Brain size predicts behavioural plasticity in guppies (Poecilia reticulata): An experiment

Understanding how animal personality (consistent between‐individual behavioural differences) arises has become a central topic in behavioural sciences. This endeavour is complicated by the fact that not only the mean behaviour of individuals (behavioural type) but also the strength of their reaction to environmental change (behavioural plasticity) varies consistently. Personality and cognitive abilities are linked, and we suggest that behavioural plasticity could also be explained by differences in brain size (a proxy for cognitive abilities), since accurate decisions are likely essential to make behavioural plasticity beneficial. We test this idea in guppies (Poecilia reticulata), artificially selected for large and small brain size, which show clear cognitive differences between selection lines. To test whether those lines differed in behavioural plasticity, we reared them in groups in structurally enriched environments and then placed adults individually into empty tanks, where we presented them daily with visual predator cues and monitored their behaviour for 20 days with video‐aided motion tracking. We found that individuals differed consistently in activity and risk‐taking, as well as in behavioural plasticity. In activity, only the large‐brained lines demonstrated habituation (increased activity) to the new environment, whereas in risk‐taking, we found sensitization (decreased risk‐taking) in both brain size lines. We conclude that brain size, potentially via increasing cognitive abilities, may increase behavioural plasticity, which in turn can improve habituation to novel environments. However, the effects seem to be behaviour‐specific. Our results suggest that brain size likely explains some of the variation in behavioural plasticity found at the intraspecific level.     

Individual variation in behavioural plasticity: direct and indirect effects of boldness,exploration and sociability on habituation to predators in lizards

Little is known about the factors causing variation in behavioural plasticity and the interplay between personality and plasticity. Habituation to predators is a special case of behavioural plasticity. We investigated the direct and indirect effects of boldness, exploration and sociability traits on the habituation ability of Iberian wall lizards, considering exposure and sex effects. Individual boldness was consistent across several non-habituation contexts, but it did not significantly affect habituation. Exploration had a strong direct effect on habituation, with more exploratory individuals being able to habituate faster than less exploratory ones, probably because of their ability to assess risk better. Individual variation in habituation was also affected by sociability, but this was an indirect effect mediated by exposure to the predator. Less social individuals avoided refuges with conspecific cues, increasing exposure to the predator and eventually habituation. Finally, the direct effects of sex (females habituated faster than males) were opposite to its indirect effects through exposure. We conclude that risk assessment, instead of the proactivity–reactivity gradient usually considered in the literature, can affect behavioural plasticity through complex interactions between direct and indirect effects, including exploratory behaviour, degree of exposure to the predator and sex, which represent novel mechanisms generating inter-individual variation in plasticity.     

Variation in Female Grey Seal (Halichoerus grypus) Reproductive Performance Correlates to Proactive-Reactive Behavioural Types

Consistent individual differences (CIDs) in behaviour, indicative of behavioural types or personalities, have been shown in taxa ranging from Cnidaria to Mammalia. However, despite numerous theoretical explanations there remains limited empirical evidence for selective mechanisms that maintain such variation within natural populations. We examined behavioural types and fitness proxies in wild female grey seals at the North Rona breeding colony. Experiments in 2009 and 2010 employed a remotely-controlled vehicle to deliver a novel auditory stimulus to females to elicit changes in pup-checking behaviour. Mothers tested twice during lactation exhibited highly repeatable individual pup-checking rates within and across breeding seasons. Observations of undisturbed mothers (i.e. experiencing no disturbance from conspecifics or experimental test) also revealed CIDs in pup-checking behaviour. However, there was no correlation between an individuals’ pup-checking rate during undisturbed observations with the rate in response to the auditory test, indicating plasticity across situations. The extent to which individuals changed rates of pup-checking from undisturbed to disturbed conditions revealed a continuum of behavioural types from proactive females, who maintained a similar rate throughout, to reactive females, who increased pup-checking markedly in response to the test. Variation in maternal expenditure (daily mass loss rate) was greater among more reactive mothers than proactive mothers. Consequently pups of more reactive mothers had more varied growth rates centred around the long-term population mean. These patterns could not be accounted for by other measured covariates as behavioural type was unrelated to a mother’s prior experience, degree of inter-annual site fidelity, physical characteristics of their pupping habitat, pup sex or pup activity. These findings are consistent with the hypothesis that variation in behavioural types is maintained by spatial and temporal environmental variation combined with limits to phenotype-environment matching.     

Effects of Emergence Time and Early Social Rearing Environment on Behaviour of Atlantic Salmon: Consequences for Juvenile Fitness and Smolt Migration

Consistent individual differences in behaviour have been well documented in a variety of animal taxa, but surprisingly little is known about the fitness and life-history consequences of such individual variation. In wild salmonids, the timing of fry emergence from gravel spawning nests has been suggested to be coupled with individual behavioural traits. Here, we further investigate the link between timing of spawning nest emergence and behaviour of Atlantic salmon (Salmo salar), test effects of social rearing environment on behavioural traits in fish with different emergence times, and assess whether behavioural traits measured in the laboratory predict growth, survival, and migration status in the wild. Atlantic salmon fry were sorted with respect to emergence time from artificial spawning nest into three groups: early, intermediate, and late. These emergence groups were hatchery-reared separately or in co-culture for four months to test effects of social rearing environment on behavioural traits. Twenty fish from each of the six treatment groups were then subjected to three individual-based behavioural tests: basal locomotor activity, boldness, and escape response. Following behavioural characterization, the fish were released into a near-natural experimental stream. Results showed differences in escape behaviour between emergence groups in a net restraining test, but the social rearing environment did not affect individual behavioural expression. Emergence time and social environment had no significant effects on survival, growth, and migration status in the stream, although migration propensity was 1.4 to 1.9 times higher for early emerging individuals that were reared separately. In addition, despite individuals showing considerable variation in behaviour across treatment groups, this was not translated into differences in growth, survival, and migration status. Hence, our study adds to the view that fitness (i.e., growth and survival) and life-history predictions from laboratory measures of behaviour should be made with caution and ideally tested in nature.     

Context is key: A comment on Herczeg et al. 2019

In the last several years, there has been a surge in the number of studies addressing the causes and consequences of among‐individual variation in cognitive ability and behavioural plasticity. Here, we use a recent publication by Herczeg et al. (2019: 32(3), 218–226) to highlight three shortcomings common to this newly emerging field. In their study, Herczeg et al. attempted to link variation in cognitive ability and behavioural plasticity by testing whether selection lines of guppies (Poecilia reticulata) that differ in relative brain size also differ in behavioural plasticity, as might be expected if the costs to plasticity are predominantly derived from the cost of developing large brains. First, residual brain size may not be a suitable proxy for ‘cognitive ability’. Recent work has shown that intraspecific variation in cognitive ability can be better understood by considering variation in the specific brain areas responsible for the relevant behaviours as opposed to whole‐brain mass. Second, the measure of behavioural plasticity, habituation, is unlikely to fulfil the assumptions that plasticity is both adaptive and costly. Finally, we point out several misconceptions regarding animal personality that continue to contribute to the choice of traits that are not well aligned with study objectives. Elucidating the mechanisms underlying among‐individual variation in cognition and behavioural plasticity within populations requires integration between behavioural ecology and comparative cognition, and the study system developed by Herczeg et al. has the potential to provide important mechanistic insights. We hope that by articulating and critically appraising the underlying assumptions that are common in these traditionally separate disciplines, a strong foundation can emerge to allow for more fruitful integration of these fields.     

Low heritabilities, but genetic and maternal correlations between red squirrel behaviours

Consistent individual differences in behaviour, and behavioural correlations within and across contexts, are referred to as animal personalities. These patterns of variation have been identified in many animal taxa and are likely to have important ecological and evolutionary consequences. Despite their importance, genetic and environmental sources of variation in personalities have rarely been characterized in wild populations. We used a Bayesian animal model approach to estimate genetic parameters for aggression, activity and docility in North American red squirrels (Tamiasciurus hudsonicus). We found support for low heritabilities (0.08-0.12), and cohort effects (0.07-0.09), as well as low to moderate maternal effects (0.07-0.15) and permanent environmental effects (0.08-0.16). Finally, we found evidence of a substantial positive genetic correlation (0.68) and maternal effects correlation (0.58) between activity and aggression providing evidence of genetically based behavioural correlations in red squirrels. These results provide evidence for the presence of heritable variation in red squirrel behaviour, but also emphasize the role of other sources of variation, including maternal effects, in shaping patterns of variation and covariation in behavioural traits.     

Environmental and genetic effects shape the development of personality traits in the mangrove killifish Kryptolebias marmoratus

Consistent individual differences in behaviour are well documented, for example, individuals can be defined as consistently bold or consistently shy. To date our understanding of the mechanisms underpinning consistent individual differences in behaviour (also termed behavioural types (BTs)) remains limited. Theoretical work suggests life‐history tradeoffs drive BT variation, however, empirical support is scarce. Moreover, whilst life‐history is known to be phenotypically plastic in response to environmental conditions during ontogeny, the extent to which such plasticity drives plasticity in behavioural traits and personality remains poorly understood. Using a natural clonal vertebrate, Kryptolebias marmoratus, we control for genetic variation and investigate developmental plasticity in life‐history and three commonly studied behavioural traits (exploration, boldness, aggression) in response to three ecologically relevant environments; conspecific presence, low food and perceived risk. Simulated predation risk was the only treatment that generated repeatable behaviour i.e. personality during ontogeny. Treatments differed in their effects on mean life‐history and behavioural scores. Specifically, low food fish exhibited reduced growth rate and exploration but did not differ from control fish in their boldness or aggression scores. Conspecific presence resulted in a strong negative effect on mean aggression, boldness and exploration during ontogeny but had minimal effect on life‐history traits. Simulated predation risk resulted in increased reproductive output but had minimal effect upon average behavioural scores. Together these results suggest that life‐history plasticity/variation may be insufficient in driving variation in personality during development. Finally, using offspring derived from each rearing environment we investigate maternal effects and find strong maternal influence upon offspring size, but not behaviour. These results highlight and support the current understanding that risk perception is important in shaping personality, and that social experience during ontogeny is a major influence upon behavioural expression.     

Individual and Sex Differences in Habituation and Neophobia in House Sparrows (Passer domesticus)

Heterogeneity in the environment favours foragers that are flexible (phenotypically plastic). However, consistent individual differences in behaviour (personality), such as in risk‐taking, might affect an individual's ability to find food, avoid predators and adjust to new conditions. It is now well known that personalities exist in many taxa, but much less is known about individual variation in plasticity. We measured the tendency to begin foraging across different levels of risk in house sparrows (Passer domesticus), using a behavioural reaction norm framework to simultaneously assess personality and plasticity. We asked whether individuals were consistently different across risk levels, and whether they differed in habituation and neophobia, both of which were treated as cases of plasticity. We found that males habituated more than females by beginning to feed sooner after repeated instances of a human disturbance in the presence of an initially unfamiliar object. Individuals of both sexes also exhibited consistent differences across trials, but did not differ in the rate of habituation beyond the difference between the sexes. When a novel object was placed in the foraging area, both sexes exhibited similar degrees of neophobia by delaying feeding. The magnitude of this change varied among birds, indicating individual differences in neophobia. Our results indicate that both personality and individual variation in plasticity exist but should be treated as independent phenomena. The presence of variation in plasticity implies that the raw material necessary for selection on neophobia exists, and that if heritable, plasticity in risk‐taking across contexts could evolve.     

A mechanistic theory of personality‐dependent movement behaviour based on dynamic energy budgets

Consistent between‐individual differences in movement are widely recognised across taxa. In addition, foraging plasticity at the within‐individual level suggests a behavioural dependency on the internal energy demand. Because behaviour co‐varies with fast‐slow life history (LH) strategies in an adaptive context, as theoretically predicted by the pace‐of‐life syndrome hypothesis, mass/energy fluxes should link behaviour and its plasticity with physiology at both between‐ and within‐individual levels. However, a mechanistic framework driving these links in a fluctuating ecological context is lacking. Focusing on home range behaviour, we propose a novel behavioural‐bioenergetics theoretical model to address such complexities at the individual level based on energy balance. We propose explicit mechanistic links between behaviour, physiology/metabolism and LH by merging two well‐founded theories, the movement ecology paradigm and the dynamic energetic budget theory. Overall, our behavioural‐bioenergetics model integrates the mechanisms explaining how (1) behavioural between‐ and within‐individual variabilities connect with internal state variable dynamics, (2) physiology and behaviour are explicitly interconnected by mass/energy fluxes, and (3) different LHs may arise from both behavioural and physiological variabilities in a given ecological context. Our novel theoretical model reveals encouraging opportunities for empiricists and theoreticians to delve into the eco‐evolutionary processes that favour or hinder the development of between‐individual differences in behaviour and the evolution of personality‐dependent movement syndromes.     

Be meek or be bold? A colony-level behavioural syndrome in ants

Consistent individual variation in animal behaviour is nearly ubiquitous and has important ecological and evolutionary implications. Additionally, suites of behavioural traits are often correlated, forming behavioural syndromes in both humans and other species. Such syndromes are often described by testing for variation in traits across commonly described dimensions (e.g. aggression and neophobia), independent of whether this variation is ecologically relevant to the focal species. Here, we use a variety of ecologically relevant behavioural traits to test for a colony-level behavioural syndrome in rock ants (Temnothorax rugatulus). Specifically, we combine field and laboratory assays to measure foraging effort, how colonies respond to different types of resources, activity level, response to threat and aggression level. We find evidence for a colony level syndrome that suggests colonies consistently differ in coping style—some are more risk-prone, whereas others are more risk-averse. Additionally, by collecting data across the North American range of this species, we show that environmental variation may affect how different populations maintain consistent variation in colony behaviour.     

What can whole genome expression data tell us about the ecology and evolution of personality?

Consistent individual differences in behaviour, aka personality, pose several evolutionary questions. For example, it is difficult to explain within-individual consistency in behaviour because behavioural plasticity is often advantageous. In addition, selection erodes heritable behavioural variation that is related to fitness, therefore we wish to know the mechanisms that can maintain between-individual variation in behaviour. In this paper, we argue that whole genome expression data can reveal new insights into the proximate mechanisms underlying personality, as well as its evolutionary consequences. After introducing the basics of whole genome expression analysis, we show how whole genome expression data can be used to understand whether behaviours in different contexts are affected by the same molecular mechanisms. We suggest strategies for using the power of genomics to understand what maintains behavioural variation, to study the evolution of behavioural correlations and to compare personality traits across diverse organisms.     

Adaptive strategies for managing uncertainty may explain personality‐related differences in behavioural plasticity

There is growing evidence that individuals within populations show consistent differences in their behaviour across contexts (personality), and that personality is associated with the extent to which individuals adjust their behaviour as function of changing conditions (behavioural plasticity). We propose an evolutionary explanation for a link between personality and plasticity based upon how individuals manage uncertainty. Individuals can employ three categories of tactics to manage uncertainty. They can 1) gather information (sample) to reduce uncertainty, 2) show strategic (state‐dependent) preferences for options that differ in their associated variances in rewards (i.e. variance‐sensitivity), or 3) invest in insurance to mitigate the consequences of uncertainty. We explicitly outline how individual differences in the use of any of these tactics can generate personality‐related differences in behavioural plasticity. For example, sampling effort is likely to co‐vary with individual activity and exploration behaviours, while simultaneously creating population variation in reactions to changes in environmental conditions. Individual differences in the use of insurance may be associated with differences in risk‐taking behaviours, such as boldness in the face of predation, thereby influencing the degree of adaptive plasticity across individuals. Population variation in responsiveness to environmental changes may also reflect individual differences in variance‐sensitivity, because stochastic change in the environment increases variances in rewards, which may both attract and benefit variance‐prone individuals, but not variance‐averse individuals. We review the existing evidence that individual variation in strategies for managing uncertainty exist, and describe how positive‐feedbacks between sampling, variance‐sensitivity and insurance can maintain and exaggerate even small initial differences between individuals in the relative use of these tactics. Given the pervasiveness of the problem of uncertainty, alternative strategies for managing uncertainty may provide a powerful explanation for consistent differences in behaviour and behavioural plasticity for a wide range of traits.     

Intraspecific variation in behaviour: effects of evolutionary history, ontogenetic experience and sex

Geographical variation in behaviour within species is common. However, how behavioural plasticity varies between and within locally adapted populations is less studied. Here, we studied behavioural plasticity induced by perceived predation risk and food availability in pond (low predation - high competition) vs. coastal marine (high predation - low competition) nine-spined sticklebacks (Pungitius pungitius) reared in a common garden experiment. Pond sticklebacks were more active feeders, more risk-taking, aggressive and explorative than marine sticklebacks. Perceived predation risk decreased aggression and risk-taking of all fish. Food restriction increased feeding activity and risk-taking. Pond sticklebacks became more risk-taking than marine sticklebacks under food shortage, whereas well-fed fish behaved similarly. Among poorly fed fish, males showed higher drive to feed, whereas among well-fed fish, females did. Apart from showing how evolutionary history, ontogenetic experience and sex influence behaviour, the results provide evidence for habitat-dependent expression of adaptive phenotypic plasticity.     

Individual differences in plasticity and sampling when playing behavioural games

When engaged in behavioural games, animals can adjust their use of alternative tactics until groups reach stable equilibria. Recent theory on behavioural plasticity in games predicts that individuals should differ in their plasticity or responsiveness and hence in their degree of behavioural adjustment. Moreover, individuals are predicted to be consistent in their plasticity within and across biological contexts. These predictions have yet to be tested empirically and so we examine the behavioural adjustment of individual nutmeg mannikins (Lonchura punctulata), gregarious ground-feeding passerines, when playing two different social foraging games: producer-scrounger (PS) and patch-choice (PC) games. We found: (i) significant individual differences in plasticity and sampling behaviour in each of the two games, (ii) individual differences in sampling behaviour were consistent over different test conditions within a game (PC) and over a six month period (PS), (iii) but neither individual plasticity nor sampling behaviour was correlated from one social foraging game to another. The rate at which birds sampled alternative tactics was positively associated with seed intake in PS trials but negatively associated in PC trials. These results suggest that games with frequency dependence of pay-offs can maintain differences in behavioural plasticity but that an important component of this plasticity is group- and/or context-specific.     

Consistency in boldness expression varies with ecological context in a jumping spider

Animals can adjust their behaviours depending on ecological context (i.e., behavioural plasticity), and an individual's response to a given context may also vary from occasion to occasion (intra‐individual variability). Recognizing the roles of both behavioural plasticity and intra‐individual variability is important in understanding how behavioural diversity is maintained within populations. However, how the ecological context itself influences the individual behavioural response and intra‐individual variability (e.g., how variable an individual is in their behavioural expression) remains largely unexplored. Here, we examine boldness expression (the duration of startle response) in a specialised spider‐eating jumping spider, Portia labiata, across three contexts following a mild disturbance: presence of a conspecific intruder (most dangerous), environmental change but no conspecific intruder, and no conspecific intruder or environmental change (safest). We found that context does not significantly influence the average boldness expression at the population level. However, each individual responded to each context differently, and the repeatability of boldness expression—the proportion of behavioural variation attributable to the between ‐individual level—is context‐dependent. We also found that in the presence of a conspecific intruder, spiders behave less predictably than in the environmental change context, but not differently from the safest context. These findings may suggest that the presence of conspecifics influences behavioural consistency in individuals, but that this may occur without influencing the population average behaviour.     

Yes,correct context is indeed the key: An answer to Haave‐Audet et al. 2019

We published a study recently testing the link between brain size and behavioural plasticity using brain size selected guppy (Poecilia reticulata) lines (2019, Journal of Evolutionary Biology, 32, 218‐226). Only large‐brained fish showed habituation to a new, but actually harmless environment perceived as risky, by increasing movement activity over the 20‐day observation period. We concluded that “Our results suggest that brain size likely explains some of the variation in behavioural plasticity found at the intraspecific level”. In a commentary published in the same journal, Haave‐Audet et al. challenged the main message of our study, stating that (a) relative brain size is not a suitable proxy for cognitive ability and (b) habituation measured by us is likely not adaptive and costly. In our response, we first show that a decade's work has proven repeatedly that relative brain size is indeed positively linked to cognitive performance in our model system. Second, we discuss how switching from stressed to unstressed behaviour in stressful situations without real risk is likely adaptive. Finally, we point out that the main cost of behavioural plasticity in our case is the development and maintenance of the neural system needed for information processing, and not the expression of plasticity. We hope that our discussion with Haave‐Audet et al. helps clarifying some central issues in this emerging research field.     

Comparing the strength of behavioural plasticity and consistency across situations: animal personalities in the hermit crab Pagurus bernhardus

Many phenotypic traits show plasticity but behaviour is often considered the 'most plastic' aspect of phenotype as it is likely to show the quickest response to temporal changes in conditions or 'situation'. However, it has also been noted that constraints on sensory acuity, cognitive structure and physiological capacities place limits on behavioural plasticity. Such limits to plasticity may generate consistent differences in behaviour between individuals from the same population. It has recently been suggested that these consistent differences in individual behaviour may be adaptive and the term 'animal personalities' has been used to describe them. In many cases, however, a degree of both behavioural plasticity and relative consistency is probable. To understand the possible functions of animal personalities, it is necessary to determine the relative strength of each tendency and this may be achieved by comparison of statistical effect sizes for tests of difference and concordance. Here, we describe a new statistical framework for making such comparisons and investigate cross-situational plasticity and consistency in the duration of startle responses in the European hermit crab Pagurus bernhardus, in the field and the laboratory. The effect sizes of tests for behavioural consistency were greater than for tests of behavioural plasticity, indicating for the first time the presence of animal personalities in a crustacean model.     

Quantitative genetics of behavioural reaction norms: genetic correlations between personality and behavioural plasticity vary across stickleback populations

Behavioural ecologists have proposed various evolutionary mechanisms as to why different personality types coexist. Our ability to understand the evolutionary trajectories of personality traits requires insights from the quantitative genetics of behavioural reaction norms. We assayed > 1000 pedigreed stickleback for initial exploration behaviour of a novel environment, and subsequent changes in exploration over a few hours, representing their capacity to adjust their behaviour to changes in perceived novelty and risk. We found heritable variation in both the average level of exploration and behavioural plasticity, and population differences in the sign of the genetic correlation between these two reaction norm components. The phenotypic correlation was not a good indicator of the genetic correlation, implying that quantitative genetics are necessary to appropriately evaluate evolutionary hypotheses in cases such as these. Our findings therefore have important implications for future studies concerning the evolution of personality and plasticity.     

Among‐individual heterogeneity in maternal behaviour and physiology affects reproductive allocation and offspring life‐history traits in the garter snake Thamnophis elegans

Accumulating evidence suggests that within‐individual plasticity of behavioural and physiological traits is limited, resulting in stable among‐individual differences in these aspects of the phenotype. Furthermore, these traits often covary within individuals, resulting in a continuum of correlated phenotypic variation among individuals within populations and species. This heterogeneity, in turn, affects individual fitness and can have cross‐generational effects. Patterns of trait covariation, among‐individual differences, and subsequent fitness consequences have long been recognized in reptiles. Here, we provide a test of patterns of among‐individual heterogeneity in behaviour and physiology and subsequent effects on reproduction and offspring fitness in the garter snake Thamnophis elegans. We find that measures of activity levels vary among individuals and are consistent within individuals in reproductive female snakes, indicating stable behavioural phenotypes. Blood hormone and glucose concentrations are not as stable within individuals, indicating that these traits do not describe consistent physiological phenotypes. Nonetheless, the major axes of variation in maternal traits describe behavioural and physiological phenotypes that interact to predict offspring body condition and mass at birth. This differential allocation of energy to offspring, in turn, strongly influences subsequent offspring growth and survival. This pattern suggests the potential for strong selection on phenotypes defined by behaviour–physiology interactions.     

Lifetime low behavioural plasticity of personality traits in the common vole (Microtus arvalis) under laboratory conditions

Individual differences in behaviour, referred to as animal personality, are consistent across time and contexts. Nevertheless, personality traits show behavioural plasticity, much like many other phenotypic traits. In the present study, we examined the relationship between personality traits and behavioural plasticity in the common vole (Microtus arvalis) under stable, long-lasting laboratory conditions. A total of 94 voles were tested in the classic open field test, designed to measure seven behavioural parameters (distance moved, grooming, immobility, rearing, running, scanning and walking duration) during a three-minute test. A total of 60 voles formed the experimental group and were tested at four different time points over their lifetime (1st, 3rd, 5th and 7th month); 34 voles formed the control group and were tested only once at the 7th month. All voles were of the same age. Based on principal component analysis (PCA), two ordination axes were determined: “exploration” and “activity.” For further analyses, “distance moved” and “scanning duration” were selected from the first axis and “walking duration” from the second. Using linear mixed-effect models (LMMs), we found highly significant random intercepts (i.e. personality traits) in all three behavioural parameters. However, evidence for behavioural plasticity was only found in the distance moved parameter, as determined from the random slope, and correlations between personality trait (intercept) and plasticity (slope) were not significant for any trait. During the experiment, variances of random effects were high and remained essentially the same, whilst the rank order of many individuals changed. Based on fixed effect slopes and a comparison with the control group, habituation was only significant for “walking duration.” The observed low behavioural plasticity could mirror stable (laboratory) conditions that result in the manifestation of original trait settings (genetic, early postnatal) or their gradual overcoming. These findings provide a starting point for further tests on free-living voles.