Abstract Metabolism and activity should be tightly linked; metabolism provides energy for activity, and general activity may often affect the rate at which animals acquire resources that drive metabolism. However, little is known about the ways metabolism and activity covary in ecologically germane contexts in natural populations. To explore this in more detail, we collected repeated measures of instantaneous metabolic rates and associated changes in activity levels in house sparrows (Passer domesticus) in response to shifts between stimulated and unstimulated contexts. Using a reaction norm approach and mixed-effects models, we examined among-individual variation in metabolism during inactivity (intercept/resting metabolic rate; RMR) and the rate of change in metabolism associated with changes in activity and time since a transition in the level of external stimuli (slope/metabolic plasticity). Individuals exhibited significantly repeatable differences in both inactive metabolism and metabolic slopes with respect to time (but not activity) during stages of the experiment designed to ramp up or ramp down responses. Individuality persisted after accounting for potential causes of variation. Our results indicate consistent differences in how individuals alter metabolic rate across time after changes in stimulation, raising questions about underlying processes contributing to and maintaining such variation. Similar analyses incorporating physiological and life history traits may reveal important relationships that allow for the maintenance of individual variation in metabolic plasticity. Our framework of treating metabolic rate as a reaction norm across time and activity uncovers ecologically relevant individual variation that holds promise for further investigations into the context-dependent link between metabolism and behavior.

Individuals face a trade-off between allocating resources to reproduction or self-maintenance, yet the drivers of the existence and strength of such trade-off have been hard to determine. Environmental conditions are thought to play a crucial role, as long-lived species are predicted to favour more precautionary life-history strategies in variable environments. However, empirical evidence remains limited. Using long-term monitoring of two black-browed albatross Thalassarche melanophris populations, we investigated variation in life-history strategies under contrasting environmental conditions, through reproductive senescence. In more variable environments, individuals displayed generally slower life histories (i.e., slow, late-onset senescence) and greater among-individual variation in life-history strategies. Interestingly, earlier and faster reproductive senescence correlated with higher lifetime reproductive success regardless of environmental variability, suggesting that either faster life histories incur higher fitness or successful reproduction accelerates reproductive senescence. These findings reveal how environmental variability shapes life-history strategies, highlighting potential responses to increasing environmental variability in a changing world.

Escape tendency and mobility behaviour of four alpine rodents do not change with altitude

Differences in growth rate and metabolism between individual animals can influence survival, reproduction and competitive social dynamics during vulnerable life stages. Acknowledging this, among-individual variability has received growing research attention. Less attention has been given to temporal variability within individuals, which can also be substantial and influence individual-level performance through time. Here, we used the estuarine fish Galaxias maculatus to understand among- and within-individual variabilities in metabolism, their relative contributions to growth and their thermal dependence. We collected five repeated measurements of metabolism (standard, routine and maximum) and mass across five months and 160 fish assigned to either 18℃ (typical summer temperature) or 23℃ (challenging summer temperature). Among-individual variability in metabolic parameters was generally low, generating low trait repeatability, and did not differ between temperature treatments. The percentage of fish achieving reproductive maturity was higher at 18℃ (98%) than 23℃ (62%). The growth rate was also higher at 18℃ and varied significantly among individuals. Individual growth rate was not correlated with standard or routine metabolic rates, but was positively correlated with maximum metabolic rate. We conclude that growth and metabolic rates are highly plastic over time, responding to a suite of influences including temperature, life stage and, in some circumstances, each other. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.

Aquatic environments are often thermally variable, and ectotherms may select thermal habitats based on their physiological state. However, it remains unclear whether immune reactions and sickness behaviours, including behavioural fever or chill, drive temperature preference and what role natural temperature shifts play in thermal choice. We used a shuttle box system to determine temperature preference in wild-caught pumpkinseed sunfish, Lepomis gibbosus, that were: (i) naturally co-infected by parasitic helminths; (ii) experimentally infected with trematodes, or (iii) immune challenged through lipopolysaccharide (LPS) endotoxin injection. We found no significant differences in preferred temperatures or activity levels based on parasite infection intensity, experimental trematode exposure or LPS injection, despite significant among-individual variation in temperature preference. However, LPS-injected fish caught in 2021 preferred higher temperatures compared with those caught in 2022, reflecting warmer water temperatures experienced in June and July of 2021. Additionally, fish from the natural infection experiment captured over three weeks in July 2021 showed increased preferred temperatures that paralleled rising water temperatures in their natural habitat. Our results suggest that variability in temperature preference reflects natural temperature shifts rather than infection status. Ultimately, this finding enhances our understanding of individual microhabitat choice while challenging the concept of behavioural fever/chill as a thermoregulatory strategy in sunfish. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.

Consistent among-individual differences in the timing of daily activities are referred to as chronotype. While chronotype has been well studied in humans and some laboratory animals, field-based research remains scarce, particularly in insects. We studied an individually tagged population of crickets (Gryllus campestris) in a meadow in northern Spain. Using a network of 140 video cameras, we quantified the daily onset and cessation of activity, predator attacks, mating events and daily mortality in adults. We found small but consistent among-individual variation in both activity onset and cessation. Early onset of activity was associated with higher probability of mating, lower probability of being attacked and reduced mortality. Ceasing activity early had less pronounced fitness associations but was associated with higher mortality risk. This suggests that the timing of daily activity patterns is under selection and may reflect variation in individual quality. Greater within-individual variability in chronotype, which has been hypothesized to reflect adaptive flexibility, was associated with higher mating frequency, an increased probability of being attacked, but a lower overall mortality risk. Our findings provide rare evidence of chronotype consistency in a wild insect and highlight the potential fitness consequences of temporal behavioural variation.

Temporal consistency and behavioural variation: a long-term study of exploration and activity in Microcebus murinus

Abstract Timing is an essential component of the phenotype, influencing individual fitness and shaping ecological interactions. Although variation in the timing of activity can be substantial, the degree to which this variation stems from within-individual variation vs. among-individual variation is poorly understood and represents a key knowledge gap with distinct ecological and evolutionary implications. Within-individual variation in timing may allow individuals to adjust to unfamiliar or shifting cues, while consistent among-individual variation in timing may lead to maladaptive behavior in novel conditions (e.g., light pollution or novel predators). We estimated activity onset from 821 captures of 254 individual white-footed mice, Peromyscus leucopus, across the upper Midwest, Northeast, Mid-Atlantic, and southern Appalachia regions of the United States. In each locale, we quantified the total phenotypic variation in activity onset and partitioned this into its within- and among-individual components. We then examined whether environmental factors, such as variable climate or habitat structure, influenced these components of variation. We found that within-individual variation in timing consistently exceeded among-individual variation, and individuals from populations at cooler, drier sites with seasonal fluctuations in precipitation and temperature exhibited more within-individual behavioral variation than mice at warmer, wetter sites with less seasonal change. Neither within- nor among- individual variation was significantly associated with habitat structure heterogeneity. Our results reveal that most variation in timing across this species’ range derives from within-individual variation and suggests that individuals may have the capacity to adjust the timing of activity onset in response to local conditions.

Abstract Animal personality, characterized by consistent individual variation in behavioral expression, and behavioral syndromes, underlying correlations between behaviors, have been studied in numerous vertebrate and invertebrate species, including ants. Yet, research on the evolutionary origins of this (co)variation has progressed more slowly than studies showing its occurrence, partly due to varying methodologies preventing comparisons among species. In this study, we applied a comparative framework to investigate patterns of average behaviors and behavioral variation in four ant species belonging to two different subfamilies: Formica fusca and Camponotus aethiops (Formicinae), Aphaenogaster senilis and Messor barbarus (Myrmicinae). We analyzed four behaviors reflecting the responses of ants in foraging or social contexts: exploratory activity, reaction-to-prey, reaction-to-brood, and sociability. All traits except sociability showed moderate to high repeatability in each species. When comparing species for each personality trait, in terms of both average behavioral expression and among-individual variance, we found evidence that phylogeny possibly influences these parameters in the reaction-to-prey test, with closely related species showing more similar behavioral profiles. Other differences among species appear to stem from ecological differences. Evidence of behavioral syndromes was found only in A. senilis, with clear correlation between reaction-to-brood and reaction-to-prey, as well as between reaction-to-brood and exploratory activity. Taken together, our findings suggest that our protocol is robust for assessing personality traits across different ant species and that phylogeny may influence the expression of inter-individual differences in certain behavioral traits. These results call for further studies exploring the evolutionary basis of animal personality within a comparative framework.

The persistence of animal personalities within populations contradicts the optimality principle, assuming a single optimal trait value for all individuals. With the advance of research on the maintenance of animal personalities, 2 types of data have emerged as particularly needed: (i) longitudinal data, allowing to distinguish population-level and cohort-level variance in personality traits, and (ii) estimates of additive genetic variance in personality traits and their genetic correlations with life history traits. While longitudinal studies are beginning to emerge, most research to date has relied on behavioral measures at a single life-stage, limiting our understanding of the interplay between ages and genotypes. To address this gap, we employed a 3-generations pedigree design in a captive guppy (Poecilia reticulata) population, measuring boldness, a behavioral trait showing among-individual variance, at different stages of ontogeny and collecting long-term survival data. This design enabled us to investigate the genetic contribution to variance in boldness and survival and examine how it changes across the lifespan in the behavioral trait. Our results show that boldness decreased with age. Further, we found support for phenotypic covariance between boldness and survival, resulting in a negative association between the traits. Additive genetic effects contributed to both boldness and survival, and we found negative genetic correlation between those traits, in line with POLS scenario. Lastly, average boldness was higher in males than females, while the contribution of additive genetic variance did not differ across sexes. Our findings highlight the complex, dynamic interplay of age, genotype, and sex in shaping individual behavior.

Abstract Animal personality, defined as consistent individual differences in behaviour across time and contexts, can significantly influence fitness and survival. However, the expression and stability of personality traits across ontogeny remain poorly understood, particularly in species undergoing complex life history transitions. Here, we examined behavioural consistency in juvenile two-spotted goby ( Pomatoschistus flavescens ) during the critical shift from a pelagic to a benthic lifestyle. Using two standardised assays—the emergence test (boldness) and open field test (exploration)—we assessed behavioural repeatability across repeated trials in a laboratory setting. Across most behavioural measures, we found low repeatability, with within-individual variance exceeding among-individual variance. Correlation analyses revealed consistent behavioural responses within contexts (e.g., among boldness measures), but no significant cross-context correlations, indicating that boldness and exploration do not form a behavioural syndrome in juveniles. These results contrast with findings in adult gobies and suggest that stable personality traits may not be present at this developmental stage. We propose that the observed behavioural flexibility may be adaptive, allowing juveniles to respond rapidly to the variable environmental and social conditions encountered during post-settlement. This ontogenetic plasticity may confer survival advantages in dynamic reef environments. Our findings underscore the importance of incorporating life stage and developmental context when investigating animal personality and behavioural organisation.

Theoretical and empirical studies agree that populations harbour extensive among-individual variation in phenotypic plasticity, but the mechanisms generating and maintaining this variation are often unknown. Endocrine systems, which can change plastically in response to environmental variation, may be shaped by natural selection, but their evolution requires heritable variation. It is currently unknown if endocrine plasticity in response to environmental challenges is heritable. We investigated this question in house sparrows, Passer domesticus, by testing glucocorticoid responsiveness to food restriction. We alternated restricted (70% of individual daily food intake) and adequate (110%) treatments twice, drawing blood samples at the end of each treatment. Based on glucocorticoid responsiveness, we classified individuals into high-plasticity, low-plasticity and medium (control) groups by selecting the 20 most responsive, least responsive and random individuals, respectively. We transferred these groups into separate aviaries and let them reproduce. In the next generation, we measured hormonal responsiveness using identical methods. Using a cross-foster design and quantitative genetic models, we partitioned the heritability of glucocorticoid responsiveness into genetic and environmental components. We found moderate heritability (h2 > 30%) of glucocorticoid plasticity in response to food availability. The environmental and residual variances of glucocorticoid responsiveness were smaller than those for the intercept. Our findings provide empirical evidence for the existence of heritable individual variation in glucocorticoid plasticity, highlighting its potential to evolve under natural selection, particularly in dynamic and rapidly changing environments.

Abstract Consistent individual differences in behavior are often expressed as personality. One such trait, boldness—reflecting an individual’s propensity to take risks—can be associated with Individual Foraging Site Fidelity (IFSF), the degree to which an individual repeatedly uses the same foraging site. In seabirds, within-pair correlations in boldness and foraging specialization may shape parental effort, enhancing reproductive success and sustaining among-individual variation. These observations suggest that a behavioral syndrome linking boldness and IFSF may manifest at the pair level as within-pair correlations across behavioral contexts; however, no study has simultaneously examined behavioral syndromes and within-pair correlations. We investigated boldness and IFSF in streaked shearwaters ( Calonectris leucomelas ) breeding on Awashima Island, Japan. Boldness was assessed using a novel object test, while IFSF was quantified through GPS tracking of foraging routes. We evaluated the boldness-IFSF syndrome, within-pair correlations in boldness and IFSF, and whether boldness similarity predicts chick growth. We found a correlation between boldness and IFSF, and partners were similar in both traits. Moreover, greater within-pair similarity in boldness was positively associated with chick growth rate. These findings could arise through three non-mutually exclusive mechanisms: adaptive partner selection, post-pairing behavioral adjustment, and non-adaptive partner selection driven by confounding factors. We could not demonstrate assortative mating based on behavioral traits or identify the mechanisms by which within-pair similarity might influence reproductive success. Nevertheless, our findings underscore the importance of within-pair behavioral correlations and highlight the value of integrating these previously distinct concepts to facilitate new discoveries.

Animal personality variation is characterized by among-individual differences in behavior that are consistent across ecological contexts and over time. However, processes influencing the amount of personality variation are not well understood. In this study, we tested 1 hypothesized mechanism through which variation in personalities may be maintained: spatial variation in natural selection. Through laboratory behavioral assays, we demonstrated that 2 personality traits—exploration and risk taking—are moderately repeatable for wavy turban snails, Megastraea undosa (mean repeatability values = 0.320 and 0.297, respectively). We also found that there could be up to a 1.7-fold difference in among-individual variation in behavior for different populations. We next measured natural selection on these behavioral traits by experimentally transporting assayed snails to field populations in a mark-recapture study to examine the relationships between behavioral traits and growth and survival. We studied 4 populations: 2 that had an abundance of slow-moving predators (whelks, sea stars) and 2 where slow-moving predators were absent and the major predators were fast-moving species (lobsters). Selection on behavioral traits varied significantly among local populations. Depending on location, patterns of selection could be predominantly stabilizing, disruptive, or correlational. Fitness surfaces were not necessarily similar for local populations with similar predator communities, and nearby locations could have strikingly different patterns of selection. Behavioral tendencies that were associated with high fitness in 1 population could be neutral or associated with low fitness in a nearby population. Such effects likely contribute to maintaining variation in animal personality within the broader population.

BackgroundAmidst the current biodiversity crisis, amphibians are particularly endangered by the emergence of infectious diseases. The skin disease chytridiomycosis is caused by the fungi Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal), which may interact with bacterial symbionts present on the amphibian epidermis. Extensive research has explored the interactions between the amphibian microbiota and Bd; yet, little is known about its interactions with Bsal. In this paper, we used the ribbed newt (Pleurodeles waltl), a model species displaying pronounced among-individual variation in response to Bsal, to (1) determine whether susceptibility to Bsal and individual microbiota vary between source groups; (2) test whether susceptibility to Bsal can be predicted from skin microbiota before exposure and (3) quantify microbiota volatility over time to determine whether Bsal infection intensity and chytridiomycosis severity correlate with the magnitude of shifts in bacterial communities caused by Bsal exposure.ResultsOur results demonstrate that newts of different origin harbor distinct microbiota even under uniform rearing conditions. We show that Bsal infection intensity and disease severity cannot be predicted from the diversity, structure, or composition of the skin microbiota of P. waltl. Instead, a strong relation between newts’ source group and their response to Bsal suggests that other factors might underpin among-individual variation in Bsal susceptibility in this species. Moreover, our results indicate that the intensity of early Bsal infection and longer-term severity of chytridiomycosis do not correlate with the magnitude of microbiota change following Bsal exposure.ConclusionThese results demonstrate a limited involvement of the microbiota in Bsal dynamics in P. waltl, suggesting that other mechanisms contribute to individual Bsal susceptibility. Further research on the relation between chytrid pathogens and their amphibian hosts will be instrumental to improve the conservation of the most endangered vertebrate class on earth.Graphical Supplementary InformationThe online version contains supplementary material available at 10.1186/s42523-025-00485-x.

Animal personalities are characterized by within-individual consistency linked to among-individual variability. Personality expression is often dependent on major life history events such as mating and the onset of reproduction. Here, we hypothesized that in facultatively polyandrous animals, multiple mating increases the females' assets (i.e., residual reproductive value - RRV), due to direct and/or indirect benefits. Based on the predictions of the asset protection principle, higher RRV should promote behaviors that reduce the risk of fitness loss and hence mediate behavioral repeatability displayed in groups. We tested our hypothesis in group-living predatory mites, Phytoseiulus persimilis. Predatory mite females were presented with one or two mates, and their postmating repeatability in activity and sociability was evaluated in groups composed of females of the same and mixed mating types. Mating frequency had little effect on activity patterns but pronounced effects on sociability traits. Polyandrous females were on average more sociable as well as more repeatable in sociability than monandrous females. These behavioral shifts reflect strategies to mitigate inter-individual conflicts within groups to enhance asset protection. Our study suggests that the mating frequency can critically influence female personality expression after mating and highlights the importance of considering mate-related variables in animal personality research.

Research Highlight: Allison, A., Conway, C., Goldberg, A., Morris, A., and Hakanson, E. (2025) Seasonal body mass dynamics mediate life-history trade-offs in a hibernating mammal. Journal of Animal Ecology. https://doi.org/10.1111/1365-2656.70160. Body mass is an important proxy of individual state, especially for animals in seasonal environments. Storage of energy, primarily as fat, decouples daily energy expenditure from current food availability, providing flexibility in how animals respond to temporal variability in conditions and resources. Hibernating mammals take this to the extreme among endotherms: By fattening extensively and entering prolonged bouts of energy-saving torpor, they can remain inactive for part of the year, which maximises their chances of survival until the productive season. Allison etal. (2025) used a large data set of annual body mass change to investigate among-individual variation in the extent and timing of pre-hibernation fattening, its ecological co-variates and survival consequences in the endangered northern Idaho ground squirrel (Urocitellus brunneus). Delayed fattening was a cost of reproduction for females. Relatively fat squirrels had increased survival over the hibernation season, which is likely explained by fatter individuals choosing to enter hibernation earlier to avoid predation risk, rather than an energetic constraint imposed by a short growing season. Allison etal. found support for hypotheses relating fattening dynamics to thermal conditions, conspecific density and possibly time limitation, but not to limited food availability or interspecific competition. These results provide insight into state-dependent behavioural decisions about activity in seasonal environments and their consequences for life-histories in an unusual model among endotherms. They are important also for informing management actions to help conserve this endangered species.

BackgroundBehavioral adaptations are considered an important factor of population success in colonizing novel environments. Individuals can be selected for specific behavioral traits during transport, introduction and especially establishment phase of the invasion process. Aside from population level average of behavioral traits, both among- and within individual variability can contribute to achieving behavioral optima for efficiently acquiring resources in new habitats. Here, we study activity/exploration behavioral traits and boldness in a novel insular population of Italian wall lizard (Podarcis siculus) with a known colonizing history and propagule pressure. We apply Bayesian mixed-effects models and variance partitioning to compare the activity/exploration behavioral traits and boldness between ancestral population from Pod Kopište island and novel population from Pod Mrčaru island.ResultsWe found no difference in average levels of activity/exploration behavioral traits (distance moved and angular velocity) or boldness between populations or sexes. Among-individual variance in both behaviors was preserved in novel population, despite small propagule size of ten individuals. Females from ancestral Pod Kopište had substantially lower within-individual variability of distance moved than males. However, females within-individual variability for this trait increased in the novel Pod Mrčaru population, while males remained the same. Females on Pod Mrčaru population also exhibited strong increase in within-individual variability in angular velocity, even surpassing the values denoted for males in that population. In contrast, within-individual variance in boldness did not differ across population by sex groups.ConclusionsOur results show that among-individual behavioral variation can be preserved even in populations founded by small propagule. Our study also demonstrates sexual dimorphism in the within-individual variability of activity/exploration behavioral traits, both within the populations and in the direction and intensity of change in a new environment. Collectively, this study highlights the importance of studying behavioral flexibility in addition to average population or individual behavioral traits and emphasizes the role of females’ activity/exploration in adaptation to new environments.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12983-025-00586-y.

Zebrafish (Danio rerio) are widely used in behavioural neuroscience as a model for studying anxiety-like and stress-related behaviours. However, substantial variability exists within and among individuals, influenced by factors such as sex, age, and environmental conditions, making the interpretation of anxiety-related behaviours challenging. Here we characterized longitudinal patterns of stability and variability in anxiety-like behaviours across individual adult zebrafish and assessed whether distinct behavioural profiles emerged over time. Using the novel tank dive test, we tracked anxiety-related behaviours in zebrafish across multiple time points over a 21-week period (90, 120, and 150 days post-fertilization). Behavioural metrics, including time spent in tank zones, swimming velocity, and immobility, were analyzed for age- and sex-related effects, repeatability, and group variation. Results indicated significant changes in anxiety-like behaviours with age, with fish spending more time in the upper zone and displaying increased swimming velocity over time. While no significant sex differences were observed in zone preference, males exhibited greater within-individual variation in time spent in the lower zone, while females demonstrated higher among-individual variation and repeatability over time. Furthermore, zebrafish were classified into high, medium, and low-anxiety groups based on cumulative behavioural scores, revealing stable individual differences in anxiety-like behaviours. These findings highlight the importance of considering age, sex and both intra- and inter-individual variation when interpreting zebrafish behaviour and provide a foundation for future research exploring selective breeding, anxiety level interactions, and pharmacological modulation of anxiety-related phenotypes.

Zebrafish (Danio rerio) are widely used in behavioural neuroscience as a model for studying anxiety-like and stress-related behaviours. However, substantial variability exists within and among individuals, influenced by factors such as sex, age, and environmental conditions, making the interpretation of anxiety-related behaviours challenging. Here we characterized longitudinal patterns of stability and variability in anxiety-like behaviours across individual adult zebrafish and assessed whether distinct behavioural profiles emerged over time. Using the novel tank dive test, we tracked anxiety-related behaviours in zebrafish across multiple time points over a 21-week period (90, 120, and 150 days post-fertilization). Behavioural metrics, including time spent in tank zones, swimming velocity, and immobility, were analyzed for age- and sex-related effects, repeatability, and group variation. Results indicated significant changes in anxiety-like behaviours with age, with fish spending more time in the upper zone and displaying increased swimming velocity over time. While no significant sex differences were observed in zone preference, males exhibited greater within-individual variation in time spent in the lower zone, while females demonstrated higher among-individual variation and repeatability over time. Furthermore, zebrafish were classified into high, medium, and low-anxiety groups based on cumulative behavioural scores, revealing stable individual differences in anxiety-like behaviours. These findings highlight the importance of considering age, sex and both intra- and inter-individual variation when interpreting zebrafish behaviour and provide a foundation for future research exploring selective breeding, anxiety level interactions, and pharmacological modulation of anxiety-related phenotypes.