The link between behavioural type and natal dispersal propensity reveals a dispersal syndrome in a large herbivore.

When individuals disperse, they modify the physical and social composition of their reproductive environment, potentially impacting their fitness. The choice an individual makes between dispersal and philopatry is thus critical, hence a better understanding of the mechanisms involved in the decision to leave the natal area is crucial. We explored how combinations of behavioural (exploration, mobility, activity and stress response) and morphological (body mass) traits measured prior to dispersal were linked to the subsequent dispersal decision in 77 roe deer Capreolus capreolus fawns. Using an unusually detailed multi-trait approach, we identified two independent behavioural continuums related to dispersal. First, a continuum of energetic expenditure contrasted individuals of low mobility, low variability in head activity and low body temperature with those that displayed opposite traits. Second, a continuum of neophobia contrasted individuals that explored more prior to dispersal and were more tolerant of capture with those that displayed opposite traits. While accounting for possible confounding effects of condition-dependence (body mass), we showed that future dispersers were less neophobic and had higher energetic budgets than future philopatric individuals, providing strong support for a dispersal syndrome in this species.

Personality-dependent dispersal: characterization, ontogeny and consequences for spatially structured populations.

Dispersal is one of the most fundamental components of ecology, and affects processes as diverse as population growth, metapopulation dynamics, gene flow and adaptation. Although the act of moving from one habitat to another entails major costs to the disperser, empirical and theoretical studies suggest that these costs can be reduced by having morphological, physiological or behavioural specializations for dispersal. A few recent studies on different systems showed that individuals exhibit personality-dependent dispersal, meaning that dispersal tendency is associated with boldness, sociability or aggressiveness. Indeed, in several species, dispersers not only develop behavioural differences at the onset of dispersal, but display these behavioural characteristics through their life cycle. While personality-dependent dispersal has been demonstrated in only a few species, we believe that it is a widespread phenomenon with important ecological consequences. Here, we review the evidence for behavioural differences between dispersers and residents, to what extent they constitute personalities. We also examine how a link between personality traits and dispersal behaviours can be produced and how personality-dependent dispersal affects the dynamics of metapopulations and biological invasions. Finally, we suggest future research directions for population biologists, behavioural ecologists and conservation biologists such as how the direction and the strength of the relationship between personality traits and dispersal vary with ecological contexts.

Carotenoid-based coloration, oxidative stress and corticosterone in common lizards.

Environmental factors including stressors, health status and social context significantly affect carotenoid-based coloration. For instance, stressors may induce the diversion of carotenoids from pigmentation pathways, potentially explaining why stressed animals often exhibit reduced coloration. However, we recently showed that high blood corticosterone concentrations, which are part of the physiological stress response, are associated with increased redness of the belly in the common lizard (Lacerta vivipara). This result clearly contrasts with the findings of many studies of carotenoid-based coloration because corticosterone is believed to increase oxidative stress. Here, we examined whether these positive effects are influenced by differences in food availability. We tested the effect of high corticosterone levels on carotenoid-based coloration, antioxidant enzyme activity and oxidative damage in common lizards subject to low and high food availability. Food restriction abolished the carotenoid-based color enhancement when corticosterone concentrations in animals were high. We discuss how carotenoid-based color can honestly signal individual quality in this species and how the increased redness induced by corticosterone could be a terminal investment in an environment where long-term survival prospects are poor but not when immediate survival is endangered.

Risky dispersal: avoiding kin competition despite uncertainty.

Leaving a population without having information about the surrounding areas is highly risky. Candidates for dispersal may reduce these risks by making decisions based on the level of connectivity between patches, e.g., through immigrants. The benefits of information acquisition may vary within a population according to the dispersal cause and the phenotype of the candidate disperser. For instance, kin-based dispersers should be prepared to accept higher dispersal cost than individuals leaving for competition with congeners, and individuals of better condition should better deal with the costs of dispersing. We investigated whether the use of information obtained from immigrants depended on the reason for dispersal and the phenotype of individuals in common lizards (Lacerta vivipara). Dispersal decisions with respect to connection status depended on the cause of dispersal and on body mass. When intraspecific competition was the driving force behind dispersal, the information carried by immigrants allowed candidate dispersers to decrease uncertainty about the success of dispersal. Therefore, larger individuals dispersed when connectivity was low, whereas smaller individuals dispersed when connectivity was high. When kin competition dominated, dispersers did not adjust their dispersal decisions on the basis of information about the existence of surrounding populations, and larger individuals dispersed whatever the connectivity. These results provide support for the hypothesis that kin competition is one of the factors driving colonization.

Multivariate phenotypes and the potential for alternative phenotypic optima in wall lizard (Podarcis muralis) ventral colour morphs.

A major goal in evolutionary biology is to determine how phenotypic variation arises and is maintained in natural populations. Recent studies examining the morphological, physiological and behavioural differences among discrete colour morphotypes (morphs) have revealed several mechanisms that maintain discrete variation within populations, including frequency-dependence, density-dependence and correlational selection. For example, trade-offs over resource allocation to morphological, physiological and behavioural traits can drive correlational selection for morph-specific phenotypic optima. Here, we describe a ventral colour polymorphism in the wall lizard (Podarcis muralis) and test the hypothesis that morphs differ along multivariate axes defined by trade-offs in morphological, physiological, and immunological traits. We show that ventral colour is a discrete trait and that morphs differ in body size, prevalence of infection by parasites and infection intensity. We also find that morphs differ along multivariate phenotypic axes and experience different multivariate selection pressures. Our results suggest that multivariate selection pressures may favour alternative optimal morph-specific phenotypes in P. muralis.

Food deprivation modifies corticosterone-dependent behavioural shifts in the common lizard.

Stressful events typically induce glucocorticoid production that suppresses unnecessary physiological and behavioural functions. The glucocorticoid production also temporally activates alternative behavioural and physiological pathways. These responses are generally adaptive changes to avoid the negative effects of stressors. However, under low food availability, these behavioural and physiological modifications might lead to energetic costs. We therefore predict that these responses should not be activated when there are energetic constraints (e.g., low food availability). We experimentally tested whether food deprivation modifies corticosterone-induced behavioural and physiological responses in captive male common lizards. We measured corticosterone-induced responses in terms of body mass, metabolic rate, activity level and basking behaviour. We found that corticosterone-induced various behavioural and physiological responses which were dependent on food availability. Well-fed lizards treated with corticosterone were active earlier, and increased their basking behaviour. These behavioural modifications did not occur in food-deprived lizards. This inactivation of stress-related behavioural changes probably allows the lizard to save energy.

Optimal level of inbreeding in the common lizard.

Mate choice with regard to genetic similarity has been rarely considered as a dynamic process. We examined this possibility in breeding populations of the common lizard (Lacerta vivipara) kept for several years in semi-natural conditions. We investigated whether they displayed a pattern of mate choice according to the genetic similarity and whether it was context-dependent. Mate choice depended on genetic similarity with the partner and also on age and condition. There was no systematic avoidance of inbreeding. Females of intermediate ages, more monogamous, did not mate with genetically similar partners, whereas younger and older females, more polyandrous, did but highest clutch proportions were associated with intermediate values of pair-relatedness. These results indicate dynamic mate choice, suggesting that individuals of different phenotypes select their partners in different ways according to their genetic similarity. We consider our results in the light of diverse and apparently contradictory theories concerning genetic compatibility, and particularly, optimal inbreeding and inclusive fitness.

Assessing the impact of climate variation on survival in vertebrate populations.

The impact of the ongoing rapid climate change on natural systems is a major issue for human societies. An important challenge for ecologists is to identify the climatic factors that drive temporal variation in demographic parameters, and, ultimately, the dynamics of natural populations. The analysis of long-term monitoring data at the individual scale is often the only available approach to estimate reliably demographic parameters of vertebrate populations. We review statistical procedures used in these analyses to study links between climatic factors and survival variation in vertebrate populations. We evaluated the efficiency of various statistical procedures from an analysis of survival in a population of white stork, Ciconia ciconia, a simulation study and a critical review of 78 papers published in the ecological literature. We identified six potential methodological problems: (i) the use of statistical models that are not well-suited to the analysis of long-term monitoring data collected at the individual scale; (ii) low ratios of number of statistical units to number of candidate climatic covariates; (iii) collinearity among candidate climatic covariates; (iv) the use of statistics, to assess statistical support for climatic covariates effects, that deal poorly with unexplained variation in survival; (v) spurious detection of effects due to the co-occurrence of trends in survival and the climatic covariate time series; and (vi) assessment of the magnitude of climatic effects on survival using measures that cannot be compared across case studies. The critical review of the ecological literature revealed that five of these six methodological problems were often poorly tackled. As a consequence we concluded that many of these studies generated hypotheses but only few provided solid evidence for impacts of climatic factors on survival or reliable measures of the magnitude of such impacts. We provide practical advice to solve efficiently most of the methodological problems identified. The only frequent issue that still lacks a straightforward solution was the low ratio of the number of statistical units to the number of candidate climatic covariates. In the perspective of increasing this ratio and therefore of producing more robust analyses of the links between climate and demography, we suggest leads to improve the procedures for designing field protocols and selecting a set of candidate climatic covariates. Finally, we present recent statistical methods with potential interest for assessing the impact of climatic factors on demographic parameters.

Social personality trait and fitness.

Several recent studies have explored various aspects of animal personality and their ecological consequences. However, the processes responsible for the maintenance of personality variability within a population are still largely unknown. We have recently demonstrated that social personality traits exist in the common lizard (Lacerta vivipara) and that the variation in sociability provides an explanation for variable dispersal responses within a given species. However, we need to know the fitness consequences of variation in sociability across environmental contexts in order to better understand the maintenance of such variation. In order to achieve this, we investigated the relationship between sociability and survival, body growth and fecundity, in one-year-old individuals in semi-natural populations with varying density. 'Asocial' and 'social' lizards displayed different fitness outcomes in populations of different densities. Asocial lizards survived better in low-density populations, while social females reproduced better. Spatiotemporal variation in environmental conditions might thus be the process underlying the maintenance of these personality traits within a population. Finally, we also discuss the position of sociability in a more general individual behavioural pattern including boldness, exploration and aggressiveness.

The role of pleiotropy vs signaller-receiver gene epistasis in life history trade-offs: dissecting the genomic architecture of organismal design in social systems.

Traditional life history theory ignores trade-offs due to social interactions, yet social systems expand the set of possible trade-offs affecting a species evolution--by introducing asymmetric interactions between the sexes, age classes and invasion of alternative strategies. We outline principles for understanding gene epistasis due to signaller-receiver dynamics, gene interactions between individuals, and impacts on life history trade-offs. Signaller-receiver epistases create trade-offs among multiple correlated traits that affect fitness, and generate multiple fitness optima conditional on frequency of alternative strategies. In such cases, fitness epistasis generated by selection can maintain linkage disequilibrium, even among physically unlinked loci. In reviewing genetic methods for studying life history trade-offs, we conclude that current artificial selection or gene manipulation experiments focus on pleiotropy. Multi-trait selection experiments, multi-gene engineering methods or multiple endocrine manipulations can test for epistasis and circumvent these limitations. In nature, gene mapping in field pedigrees is required to study social gene epistases and associated trade-offs. Moreover, analyses of correlational selection and frequency-dependent selection are necessary to study epistatic social system trade-offs, which can be achieved with group-structured versions of Price's (1970) equation.

Corticosterone manipulation reveals differences in hierarchical organization of multidimensional reproductive trade-offs in r-strategist and K-strategist females.

Life history trade-offs are often hierarchical with decisions at one level affecting lower level trade-offs. We investigated trade-off structure in female side-blotched lizards (Uta stansburiana), which exhibit two evolved strategies: yellow-throated females are K-strategists and orange-throated are r-strategists. Corticosterone treatment was predicted to differentially organize these females' reproductive decisions. Corticosterone-treated yellow females suppressed reproduction but survived well, and augmented egg mass without decreasing clutch size. Conversely, corticosterone enhanced mortality and reproductive rates in orange females, and increased egg mass only after lengthy exposure. Corticosterone did not affect post-laying condition, suggesting that corticosterone increased egg mass through enhanced energy acquisition (income breeding). Corticosterone enhanced survival of lightweight females, but decreased survival of heavy females, introducing a foraging vs. predation trade-off. We conclude that rather than being a direct, functional relationship, observed trade-offs between offspring size and number represent evolved differences in hierarchical organization of multidimensional trade-offs, particularly in response to stress.

Determinants of male fitness: disentangling intra- and inter-sexual selection.

Both intra- and inter-sexual selection may crucially determine a male's fitness. Their interplay, which has rarely been experimentally investigated, determines a male's optimal reproductive strategy and thus is of fundamental importance to the understanding of a male's behaviour. Here we investigated the relative importance of intra- and inter-sexual selection for male fitness in the common lizard. We investigated which male traits predict a male's access to reproduction allowing for both selective pressures and comparing it with a staged mating experiment excluding all types of intra-sexual selection. We found that qualitatively better males were more likely to reproduce and that sexual selection was two times stronger when allowing for both selective pressures, suggesting that inter- and intra-sexual selection determines male fitness and confirming the existence of multi-factorial sexual selection. Consequently, to optimize fitness, males should trade their investment between the traits, which are important for inter- and intra-sexual selection.

Social information and emigration: lessons from immigrants.

'Should I stay or should I go?' is a fundamental question facing any candidate for emigration, as emigrating without outside information has major costs. Most studies on this topic have concentrated on risk-reducing strategies (e.g. exploration) developed after leaving the natal habitat. The idea that information might be acquired before leaving has not been investigated. Immigrants carrying information about their origins could provide such information to potential emigrants in their initial habitat. We manipulated the density of common lizard (Lacerta vivipara) populations, to investigate whether immigrants originating from these populations transmitted such information to the population they joined. Emigration of the residents of this new population clearly depended on the origin of the immigrant. Immigrants are therefore a source of information, in this case about surrounding population densities, and may have a major effect on dispersal and species persistence in a fragmented habitat.

Mother-offspring competition promotes colonization success.

Colonization is the crucial process underlying range expansions, biological invasions, and metapopulation dynamics. Which individuals leave their natal population to colonize empty habitats is a crucial question and is presently unresolved. Dispersal is the first step in colonization. However, not all dispersing individuals are necessarily good colonizers. Indeed, in some species, the phenotype of dispersers differs depending on the selective pressures that induce dispersal. In particular, kin-based interactions, a factor driving social evolution, should induce different social response profiles in nondispersing and dispersing individuals. Kin competition (defined here as between the mother and offspring) has been proven to produce dispersers with a particular phenotype that may enhance their colonizing ability. By using the common lizard (Lacerta vivipara), we conducted a multipopulation experiment to study the effect of kin competition on dispersal and colonization success. We manipulated mother-offspring interactions, which are the most important component of kin competition in the studied species, at the family and population levels and measured the consequences on colonization success. We demonstrate that mother-offspring competition at the population level significantly influences colonization success. Increased competition at the population level enhanced the colonization rate of the largest juveniles as well as the growth and survival of the colonizers. Based on these results, we calculated that kin-induced colonization halves the extinction probability of a newly initiated population. Because interactions between relatives are likely to affect the ability of a species to track habitat modifications, kin-based dispersal should be considered in the study of invasion dynamics and metapopulation functioning.

Colour variation and alternative reproductive strategies in females of the common lizard Lacerta vivipara.

Within-sex colour variation is a widespread phenomenon in animals that often plays a role in social selection. In males, colour variation is typically associated with the existence of alternative reproductive strategies. Despite ecological conditions theoretically favourable to the emergence of such alternative strategies in females, the social significance of colour variation in females has less commonly been addressed, relative to the attention given to male strategies. In a population of the common lizard, females display three classes of ventral colouration: pale yellow, orange and mixed. These ventral colours are stable through individual's life and maternally heritable. Females of different ventral colourations displayed different responses of clutch size, clutch hatching success and clutch sex-ratio to several individual and environmental parameters. Such reaction patterns might reflect alternative reproductive strategies in females. Spatial heterogeneity and presence of density- and frequency-dependent feedbacks in the environment could allow for the emergence of such alternative strategies in this population and the maintenance of colour variation in females.

Social personalities influence natal dispersal in a lizard.

Animal personalities are common across taxa and have important evolutionary and ecological implications. Such consistent individual differences correlate with important life-history traits such as dispersal. Indeed, some environmental conditions are supposed to determine dispersers with a specific personality. For example, an increased density should promote the departure of individuals with less social tolerance. Therefore, we hypothesized that dispersers from high-density populations should primarily be asocial individuals, whereas dispersers from low-density populations should be social individuals. In the common lizard (Lacerta vivipara), we measured attraction towards the odour of conspecifics on juveniles at birth as a metric of social tolerance. We then released these juveniles into populations of different densities and measured dispersal and settlement behaviours with regard to social tolerance. One year later, we again measured the social tolerance of surviving individuals. The social tolerance is constant across time and strongly reflects the individual's dispersal and settlement patterns with respect to population density. These results strongly suggest that social personalities exist and influence dispersal decisions. Further studies will help to elucidate the proximate and ultimate determinants of social personalities.

Ontogenic sources of variation in sexual size dimorphism in a viviparous lizard.

To elucidate the developmental aspects of the evolution of sexual size dimorphism (SSD), an understanding of the sex-specific ontogeny of body size is critical. Here, we evaluate the relative importance of genetic and environmental determinants of SSD in juvenile common lizards (Lacerta vivipara). We examined the prenatal and post-natal effects of population density and habitat humidity on SSD, as well as the maternal effects of food availability, corticosterone level, humidity and heat regime during gestation. Analyses indicated strong prenatal and post-natal plasticity in body size per se and yielded three main results with respect to SSD. First, SSD in juvenile common lizards matches qualitatively the SSD observed in adults. Secondly, SSD was influenced by none of the prenatal factors investigated here, suggesting poor sex-biased maternal effects on offspring size. Thirdly, SSD was sensitive to post-natal habitat humidity, which positively affected growth rate more strongly in females than in males. Thus, natural variation in SSD in juvenile common lizards appears to be primarily determined by a combination of sex-biased genetic factors and post-natal conditions. We discuss the possibility that viviparity may constrain the evolution of sex-biased maternal effects on offspring size.

Experimental enhancement of corticosterone levels positively affects subsequent male survival.

Corticosterone is an important hormone of the stress response that regulates physiological processes and modifies animal behavior. While it positively acts on locomotor activity, it may negatively affect reproduction and social activity. This suggests that corticosterone may promote behaviors that increase survival at the cost of reproduction. In this study, we experimentally investigate the link between corticosterone levels and survival in adult common lizards (Lacerta vivipara) by comparing corticosterone-treated with placebo-treated lizards. We experimentally show that corticosterone enhances energy expenditure, daily activity, food intake, and it modifies the behavioral time budget. Enhanced appetite of corticosterone-treated individuals compensated for increased energy expenditure and corticosterone-treated males showed increased survival. This suggests that corticosterone may promote behaviors that reduce stress and it shows that corticosterone per se does not reduce but directly or indirectly increases longer-term survival. This suggests that the production of corticosterone as a response to a stressor may be an adaptive mechanism that even controls survival.

Female common lizards (Lacerta vivipara) do not adjust their sex-biased investment in relation to the adult sex ratio.

Sex allocation theory predicts that facultative maternal investment in the rare sex should be favoured by natural selection when breeders experience predictable variation in adult sex ratios (ASRs). We found significant spatial and predictable interannual changes in local ASRs within a natural population of the common lizard where the mean ASR is female-biased, thus validating the key assumptions of adaptive sex ratio models. We tested for facultative maternal investment in the rare sex during and after an experimental perturbation of the ASR by creating populations with female-biased or male-biased ASR. Mothers did not adjust their clutch sex ratio during or after the ASR perturbation, but produced sons with a higher body condition in male-biased populations. However, this differential sex allocation did not result in growth or survival differences in offspring. Our results thus contradict the predictions of adaptive models and challenge the idea that facultative investment in the rare sex might be a mechanism regulating the population sex ratio.

Age-specific mating strategies and reproductive senescence.

While males gain obvious direct advantages from multiple mating, the reproductive capacity of females is more constrained. The reason why polyandry evolved in females is therefore open to many conjectures. One hypothesis postulates that females gain indirect benefits by increasing the probability of siring young from high quality males. To explore this hypothesis, we used the natural variation of the reproductive value that males and females undergo through age. The age-related variation of phenotypic performance might then induce variations in mating strategies in males and females. Using the common lizard (Lacerta vivipara) as our model system, we showed that reproductive immaturity and senescence created variability in both male and female reproductive success (including survival of offspring). Consistent with theory, males at their best-performing phenotype adopted a polygynous strategy. These males were of an intermediate age and they produced offspring of higher viability than younger and older males. In contrast, females at their best performing phenotype, also of an intermediate age, were less polyandrous than other less-performing females. Middle-aged females tended to mate with males of an intermediate age and produced litters with higher viability independently from their reproductive strategy. Males of an intermediate age enhanced their fitness by additional matings with young or old females. Young and old females increased their fitness by being more polyandrous. Polyandry therefore appears as means to seek for good males. A positive correlation between males and their partners' fitness disagree with the idea that polyandry is the result of a sexual conflict in this species.