Among-individual Correlations Research Articles

Are comparable studies really comparable? Suggestions from a problem-solving experiment on urban and rural great tits

Performance in tests of various cognitive abilities has often been compared, both within and between species. In intraspecific comparisons, habitat effects on cognition has been a popular topic, frequently with an underlying assumption that urban animals should perform better than their rural conspecifics. In this study, we tested problem-solving ability in great tits Parus major, in a string-pulling and a plug-opening test. Our aim was to compare performance between urban and rural great tits, and to compare their performance with previously published problem solving studies. Our great tits perfomed better in string-pulling than their conspecifics in previous studies (solving success: 54%), and better than their close relative, the mountain chickadee Poecile gambeli, in the plug-opening test (solving success: 70%). Solving latency became shorter over four repeated sessions, indicating learning abilities, and showed among-individual correlation between the two tests. However, the solving ability did not differ between habitat types in either test. Somewhat unexpectedly, we found marked differences between study years even though we tried to keep conditions identical. These were probably due to small changes to the experimental protocol between years, for example the unavoidable changes of observers and changes in the size and material of test devices. This has an important implication: if small changes in an otherwise identical set-up can have strong effects, meaningful comparisons of cognitive performance between different labs must be extremely hard. In a wider perspective this highlights the replicability problem often present in animal behaviour studies.

Individual behaviour, growth, survival and vulnerability to hunting in a large mammal.

Humans have exploited wild animals for thousands of years. Recent studies indicate that harvest-induced selection on life-history and morphological traits may lead to ecological and evolutionary changes. Less attention has been given to harvest-induced selection on behavioural traits, especially in terrestrial systems. We assessed in a wild population of large terrestrial mammals whether decades of hunting led to harvest-induced selection on trappability, a proxy of risk-taking behaviour. We investigated links between trappability, horn growth and survival across individuals in early life and quantified the correlations between early-life trappability and horn growth with availability to hunters and probability of being shot. We found positive among-individual correlations between early-life trappability and horn growth, early-life trappability and survival and early-life horn growth and survival. Faster growing individuals were more likely to be available to hunters and shot at a young age. We found no correlations between early-life trappability and availability to hunters or probability of being shot. Our results show that correlations between behaviour and growth can occur in wild terrestrial population but may be context dependent. This result highlights the difficulty in formulating general predictions about harvest-induced selection on behaviour, which can be affected by species ecology, harvesting regulations and harvesting methods used. Future studies should investigate mechanisms linking physiological, behavioural and morphological traits and how this effects harvest vulnerability to evaluate the potential for harvest to drive selection on behaviour in wild animal populations.

Consistent individual differences in cattle grazing patterns

Cattle grazing patterns can have a significant impact on the economic and environmental sustainability of rangelands and cattle production. Grazing patterns appear to be driven by consistent individual differences in cattle, deemed ‘grazing personalities’. Optimal distribution of cattle on rangelands can be manipulated by alteration of the environment, like adding water, supplement, or fencing to targeted sites, but few studies address how these management tools are impacting grazing patterns of individual cattle. Our study sought to fill this gap in knowledge by investigating consistency of grazing patterns among fifty cattle, fitted with GPS collars, across two years given the addition of a water site at higher elevation prior to year two. We used Bayesian multivariate mixed models to evaluate consistency, evidenced by among-individual correlations across years with confidence intervals that do not cross 0. We found that cattle were consistent in all rangeland use metrics across years after the addition of the new water site: elevation (Estimate = 0.38, CI = [0.07,0.64]), slope (Estimate = 0.59, CI = [0.21,0.86]), distance to water (Estimate = 0.38 CI = [0.03,0.67]), distance to supplement (Estimate = 0.42, CI = [0.04,0.72]), distance to loafing sites (Estimate = 0.68, CI = [0.24,0.95]), except for distance traveled (Estimate = 0.16,CI = [−0.18, 0.48]). Cows that ventured higher on rangeland in year one (Estimate = 0.84 CI = [0.59,0.98]) and year two (Estimate = 0.93 CI = [0.79,0.99]) and further from watering sites in year two (Estimate = 0.72 CI = [0.41,0.92]) also had more within-individual variability in their grazing patterns than those that stayed at lower elevation and closer to water. These results indicate differences in plasticity or variability can be linked to grazing personalities. There was strong evidence that temperature affected grazing distribution such that on hotter days, cows clumped closer to water and loafing sites, were found at lower elevation, and traveled less far, not reaching supplement sites that were placed at the higher elevations. Based on results of this study, ranchers should consider the effectiveness of management tools on individual cattle and conduct cost-benefit analyses of tools implemented for specific behavior types of cattle. Cattle with behavior types demonstrating more within-cow variability and more ‘hill-climbing’ will impact how cattle graze especially patchy landscapes where resources are distributed across elevation, thus ranchers could potentially match cattle types to rangeland characteristics.

The role of plasticity, trade-offs, and feedbacks in shaping behavioral correlations

Abstract How behaviors vary among individuals and covary with other behaviors has been a major topic of interest over the last two decades, particularly in research on animal personality, behavioral syndromes, and trade-offs with life-history traits. Unfortunately, proposed theoretical and conceptual frameworks explaining the seemingly ubiquitous observation of behavioral (co)variation have rarely successfully generalized. For example, the “pace-of-life syndrome hypothesis” proposes that behaviors, life-history, and physiological traits should be correlated in a predictable manner. However, these predictions are not consistently upheld. Two observations perhaps explain this failure: First, phenotypic correlations between behaviors are more strongly influenced by correlated and reversible plastic changes in behavior than by among-individual correlations which stem from the joint effects of genetics and developmental plasticity. Second, while trait correlations are frequently assumed to arise via trade-offs, the observed pattern of correlations is not consistent with simple pair-wise trade-offs. A possible resolution to the apparent inconsistency between observed correlations and a role for trade-offs is provided by state-behavior feedbacks. This is critical because the inconsistency between data and theory represents a major failure in our understanding of behavioral evolution. These two primary observations emphasize the importance of an increased research focus on correlated reversible plasticity in behavior—frequently estimated and then disregarded as within-individual covariances.

Jekyll and Hyde: Day–night personality differences in the water scorpionNepa hoffmanni

AbstractDifferences between daytime and night-time behaviors are common in insects due to potential trade-offs between predator avoidance and foraging. While diel effects on average behavior are well documented, it remains unclear whether individuals differ in how they adjust their behavior between daytime and night-time. In this study, we focused on two light dependent behaviors, activity and tonic immobility, in the water scorpion (Nepa hoffmanni). Each individual was assayed for each behavior six times during the day and six times at night. Then, we explored diel differences in (1) average behavior, (2) among-individual variation in behavior, (3) individual rank order of behavioral expression, and (4) the sign and magnitude of behavioral syndromes (among-individual correlations between behaviors). At night, N. hoffmanni was more active and exhibited shorter periods of tonic immobility. Among-individual variation in activity was greater at night than during the day, but among-individual variation in tonic immobility did not differ between day and night. Individuals also differed in their diel plasticity in activity but not in tonic immobility. Thus, both (1) day–night differences in among-individual variation and (2) individual variation in diel plasticity contributed to the light-dependent sign of the behavioral syndromes. Our findings highlight the need for individual-level studies to fully elucidate diel influences on animal behavior. We suggest that day–night differences in behavioral syndromes as well as in selection might be key to behavioral evolution, especially in animals active during both day and night.

Animal personality: a comparison of standardized assays and focal observations in North American red squirrels

As interest in animal personality research grows, methodologies for quantifying consistent among-individual differences in behaviour are expanding. Two of the most common methods for quantifying animal personality are standardized behavioural assays and focal animal sampling. We evaluated whether assays and focals provided similar animal personality measures in a wild population of free-ranging North American red squirrels, Tamiasciurus hudsonicus , by comparing the among-individual correlations of traits between methods. Assays described two behavioural axes—one associated with movement behaviours and another associated with antagonistic behaviours towards conspecifics. Focals described two additional behavioural axes—one associated with movement and territorial behaviours, and another associated with the trade-off between vigilant and feeding behaviours. Although we found evidence of high trait repeatability for behavioural axes measured with assays, both focal behavioural axes had low trait repeatability regardless of whether among-individual differences in the social environment were controlled for (i.e. to account for ‘pseudopersonality’). We also found no among-individual correlations between assay and focal behavioural axes. The lack of correlation between methods may be because the dominant axes of variation differ between methods or because of the low trait repeatability of focals due to high behavioural plasticity. Given this, we conclude that assays will likely remain the mainstay for measuring consistent among-individual differences in behaviour given their ability to standardize for environmental conditions. • We evaluated whether assay and focal observations measure similar behavioural axes. • Assays captured behaviours associated with movement and antagonistic behaviour. • Focals captured behaviours associated with territoriality and vigilance/feeding. • We found no among-individual correlations between methodologies. • Assays remain the mainstay for measuring personality by controlling for environment.

Individual (Co)variation in Resting and Maximal Metabolic Rates in Wild Mice

Basal metabolic rate (BMR) represents the lowest level of aerobic metabolism in a resting, postabsorptive endotherm as measured within the thermoneutral zone. By contrast, maximal metabolic rate (V˙O2max) reflects the upper limit of aerobic metabolism achieved during intensive exercise. As BMR and V˙O2max define the boundaries of the possible levels of aerobic metabolism expressed by a normothermic individual, a key question is whether BMR and V˙O2max are correlated. In the present study, we took repeated paired measurements of thermoneutral resting metabolic rate (RMRt) and V˙O2max on 165 white-footed mice (Peromyscus leucopus). Over a single summer (May–October), repeatability (R ± SE) was low but statistically significant (P<0.05) for both RMRt and V˙O2max (R=0.15±0.08 for RMRt; R=0.27±0.12 for V˙O2max). Willingness to run during the forced-exercise trials was also significantly repeatable (R=0.16±0.08). At the residual level (within individual), RMRt and V˙O2max tended to be positively correlated (re=0.23±0.11, P=0.051), suggesting the presence of correlated phenotypic plasticity. By contrast, RMRt and V˙O2max were significantly negatively correlated at the among-individual level (rind=−0.84±0.40). To the extent that variation in RMRt reflects variation in BMR, the negative among-individual correlation does not corroborate the idea that a costly metabolic machinery is needed to support a high V˙O2max. Future research should investigate the (genetic) relationship between RMRt (and BMR) and other energetically expensive behaviors and activities to better understand how energy is allocated within individuals.

Territoriality in Drosophila: indirect effects and covariance with body mass and metabolic rate

Abstract Territoriality (i.e., defense of a resource) is the outcome of behavioral interactions that can result in selective advantages in many vertebrates and invertebrates. Since territoriality is expressed in a social context, an individuals’ territoriality may change according to the phenotype of the opponents that they are confronted with (termed “indirect effects”). Defending a territory may also confer energetic costs to individuals, which could be reflected in their standard metabolic rate (SMR), a key component of an ectotherms’ energy budget. Here, we measured territoriality using dyadic contests, body mass, and SMR using flow-through respirometry, twice in each of 192 adult male Drosophila melanogaster. Territoriality, body mass, and (whole-animal) SMR were all significantly repeatable. However, essentially all the among-individual variation in SMR was shared with body mass, as indicated by a very strong among-individual correlation (rind) between body mass and SMR. The among-individual correlation between territoriality and SMR also tended to be positive, suggesting the presence of underlying metabolic costs to territoriality. Although indirect effects on territoriality were present but weak (accounting for 8.4% of phenotypic variance), indirect effects on territoriality were negatively and significantly correlated with body mass. This indicates that larger individuals tended to suppress their opponents territoriality. Variation among individuals in their ability to suppress territoriality in others was not associated with their own territoriality or SMR.

Personality and behavioral syndromes in two Peromyscus species: presence, lack of state dependence, and lack of association with home range size

One common theme of adaptive hypotheses for the existence of stable individual differences in behavior (personality) or persistent correlations among behaviors (behavioral syndromes) is an association between intrinsic state (e.g., body size, mass, metabolism) and the behavioral traits of interest. Empiricists are tasked with assessing whether there is an association between intrinsic state and behavior, but the statistical methods to appropriately quantify the among-individual correlation between intrinsic state and behavior have only recently become widely known. We conducted a multiyear study in wild mice of two Peromyscus species (Peromyscus leucopus noveboracensis and Peromyscus maniculatus gracilis) to assess the existence of stable individual differences in four different behaviors (presence of animal personality, as assessed by quantifying repeatabilities) and one measure of intrinsic state (body mass), the degree of association between these four behaviors (presence of behavioral syndromes), and the association of these behavioral traits with body mass using multivariate methods that allowed us to estimate the within-individual (residual) component and the among-individual component. We used standardized behavioral tests to measure struggle time (time spent struggling when removed from the trap) and exploration time (time spent exploring a novel surface) and used open-field trials to measure whether or not an individual entered the open field plus the total time it spent active in the open field. In P. leucopus, we assessed whether coarse but quick methods of assessing animal personality (struggle or exploration time) correlate with behavior using more in-depth estimates obtained by open-field trials. Additionally, we tested the ecological relevance of our personality measures by assessing their association with home range size in P. leucopus. In both species, struggle time, exploration time, and body mass were significantly repeatable and there was a significant among-individual correlation between struggle time and exploration time. However, in both species, there was no evidence for an among-individual correlation between our measure of intrinsic state (body mass) and personality traits. In one species (P. leucopus), we found that individuals that spent more time struggling or exploring a novel surface were also more likely to emerge into an open field and spent more time being active in an open field, but these four behavioral measures were not associated with home range size. Our results suggest among-individual correlation among these different behaviors but no among-individual correlation between behavior and intrinsic state. Over the past 15 years, it has become clear that nonhuman animals exhibit “personalities”: some individuals consistently differ in their behavior, and there is limited flexibility in their behavior when they experience environmental change. Biologists have now been searching for why and how animal personalities exist and their ecological and evolutionary consequences. Some explanations have focused on how condition or “internal state” may cause variation in animal personalities. In this study, we show that two species of wild mice exhibit personalities and that their personality traits are associated with one another, in that mice that were less docile were also more active, exploratory, and bold. We then show that their personality traits were not linked with body mass, suggesting a lack of association between internal state and personality. Our results provide important insights into the causes of personality in nonhuman animals.

Context-dependent trait covariances: how plasticity shapes behavioral syndromes.

The study of behavioral syndromes aims to understand among-individual correlations of behavior, yielding insights into the ecological factors and proximate constraints that shape behavior. In parallel, interest has been growing in behavioral plasticity, with results commonly showing that animals vary in their behavioral response to environmental change. These two phenomena are inextricably linked—behavioral syndromes describe cross-trait or cross-context correlations, while variation in behavioral plasticity describes variation in response to changing context. However, they are often discussed separately, with plasticity analyses typically considering a single trait (univariate) across environments, while behavioral trait correlations are studied as multiple traits (multivariate) under one environmental context. Here, we argue that such separation represents a missed opportunity to integrate these concepts. Through observations of multiple traits while manipulating environmental conditions, we can quantify how the environment shapes behavioral correlations, thus quantifying how phenotypes are differentially constrained or integrated under different environmental conditions. Two analytical options exist which enable us to evaluate the context dependence of behavioral syndromes—multivariate reaction norms and character state models. These models are largely two sides of the same coin, but through careful interpretation we can use either to shift our focus to test how the contextual environment shapes trait covariances.

Environmental complexity during early life shapes average behavior in adulthood

AbstractPersonality has been identified in a range of animal taxa during the last few decades, with important ecological and evolutionary implications. Investigating the effects of environmental factors during early life can provide important insights into the ontogeny of animal personality. We reared newborn mosquitofish, Gambusia affinis, in tanks of different structural complexities, and measured their behavioral traits (i.e., shyness, exploration, and sociability) when they reached sexual maturity. Univariate linear mixed-effects models were fitted to test the effects of environmental complexity and sex on population-average behavior, whereas multivariate models were fitted to quantify behavioral repeatability (i.e., personality) and among-individual correlations (i.e., behavioral syndromes). On average, females were shyer and more social than males, and the fish reared in complex environments were shyer, less explorative, and more social than those reared in open environments. Among-individual differences were consistently large across trials for all behaviors, indicating that personality variation was present in mosquitofish of both sexes reared in different environments. Repeatability did not differ among behaviors, and there were no differences in repeatability in any behavior between sexes or among environments. A negative correlation between shyness and exploration was found in mosquitofish from all treatments at both phenotypic and among-individual levels, with the latter indicating a strong shyness–exploration behavioral syndrome. Our study provides robust evidence that average levels of personality might vary when mosquitofish are raised in different levels of structural complexity during early life.

Behavioural repeatability in the Big Brown Bat, Eptesicus fuscus

Abstract Work on animal personality, characterized by high behavioural repeatability, shows that animals exhibit limits to plasticity, with an individual’s range of variation only covering a portion of the range observed across the entire population. This work is particularly important for understanding the underlying genetic basis of behaviours. While research on behavioural repeatability has spanned many taxa, little work has focused on bats. Our objective was to determine if big brown bats (Eptesicus fuscus) exhibit moderate to high repeatability of different behavioural traits and if behavioural syndromes (among-individual correlations between behaviours) are present. Using a captive population, repeatability was assessed for aggression, exploration, and activity. Moderate repeatabilities were observed for all behaviours, indicating that individuals exhibit consistent behaviour over time. There were no significant correlations among behaviours. Future research aimed at assessing how changes in social structure impact behavioural repeatability will be valuable for understanding behavioural plasticity in social bat species.

Criteria for acceptable studies of animal personality and behavioural syndromes

Criteria for acceptable studies of animal personality and behavioural syndromes

Impacts of caudal autotomy on personality

Caudal autotomy, the voluntary shedding of a tail, is a last-ditch strategy used by many lizard species to escape from predators. There are several costs associated with caudal autotomy that may cause lizards to change their behaviour during tail regeneration. These behavioural changes may be dependent upon individual differences in response to autotomy (e.g. trait or state-dependent differences) and/or the degree of tail loss, as many lizards have the capacity to only partially shed their tail which probably entails fewer costs relative to complete autotomy. However, no study, to our knowledge, has considered how caudal autotomy, or the extent of autotomy, affects individual behavioural variation. Accordingly, we investigated the effects of both partial and complete tail loss on individual behavioural variation in delicate skinks, Lampropholis delicata. We found that lizards that experienced complete tail loss, on average, became less active and explorative and had much slower sprint speeds following autotomy. These lizards also became more predictable and consistent in their behaviour, exhibiting a notable decrease in their within-individual behavioural variance. In contrast, we did not see any significant behavioural effects in lizards that experienced partial tail loss. We also found a positive among-individual correlation between activity/exploration and neophilia, but tail loss had no effect on the structure of this syndrome. Our results suggest that complete tail loss may impose effects on traits more closely associated with locomotion and predator escape ability, while also constraining an individual's capacity for differential behavioural expression.

Sex-Specific Among-Individual Covariation in Locomotor Activity and Resting Metabolic Rate in Drosophila melanogaster.

A key endeavor in evolutionary physiology is to identify sources of among- and within-individual variation in resting metabolic rate (RMR). Although males and females often differ in whole-organism RMR due to sexual size dimorphism, sex differences in RMR sometimes persist after conditioning on body mass, suggesting phenotypic differences between males and females in energy-expensive activities contributing to RMR. One potential difference is locomotor activity, yet its relationship with RMR is unclear and different energy budget models predict different associations. We quantified locomotor activity (walking) over 24 h and RMR (overnight) in 232 male and 245 female Drosophila melanogaster that were either mated or maintained as virgins between two sets of measurements. Accounting for body mass, sex, and reproductive status, RMR and activity were significantly and moderately repeatable (RMR: ; activity: ). RMR and activity were positively correlated among () but not within () individuals. Moreover, activity varied throughout the day and between the sexes. Partitioning our analysis by sex and activity by time of day revealed that all among-individual correlations were positive and significant in males but nonsignificant or even significantly negative in females. Such differences in the RMR-activity covariance suggest fundamental differences in how the sexes manage their energy budget.

Characterizing the (co)variance of personality traits in female Trinidadian guppies (Poecilia reticulata)

A fundamental question in behavioural and evolutionary ecology is to what degree individual phenotypic traits are variable over time and across ecological contexts and whether they are correlated with each other, as (co)variation in traits determines in part their responses to selection and rate of evolution. A first step to addressing this question is to characterize variation in behavioural traits within and among individuals and their covariation in particular ecological contexts. Here, we ascertained whether adult female guppies (Poecilia reticulata), descendants of wild guppies in Trinidad, are consistent or repeatable in their behaviour over time, such that they exhibit individual ‘personalities’ or behavioural types, and whether their personality traits are inter-correlated. Using a repeated-measures design, we twice assayed individual females for each of three ecologically important behavioural traits, Exploration (of a novel environment), Sociability (shoaling with conspecifics) and Boldness (risk taking), under standardized laboratory conditions. We characterized the among-individual (co)variance structure of each behavioural trait measure, and estimated their respective repeatability and the among-individual correlations between these traits using mixed-effects models. All three traits were found to be significantly repeatable and thus can be considered personality traits. None of the estimated correlations between all possible pairings of the three traits were significantly different from zero. Although adult female Trinidadian guppies from our study population exhibited repeatable personality traits, these traits were not inter-correlated (i.e. did not constitute a behavioural syndrome) and were thus independent of each other. Our findings have important implications for the maintenance of behavioural variation within populations and trait evolution.

Performance trade-offs in wild mice.

Various aspects of performance (e.g., speed, strength, endurance) are thought to be important determinants of the success of animals in natural activities such as foraging, mating, and escaping from predators. However, it is generally known that morphological properties enhancing one type of performance (e.g., strength) can lead to a reduction in another (e.g., speed). Such performance trade-offs have been quantified at the inter-specific level, but evidence at the individual level remains equivocal. To test for the presence of a performance trade-off, we repeatedly captured a total of 189 wild white-footed mice (Peromyscus leucopus) and measured their grip strength and sprint speed. Using the maximum performance score obtained for each individual across all their repeated tests, we obtained a counter-intuitive (and biased) positive and highly significant phenotypic correlation. Using a bivariate mixed model, we detected a significant negative among-individual correlation between grip strength and sprint speed. By contrast, the within-individual correlation was positive but non-significant, thus illustrating the importance of properly partitioning the correlations at the among- and within-individual levels when testing for the presence of a performance trade-off. This study is one of the first to detect a performance trade-off at the among-individual level in a wild animal population. Such a trade-off may be caused by individual differences in muscle physiology and scapular morphology resulting from genetic differences and/or plastic responses to differential use of the arboreal vs. terrestrial parts of the environment.

No evidence for behavioural syndrome and genetic basis for three personality traits in a wild bird population

Personality traits and their correlations have been shown to be linked with life history strategies and fitness in various species. Among-individual correlations (i.e. behavioural syndromes) between personality traits can affect the evolutionary responses of these traits to environmental variation. Understanding the genetic and ecological determinants of personality traits and their interactions as behavioural syndromes in the wild is thus needed to shed light on the mechanisms shaping their evolution. Partitioning the observed (co)variance in these traits, however, requires large numbers of repeated behavioural measures on many individuals of known relatedness level. In the absence of such data, it is thus often assumed that phenotypic (co)variances inform about (i) underlying among-individual (co)variances (i.e. ignoring within-individual (co)variances) and (2) underlying genetic (co)variances. We tested these assumptions using three personality traits collected during 3 years on a long-term monitored breeding population of collared flycatchers, Ficedula albicollis. We partitioned the observed phenotypic (co)variance of aggressiveness, boldness and neophobia into genetic, permanent environment and parental components, and we estimated the repeatability, and heritability of these traits and their among-individual correlations. All three traits were repeatable between years (at least on the latent scale) but none were heritable. Permanent environment effects explained 15% of the phenotypic variance in aggressiveness, and parental effects explained 25% of the phenotypic variance in neophobia, in line with previous studies in wild populations. The three traits showed phenotypic correlations but no among-individual correlations and no additive genetic covariance. Thus, our results did not support the assumptions that phenotypic covariance reflects behavioural syndromes and genetic covariance. We discuss the reasons for the absence of heritability and among-individual and genetic covariance between these three personality traits in light of the possible selective pressures acting on this population.

Energy metabolism and personality in wild-caught fall field crickets

Standard metabolic rate (SMR) is known to be highly variable across levels of biological organisation (e.g., species, populations, among individuals, within individuals). Some of the variation in SMR can be attributed to factors such as diet, temperature, and body mass, yet much of the residual variation in SMR remains unexplained. Intuitively, we can expect SMR to co-vary with “personality”, but the rapidly accumulating empirical evidence on this topic remains equivocal. The goal of this study was to test for a link between SMR and a behavioural syndrome at the among-individual level in wild-caught fall field crickets (Gryllus pennsylvanicus). Paired measurements of SMR and two behavioural traits were repeatedly taken over a two-month period, thus allowing to estimate the among-individual correlations (rind) separately from the residual (within-individual) correlations. The two behavioural traits (latency to exit a refuge in a novel environment and “freezing” time following a stressful stimulus) were significantly and moderately repeatable and were found to be part of a syndrome, as indicated by a strong and positive among-individual correlation (rind = 0.82 ± 0.27). Yet, only latency to exit was significantly and positively correlated with SMR (rind = 0.45 ± 0.21), suggesting that the link between boldness and SMR may be driven by individual differences in responses to novelty and not to simulated predatory cues. Since we found that bold individuals (short latency to exit) consistently had a lower SMR than shy individuals (long latency to exit), our results go against the pace-of-life syndrome hypothesis. Instead, our results suggest the presence of constrains in the energy budgets of crickets, which generated an allocation trade-off between energy spent on physical activity vs. maintenance costs (SMR).

On the usage of single measurements in behavioural ecology research on individual differences

Since the early 21st century, behavioural ecologists have increasingly focused on the ecology and evolution of repeatable individual differences in behaviour (aka ‘animal personality’). More and more studies have investigated correlations between behaviour(s) and other phenotypic traits, thereby seeking to understand among-individual variation in behaviour from either a proximate or an ultimate perspective. Statistically, such studies require the estimation of among-individual correlations, necessitating study designs where suites of labile traits are repeatedly assayed. Most published studies, by contrast, instead assume that among-individual correlations can be approximated using data of suites of phenotypic traits measured only once. Such studies take an ‘individual gambit’ as they assume that phenotypic correlations between two traits measured once match among-individual correlations. A literature survey shows that this assumption was made in 62% of empirical studies; this is a worrying trend as a mismatch between the research question and the study (or statistical) design can lead to biased conclusions about biological patterns. In this paper we use a visual approach to illustrate these concerns for a broad audience, thereby complementing previous papers using a jargon more suitable for the statistically oriented. We thereby seek to reiterate the notion that reliable answers to any scientific question require specific types of data. While this commentary underlines the importance of spelling out key assumptions in discussions of (suboptimal) data in scientific papers on individuality, our main message is that researchers should typically be able to avoid publishing studies with conclusions heavily hinging on unverifiable assumptions. This is because researchers normally have the freedom to decide a priori to focus their research towards questions for which appropriate data can arguably be collected. Doing so would greatly facilitate our ability to robustly address key questions of biological interest, such as the ecology and evolution of individuality.