Between-individual Correlations Research Articles

Long-term effects of widespread pharmaceutical pollution on trade-offs between behavioural, life-history and reproductive traits in fish.

In our rapidly changing world, understanding how species respond to shifting conditions is of paramount importance. Pharmaceutical pollutants are widespread in aquatic ecosystems globally, yet their impacts on animal behaviour, life-history and reproductive allocation remain poorly understood, especially in the context of intraspecific variation in ecologically important traits that facilitate species' adaptive capacities. We test whether a widespread pharmaceutical pollutant, fluoxetine (Prozac), disrupts the trade-off between individual-level (co)variation in behavioural, life-history and reproductive traits of freshwater fish. We exposed the progeny of wild-caught guppies (Poecilia reticulata) to three field-relevant levels of fluoxetine (mean measured concentrations: 0, 31.5 and 316 ng/L) for 5 years, across multiple generations. We used 12 independent laboratory populations and repeatedly quantified activity and risk-taking behaviour of male guppies, capturing both mean behaviours and variation within and between individuals across exposure treatments. We also measured key life-history traits (body condition, coloration and gonopodium size) and assessed post-copulatory sperm traits (sperm vitality, number and velocity) that are known to be under strong sexual selection in polyandrous species. Intraspecific (co)variation of these traits was analysed using a comprehensive, multivariate statistical approach. Fluoxetine had a dose-specific (mean) effect on the life-history and sperm trait of guppies: low pollutant exposure altered male body condition and increased gonopodium size, but reduced sperm velocity. At the individual level, fluoxetine reduced the behavioural plasticity of guppies by eroding their within-individual variation in both activity and risk-taking behaviour. Fluoxetine also altered between-individual correlations in pace-of-life syndrome traits: it triggered the emergence of correlations between behavioural and life-history traits (e.g. activity and body condition) and between life-history and sperm traits (e.g. gonopodium size and sperm vitality), but collapsed other between-individual correlations (e.g. activity and gonopodium size). Our results reveal that chronic exposure to global pollutants can affect phenotypic traits at both population and individual levels, and even alter individual-level correlations among such traits in a dose-specific manner. We discuss the need to integrate individual-level analyses and test behaviour in association with life-history and reproductive traits to fully understand how animals respond to human-induced environmental change.

Functional connectome fingerprinting across the lifespan.

Systematic changes have been observed in the functional architecture of the human brain with advancing age. However, functional connectivity (FC) is also a powerful feature to detect unique "connectome fingerprints," allowing identification of individuals among their peers. Although fingerprinting has been robustly observed in samples of young adults, the reliability of this approach has not been demonstrated across the lifespan. We applied the fingerprinting framework to the Cambridge Centre for Ageing and Neuroscience cohort (n = 483 aged 18 to 89 years). We found that individuals are "fingerprintable" (i.e., identifiable) across independent functional MRI scans throughout the lifespan. We observed a U-shape distribution in the strength of "self-identifiability" (within-individual correlation across modalities), and "others-identifiability" (between-individual correlation across modalities), with a decrease from early adulthood into middle age, before improving in older age. FC edges contributing to self-identifiability were not restricted to specific brain networks and were different between individuals across the lifespan sample. Self-identifiability was additionally associated with regional brain volume. These findings indicate that individual participant-level identification is preserved across the lifespan despite the fact that its components are changing nonlinearly.

Efficient penalized generalized linear mixed models for variable selection and genetic risk prediction in high-dimensional data.

Sparse regularized regression methods are now widely used in genome-wide association studies (GWAS) to address the multiple testing burden that limits discovery of potentially important predictors. Linear mixed models (LMMs) have become an attractive alternative to principal components (PCs) adjustment to account for population structure and relatedness in high-dimensional penalized models. However, their use in binary trait GWAS rely on the invalid assumption that the residual variance does not depend on the estimated regression coefficients. Moreover, LMMs use a single spectral decomposition of the covariance matrix of the responses, which is no longer possible in generalized linear mixed models (GLMMs). We introduce a new method called pglmm, a penalized GLMM that allows to simultaneously select genetic markers and estimate their effects, accounting for between-individual correlations and binary nature of the trait. We develop a computationally efficient algorithm based on penalized quasi-likelihood estimation that allows to scale regularized mixed models on high-dimensional binary trait GWAS. We show through simulations that when the dimensionality of the relatedness matrix is high, penalized LMM and logistic regression with PC adjustment fail to select important predictors, and have inferior prediction accuracy compared to pglmm. Further, we demonstrate through the analysis of two polygenic binary traits in a subset of 6731 related individuals from the UK Biobank data with 320K SNPs that our method can achieve higher predictive performance, while also selecting fewer predictors than a sparse regularized logistic lasso with PC adjustment. Our Julia package PenalizedGLMM.jl is publicly available on github: https://github.com/julstpierre/PenalizedGLMM. Supplementary data are available at Bioinformatics online.

Temperature and body size influence personality and behavioural syndromes in an invasive crayfish

The presence of repeatable behaviours that vary between individuals (animal personality) and the mechanisms that underpin these behaviours are of significant ecological and evolutionary importance. With current climatic trends predicted to facilitate biological invasions, we sought to examine how personality and behavioural syndromes may influence invasion potential in the face of a changing climate. We assessed a population of an invasive crayfish, Cherax destructor , in a laboratory setting for (1) the presence of temporally and contextually repeatable behavioural differences between individuals along three axes: (i) activity, (ii) boldness/shyness and (iii) exploration/avoidance, (2) correlations between body size, sex and behavioural traits at the between-individual level, (3) the influence of predicted near-future increases in temperature on personality and (4) the presence of behavioural syndromes (between-individual correlations among behaviours). Individuals exhibited repeatable variation along the three behavioural axes, providing support for the presence of animal personality. Body size was negatively correlated with exploration and activity, but sex was unrelated to personality across the three axes. Exploration increased with temperature, whereas the effect of temperature on activity was dependent on the treatment order. Finally, activity and exploration were positively correlated, indicative of a behavioural syndrome. This study provides evidence of state-dependent personality and support for the existence of behavioural syndromes which may facilitate biological invasions. Our work also provides evidence to suggest that predicted future increases in temperature will promote exploratory behaviour in invasive populations of C. destructor . • Invasive Cherax destructor exhibited personality along three behavioural axes. • Body size was negatively correlated with both active and exploratory behaviour. • Exploration of a novel environment increased at predicted near-future temperatures. • Correlations between behaviours provide evidence of an invasive syndrome. • Increasing exploration with temperature may increase future invasive potential.

Individual differences in spatial learning are correlated across tasks but not with stress response behaviour in guppies

Cognition is vital for carrying out behaviours required for survival and reproduction. Cognitive performance varies between species, but also between individuals within populations. While variation is a prerequisite for natural selection, selection does not act on traits in isolation. The extent to which cognitive traits covary with other aspects of phenotype (e.g. personality traits) may be important in shaping evolutionary dynamics. Here we adopted a multivariate approach to test spatial learning in male Poecilia reticulata and asked whether differences in cognitive performance are associated with (repeatable) differences in stress response behaviour. Functional links between cognitive traits and ‘stress coping style’ have been hypothesized. Furthermore, individual level studies of cognitive performance typically rely on multiple testing paradigms that may themselves be a stressor. There is a risk that variation in stress responsiveness is itself a cause of apparent, but artefactual variance in cognitive ability. Fish repeatedly experienced two spatial learning tasks (maze layouts) and an acute stress response test (open field trial). We found repeatable differences between individuals in performance within and across maze layouts. On average, performance improved with experience in the first maze, consistent with spatial learning, but not in the second. Individuals varied in the trajectory of mean performance with trial number in both mazes, suggesting they differ in ‘learning rate’. Acute stress response behaviour was repeatable but predicted neither average time to solve the maze nor learning rate. We found no support for between-individual correlation between acute stress response and cognitive performance. However, we highlight the possibility that cumulative, chronic stress effects may nevertheless cause declines in performance for some individuals (leading to lack of improvement in mean time to solve the second maze). If so, this may represent a pervasive but difficult challenge for our ability to robustly estimate learning rates in studies of animal cognition.

Quantifying Internal Load in Pre-Professional Contemporary Dancers: The Association Between Objective Heart-Rate Derived Outcome Measures and Subjective Dancers' and Teachers' Perceptions.

Monitoring heart rate is one of the most common methods used to quantify internal training load (ITL) in athletes. The aim of this study was to determine whether subjective measurements can be used as a simple, non-invasive, and inexpensive method for assessing ITL in pre-professional contemporary dancers. <br/>Method: A total of 16 first-year contemporary dance students participated in this study. Students wore heart rate monitors during 56 training sessions. After each session, students completed the rate of perceived exertion (RPE) scale and teachers completed the rate of observed exertion (ROE) scale. For each session, we calculated the session-RPEs (sRPE), session-ROEs (sROE), and heart-rate-derived ITLs [i. e., Banister TRaining IMPulse method (TRIMP) and Edwards TRIMP]. Pearson correlation coefficients were calculated to determine the association between sRPE, sROE, Banister TRIMP, and Edwards TRIMP. <br/>Results: Between-individual correlation between Banister TRIMP and subjective dancers' (sRPE) and teachers' perceptions (sROE) were moderate (r = .49, p < 0.01) to large (r = .57, p < 0.01). Between-individual correlations between Edwards TRIMP and the subjective perceptions (sRPE and sROE) were very large (r = .74, p < 0.01; and r = .79, p < 0.01). There was a statistically significant large (r = 0.52, p < 0.01) to almost perfect (r = 0.93, p < 0.01) within-individual correlation between the sRPE and the two measured TRIMP methods. There was a statistically significant small (r = .29, p < 0.01) to almost perfect (r = .92, p < 0.01) within-individual correlation between the sROE and the two measured TRIMP methods. <br/>Conclusion: The results of this study suggest that the sRPE is a simple, non-invasive, inexpensive, and valid method for quantifying ITL in pre-professional dancers.

A comparison of analytical strategies for cluster randomized trials with survival outcomes in the presence of competing risks.

While statistical methods for analyzing cluster randomized trials with continuous and binary outcomes have been extensively studied and compared, little comparative evidence has been provided for analyzing cluster randomized trials with survival outcomes in the presence of competing risks. Motivated by the Strategies to Reduce Injuries and Develop Confidence in Elders trial, we carried out a simulation study to compare the operating characteristics of several existing population-averaged survival models, including the marginal Cox, marginal Fine and Gray, and marginal multi-state models. For each model, we found that adjusting for the intraclass correlations through the sandwich variance estimator effectively maintained the type I error rate when the number of clusters is large. With no more than 30 clusters, however, the sandwich variance estimator can exhibit notable negative bias, and a permutation test provides better control of type I error inflation. Under the alternative, the power for each model is differentially affected by two types of intraclass correlations-the within-individual and between-individual correlations. Furthermore, the marginal Fine and Gray model occasionally leads to higher power than the marginal Cox model or the marginal multi-state model, especially when the competing event rate is high. Finally, we provide an illustrative analysis of Strategies to Reduce Injuries and Develop Confidence in Elders trial using each analytical strategy considered.

Factors influencing the jump momentum – sprint momentum correlation: a data simulation

ABSTRACT Jump take-off momentum has previously been proposed as an alternative test to predict sprint momentum. This study used a data simulation to replicate and systematically investigate relationships reported in previous studies between body mass, vertical jump performance, and sprint performance. Results were averaged for 1000 simulated data sets in each condition, and the effects of various parameters on correlations between jump momentum and sprint momentum were determined. The ability of jump take-off momentum to predict sprint momentum is greatest under relatively high inter-individual variation in body mass and relatively low inter-individual variation in jump height. This is largely due to the increased emphasis on body mass in these situations. Even under zero or a small negative (r = −0.30) correlation between jump height and sprint velocity, the correlation between the two momenta remained very large (r ≥ 0.76) on average. There were no investigated conditions under which jump momentum was most frequently a significantly (p < 0.05) greater predictor of sprint momentum compared to simply using body mass alone. Furthermore, between-individual correlations should not be used to make inferences or predictions for within-individual applications (e.g. predicting or evaluating the effects of a longitudinal training intervention). It is recommended that any rationale for calculating and/or monitoring jump take-off momentum should be separate from its ability to predict sprint momentum. Indeed, body mass alone may be a better predictor of sprint momentum.

Nest ornaments and feather composition form an extended phenotype syndrome in a wild bird

Many species throughout the animal kingdom construct nests for reproduction. A nest is an extended phenotype—a non-bodily attribute—of the individual building it. In some bird species, including our study population of blue tits (Cyanistes caeruleus), conspicuous feathers or other material are placed on top of the nest. These so-called nest ornaments do not contribute to nest insulation, but are hypothesised to have a signalling function. Here, we apply the concept of behavioural syndromes, with focus on between-individual variation (repeatability) and between-individual correlations, to the study of avian nest construction. We find that nest ornamentation is a moderately repeatable trait in female blue tits, which suggests it is an extended phenotype of the female. Furthermore, the tendency to ornament the nest covaries across females with another aspect of her extended phenotype, the composition of the nest lining material, and these two traits thus form an extended phenotype syndrome. Assuming the correlation is reflected on a genetic level, it implies that nest ornamentation and composition of the nest lining do not evolve in isolation; one aspect may be an evolutionary by-product of selection on the other aspect and their overall flexibility to respond to change is reduced.Significance statementThe avian nest is an extended phenotype (a non-bodily attribute) of its builder with potentially multiple functions in terms of insulation and signalling. In particular, many bird species’ nests contain nest ornaments, feathers or other materials that are placed on top of the nest and that stand out from the nest material due to their colour and/or size. We quantified between-individual variation (repeatability) of nest ornamentation behaviour in a wild population of blue tits and between-individual covariation (syndrome) of nest ornamentation to other features of nest construction. We find that nest ornamentation is a repeatable trait limited to females in our study population. The tendency to ornament the nest covaries across females with another aspect of her extended nest phenotype, the composition of the nest lining material. These correlated traits thus form an extended phenotype syndrome. It hence becomes crucial to recognise that a study of a single aspect of nest construction in isolation captures only a part of the complexity, as one aspect may have evolved as a correlated response of selection on the other aspect. Moreover, such a syndrome implies limited flexibility in the range of adaptive response.

Muscle size-scaled shear elastic modulus: A muscle force index independent of maximal voluntary contraction, assessed during elbow extension

Shear elastic modulus (G) can differ among individuals due to muscle size and other factors, even for constant muscle force. Inter-individual comparisons of G usually require normalization by maximal voluntary contraction (MVC), but MVC procedures may not be appropriate for certain clinical populations including those presenting with pain or other compromised functionality. This study aimed to test whether muscle size-scaled G, which does not require MVC testing, would yield stronger correlation with absolute torque than unscaled G. Twelve-healthy males performed isometric elbow extension across a range of torque magnitudes (from 5 Nm until 60% MVC). G of the triceps brachii was measured using shear wave elastography during each trial. Cross-sectional area (CSA) and muscle thickness (MT) of the triceps brachii were measured at rest. Scaled G was calculated as a product of G and CSA or MT (“G-CSA” and “G-MT”, respectively). Within-individual linear regressions were conducted between absolute torque and the three force indicator variables. The regression slopes’ coefficient of variation (CV) was calculated for each indicator across individuals. Between-individual correlation coefficients were calculated, after pooling all data across individuals into a single regression analysis for each indicator. Linear regression found that inter-individual slope variation increased in the following order: G-CSA, G-MT, and unscaled G (CV = 0.15, 0.18, and 0.29, respectively). Pooled-individual correlation coefficients were significantly higher in G-CSA and G-MT than in unscaled G (r = 0.948, 0.924, and r = 0.783, respectively). These results suggest that muscle size-scaled G may be more appropriate than unscaled G when comparing shear moduli across individuals.

Modeling within-household associations in household panel studies

Household panel data provide valuable information about the extent of similarity in coresidents’ attitudes and behaviours. However, existing analysis approaches do not allow for the complex association structures that arise due to changes in household composition over time. We propose a flexible marginal modeling approach where the changing correlation structure between individuals is modeled directly and the parameters estimated using second-order generalized estimating equations (GEE2). A key component of our correlation model specification is the “superhousehold”, a form of social network in which pairs of observations from different individuals are connected (directly or indirectly) by coresidence. These superhouseholds partition observations into clusters with nonstandard and highly variable correlation structures. We thus conduct a simulation study to evaluate the accuracy and stability of GEE2 for these models. Our approach is then applied in an analysis of individuals’ attitudes towards gender roles using British Household Panel Survey data. We find strong evidence of between-individual correlation before, during and after coresidence, with large differences among spouses, parent–child, other family, and unrelated pairs. Our results suggest that these dependencies are due to a combination of nonrandom sorting and causal effects of coresidence.

Behavioral repeatability and choice performance in wild free-flying nectarivorous bats (Glossophaga commissarisi)

Animal individuals show patterns of behavior that are stable within individuals but different among individuals. Such individual differences are potentially associated with differences in foraging efficiency and in fitness. Furthermore, behavioral responses may be correlated in specific suites of so-called behavioral syndromes that are consistent across different contexts and with time. Here, we present a field investigation on individual differences between wild, free-flying nectarivorous bats (Glossophaga commissarisi) in the foraging context. We further investigated how individual differences affect choice performance, and we examined their interdependence within hypothesized behavioral syndrome structures. Free-ranging bats were individually identified as they visited an array of 24 artificial flowers with nectar of high or low sugar concentration. We found that three behavioral measures of foraging behavior were individually stable over the two-month observation period. We investigated the link between individual behavioral measures and measures of choice performance using generalized linear mixed models. Individual measures of choice performance showed significant repeatability, and we found evidence that bats making more visits per bout tend to be slower in learning to avoid unprofitable flowers. We used a multi-response generalized linear mixed model to estimate between-individual correlations and compare hypothesized syndrome structures. There were no clear patterns of between-individual correlations among the behavioral measures in our study, despite the measures exhibiting significant repeatability. This may indicate that foraging behavior depends on multiple individual behavior dimensions that are not adequately described by simple models of behavioral syndromes. Nectar-feeding bats, like other animals including humans, have their own peculiar ways of consuming food that differ among individuals of the same species. We characterized the feeding habits of individuals in a population of wild, free-flying bats that were trained to gather nectar from computer-automated artificial flowers in a Costa Rican rainforest. Individual bats responded to the experimental conditions in different ways, but consistently over the two-month observation period. For example, some bats frequently returned to the same flowers, while others tended to meticulously probe most flowers they encountered before returning to a previously visited location. Interestingly, bats also consistently differed in how fast they learned to avoid flowers with dilute nectar and the faster learners were the bats that made only a few visits on each feeding trip. This suggests that individual foraging strategies might be associated with differences in foraging efficiency.

Don't poke the bear: using tracking data to quantify behavioural syndromes in elusive wildlife

Animal personality traits and the emergence of behavioural syndromes, i.e. between-individual correlation of behaviours, are commonly quantified from behavioural observations in controlled environments. Subjecting large and elusive wildlife to controlled test situations is, however, rarely possible, suggesting that ecologists should exploit alternative measures of behaviours for quantifying differences between individuals. Our goal was to test whether movement and space use data can be used to quantify behavioural syndromes in the wild. We quantified six behaviours from GPS and dual motion sensor tracking devices of 46 adult female brown bears followed in southcentral Sweden over the summer and early autumn. As well as daily travel distance, an indicator for activity, and daily displacement, an indicator for exploration, we quantified four behaviours that increase a bear's likelihood of encountering humans and could thus serve as indicators for boldness: diurnality, selection for roads and selection for two open habitat types, bogs and clearcuts, with low lateral cover. We tested (1) whether behaviours showed repeatable between-individual variation (animal personality) and (2) whether behaviours were correlated between individuals and thus formed a behavioural syndrome. Repeatability of behaviours ranged from 0.16 to 0.61 confirming between-individual variation in movement, activity and space use. A multivariate mixed model revealed significant positive correlations between travel distance, displacement and diurnality, suggesting the existence of an activity–exploration and potentially partial boldness syndrome in our bear population. Selection for exposed or human-frequented habitats were uncorrelated with the activity–exploration syndrome and with each other, albeit there was a trend for stronger road avoidance by bears that readily used clearcuts. We show that large tracking data sets can be used to quantify between-individual correlation in spatial behaviours. We suggest that delineating behavioural types from wildlife tracking data will be of increasing interest because of the importance of animal personality for ecological processes, wildlife conservation and human–wildlife coexistence.

Depression, anxiety and delinquency: Results from the Pittsburgh Youth Study

PurposeThe main aim of this research is to investigate to what extent within-individual changes in anxiety and depression are related to within-individual changes in theft and violence. MethodsThe youngest sample of the Pittsburgh Youth Study (PYS), a prospective longitudinal survey of 503 boys followed up from age 7 onwards, was analyzed. Depression and anxiety were measured for boys from ages 11 to 16 as were moderate and serious forms of self-reported theft and violence. A hierarchical linear random effects model was used to investigate anxiety and depression as potential causes or outcomes of these forms of delinquency. ResultsThe results showed that the between-individual correlations were consistently higher than the corresponding within-individual correlations, and provided little evidence to discern the directionality of the potential relationships between depression, anxiety and delinquency. Using a random effects approach, there was limited evidence that prior depression or anxiety was related to later offending, but there was evidence that offending (particularly theft and serious violence) was associated with later increases in anxiety, and to a lesser extent depression. ConclusionsThis study indicates that depression and anxiety were outcomes of offending. If replicated, this would suggest that evidence-based interventions which reduced offending would have a desirable influence in reducing depression and anxiety. However, interventions for depression should still form part of responsive interventions. More research which explores within-individual changes in longitudinal studies with repeated measures is needed.

Long working hours and sickness absence\u2014a fixed effects design

BackgroundWhile long working hours seem to lead to impaired health, several studies have also shown that long working hours are related to lower levels of sickness absence. Previous studies on the relationship between long working hours and sickness absence have compared those who work long hours to those who do not, looking only at between-individual correlations. Those results might therefore reflect relatively stable differences between employees who typically work long hours and employees who typically do not. The aim of the present study is to examine within-individual correlations between long working hours and sickness absence.MethodsRecords from the Human Resources department in a large Norwegian hospital from 2012 to 2015 provided objective data on both working hours and sickness absence. Two analyses were performed: a prospective cohort analysis to replicate the results from previous between-individual analyses and a second analysis of within-individual correlations using a fixed effect design.ResultsIn line with existing research, both between-individual and within-individual analyses showed a negative relationship between long working hours (> 48 h/week) and short-term sickness absence (1–8 days) and no significant difference in incidence of long-term sickness absence (> 8 days).ConclusionsThe results indicate that the negative relationship between long working hours and sickness absence is not due only to relatively stable individual differences between those who typically work long hours and those who do not. The results from both analyses therefore still contrast with previous research showing a negative relationship between long working hours and other health indicators.

Environment-dependence of behavioural consistency in adult male European green lizards (Lacerta viridis).

Understanding the background mechanisms affecting the emergence and maintenance of consistent between-individual variation within population in single (animal personality) or across multiple (behavioural syndrome) behaviours has key importance. State-dependence theory suggests that behaviour is ‘anchored’ to individual state (e.g. body condition, gender, age) and behavioural consistency emerges through behavioural-state feedbacks. A number of relevant state variables are labile (e.g. body condition, physiological performance) and expected to be affected by short-term environmental change. Yet, whether short-term environmental shifts affect behavioural consistency during adulthood remains questionable. Here, by employing a full-factorial laboratory experiment, we explored if quantity of food (low vs. high) and time available for thermoregulation (3h vs. 10h per day) had an effect on activity and risk-taking of reproductive adult male European green lizards (Lacerta viridis). We focussed on different components of behavioural variation: (i) strength of behavioural consistency (repeatability for animal personality; between-individual correlation for behavioural syndrome), (ii) behavioural type (individual mean behaviour) and (iii) behavioural predictability (within-individual behavioural variation). Activity was repeatable in all treatments. Risk-taking was repeatable only in the low basking treatments. We found significant between-individual correlation only in the low food × long basking time group. The treatments did not affect behavioural type, but affected behavioural predictability. Activity predictability was higher in the short basking treatment, where it also decreased with size (≈ age). Risk-taking predictability in the short basking treatment increased with size under food limitation, but decreased when food supply was high. We conclude that short-term environmental change can alter various components of behavioural consistency. The effect could be detected in the presence/absence patterns of animal personality and behavioural syndrome and the level of individual behavioural predictability, but not in behavioural type.

Body size predicts between-individual differences in exploration behaviour in the southern corroboree frog

Proximate mechanisms underpinning between-individual variation in repeatedly expressed behaviours (animal personality) remain poorly understood. Recent theoretical models have focused on the concept of adaptive state-dependent behaviour, proposing that repeatable differences in behaviour emerge due to individual differences in repeatable state variables such as metabolic rate, age, sex or body size. Few studies have attempted to investigate the effect of state on personality empirically, and evidence for links between individual variation in state and personality remains equivocal. We used a captive colony of southern corroboree frogs, Pseudophryne corroboree , to (1) test for innate, temporally repeatable behavioural differences (animal personality) along the activity, exploration/avoidance and boldness/shyness behavioural axes, (2) test for behavioural syndromes (between-individual correlations between behavioural traits) and (3) determine whether behavioural traits are correlated with body size at the between-individual level. Individuals exhibited repeatable variation along all three behavioural axes, but between-individual correlations did not deviate from zero, providing no evidence for behavioural syndromes. Body size explained 40% of the between- individual variance in exploration behaviour, with larger frogs exhibiting greater mobility and travelling further in a novel environment. These associations indicate that there is potential for innate body size variation in P. corroboree to act as an important state variable underpinning repeatable between-individual behavioural differences. Future research may test this idea experimentally. Continued investigation of state-dependent individual behaviour in P. corroboree and other animals is likely to provide important insights into the proximate causes of animal personality.

Phenotypic correlations capture between-individual correlations underlying behavioral syndromes

The presence of variation in behavior on the between-individual level is considered the hallmark of personality. In contrast, behavioral syndromes are commonly recognized when documented on the phenotypic level, which is a mix of between-individual and residual (within-individual) correlations. Phenotypic and between-individual correlations need not align, and investigations on different levels of the same syndrome may hence lead to opposite inferences. Between-individual correlations, arguably, provide stronger evidence for an intrinsically determined behavioral syndrome than phenotypic correlations. We compiled 109 literature estimates of between-individual and phenotypic correlations between behaviors from 30 studies, performed on 22 species covering a wide range of taxa. Contrary to our expectation, the phenotypic correlation in behaviors was, on average, a reasonable predictor of the between-individual correlation in terms of magnitude and sign. Although our finding does not warrant the use of the phenotypic correlation as a suitable approximation of the between-individual correlation for any particular study, it does suggest that the phenotypic correlations used to infer the majority of behavioral syndromes to date have provided a reasonable characterization of syndrome associative strength. Aspects of personality often correlate and are then said to form a behavioral syndrome. Syndromes are interesting because they signal that multiple behaviors may share causal drivers. Most syndromes to date are based on correlations of measured behaviors (phenotypes), but any pattern on the phenotypic level is likely influenced by uncontrolled associations between the focal behaviors which are expected to reduce the phenotypic correlation relative to the correlation on the underlying individual level. If so, behavioral ecologists may have underestimated the associative strength of behavioral syndromes. Based on over 100 published estimates of individual-level and phenotypic correlations between aspects of personality, we show that the phenotypic correlation provides a reasonable estimate of the magnitude and sign of individual-level correlations.

Exploration and sociability in a highly gregarious bird are repeatable across seasons and in the long term but are unrelated

Personality traits and behavioural syndromes are often assumed to relate to life history strategies and lifetime fitness variation and hence may be generally under selection. Key in this regard is the, often untested, assumption that individual differences in (correlated) behaviours are maintained across contexts and over an individual's lifetime. Here, we tested this assumption, using a population of 30 captive male starlings, Sturnus vulgaris, a highly gregarious avian species. We repeatedly assayed novel environment exploration and different aspects of sociability towards a female conspecific, across seasonal contexts (spring and autumn) and across a 2-year period, which represents a substantial portion of a starling's life span. We found that, regardless of plasticity at the population level, both exploration behaviour and sociability traits investigated were moderately repeatable across seasons and years, with no significant differences between repeatability estimates over different timescales. However, no evidence was found for significant between-individual correlations between the investigated traits, including different aspects of sociability. Taken together, our results provide empirical evidence that exploration and sociability are personality traits that are stable across seasons and in the long term but do not form behavioural syndromes. Given the recent evidence that personality traits are often heritable, the traits assessed in our study might have the potential to evolve independently under selection. This long-term consistency in exploration and sociability might have important implications for the social organization within complex social environments and influence a wide variety of ecologically relevant processes.

Among-year variation in the repeatability, within- and between-individual, and phenotypic correlations of behaviors in a natural population.

When mean behaviors correlate among individuals, they form behavioral syndromes. One way to understand the evolution of such a group-level phenomenon is to compare horizontally patterns of correlations among populations (or species) or follow longitudinally the same population over years in the light of parallel differences in the environment. We applied the longitudinal approach to 8-year field data and analyzed phenotypic correlations, and their within- and between-individual components, among three behaviors (novelty avoidance, aggression, and risk-taking) in male collared flycatchers, Ficedula albicollis, in a meta-analytic framework. The phenotypic correlation between novelty avoidance and aggression varied heterogeneously (it was positive in some years, while it was negative in other years), while the other pair-wise correlations were consistently positive over the study period. We investigated four potential socio-ecological factors, and found evidence that the among-year alterations in the demographic structure of the population (density, age composition) can be responsible for the heterogeneous effect sizes. Comparing within- and between-individual correlations across pairs of traits, we found that the correlation between aggression and risk-taking at the among-individual level was the strongest suggesting that this relationship has the highest potential to form a behavioral syndrome. Within-year repeatabilities varied among traits, but were systematically higher than between-year repeatabilities. Our study highlights on an empirical basis that there can be several biological and statistical reasons behind detecting a phenotypic correlation in a study, but only few of these imply that fixed behavioral syndromes are maintained in a natural population. In fact, some correlations seem to be shaped by environmental fluctuations.