and elevated by high intraspecific competition. Correlations between personality traits also differed between spring and river fish. River populations showed correlations between dispersal and novel environment behaviours, revealing an adaptive behavioural syndrome (related to dispersal and exploration) that was not found in spring populations. This illustrates the adaptive significance of heritable behavioural variation within and between populations, and their importance to animals persisting across contrasting habitats.

Hormonal responsiveness to stress is negatively associated with vulnerability to angling capture in fish

Differences in behavior and physiology amongst individuals often alter relative fitness levels in the environment. However, the ideal behavioral/physiological phenotype in a given environment may be altered by human activity, leading to an evolutionary response in the affected population. One example of this process can be found in fisheries (including recreational freshwater fisheries), where selective capture and harvest of individuals with certain phenotypes can drive evolutionary change. While some life history traits and behavioral tendencies influencing capture likelihood have been studied, the physiological mechanisms driving this vulnerability remain poorly understood. To address this, we assessed how two major physiological characteristics (hormonal responsiveness to stress and metabolic phenotype) and one behavioral characteristic (boldness) impact the likelihood of an individual being captured by anglers. Largemouth bass, Micropterus salmoides, derived from a population artificially selected for differential angling vulnerability were assessed for boldness and for stress responsiveness (as indicated by plasma cortisol levels) following an air-exposure challenge. Largemouth bass were then stocked into a pond where experimental angling trials took place, and a subset of captured and uncaptured fish were afterwards assessed for metabolic phenotype. The results showed that stress responsiveness was the primary driver of angling vulnerability, with individuals that experienced lower rises in cortisol following the air-exposure challenge more likely to be captured. Neither boldness nor metabolic phenotype influenced capture probability. The results from this study indicate that fisheries-induced selective pressure may act on physiology, potentially altering stress responsiveness and its associated behaviors in populations exploited by recreational anglers.

Personality and Cognition: Sociability Negatively Predicts Shoal Size Discrimination Performance in Guppies

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

Demystifying animal 'personality' (or not): why individual variation matters to experimental biologists

Animal 'personality', defined as repeatable inter-individual differences in behaviour, is a concept in biology that faces intense controversy. Critics argue that the field is riddled with terminological and methodological inconsistencies and lacks a sound theoretical framework. Nevertheless, experimental biologists are increasingly studying individual differences in physiology and relating these to differences in behaviour, which can lead to fascinating insights. We encourage this trend, and in this Commentary we highlight some of the benefits of estimating variation in (and covariation among) phenotypic traits at the inter- and intra-individual levels. We focus on behaviour while drawing parallels with physiological and performance-related traits. First, we outline some of the confusion surrounding the terminology used to describe repeatable inter-individual differences in behaviour. Second, we argue that acknowledging individual behavioural differences can help researchers avoid sampling and experimental bias, increase explanatory power and, ultimately, understand how selection acts on physiological traits. Third, we summarize the latest methods to collect, analyse and present data on individual trait variation. We note that, while measuring the repeatability of phenotypic traits is informative in its own right, it is only the first step towards understanding how natural selection and genetic architecture shape intra-specific variation in complex, labile traits. Thus, understanding how and why behavioural traits evolve requires linking repeatable inter-individual behavioural differences with core aspects of physiology (e.g. neurophysiology, endocrinology, energy metabolism) and evolutionary biology (e.g. selection gradients, heritability).

Stress-induced peak (but not resting) metabolism correlates with mating display intensity in male guppies

Recent empirical and conceptual papers have highlighted the potential for metabolism to act as a proximate mechanism for behavior that could explain animal personality (consistency over time). Under this hypothesis, individuals with consistently high levels of behavioral activity should also have high resting metabolic rate (RMR) as it can reflect capacity to process food and generate energy. We tested for the predicted positive covariance between RMR and three behaviors that differ in energy demands in 30 male guppies, using multivariate mixed models; we repeatedly measured their activity (10 times each), courtship displays (nine times), voracity (10 times), and metabolism (four‐times). Resting metabolic rate (measured overnight in respirometry trials) did not consistently differ among males, whereas initial peak metabolism measured during those same trials (R = 0.42), and all behaviors were repeatable (R = 0.33–0.51). RMR declined over time suggesting habituation to the protocol, whereas peak metabolism did not. Initial peak metabolism was negatively correlated with courtship display intensity, and voracity was positively correlated with activity, but all other among‐individual correlations were not significant. We conclude that RMR does not provide a proximate explanation for consistent individual differences in behavior in male guppies, and therefore the potential for independent evolution of these physiological and behavioral traits seems possible. Finally, we identify peak metabolism as a potential measure of the stress response to confinement, which highlights the value of considering various aspects of metabolic rates recording during respirometry trials.

Individual consistency in the behaviors of newly-settled reef fish

Flexibility in behavior is advantageous for organisms that transition between stages of a complex life history. However, various constraints can set limits on plasticity, giving rise to the existence of personalities that have associated costs and benefits. Here, we document a field and laboratory experiment that examines the consistency of measures of boldness, activity, and aggressive behavior in the young of a tropical reef fish, Pomacentrus amboinensis (Pomacentridae) immediately following their transition between pelagic larval and benthic juvenile habitats. Newly-settled fish were observed in aquaria and in the field on replicated patches of natural habitat cleared of resident fishes. Seven behavioral traits representing aspects of boldness, activity and aggression were monitored directly and via video camera over short (minutes), medium (hours), and long (3 days) time scales. With the exception of aggression, these behaviors were found to be moderately or highly consistent over all time scales in both laboratory and field settings, implying that these fish show stable personalities within various settings. Our study is the first to examine the temporal constancy of behaviors in both field and laboratory settings in over various time scales at a critically important phase during the life cycle of a reef fish.