Adaptive individual differences within single populations

Are shy individuals less behaviorally variable? Insights from a captive population of mouse lemurs

Increasingly, individual variation in personality has become a focus of behavioral research in animal systems. Boldness and shyness, often quantified as the tendency to explore novel situations, are seen as personality traits important to the fitness landscape of individuals. Here we tested for individual differences within and across contexts in behavioral responses of captive mouse lemurs (Microcebus murinus) to novel objects, novel foods, and handling. We report consistent differences in behavioral responses for objects and handling. We also found that the responses to handling and novel objects were correlated and repeatable. Lastly, we show that shyer individuals may show less variability in their behavioral responses. This study provides new information on the potential for behavioral syndromes in this species and highlights differences in the degree to which behavioral types (e.g., shy/bold) vary in their behavioral responses.

Social niche specialization under constraints: personality, social interactions and environmental heterogeneity

Several personality traits are mainly expressed in a social context, and others, which are not restricted to a social context, can be affected by the social interactions with conspecifics. In this paper, we focus on the recently proposed hypothesis that social niche specialization (i.e. individuals in a population occupy different social roles) can explain the maintenance of individual differences in personality. We first present ecological and social niche specialization hypotheses. In particular, we show how niche specialization can be quantified and highlight the link between personality differences and social niche specialization. We then review some ecological factors (e.g. competition and environmental heterogeneity) and the social mechanisms (e.g. frequency-dependent, state-dependent and social awareness) that may be associated with the evolution of social niche specialization and personality differences. Finally, we present a conceptual model and methods to quantify the contribution of ecological factors and social mechanisms to the dynamics between personality and social roles. In doing so, we suggest a series of research objectives to help empirical advances in this research area. Throughout this paper, we highlight empirical studies of social niche specialization in mammals, where available.

Scavengers on the move: behavioural changes in foraging search patterns during the annual cycle

BACKGROUND

Optimal foraging theory predicts that animals will tend to maximize foraging success by optimizing search strategies. However, how organisms detect sparsely distributed food resources remains an open question. When targets are sparse and unpredictably distributed, a Lévy strategy should maximize foraging success. By contrast, when resources are abundant and regularly distributed, simple brownian random movement should be sufficient. Although very different groups of organisms exhibit Lévy motion, the shift from a Lévy to a brownian search strategy has been suggested to depend on internal and external factors such as sex, prey density, or environmental context. However, animal response at the individual level has received little attention.

METHODOLOGY/PRINCIPAL FINDINGS

We used GPS satellite-telemetry data of Egyptian vultures Neophron percnopterus to examine movement patterns at the individual level during consecutive years, with particular interest in the variations in foraging search patterns during the different periods of the annual cycle (i.e. breeding vs. non-breeding). Our results show that vultures followed a brownian search strategy in their wintering sojourn in Africa, whereas they exhibited a more complex foraging search pattern at breeding grounds in Europe, including Lévy motion. Interestingly, our results showed that individuals shifted between search strategies within the same period of the annual cycle in successive years.

CONCLUSIONS/SIGNIFICANCE

Results could be primarily explained by the different environmental conditions in which foraging activities occur. However, the high degree of behavioural flexibility exhibited during the breeding period in contrast to the non-breeding period is challenging, suggesting that not only environmental conditions explain individuals' behaviour but also individuals' cognitive abilities (e.g., memory effects) could play an important role. Our results support the growing awareness about the role of behavioural flexibility at the individual level, adding new empirical evidence about how animals in general, and particularly scavengers, solve the problem of efficiently finding food resources.

Allele-specific associations of 5-HTTLPR/rs25531 with ADHD and autism spectrum disorder

BACKGROUND

The aims of the present study were to examine the association between a common serotonin transporter gene (SLC6A4) polymorphism 5-HTTLPR/rs25531 with severity of attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) symptoms.

METHODS

Mothers and teachers completed a validated DSM-IV-referenced rating scale for ADHD and ASD symptoms in 118 children with ASD.

RESULTS

Analyses indicated that children with at least one copy of the S or L(G) allele obtained significantly more severe maternal ratings of hyperactivity (p=0.001; ηp(2)=0.097) and impulsivity (p=0.027; ηp(2)=0.044) but not inattention (p=0.061; ηp(2)=0.032), controlling for ASD severity, than children homozygous for the L(A) allele. Conversely, mothers' ratings indicated that children with L(A)/L(A) genotype had more severe ASD social deficits than S or L(G) allele carriers (p=0.003; ηp(2)=0.081), controlling for ADHD symptom severity. Teachers' ratings though consistent with mothers' ratings of hyperactivity and social deficits were marginally significant (p=0.07/p=0.09). There was some evidence that the magnitude of parent-teacher agreement regarding symptom severity varied as a function of the child's genotype.

CONCLUSION

The 5-HTTLPR/rs25531 polymorphism or its correlates may modulate severity of ADHD and ASD symptoms in children with ASD, but in different ways. These tentative, hypothesis-generating findings require replication with larger independent samples.

Experimental evidence for adaptive personalities in a wild passerine bird

Individuals of the same species differ consistently in risky actions. Such 'animal personality' variation is intriguing because behavioural flexibility is often assumed to be the norm. Recent theory predicts that between-individual differences in propensity to take risks should evolve if individuals differ in future fitness expectations: individuals with high long-term fitness expectations (i.e. that have much to lose) should behave consistently more cautious than individuals with lower expectations. Consequently, any manipulation of future fitness expectations should result in within-individual changes in risky behaviour in the direction predicted by this adaptive theory. We tested this prediction and confirmed experimentally that individuals indeed adjust their 'exploration behaviour', a proxy for risk-taking behaviour, to their future fitness expectations. We show for wild great tits (Parus major) that individuals with experimentally decreased survival probability become faster explorers (i.e. increase risk-taking behaviour) compared to individuals with increased survival probability. We also show, using quantitative genetics approaches, that non-genetic effects (i.e. permanent environment effects) underpin adaptive personality variation in this species. This study thereby confirms a key prediction of adaptive personality theory based on life-history trade-offs, and implies that selection may indeed favour the evolution of personalities in situations where individuals differ in future fitness expectations.

Anger and irritability symptoms among youth with ODD: cross-informant versus source-exclusive syndromes

We examined differences in co-occurring psychological symptoms and background characteristics among clinically referred youth with oppositional defiant disorder (ODD) with and without anger/irritability symptoms (AIS) according to either parent or teacher (source-exclusive) and both informants (cross-informant), youth with noncompliant symptoms (NS) of ODD, and non-ODD clinic controls. Parents and teachers evaluated 1127 youth (ages 6-18) with a DSM-IV-referenced rating scale to assess ODD and co-occurring psychological symptoms. Parents also completed a background questionnaire (demographic, developmental, treatment, relationship, and academic characteristics) and teachers rated school functioning. Source-exclusive AIS groups were associated with different clinical features, and there was some evidence that cross-informant youth had more mental health concerns than source-exclusive groups. Findings varied to some extent among older (12-18 years) versus younger (6-11 years) youth. In general, the NS group (youth without AIS) was the most similar to clinic controls. AIS and NS are likely candidates for component phenotypes in ODD and continued research into their pathogenesis may have important implications for nosology, etiology, and intervention.

Linking personality to larval energy reserves in rainbow trout (Oncorhynchus mykiss)

There is a surging interest in the evolution, ecology and physiology of personality differences. However, most of the studies in this research area have been performed in adult animals. Trait variations expressed early in development and how they are related to the ontogeny of an animal’s personality are far less studied. Genetic differences as well as environmental factors causing functional variability of the central serotonergic system have been related to personality differences in vertebrates, including humans. Such gene-environment interplay suggests that the central serotonergic system plays an important role in the ontogeny of personality traits. In salmonid fishes, the timing of emergence from spawning nests is related to energy reserves, aggression, and social dominance. However, it is currently unknown how the size of the yolk reserve is reflected on aggression and dominance, or if these traits are linked to differences in serotonergic transmission in newly emerged larvae. In this study we investigated the relationship between yolk reserves, social dominance, and serotonergic transmission in newly emerged rainbow trout (Oncorhynchus mykiss) larvae. This was conducted by allowing larvae with the same emergence time, but with different yolk sizes, to interact in pairs for 24 h. The results show that individuals with larger yolks performed more aggressive acts, resulting in a suppression of aggression in individuals with smaller yolks. A higher brain serotonergic activity confirmed subordination in larvae with small yolks. The relationship between social dominance and yolk size was present in siblings, demonstrating a link between interfamily variation in energy reserves and aggression, and suggests that larger yolk reserves fuel a more aggressive personality during the initial territorial establishment in salmonid fishes. Furthermore, socially naïve larvae with big yolks had lower serotonin levels, suggesting that other factors than the social environment causes variation in serotonergic transmission, underlying individual variation in aggressive behavior.

An evolutionary ecology of individual differences

Individuals often differ in what they do. This has been recognised since antiquity. Nevertheless, the ecological and evolutionary significance of such variation is attracting widespread interest, which is burgeoning to an extent that is fragmenting the literature. As a first attempt at synthesis, we focus on individual differences in behaviour within populations that exceed the day-to-day variation in individual behaviour (i.e. behavioural specialisation). Indeed, the factors promoting ecologically relevant behavioural specialisation within natural populations are likely to have far-reaching ecological and evolutionary consequences. We discuss such individual differences from three distinct perspectives: individual niche specialisations, the division of labour within insect societies and animal personality variation. In the process, while recognising that each area has its own unique motivations, we identify a number of opportunities for productive 'cross-fertilisation' among the (largely independent) bodies of work. We conclude that a complete understanding of evolutionarily and ecologically relevant individual differences must specify how ecological interactions impact the basic biological process (e.g. Darwinian selection, development and information processing) that underpin the organismal features determining behavioural specialisations. Moreover, there is likely to be co-variation amongst behavioural specialisations. Thus, we sketch the key elements of a general framework for studying the evolutionary ecology of individual differences.

Performance, personality, and energetics: correlation, causation, and mechanism

The study of phenotypic evolution should be an integrative endeavor that combines different approaches and crosses disciplinary and phylogenetic boundaries to consider complex traits and organisms that historically have been studied in isolation from each other. Analyses of individual variation within populations can act to bridge studies focused at the levels of morphology, physiology, biochemistry, organismal performance, behavior, and life history. For example, the study of individual variation recently facilitated the integration of behavior into the concept of a pace-of-life syndrome and effectively linked the field of energetics with research on animal personality. Here, we illustrate how studies on the pace-of-life syndrome and the energetics of personality can be integrated within a physiology-performance-behavior-fitness paradigm that includes consideration of ecological context. We first introduce key concepts and definitions and then review the rapidly expanding literature on the links between energy metabolism and personality traits commonly studied in nonhuman animals (activity, exploration, boldness, aggressiveness, sociability). We highlight some empirical literature involving mammals and squamates that demonstrates how emerging fields can develop in rather disparate ways because of historical accidents and/or particularities of different kinds of organisms. We then briefly discuss potentially interesting avenues for future conceptual and empirical research in relation to motivation, intraindividual variation, and mechanisms underlying trait correlations. The integration of performance traits within the pace-of-life-syndrome concept has the potential to fill a logical gap between the context dependency of selection and how energetics and personality are expected to interrelate. Studies of how performance abilities and/or aspects of Darwinian fitness relate to both metabolic rate and personality traits are particularly lacking.

Fecal glucocorticoid metabolites in wild yellow-bellied marmots: experimental validation, individual differences and ecological correlates

Natural selection is expected to shape phenotypic traits that permit organisms to respond appropriately to the environments in which they live. One important mechanism by which animals cope with changes in their environment is through physiological responses to stressors mediated by glucocorticoid hormones. Here we perform biological and physiological validations of a minimally-invasive technique for assessing fecal corticosterone metabolites (FCMs) in captive and wild groups of yellow-bellied marmots (Marmota flaviventris). Then we draw from ten years of data on these obligate hibernators at the Rocky Mountain Biological Laboratory in Colorado, USA to assess the extent to which seasonal and daily changes explain naturalistic variation in baseline levels of FCMs. Interestingly, beyond important population-level variation with respect to year, season, time of day, sex, age and reproductive state, we found repeatable inter-individual differences in FCMs, suggesting this hormonal trait might be a meaningful target of selection. FCM levels were 68% lower in captive than wild marmots, suggesting that the natural environment in which these animals occur is generally more challenging or less predictable than life in captivity. Most live-trapping events failed to represent stressors for wild marmots such that repeated measurements of traits were possible with minimal "stress" to subjects. We also document the natural ranges of annual and seasonal variation necessary for understanding the extent to which anthropogenic assaults represent stressors for wild mammals. Taken together, this study provides a foundation for understanding the evolution of hormonal traits and has important welfare and conservation implications for field biologists.

Diversifying and correlational selection on behavior toward conspecific and heterospecific competitors in brook stickleback (Culaea inconstans)

Behaviors toward heterospecifics and conspecifics may be correlated because of shared mechanisms of expression in both social contexts (nonadaptive covariation) or because correlational selection favors adaptive covariation. We evaluated these hypotheses by comparing behavior toward conspecifics and heterospecifics in brook stickleback (Culaea inconstans) from three populations sympatric with and three allopatric from a competitor, the ninespine stickleback (Pungitius pungitius). Behavioral traits were classified into three multivariate components: overt aggression, sociability, and activity. The correlation of behavior between social contexts for both overt aggression and activity varied among populations in a way unrelated to sympatry with ninespine stickleback, while mean aggression was reduced in sympatry. Correlations in allopatric populations suggest that overt aggression and activity may genetically covary between social contexts for nonadaptive reasons. Sociability was rarely correlated in allopatry but was consistently correlated in sympatry despite reduced mean sociability, suggesting that correlational selection may favor a sociability syndrome in brook stickleback when they coexist with ninespine stickleback. Thus, interspecific competition may impose diversifying selection on behavior among populations, although the causes of correlated behavior toward conspecifics and heterospecifics and whether it can evolve in one social context independent of the other may depend on the type of behavior.

"Personality" in laboratory mice used for biomedical research: a way of understanding variability?

The mouse, including countless lines of transgenic and knockout mice, has become the most prominent model organism in biomedical research. Behavioral characterization is often conducted in batteries of short tests on locomotion, anxiety, learning and memory, etc. In such tests, any individual differences within groups are usually considered to be disturbing variance. In order to reduce variance in experimental animal research enormous efforts of standardization have been made. While a substantial reduction of variability has been reached compared to the earlier years of experimental animal studies a considerable amount of inter-individual differences still seems to escape standardization. This effect is demonstrated and evaluated by re-analyzing data from two experiments conducted in our laboratory with inbred mice. Interestingly, behavioral patterns of individual animals seem to be correlated across context and time. In evolutionary biology, "animal personalities" have been discussed recently to comprise such stable patterns. We argue here, that nonrandom behavioral correlations across contexts and time might underlie the variability commonly found in biomedical mouse studies.

Symptoms of autism and schizophrenia spectrum disorders in clinically referred youth with oppositional defiant disorder

Examined autism spectrum disorder (ASD) and schizophrenia spectrum disorder (SSD) symptoms in a clinically referred, non-ASD sample (N=1160; ages 6-18) with and without oppositional defiant disorder (ODD). Mothers and teachers completed DSM-IV-referenced symptom checklists. Youth with ODD were subdivided into angry/irritable symptom (AIS) or noncompliant symptom (NS) subtypes. Two different classification strategies were used: within-informant (source-specific) and between-informant (source-exclusive). For the source-specific strategy, youth were classified AIS, NS, or Control (C) according to mothers' and teachers' ratings separately. A second set of analyses focused on youth classified AIS according to mother or teacher report but not both (source-exclusive) versus both mother and teacher (cross-informant) AIS. Results indicated the mother-defined source-specific AIS groups generally evidenced the most severe ASD and SSD symptoms (AIS>NS>C), but this was more pronounced among younger youth. Teacher-defined source-specific ODD groups exhibited comparable levels of symptom severity (AIS, NS>C) with the exception of SSD (AIS>NS>C; younger youth). Source-exclusive AIS groups were clearly differentiated from each other, but there was little evidence of differential symptom severity in cross-informant versus source-exclusive AIS. These findings were largely dependent on the informant used to define the source-exclusive groups. AIS and NS groups differed in their associations with ASD and SSD symptoms. Informant discrepancy provides valuable information that can inform nosological and clinical concerns and has important implications for studies that use different strategies to configure clinical phenotypes.

Animal personalities: consequences for ecology and evolution

Personality differences are a widespread phenomenon throughout the animal kingdom. Past research has focused on the characterization of such differences and a quest for their proximate and ultimate causation. However, the consequences of these differences for ecology and evolution received much less attention. Here, we strive to fill this gap by providing a comprehensive inventory of the potential implications of personality differences, ranging from population growth and persistence to species interactions and community dynamics, and covering issues such as social evolution, the speed of evolution, evolvability, and speciation. The emerging picture strongly suggests that personality differences matter for ecological and evolutionary processes (and their interaction) and, thus, should be considered a key dimension of ecologically and evolutionarily relevant intraspecific variation.

Personality predicts individual responsiveness to the risks of starvation and predation

Theory suggests that individual personality is tightly linked to individual life histories and to environmental variation. The reactive-proactive axis, for example, is thought to reflect whether individuals prioritize productivity or survival, mutually exclusive options that can be caused by conflicts between foraging and anti-predation behaviour. Evidence for this trade-off hypothesis, however, is limited. Here, we tested experimentally whether exploration behaviour (EB), an assay of proactivity, could explain how great tits (Parus major) respond to changes in starvation and predation risk. Individuals were presented with two feeders, holding good or poor quality food, which interchanged between safe and dangerous positions 10 m apart, across two 24 h treatments. Starvation risk was assumed to be highest in the morning and lowest in the afternoon. The proportion of time spent feeding on good quality food (PTG) rather than poor quality food was repeatable within treatments, but individuals varied in how PTG changed with respect to predation- and starvation-risk across treatments. This individual plasticity variation in foraging behaviour was linked to EB, as predicted by the reactive-proactive axis, but only among individuals in dominant social classes. Our results support the trade-off hypothesis at the level of individuals in a wild population, and suggest that fine-scale temporal and spatial variation may play important roles in the evolution of personality.

On the evolution of personalities via frequency-dependent selection

Personality differences can be found in a wide range of species across the animal kingdom, but why natural selection gave rise to such differences remains an open question. Frequency-dependent selection is a potent mechanism explaining variation; it does not explain, however, the other two key features associated with personalities, consistency and correlations. Using the hawk-dove game and a frequency-dependent foraging game as examples, we here show that this changes fundamentally whenever one takes into account the physiological architecture underlying behavior (e.g., metabolism). We find that the inclusion of physiology changes the evolutionary predictions concerning consistency and correlations: while selection gives rise to inconsistent individuals and stochastically fluctuating behavioral correlations in scenarios that neglect physiology, we find high levels of behavioral consistency and tight and stable trait correlations in scenarios that incorporate physiology. The coevolution of behavioral and physiological traits also gives rise to adaptive physiological differences that are systematically associated with behavioral differences. As well as providing a framework for understanding behavioral consistency and behavioral correlations, our work thus also provides an explanation for systematic physiological differences within populations, a phenomenon that appears to exist in a wide range of species but that, up to now, has been poorly understood.

Evolutionary dynamics of fearfulness and boldness: a stochastic simulation model

A stochastic simulation model is investigated for the evolution of anti-predator behavior in birds. The main goal is to reveal the effects of population size, predation threats, and energy lost per escape on the evolutionary dynamics of fearfulness and boldness. Two pure strategies, fearfulness and boldness, are assumed to have different responses for the predator attacks and nonlethal disturbance. On the other hand, the co-existence mechanism of fearfulness and boldness is also considered. For the effects of total population size, predation threats, and energy lost per escape, our main results show that: (i) the fearful (bold) individuals will be favored in a small (large) population, i.e. in a small (large) population, the fearfulness (boldness) can be considered to be an ESS; (ii) in a population with moderate size, fearfulness would be favored under moderate predator attacks; and (iii) although the total population size is the most important factor for the evolutionary dynamics of both fearful and bold individuals, the small energy lost per escape enables the fearful individuals to have the ability to win the advantage even in a relatively large population. Finally, we show also that the co-existence of fearful and bold individuals is possible when the competitive interactions between individuals are introduced.

Behavioural syndrome in a solitary predator is independent of body size and growth rate

Models explaining behavioural syndromes often focus on state-dependency, linking behavioural variation to individual differences in other phenotypic features. Empirical studies are, however, rare. Here, we tested for a size and growth-dependent stable behavioural syndrome in the juvenile-stages of a solitary apex predator (pike, Esox lucius), shown as repeatable foraging behaviour across risk. Pike swimming activity, latency to prey attack, number of successful and unsuccessful prey attacks was measured during the presence/absence of visual contact with a competitor or predator. Foraging behaviour across risks was considered an appropriate indicator of boldness in this solitary predator where a trade-off between foraging behaviour and threat avoidance has been reported. Support was found for a behavioural syndrome, where the rank order differences in the foraging behaviour between individuals were maintained across time and risk situation. However, individual behaviour was independent of body size and growth in conditions of high food availability, showing no evidence to support the state-dependent personality hypothesis. The importance of a combination of spatial and temporal environmental variation for generating growth differences is highlighted.

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.

Frequency-dependent payoffs and sequential decision-making favour consistent tactic use

Although natural selection should have favoured individuals capable of adjusting the weight they give to personal and social information according to circumstances, individuals generally differ consistently in their individual weighting of both types of information. Such individual differences are correlated with personality traits, suggesting that personality could directly affect individuals' ability to collect personal or social information. Alternatively, the link between personality and information use could simply emerge as a by-product of the sequential decision-making process in a frequency-dependent context. Indeed, when the gains associated with behavioural options depend on the choices of others, an individual's sequence of arrival could constrain its choice of options leading to the emergence of correlated behaviours. Any factor such as personality that affects decision order could thus be correlated with information use. To test this new explanation, we developed an individual-based model that simulates a group of animals engaged in a game of sequential frequency-dependent decision: a producer-scrounger game. Our results confirm that the sequence of decision, in this case enforced by the order in which animals enter a foraging area, consistently influences their mean tactic use and their individual plasticity, an outcome reminiscent of the correlation reported between personality and social information use.

Sex ratio dynamics and fluctuating selection on personality

Fluctuating selection has often been proposed as an explanation for the maintenance of genetic variation in personality. Here I argue that the temporal dynamics of the sex ratio can be a powerful source of fluctuating selection on personality traits, and develop this hypothesis with respect to humans. First, I review evidence that sex ratios modulate a wide range of social processes related to mating and parenting. Since most personality traits affect mating and parenting behavior, changes in the sex ratio can be expected to result in variable selection on personality. I then show that the temporal dynamics of the sex ratio are intrinsically characterized by fluctuations at various timescales. Finally, I address a number of evolutionary genetic challenges to the hypothesis. I conclude that the sex ratio hypothesis is a plausible explanation of genetic variation in human personality, and may be fruitfully applied to other species as well.

Promiscuity, paternity and personality in the great tit

Understanding causes of variation in promiscuity within populations remain a major challenge. While most studies have focused on quantifying fitness costs and benefits of promiscuous behaviour, an alternative possibility--that variation in promiscuity within populations is maintained because of linkage with other traits-has received little attention. Here, we examine whether promiscuity in male and female great tits (Parus major)--quantified as extra-pair paternity (EPP) within and between nests--is associated with variation in a well-documented personality trait: exploration behaviour in a novel environment. Exploration behaviour has been shown to correlate with activity levels, risk-taking and boldness, and these are behaviours that may plausibly influence EPP. Exploration behaviour correlated positively with paternity gained outside the social pair among males in our population, but there was also a negative correlation with paternity in the social nest. Hence, while variation in male personality predicted the relative importance of paternity gain within and outside the pair bond, total paternity gained was unrelated to exploration behaviour. We found evidence that males paired with bold females were more likely to sire extra-pair young. Our data thus demonstrate a link between personality and promiscuity, with no net effects on reproductive success, suggesting personality-dependent mating tactics, in contrast with traditional adaptive explanations for promiscuity.

Stress-induced rise in body temperature is repeatable in free-ranging Eastern chipmunks (Tamias striatus)

In response to handling or other acute stressors, most mammals, including humans, experience a temporary rise in body temperature (T(b)). Although this stress-induced rise in T(b) has been extensively studied on model organisms under controlled environments, individual variation in this interesting phenomenon has not been examined in the field. We investigated the stress-induced rise in T(b) in free-ranging eastern chipmunks (Tamias striatus) to determine first if it is repeatable. We predicted that the stress-induced rise in T(b) should be positively correlated to factors affecting heat production and heat dissipation, including ambient temperature (T(a)), body mass (M(b)), and field metabolic rate (FMR). Over two summers, we recorded both T(b) within the first minute of handling time (T(b1)) and after 5 min of handling time (T(b5)) 294 times on 140 individuals. The mean ∆T(b) (T(b5) - T(b1)) during this short interval was 0.30 ± 0.02°C, confirming that the stress-induced rise in T(b) occurs in chipmunks. Consistent differences among individuals accounted for 40% of the total variation in ∆T(b) (i.e. the stress-induced rise in T(b) is significantly repeatable). We also found that the stress-induced rise in T(b) was positively correlated to T(a), M(b), and mass-adjusted FMR. These results confirm that individuals consistently differ in their expression of the stress-induced rise in T(b) and that the extent of its expression is affected by factors related to heat production and dissipation. We highlight some research constraints and opportunities related to the integration of this laboratory paradigm into physiological and evolutionary ecology.

Exploration behaviour is not associated with chick provisioning in great tits

In biparental systems, members of the same pair can vary substantially in the amount of parental care they provide to offspring. The extent of this asymmetry should depend on the relative costs and benefits of care. Individual variation in personality is likely to influence this trade-off, and hence is a promising candidate to explain differences in care. In addition, plasticity in parental care may also be associated with personality differences. Using exploration behaviour (EB) as a measure of personality, we investigated these possibilities using both natural and experimental data from a wild population of great tits (Parus major). Contrary to predictions, we found no association between EB and natural variation in provisioning behaviour. Nor was EB linked to responsiveness to experimentally increased brood demand. These results are initially surprising given substantial data from other studies suggesting personality should influence investment in parental care. However, they are consistent with a recent study showing selection on EB is weak and highly context-specific in the focal population. This emphasises the difficulty faced by personality studies attempting to make predictions based on previous work, given that personalities often vary among populations of the same species.