Pace-of-life Syndrome Research Articles

Individual behavior, behavioral stability, and pace of life within and among five shrew species

Phenotypic variation in behavior exists among species and populations, as well as among and within individuals. The pace-of-life syndrome hypothesis predicts covariation between life-history strategies, ranging from slow to fast, and behavior, ranging from shy, inactive, and flexible to bold, active, and less flexible. This covariation is expected to exist at multiple hierarchical levels, from the species down to the individual. We predict that fast-lived species will differ in average levels of behavior, and additionally show lower within-individual and among-individual variation than slow-lived ones. Shrews represent a highly suitable model to test these predictions, as they comprise a range of genera which differ tremendously in life-history strategy and metabolism. We performed repeated tests of boldness and aggression on 155 wild-caught individuals of five species of shrews, two species of the slow-lived genus Crocidura, two of the fast-lived genus Sorex, and one of the intermediate-paced genus Neomys. To compare not only average levels of behavior but also its variance components between those groups, we calculated coefficients of variation at within- and among-individual levels. Our results support our first prediction that, following the framework of pace-of-life-syndromes, fast-lived species should exhibit bolder behavior than slow-lived ones. However, our prediction of lower within- and among-individual variation in fast-lived species was not supported. Instead, our data suggest that other ecological factors might influence the expression of behavioral variation in shrew species, such as the variability in habitat choice and differences in anti-predator strategies. The behavior and life history of animals are often structured into so-called pace-of-life syndromes (POLS), with slow-lived individuals being rather shy, inactive, and flexible and fast-lived individuals rather bold, active, and less flexible. Comparing the behavior in five species of shrews, we tested if such a gradient can also be found on the species level. While the average levels of species’ behavior indeed matched their pace of life, their individual behavior and behavioral stability did not. It was rather explained by an interplay of ecological and physiological factors, among them the variability in habitat choices and differences in anti-predator strategies. Our study shows that behavioral variation cannot be explained by just one factor like POLS at different hierarchical levels, but rather by a combination of factors including the animals’ life-history and ecological and physiological background.

Ecosystem consequences of multi-trait response to environmental changes in Japanese medaka, Oryzias latipes.

Intraspecific trait variation has large effects on the ecosystem and is greatly affected by human activities. To date, most studies focused on single-trait analyses, while considering multiple traits is expected to better predict how an individual interacts with its environment. Here, we used a mesocosm experiment with fish Oryzias latipes to test whether individual growth, boldness and functional traits (feeding rate and stoichiometric traits) formed one functional pace-of-life syndrome (POLS). We then tested the effects of among-individual mean and variance of fish functional POLSs within mesocosms on invertebrate community (e.g. zoobenthos and zooplankton abundances) and ecosystem processes (e.g. ecosystem metabolism, algae stock, nutrient concentrations). Stoichiometric traits correlated with somatic growth and behaviours, forming two independent functional POLS (i.e. two major covariance axes). Mean values of the first syndrome were sex- and environment-dependent and were associated with (i) long-term (10 generations; 4 years) selection for small or large body size resulting in contrasting life histories and (ii) short-term (6 weeks) effects of experimental treatments on resource availability (through manipulation of light intensity and interspecific competition). Specifically, females and individuals from populations selected for a small body size presented fast functional POLS with faster growth rate, higher carbon body content and lower boldness. Individuals exposed to low resources (low light and high competition) displayed a slow functional POLS. Higher mesocosm mean and variance values in the second functional POLS (i.e. high feeding rate, high carbon:nitrogen body ratio, low ammonium excretion rate) were associated to decreased prey abundances, but did not affect any of the ecosystem processes. We highlighted the presence of functional multi-trait covariation in medaka, which were affected by sex, long-term selection history and short-term environmental conditions, that ultimately had cascading ecological consequences. We stressed the need for applying this approach to better predict ecosystem response to anthropogenic global changes.

Among-population divergence in personality is linked to altitude in plateau pikas (Ochotona curzoniae)

BackgroundAnimals inhabiting high altitudes consistently show slow life-histories. The pace-of-life syndrome (POLS) hypothesis posits behavioural, physiological and/or morphological traits that mediate the trade-off between current and future reproduction or survival, which have coevolved along a slow-fast life history continuum. Previous studies have shown that the life histories of plateau pikas varied across altitude, high-altitude individuals showed slow pace of life which were characterized by few litters per year with small litter sizes. Thus, we hypothesized that pikas populations at higher altitudes would also express personalities characteristic associated with slow life history, such as high sociability, low activity or aggressiveness. We tested this hypothesis by comparing the activity and docility of three plateau pika (Ochotona curzoniae) populations distributed along an altitudinal gradient of the Tibetan Plateau. We predicted that high-altitude pika would be more docile and less active.ResultsThe behaviour of 556 pikas, from which 120 individuals were measured at least twice, was quantified. We observed that plateau pikas at high altitudes were less active and more docile than pika at lower altitudes. Activity and docility were significantly and negatively correlated in populations from high altitudes but not in populations from low altitudes.ConclusionsOur results support the POLS hypothesis, highlight the existence of personality variation among populations distributed along an altitudinal gradient and emphasise the importance of environmental selection on personality divergence.

Evidence for risk-taking behavioural types and potential effects on resource acquisition in a parasitoid wasp

Consistent individual differences in behaviour, also called behavioural types, are often observed in animal populations and can correlate to form behavioural syndromes. Behavioural types and syndromes have consequences in various fields of ecology, from species interactions to ecosystem services, notably through the role they can play in the slow–fast life history continuum. Individuals differ in their life history strategies along this continuum according to their relative investment in reproduction or survival, and the pace-of-life syndrome (POLS) hypothesis predicts a coevolution between these life history strategies and behavioural types linked to risk taking. In this framework, the aim of our study was thus to investigate the existence of behavioural types and their links with life history traits in the parasitoid hymenopteran Venturia canescens . We considered two behaviours, exploration and activity, that are linked to risk taking and resource acquisition, and three life history traits, longevity, egg load and offspring production rate. We found consistent individual differences in both exploration and activity. However, contrary to the predictions of the POLS, the behavioural types did not correlate with each other or with the life history traits tested. We thus found no evidence of a POLS in V. canescens . However, the existence of behavioural types in female parasitoids may be of utmost importance for biological control efficiency.

Covariations between personality behaviors and metabolic/performance traits in an Asian agamid lizard (Phrynocephalus vlangalii).

Ecological factors related to predation risks and foraging play major roles in determining which behavioral traits may mediate life history trade-offs and, therefore, the pace-of-life syndrome (POLS) structure among behavioral, physiological, and life-history traits. It has been proposed that activity/exploration or risk-taking behaviors are more likely to impact resource acquisition for organisms (individuals, populations, and species) foraging on clumped and ephemeral food sources than for organisms foraging on abundant and evenly distributed resources. In contrast, vigilance or freezing behavior would be expected to covary with the pace of life when organisms rely on food items requiring long bouts of handling. Nevertheless, it remains unclear how general this pattern is. We tested this hypothesis by examining the associations between exploration/risk-taking behaviors and metabolic/performance traits for the viviparous agamid lizard, Phrynocephalus vlangalii. This species forages on sparse and patchy food sources. The results showed positive correlations between exploration and endurance capacity, and between bite force and risk-taking willingness. Our current findings, in conjunction with our previous work showed no correlations between freezing behavior and performance in this species, support the idea that behaviors in life-history trade-offs are natural history-dependent in P. vlangalii, and provide evidence that behavioral types play functional roles in life history trade-offs to supporting POLS hypothesis.

Sex-specific pace-of-life syndromes

Abstract The pace-of-life syndrome (POLS) hypothesis considers an animal’s behavior, physiology, and life history as nonindependent components of a single integrated phenotype. However, frequent deviations from the expected correlations between POLS traits suggest that these relationships may be context, and potentially, sex dependent. To determine whether the sexes express distinct POLS trait covariance structures, we observed the behavior (mobility, latency to emerge from a shelter), physiology (mass-specific metabolic rate), and life history (life span, development time) of male and female Australian field crickets (Teleogryllus oceanicus). Path analysis modeling suggested that POLS trait covariation differed between the sexes. Although neither sex displayed the complete integration of traits predicted by the POLS hypothesis, females did display greater overall integration with a significant negative correlation between metabolic rate and risk-taking behavior but with life-history traits varying independently. In males, however, there was no clear association between traits. These results suggest that T. oceanicus do indeed display sex-specific trait covariance structures, emphasizing the importance of acknowledging sex in assessments of POLS.

Implications of slow pace-of-life for nesting behavior in an armored ectotherm

Pace-of-life syndrome (POLS) theory predicts that physiological and behavioral traits coevolve and should fall predictably along a fast-slow life history gradient. Although this theory represents an attractive theoretical framework for exploring ramifications of life history for behavior and physiology, empirical support has been mixed for this theory, and more work is required to determine how well POLS theory explains existing patterns of behavior, particularly for extremely fast or slow life histories. We investigated factors that affect adult risk exposure and nest success during nesting excursions for the Blanding’s turtle, a long-lived ectotherm. We used radio telemetry to track gravid animals to nesting areas while measuring temperature and predation risk across the study site and monitored nest success to connect behavior to current reproductive success. Turtles responded more strongly to thermal gradients than predation risk when moving to nest sites, consistent with their armored morphology and ectothermic physiology, and generally selected relatively warm microclimates during these excursions. Nests placed further from wetland edges were more successful; unexpectedly, these same areas generally had relatively high predator activity, indicating that successful nesting areas may be riskier to adults. Accordingly, turtles did not appear to select for nest sites far from wetlands in areas likely to produce successful nests, instead placing nest sites in areas where predator species richness and activity were lower. Consistent with POLS theory, our study demonstrates that long-lived organisms engage in behavioral strategies that prioritize their own survival at the expense of current reproduction.

Ecological Stoichiometry: A Link Between Developmental Speed and Physiological Stress in an Omnivorous Insect.

The elemental composition of organisms belongs to a suite of functional traits that may adaptively respond to fluctuating selection pressures. Life history theory predicts that predation risk and resource limitations impose selection pressures on organisms’ developmental time and are further associated with variability in energetic and behavioral traits. Individual differences in developmental speed, behaviors and physiology have been explained using the pace-of-life syndrome (POLS) hypothesis. However, how an organism’s developmental speed is linked with elemental body composition, metabolism and behavior is not well understood. We compared elemental body composition, latency to resume activity and resting metabolic rate (RMR) of western stutter-trilling crickets (Gryllus integer) in three selection lines that differ in developmental speed. We found that slowly developing crickets had significantly higher body carbon, lower body nitrogen and higher carbon-to-nitrogen ratio than rapidly developing crickets. Slowly developing crickets had significantly higher RMR than rapidly developing crickets. Male crickets had higher RMR than females. Slowly developing crickets resumed activity faster in an unfamiliar relative to a familiar environment. The rapidly developing crickets did the opposite. The results highlight the tight association between life history, physiology and behavior. This study indicates that traditional methods used in POLS research should be complemented by those used in ecological stoichiometry, resulting in a synthetic approach that potentially advances the whole field of behavioral and physiological ecology.

Proactive avoidance behaviour and pace-of-life syndrome in Atlantic salmon.

Individuals in a fish population differ in key life-history traits such as growth rate and body size. This raises the question of whether such traits cluster along a fast-slow growth continuum according to a pace-of-life syndrome (POLS). Fish species like salmonids may develop a bimodal size distribution, providing an opportunity to study the relationships between individual growth and behavioural responsiveness. Here we test whether proactive characteristics (bold behaviour coupled with low post-stress cortisol production) are related to fast growth and developmental rate in Atlantic salmon, Salmo salar. Boldness was tested in a highly controlled two-tank hypoxia test were oxygen levels were gradually decreased in one of the tanks. All fish became inactive close to the bottom at 70% oxygen saturation. At 40% oxygen saturation level a fraction of the fish actively sought to avoid hypoxia. A proactive stress coping style was verified by lower cortisol response to a standardized stressor. Two distinct clusters of bimodal growth trajectories were identified, with fast growth and early smoltification in 80% of the total population. There was a higher frequency of proactive than reactive individuals in this fast-developing fraction of fish. The smolts were associated with higher post-stress plasma cortisol than parr, and the proactive smolts leaving hypoxia had significant lower post-stress cortisol than the stayers. The study demonstrated a link between a proactive coping and fast growth and developmental ratio and suggests that selection for domestic production traits promotes this trait cluster.

Pace‐of‐life: Relationships among locomotor activity, life history, and circadian rhythm in the assassin bug, Amphibolus venator

AbstractThe pace‐of‐life syndrome (POLS) hypothesis means that animal behavior is correlated with life history strategies. Studies have reported that the free‐running period of the circadian rhythm (length of the period) is correlated with life history strategies in some animals. Although the length of the circadian rhythm may be associated with the POLS hypothesis, few studies have investigated the relationships among animal behavior, life history traits, and circadian rhythm. We tested the POLS hypothesis in the assassin bug, Amphibolus venator, which shows individual variation in locomotor activity. We found higher repeatability of differences in locomotor activity between individuals. Moreover, we found a trade‐off between locomotor activity and developmental period such that active individuals developed faster. However, locomotor activity was not correlated with the length of the circadian rhythm in A. venator. Therefore, this study suggests that the length of the circadian rhythm in A. venator does not support the POLS hypothesis.

Do Metabolic Traits, Vulnerability to Angling, or Capture Method Explain Boldness Variation in Eurasian Perch?

The pace-of-life syndrome (POLS) concept predicts that individuals with high baseline metabolic rates demonstrate high boldness, aggressiveness, and activity, especially in food acquisition, with associated relatively greater energy requirements. In fishes, these behaviors may increase individual vulnerability to angling. To test the predictions of the POLS concept, we quantified individual standard metabolic rate (SMR) and boldness in both wild-caught and hatchery-reared Eurasian perch (Perca fluviatilis). We found both SMR and boldness to be repeatable traits but detected no correlation between them. Individual vulnerability to angling was assessed in the hatchery-reared perch, but we found no difference in boldness or SMR between vulnerable and nonvulnerable perch. Wild-caught perch were ice fished using either natural or artificial bait, and we observed no differences in boldness or SMR with respect to bait type or capture order. Our findings do not support the predictions of the POLS concept and, consistent with earlier studies in perch, suggest that angling may not drive selection against boldness in this species.

Pace of life syndrome under warming and pollution: integrating life history, behavior, and physiology across latitudes

AbstractTo fully comprehend and predict the impact of drivers of global change such as climate warming and pollution, integrated multi‐trait approaches are needed. As organismal traits are often correlated, responses to stressors are expected to induce coordinated changes in many traits. A promising framework to study this is the pace of life syndrome (POLS), which predicts the integration of life history, behavioral, and physiological traits along a fast‐slow continuum. Using an integrative multi‐trait approach, we evaluated the presence of a POLS both within and across latitudes and how POLS patterns are affected by warming and metal pollution. We studied this in Ischnura elegans damselfly larvae of replicated low‐ and high‐latitude populations that strongly differ in voltinism (three to four generations per year vs. one generation every two years) reared in a common‐garden experiment at two temperatures. Across latitudes, life history, behavior, and physiology covaried in accordance with the POLS, with the fast‐paced low‐latitude damselflies characterized by a fast growth rate, high activity, and more explorative and risk‐taking behavior, fast metabolic rate, and low investment in immune function (activity of phenoloxidase). This fast POLS strategy was associated with a higher sensitivity to metal exposure and a higher vulnerability to predation. Warming caused opposite responses between the latitudes consistent with differential thermal adaptation in growth rate, behavior, and oxidative stress parameters. Despite this, damselflies of both latitudes showed a consistent pattern in phenotypic correlations among traits that, moreover, was not affected by warming and metal exposure. Within latitudes, there was no full support for the POLS. More active larvae were more explorative and risk taking, which aligned with the fast‐slow life history axis, but less strong than at the across‐latitude level. Physiological traits were also integrated within latitudes, yet there was no unambiguous coupling with the fast‐slow life history continuum. The consistent syndrome structure, if underpinned by genetic correlations, may restrict the independent evolution of individual traits, yet may not necessarily constrain adaptive evolution of integrated trait sets. This is because the covariance pattern was to a large extent similar across latitudes and within latitudes, suggesting adaptive trait integration guiding adaptive evolution of trait sets along the fast‐slow continuum.

Ecological conditions drive pace-of-life syndromes by shaping relationships between life history, physiology and behaviour in two populations of Eastern mosquitofish

The pace-of-life syndrome (POLS) hypothesis predicts variation in behaviour and physiology among individuals to be associated with variation in life history. Thus, individuals on the “fast” end of POLS continuum grow faster, exhibit higher metabolism, are more risk prone, but die earlier than ones on the “slow” end. Empirical support is nevertheless mixed and modelling studies suggested POLS to vary along selection gradients. Therefore, including ecological variation when testing POLS is vastly needed to determine whether POLS is a fixed construct or the result of specific selection processes. Here, we tested POLS predictions between and within two fish populations originating from different ecological conditions. We observed opposing life histories between populations, characterized by differential investments into growth, fecundity, and functional morphology under identical laboratory conditions. A slower life history was, on average, associated with boldness (latency to emergence from a refuge), high activity (short freezing time and long distance travelled), and increased standard metabolism. Correlation structures among POLS traits were not consistent between populations, with the expression of POLS observed in the slow-growing but not in the fast-growing population. Our results suggest that POLS traits can evolve independently from one another and that their coevolution depends upon specific ecological processes.

Evaluating cognition and thermal physiology as components of the pace-of-life syndrome

The pace-of-life syndrome (POLS) suggests that behavioral traits are correlated and integrate within a fast–slow physiological continuum. At the fast extreme, individuals having higher metabolic rates are more active, exploratory, and bold with the opposite suite of traits characterizing those at the slow physiological extreme. A recent framework suggests that behavioral types may also differ consistently in their cognitive style. Accordingly, we propose that cognition could be further incorporated into the POLS framework comprised of behavioral and thermal physiological traits. Under this premise, fast behavioral types having high thermal traits are predicted to acquire a novel task faster but at the cost of accuracy while slow behavioral types with low thermal traits would be more attentive, responding to cues at a slower rate leading to higher accuracy and flexibility. This was tested by measuring physiological and behavioral traits in delicate skinks (Lampropholis delicata) and testing their learning ability. Correlations were detected between cognition and behavior but not thermal physiology. Contrary to our predictions, individual positioning along these axes opposed our predicted directions along the fast–slow continuum. Fast lizards preferring lower body temperatures expressed higher activity, exploration, sociality, and boldness levels, and learned the discrimination learning task at a slower rate but made the most errors. Additionally, modelling results indicated that neither thermal physiology, behavior, or their interaction influenced cognitive performance. Although the small number of animals completing the final stages of the learning assays limits the strength of these findings. Thus, we propose that future research involving a greater sample size and number of trials be conducted so as to enhance our understanding into how the integration of cognitive style, behavior, and physiology may influence individual fitness within natural populations.

Life-history evolution under fluctuating density-dependent selection and the adaptive alignment of pace-of-life syndromes.

We present a novel perspective on life-history evolution that combines recent theoretical advances in fluctuating density-dependent selection with the notion of pace-of-life syndromes (POLSs) in behavioural ecology. These ideas posit phenotypic co-variation in life-history, physiological, morphological and behavioural traits as a continuum from the highly fecund, short-lived, bold, aggressive and highly dispersive 'fast' types at one end of the POLS to the less fecund, long-lived, cautious, shy, plastic and socially responsive 'slow' types at the other. We propose that such variation in life histories and the associated individual differences in behaviour can be explained through their eco-evolutionary dynamics with population density - a single and ubiquitous selective factor that is present in all biological systems. Contrasting regimes of environmental stochasticity are expected to affect population density in time and space and create differing patterns of fluctuating density-dependent selection, which generates variation in fast versus slow life histories within and among populations. We therefore predict that a major axis of phenotypic co-variation in life-history, physiological, morphological and behavioural traits (i.e. the POLS) should align with these stochastic fluctuations in the multivariate fitness landscape created by variation in density-dependent selection. Phenotypic plasticity and/or genetic (co-)variation oriented along this major POLS axis are thus expected to facilitate rapid and adaptively integrated changes in various aspects of life histories within and among populations and/or species. The fluctuating density-dependent selection POLS framework presented here therefore provides a series of clear testable predictions, the investigation of which should further our fundamental understanding of life-history evolution and thus our ability to predict natural population dynamics.

Seasonal effects on behavior and immunity in a precocial rodent

Environmental variation strongly influences individual life histories, behavior, and immunity. For many animals living in non-tropical regions, environmental variation due to season is very important and can be reliably predicted by photoperiod. By manipulating the pre- and post-natal photoperiod for cavies (Cavia aperea), we simulated different birth seasons that go along with predictable differences in life history. Offspring born into simulated spring conditions mature earlier, indicating a fast pace of life compared to offspring born into simulated fall. We investigated the development of two innate (titers of natural antibodies and complement) and one adaptive (immunoglobulin G) immune effectors across multiple life stages. In addition, we tested if three behaviors, indicating risk-taking and stress-coping, differed between seasons. Finally, we tested if behaviors and immune traits correlate with each other as predicted by the pace-of-life syndrome hypothesis. The season of birth influenced average trait expression of both immunity and behavior. Boldness, indicating risk-taking behavior, correlated positively with the titers of natural antibodies, fitting the predictions of the pace-of-life syndrome (POLS). However, fearlessness, indicating how an individual copes with a stressful situation, correlated positively with immunoglobulin G concentrations, against the predictions of the POLS. Together, our results show that behaviors and immune traits react to differences in expected life histories, but do not show the predicted covariation. We conclude that proximate mechanisms and different functions of traits need to be considered to predict the nature of behavior-immune associations. Individual life histories, behavior, and immunity are strongly influenced by the environment individual’s experience during early life. Recent theory predicts correlations between behavior and immunity to arise as a consequence of different life histories. By simulating different birth seasons, we induced different life history expectations as well as a season-dependent development of immunity and behavior. However, correlations between behavior and immunity do not fit theoretical predictions, highlighting the need for more detailed theory.

Linking organismal growth, coping styles, stress reactivity, and metabolism via responses against a selective serotonin reuptake inhibitor in an insect

Evidence suggests that brain serotonin (5-HT) is one of the central mediators of different types of animal personality. We tested this assumption in field crickets Gryllus integer using a selective serotonin reuptake inhibitor (SSRI). Crickets were selected for slow and rapid development and tested for their coping styles under non-stressful conditions (time spent exploring a novel object). Resting metabolic rate, maximum metabolic rate and latency to resume activity were measured under stressful conditions (stress reactivity). Measurements were taken (i) before and (ii) during the SSRI treatment. Before the SSRI treatment, a strong negative correlation was observed between coping style and stress reactivity, which suggests the existence of a behavioral syndrome. After the SSRI treatment, the syndrome was no longer evident. The results of this study show that 5-HT may be involved in regulating behavior not only along a stress reactivity gradient but also along a coping styles axis. The relationship between personality and the strength and direction of 5-HT treatment on observed behaviors indicates trait-like individual differences in 5-HT signaling. Overall, these findings do not support recent ideas arising from the pace-of-life syndrome (POLS) hypothesis, which predict higher exploration and metabolic rates in rapidly developing bold animals.

Domestication affects exploratory behaviour of pikeperch (Sander lucioperca L.) during the transition to pelleted food.

Genetic selection for body size during domestication of animal species can inadvertently affect a number of physiological and behavioural traits. The pace-of-life syndrome (POLS) hypothesis predicts that domestication in an artificial environment lacking predators and providing abundant resources prefers proactive individuals with strong feeding motivation, high levels of aggression and risk taking, with low hypothalamus–pituitary–adrenal (HPA) axis responsiveness. In the present experiment we weaned fingerling pike-perch from live feed and habituated them to formulated feed. We recorded the number of weeks needed for the fish to accept pellets, their body length at the age of 100 days, their boldness in a novel object test and their HPI axis responsiveness. Individuals accepting the artificial feed within the first week grew larger than fish habituating later; therefore early weaners would be kept and bred in routine aquaculture procedures. Contrary to predictions of POLS hypothesis, fish weaning earlier and thus growing faster were less bold and had higher HPI axis responsiveness than fish accepting the pellets later or never. As live feed is preferred to artificial pellets, less competitive individuals may switch to pellets earlier. Inadvertent selection for stress sensitive fish may have an effect on production in aquaculture as well as on natural population after intensive restocking.

Pace-of-life syndromes: a framework for the adaptive integration of behaviour, physiology and life history

This introduction to the topical collection on Pace-of-life syndromes: a framework for the adaptive integration of behaviour, physiology, and life history provides an overview of conceptual, theoretical, methodological, and empirical progress in research on pace-of-life syndromes (POLSs) over the last decade. The topical collection has two main goals. First, we briefly describe the history of POLS research and provide a refined definition of POLS that is applicable to various key levels of variation (genetic, individual, population, species). Second, we summarise the main lessons learned from current POLS research included in this topical collection. Based on an assessment of the current state of the theoretical foundations and the empirical support of the POLS hypothesis, we propose (i) conceptual refinements of theory, particularly with respect to the role of ecology in the evolution of (sexual dimorphism in) POLS, and (ii) methodological and statistical approaches to the study of POLS at all major levels of variation. This topical collection further holds (iii) key empirical examples demonstrating how POLS structures may be studied in wild populations of (non)human animals, and (iv) a modelling paper predicting POLS under various ecological conditions. Future POLS research will profit from the development of more explicit theoretical models and stringent empirical tests of model assumptions and predictions, increased focus on how ecology shapes (sex-specific) POLS structures at multiple hierarchical levels, and the usage of appropriate statistical tests and study designs. As an introduction to the topical collection, we summarise current conceptual, theoretical, methodological and empirical progress in research on pace-of-life syndromes (POLSs), a framework for the adaptive integration of behaviour, physiology and life history at multiple hierarchical levels of variation (genetic, individual, population, species). Mixed empirical support of POLSs, particularly at the within-species level, calls for an evaluation and refinement of the hypothesis. We provide a refined definition of POLSs facilitating testable predictions. Future research on POLSs will profit from the development of more explicit theoretical models and stringent empirical tests of model assumptions and predictions, increased focus on how ecology shapes (sex-specific) POLSs structures at multiple hierarchical levels and the usage of appropriate statistical tests and study designs.

Evolution of sex-specific pace-of-life syndromes: causes and consequences

Males and females commonly differ in their life history optima and, consequently, in the optimal expression of life history, behavioral and physiological traits involved in pace-of-life syndromes (POLS). Sex differences in mean trait expression typically result if males and females exhibit different fitness optima along the same pace-of-life continuum, but the syndrome structure may also differ for the sexes. Due to sex-specific selective pressures imposed by reproductive roles and breeding strategies, the sexes may come to differ in the strength of correlation among traits, or different traits may covary in males and females. Ignorance of these selective forces operating between and within the sexes may lead to flawed conclusions about POLS manifestation in the species, and stand in the way of understanding the evolution, maintenance, and variability of POLS. We outline ways in which natural and sexual selection influence sex-specific trait evolution, and describe potential ultimate mechanisms underlying sex-specific POLS. We make predictions on how reproductive roles and the underlying sexual conflict lead to sex-specific trait covariances. These predictions lead us to conclude that sexual dimorphism in POLS is expected to be highly prevalent, allow us to assess possible consequences for POLS evolution, and provide guidelines for future studies.