Adaptive individual differences within single populations

Scale and state dependence of the relationship between personality and dispersal in a great tit population

1. Dispersal is a key process in population biology and ecology. Although the general ecological conditions that lead to dispersal have been well studied, the causes of individual variation in dispersal are less well understood. A number of recent studies suggest that heritable temperament - or personality - traits are correlated with dispersal in the wild but the extent to which these 'personality-dispersal syndromes' are general, how they depend on an individual's state and on spatial scale and whether they are temporally stable, both within and across individuals, remains unclear. 2. Here, we examine the relationship between exploration behaviour - an axis of personality that appears to be important in animals generally - and a variety of dispersal processes over 6 years in a population of the great tit Parus major. 3. Exploration behaviour was higher in immigrant than in locally born juveniles, but the difference was much larger for individuals with a small body mass, though independent of sex, representing one of the first examples of a state-dependent effect in a personality-dispersal syndrome. 4. Despite a temporal trend in exploration behaviour at the population level, the difference between immigrants and locally born birds remained stable over time, both across and within individuals. This suggests that the personality difference between immigrants and locally born birds is established early in development, but that the process of immigration interacts with both personality and state. 5. We found that the number of immigrant parents a locally born bird had did not influence exploration behaviour, suggesting either the difference between immigrants and residents was environmental or that the effect is overridden by local environmental sources of variation. 6. In contrast to previous work, we found no evidence for links between personality and natal dispersal distance within the population, either in terms of an individual's own exploration behaviour or that of its parents. 7. Our results suggest that there are links between individual differences in personality and dispersal, but that these can be dependent on differences in state among individuals and on the scale over which dispersal is measured. Future work should aim to understand the differences between dispersal within and between populations and the ways in which personality and state interact to determine the outcome of these processes.

Developmental perspectives on personality: implications for ecological and evolutionary studies of individual differences

Developmental processes can have major impacts on the correlations in behaviour across contexts (contextual generality) and across time (temporal consistency) that are the hallmarks of animal personality. Personality can and does change: at any given age or life stage it is contingent upon a wide range of experiential factors that occurred earlier in life, from prior to conception through adulthood. We show how developmental reaction norms that describe the effects of prior experience on a given behaviour can be used to determine whether the effects of a given experience at a given age will affect contextual generality at a later age, and to illustrate how variation within individuals in developmental plasticity leads to variation in contextual generality across individuals as a function of experience. We also show why niche-picking and niche-construction, behavioural processes which allow individuals to affect their own developmental environment, can affect the contextual generality and the temporal consistency of personality. We conclude by discussing how an appreciation of developmental processes can alert behavioural ecologists studying animal personality to critical, untested assumptions that underlie their own research programmes, and outline situations in which a developmental perspective can improve studies of the functional significance and evolution of animal personality.

Evolutionary and ecological approaches to the study of personality

This introduction to the themed issue on Evolutionary and ecological approaches to the study of personality provides an overview of conceptual, theoretical and methodological progress in research on animal personalities over the last decade, and places the contributions to this volume in context. The issue has three main goals. First, we aimed to bring together theoreticians to contribute to the development of models providing adaptive explanations for animal personality that could guide empiricists, and stimulate exchange of ideas between the two groups of researchers. Second, we aimed to stimulate cross-fertilization between different scientific fields that study personality, namely behavioural ecology, psychology, genomics, quantitative genetics, neuroendocrinology and developmental biology. Third, we aimed to foster the application of an evolutionary framework to the study of personality.

An explanatory framework for adaptive personality differences

We develop a conceptual framework for the understanding of animal personalities in terms of adaptive evolution. We focus on two basic questions. First, why do behavioural types exhibit limited behavioural plasticity, that is, behavioural correlations both across contexts and over time? Second, how can multiple behavioural types coexist within a single population? We emphasize differences in 'state' among individuals in combination with state-dependent behaviour. Some states are inherently stable and individual differences in such states can explain stable differences in suites of behaviour if it is adaptive to make behaviour in various contexts dependent on such states. Behavioural stability and cross-context correlations in behaviour are more difficult to explain if individual states are potentially more variable. In such cases stable personalities can result from state-dependent behaviour if state and behaviour mutually reinforce each other by feedback mechanisms. We discuss various evolutionary mechanisms for the maintenance of variation (in states and/or behaviour), including frequency-dependent selection, spatial variation with incomplete matching between habitat and phenotype, bet-hedging in a temporally fluctuating environment, and non-equilibrium dynamics. Although state differences are important, we also discuss how social conventions and social signalling can give rise to adaptive personality differences in the absence of state differences.

Personality: bridging the literatures from human psychology and behavioural ecology

The concept of personality has recently begun to attract a great deal of interest in behavioural ecology. However, there is also a large and mature literature on personality within human psychology. These two bodies of work have developed independently and at present make rather little reference to one another. The current paper has two main objectives. First, we seek to acquaint behavioural ecologists with the principal ideas and issues found in the human personality psychology literature. Second, we explore how ideas from the behavioural ecology literature might help advance research in human personality psychology. We suggest strong potential for convergence between the two literatures in the near future. Common themes of this future unified science of personality include the conception of personality traits as reaction norms, a commitment to the importance of direct measurement of behaviour, investigation of both proximate and ultimate explanations for personality variation, and a concern with the impact of personality variation on survival and reproductive success.

Recent models for adaptive personality differences: a review

In this paper we review recent models that provide adaptive explanations for animal personalities: individual differences in behaviour (or suites of correlated behaviours) that are consistent over time or contexts. We start by briefly discussing patterns of variation in behaviour that have been documented in natural populations. In the main part of the paper we discuss models for personality differences that (i) explain animal personalities as adaptive behavioural responses to differences in state, (ii) investigate how feedbacks between state and behaviour can stabilize initial differences among individuals and (iii) provide adaptive explanations for animal personalities that are not based on state differences. Throughout, we focus on two basic questions. First, what is the basic conceptual idea underlying the model? Second, what are the key assumptions and predictions of the model? We conclude by discussing empirical features of personalities that have not yet been addressed by formal modelling. While this paper is primarily intended to guide empiricists through current adaptive theory, thereby stimulating empirical tests of these models, we hope it also inspires theoreticians to address aspects of personalities that have received little attention up to now.

Individual-learning ability predicts social-foraging strategy in house sparrows

Social foragers can use either a 'producer' strategy, which involves searching for food, or a 'scrounger' strategy, which involves joining others' food discoveries. While producers rely on personal information and past experience, we may ask whether the tendency to forage as a producer is related to being a better learner. To answer this question, we hand-raised house sparrow (Passer domesticus) nestlings that upon independence were given an individual-learning task that required them to associate colour signal and food presence. Following the testing phase, all fledglings were released into a shared aviary, and their social-foraging tendencies were measured. We found a significant positive correlation between individual's performance in the individual-learning task and subsequent tendency to use searching (producing) behaviour. Individual-learning score was negatively correlated with initial fear of the test apparatus and with body weight. However, the correlation between individual learning and searching remained significant after controlling for these variables. Since it was measured before the birds entered a social group, individual-learning ability could not be the outcome of being a producer. However, the two traits may be initially associated, or individual learning could facilitate producing behaviour. To our knowledge, this is the first evidence that associates individual-learning abilities with social-foraging strategies in animal groups.

Increased exposure to predators increases both exploration and activity level in Brachyrhaphis episcopi

Two temperament traits, tendency to explore and activity level, were measured in a tropical poeciliid fish, the Panamanian bishop Brachyrhaphis episcopi. Open-field arena tests were used to quantify how predation pressure shapes activity levels and exploratory behaviours. Fish behaviour differed between high and low-predation populations. Fish that experienced higher levels of predation were both more explorative and more active. There were also some individual differences within populations; fish varied in their levels of exploration and activity in a novel open arena, but these differences were not related to sex or size. Together with previous studies on this species, these results indicate that there is a behavioural syndrome associated with predation pressure. Fish from high-predation populations are bolder, more explorative and more active than those from low-predation populations.

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.

Individual behaviour in firebugs (Pyrrhocoris apterus)

The concept of animal personalities has recently become of major interest as researchers began to wonder why animals within a given population show consistent behaviour across situations and contexts, what led to the evolution of such behavioural inflexibility and what mechanisms might underlie the phenomenon. A recent model explains individual differences in a population as the result of trade-off between present and future reproduction. We tested this model on the two wing morphs, i.e. short-winged (brachypterous) and long-winged (macropterous) specimens of the firebug (Pyrrhocoris apterus). Since it has been already demonstrated that the two wing morphs differ in their life-history strategies, this species is an ideal subject to test whether the specimens with different life-history strategies have different personalities as well. The results show that individuals behave consistently over time and across contexts, meaning observed bugs do have personalities. We also have found that in females, the two wing morphs have different personalities supporting the theoretical predictions, i.e. winged ones, which are supposed to have lower future reproductive value, are braver and more exploratory. We found no difference between the morphs in males. Differences in reproductive investment might explain this discrepancy between the sexes.

On the coevolution of social responsiveness and behavioural consistency

Recent research focuses on animal personalities, that is individual differences in behaviour that are consistent across contexts and over time. From an adaptive perspective, such limited behavioural plasticity is surprising, since a more flexible structure of behaviour should provide a selective advantage. Here, we argue that consistency can be advantageous because it makes individuals predictable. Predictability, however, can only be advantageous if at least some individuals in the population respond to individual differences. Consequently, the evolution of consistency and responsiveness are mutually dependent. We present a general analysis of this coevolutionary feedback for scenarios that can be represented as matrix games with two pure strategies (e.g. hawk-dove game, snowdrift game). We first show that responsive strategies are favoured whenever some individual differences are present in the population (e.g. due to mutation and drift). We then show that the presence of responsive individuals can trigger a coevolutionary process between responsiveness and consistency that gives rise to populations in which responsive individuals coexist with unresponsive individuals who show high levels of adaptive consistency in their behaviour. Next to providing an adaptive explanation for consistency, our results also link two key features associated with personalities, individual differences in responsiveness and behavioural consistency.

Learning in a game context: strategy choice by some keeps learning from evolving in others

Behavioural decisions in a social context commonly have frequency-dependent outcomes and so require analysis using evolutionary game theory. Learning provides a mechanism for tracking changing conditions and it has frequently been predicted to supplant fixed behaviour in shifting environments; yet few studies have examined the evolution of learning specifically in a game-theoretic context. We present a model that examines the evolution of learning in a frequency-dependent context created by a producer-scrounger game, where producers search for their own resources and scroungers usurp the discoveries of producers. We ask whether a learning mutant that can optimize its use of producer and scrounger to local conditions can invade a population of non-learning individuals that play producer and scrounger with fixed probabilities. We find that learning provides an initial advantage but never evolves to fixation. Once a stable equilibrium is attained, the population is always made up of a majority of fixed players and a minority of learning individuals. This result is robust to variation in the initial proportion of fixed individuals, the rate of within- and between-generation environmental change, and population size. Such learning polymorphisms will manifest themselves in a wide range of contexts, providing an important element leading to behavioural syndromes.

Association between DRD4 gene polymorphism and personality variation in great tits: a test across four wild populations

Polymorphisms in the dopamine receptor D4 gene (DRD4) have been related to individual variation in novelty-seeking or exploratory behaviour in a variety of animals, including humans. Recently, the human DRD4 orthologue was sequenced in a wild bird, the great tit (Parus major) and a single nucleotide polymorphism in exon 3 of this gene (SNP830) was shown to be associated with variation in exploratory behaviour of lab-raised individuals originating from a single wild population. Here we test the generality of this finding in a large sample of free-living individuals from four European great tit populations, including the originally sampled population. We demonstrate that the association between SNP830 genotype and exploratory behaviour also exists in free-living birds from the original population. However, in the other three populations we found only limited evidence for an association: in two populations the association appeared absent; while in one there was a nonsignificant tendency. We could not confirm a previously demonstrated interaction with another DRD4 polymorphism, a 15 bp indel in the promoter region (ID15). As yet unknown differences in genetic or environmental background could explain why the same genetic polymorphism (SNP830) has a substantial effect on exploratory behaviour in one population, explaining 4.5-5.8% of the total variance-a large effect for a single gene influencing a complex behavioural trait-but not in three others. The confirmation of an association between SNP830 genotype and personality-related behaviour in a wild bird population warrants further research into potential fitness effects of the polymorphism, while also the population differences in the strength of the association deserve further investigation. Another important future challenge is the identification of additional loci influencing avian personality traits in the wild.

Sexual selection and animal personality

Consistent individual behavioural tendencies, termed "personalities", have been identified in a wide range of animals. Functional explanations for personality have been proposed, but as yet, very little consideration has been given to a possible role for sexual selection in maintaining differences in personality and its stability within individuals. We provide an overview of the available literature on the role of personality traits in intrasexual competition and mate choice in both human and non-human animals and integrate this into a framework for considering how sexual selection can generate and maintain personality. For this, we consider the evolution and maintenance of both main aspects of animal personality: inter-individual variation and intra-individual consistency.

Risk-taking behaviour variation over time in sea bass Dicentrarchus labrax: effects of day-night alternation, fish phenotypic characteristics and selection for growth

Differences in bold and shy personality on sea bass Dicentrarchus labrax were investigated between a population (wild) produced from wild-brood fish and a population (selected) produced from selected-brood fish. During the experiment (112 days), fish were reared under self-feeding condition to characterize the feeding behaviour of each individual fish. Three risk-taking tests (T1, T2 and T3 of 24 h with day-night alternation) were carried out at >1 month intervals on 180 fish of each strain in order to monitor D. labrax behaviour over time and in relation to the light:dark period. A risk-taking score was evaluated via a preference choice between a safe zone (without food) and a risky zone (potentially with food) by recording the number and the duration of individual passages through an opening in an opaque divider. Results showed that fish performed passages preferentially during the night period and that wild fish were generally bolder than selected fish during T1 and T2 but showed a decrease in risk taking during T3, contrary to selected fish which showed a constant increase in their risk-taking behaviour. The phenotypic characteristics of the bold fish were different in the two strains: wild bold fish were the smallest within the wild strain and selected bold fish presented the higher growth rate within the selected strain. For both strains, these bold fish were also generally characterized by a high feed-demand activity. Fish hunger state thus seemed to be the highest motivation for risk-taking behaviour under the present conditions. Furthermore, behavioural variations over tests such as higher risk taking (number of passages) and faster exploratory responses (higher score emergence) could be interpreted as relevant indicators of the learning process and habituation. According to the results, however, no real difference in coping strategy between strains could be observed at this first stage of domestication and selection.

Behavioural reaction norms: animal personality meets individual plasticity

Recent studies in the field of behavioural ecology have revealed intriguing variation in behaviour within single populations. Increasing evidence suggests that individual animals differ in their average level of behaviour displayed across a range of contexts (animal 'personality'), and in their responsiveness to environmental variation (plasticity), and that these phenomena can be considered complementary aspects of the individual phenotype. How should this complex variation be studied? Here, we outline how central ideas in behavioural ecology and quantitative genetics can be combined within a single framework based on the concept of 'behavioural reaction norms'. This integrative approach facilitates analysis of phenomena usually studied separately in terms of personality and plasticity, thereby enhancing understanding of their adaptive nature.

Melanin-based skin spots reflect stress responsiveness in salmonid fish

Within animal populations, genetic, epigenetic and environmental factors interact to shape individual neuroendocrine and behavioural profiles, conferring variable vulnerability to stress and disease. It remains debated how alternative behavioural syndromes and stress coping styles evolve and are maintained by natural selection. Here we show that individual variation in stress responsiveness is reflected in the visual appearance of two species of teleost fish; rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). Salmon and trout skin vary from nearly immaculate to densely spotted, with black spots formed by eumelanin-producing chromatophores. In rainbow trout, selection for divergent hypothalamus-pituitary-interrenal responsiveness has led to a change in dermal pigmentation patterns, with low cortisol-responsive fish being consistently more spotted. In an aquaculture population of Atlantic salmon individuals with more spots showed a reduced physiological and behavioural response to stress. Taken together, these data demonstrate a heritable behavioural-physiological and morphological trait correlation that may be specific to alternative coping styles. This observation may illuminate the evolution of contrasting coping styles and behavioural syndromes, as occurrence of phenotypes in different environments and their response to selective pressures can be precisely and easily recorded.

Heterogeneous selection on a heritable temperament trait in a variable environment

1. Temperament traits increasingly provide a focus for investigating the evolutionary ecology of behavioural variation. Here, we examine the underlying causes and selective consequences of individual variation in the temperament trait 'exploration behaviour in a novel environment' (EB, based on an 8-min assay) in a free-ranging population of a passerine bird, the great tit Parus major. 2. First, we conducted a quantitative genetic analysis on EB using a restricted maximum likelihood-based animal model with a long-term pedigree. Although repeatability was relatively high, EB was only moderately heritable and permanent environment (V(PE)) effects contributed as much to phenotypic variance as additive genetic effects. 3. We then asked whether heterogeneous selection acted on EB at various temporal and spatial scales. Using estimates of lifetime reproductive success, we found evidence of weak negative directional selection acting on EB amongst females which was driven by selection through recruitment, but not fecundity, in one of the four breeding years. There was no evidence of any selection on EB through survival. 4. Heterogeneous selection on EB within seasons was also observed amongst males through fecundity along two fine-scale environmental gradients--local breeding density and habitat quality; we are unaware of any previous equivalent demonstrations. 5. All of these analyses were repeated on a second measure of exploration behaviour (EB(2), measured during a 2-min assay) to facilitate comparison with other studies. EB and EB(2) were strongly correlated to one another at the genetic level, but were only moderately correlated at the phenotypic level and V(PE) was undetected in EB(2). Selection on EB(2) was similar to that on EB; we conclude that both traits are broadly equivalent from an evolutionary perspective. 6. Our analyses suggest that to the extent that the temperament trait 'exploration behaviour' is subject to natural selection in this population, this selection is highly context dependent and most evident along two environmental gradients. Furthermore, the strong V(PE) effect detected suggests that understanding the causes and consequences of variation in this trait will require studies firmly embedded in an environmental context.

Evolution of trust and trustworthiness: social awareness favours personality differences

Interest in the evolution and maintenance of personality is burgeoning. Individuals of diverse animal species differ in their aggressiveness, fearfulness, sociability and activity. Strong trade-offs, mutation–selection balance, spatio-temporal fluctuations in selection, frequency dependence and good-genes mate choice are invoked to explain heritable personality variation, yet for continuous behavioural traits, it remains unclear which selective force is likely to maintain distinct polymorphisms. Using a model of trust and cooperation, we show how allowing individuals to monitor each other's cooperative tendencies, at a cost, can select for heritable polymorphisms in trustworthiness. This variation, in turn, favours costly ‘social awareness’ in some individuals. Feedback of this sort can explain the individual differences in trust and trustworthiness so often documented by economists in experimental public goods games across a range of cultures. Our work adds to growing evidence that evolutionary game theorists can no longer afford to ignore the importance of real world inter-individual variation in their models.

Behavioral Profile Predicts Dominance Status in Mountain Chickadees

Individual variation in stable behavioral traits may explain variation in ecologically-relevant behaviors such as foraging, dispersal, anti-predator behavior, and dominance. We investigated behavioral variation in mountain chickadees (Poecile gambeli), a North American parid that lives in dominance-structured winter flocks, using two common measures of behavioral profile: exploration of a novel room and novel object exploration. We related those behavioral traits to dominance status in male chickadees following brief, pair-wise encounters. Low-exploring birds (birds that visited less than four locations in the novel room) were significantly more likely to become dominant in brief, pairwise encounters with high-exploring birds (i.e., birds that visited all perching locations within a novel room). On the other hand, there was no relationship between novel object exploration and dominance. Interestingly, novel room exploration was also not correlated with novel object exploration. These results suggest that behavioral profile may predict the social status of group-living individuals. Moreover, our results contradict the idea that novel object exploration and novel room exploration are always interchangeable measures of individuals' sensitivity to environmental novelty.

Hand and paw preferences in relation to the lateralized brain

Hand preferences of primates are discussed as part of the broad perspective of brain lateralization in animals, and compared with paw preferences in non-primates. Previously, it has been suggested that primates are more likely to express a species-typical hand preference on complex tasks, especially in the case of coordinated hand use in using tools. I suggest that population-level hand preferences are manifested when the task demands the obligate use of the processing specialization of one hemisphere, and that this depends on the nature of the task rather than its complexity per se. Depending on the species, simple reaching tasks may not demand the obligate use of a specialized hemisphere and so do not constrain limb/hand use. In such cases, individuals may show hand preferences that are associated with consistent differences in behaviour. The individual's hand preference is associated with the expression of behaviour controlled by the hemisphere contralateral to the preferred hand (fear and reactivity in left-handed individuals versus proactivity in right-handed individuals). Recent findings of differences in brain structure between left- and right-handed primates (e.g. somatosensory cortex in marmosets) have been discussed and related to potential evolutionary advances.

Evolutionary emergence of responsive and unresponsive personalities

In many animal species, individuals differ consistently in suites of correlated behaviors, comparable with human personalities. Increasing evidence suggests that one of the fundamental factors structuring personality differences is the responsiveness of individuals to environmental stimuli. Whereas some individuals tend to be highly responsive to such stimuli, others are unresponsive and show routine-like behaviors. Much research has focused on the proximate causes of these differences but little is known about their evolutionary origin. Here, we provide an evolutionary explanation. We develop a simple but general evolutionary model that is based on two key ingredients. First, the benefits of responsiveness are frequency-dependent; that is, being responsive is advantageous when rare but disadvantageous when common. This explains why responsive and unresponsive individuals can coexist within a population. Second, positive-feedback mechanisms reduce the costs of responsiveness; that is, responsiveness is less costly for individuals that have been responsive before. This explains why individuals differ consistently in their responsiveness, across contexts and over time. As a result, natural selection gives rise to stable individual differences in responsiveness. Whereas some individuals respond to environmental stimuli in all kinds of contexts, others consistently neglect such stimuli. Interestingly, such differences induce correlations among all kinds of other traits (e.g., boldness and aggressiveness), thus providing an explanation for environment-specific behavioral syndromes.