Shy-bold Continuum Research Articles

Personality drives activity and space use in a mammalian herbivore

BackgroundAnimal personality has emerged as a key concept in behavioral ecology. While many studies have demonstrated the influence of personality traits on behavioral patterns, its quantification, especially in wild animal populations, remains a challenge. Only a few studies have established a link between personality and recurring movements within home ranges, although these small-scale movements are of key importance for identifying ecological interactions and forming individual niches. In this regard, differences in space use among individuals might reflect different exploration styles between behavioral types along the shy-bold continuum.MethodsWe assessed among-individual differences in behavior in the European hare (Lepus europaeus), a characteristic mammalian herbivore in agricultural landscapes using a standardized box emergence test for captive and wild hares. We determined an individuals’ degree of boldness by measuring the latencies of behavioral responses in repeated emergence tests in captivity. During capture events of wild hares, we conducted a single emergence test and recorded behavioral responses proven to be stable over time in captive hares. Applying repeated novel environment tests in a near-natural enclosure, we further quantified aspects of exploration and activity in captive hares. Finally, we investigated whether and how this among-individual behavioral variation is related to general activity and space use in a wild hare population. Wild and captive hares were treated similarly and GPS-collared with internal accelerometers prior to release to the wild or the outdoor enclosure, respectively. General activity was quantified as overall dynamic body acceleration (ODBA) obtained from accelerometers. Finally, we tested whether boldness explained variation in (i) ODBA in both settings and (ii) variation in home ranges and core areas across different time scales of GPS-collared hares in a wild population.ResultsWe found three behavioral responses to be consistent over time in captive hares. ODBA was positively related to boldness (i.e., short latencies to make first contact with the new environment) in both captive and wild hares. Space use in wild hares also varied with boldness, with shy individuals having smaller core areas and larger home ranges than bold conspecifics (yet in some of the parameter space, this association was just marginally significant).ConclusionsAgainst our prediction, shy individuals occupied relatively large home ranges but with small core areas. We suggest that this space use pattern is due to them avoiding risky, and energy-demanding competition for valuable resources. Carefully validated, activity measurements (ODBA) from accelerometers provide a valuable tool to quantify aspects of animal personality along the shy-bold continuum remotely. Without directly observing—and possibly disturbing—focal individuals, this approach allows measuring variability in animal personality, especially in species that are difficult to assess with experiments. Considering that accelerometers are often already built into GPS units, we recommend activating them at least during the initial days of tracking to estimate individual variation in general activity and, if possible, match them with a simple novelty experiment. Furthermore, information on individual behavioral types will help to facilitate mechanistic understanding of processes that drive spatial and ecological dynamics in heterogeneous landscapes.

Does genetic variation on the shy–bold continuum influence carnivore attacks on people? Evidence from the brown bear

AbstractLittle is known about the heritable behavioural traits of attacks by large carnivores on people. During the last 30 years attacks by brown bears Ursus arctos on people in the Cantabrian Mountains of Spain have been disproportionately concentrated in the eastern subpopulation. Excluding factors such as the existence of a single unusually bold bear, a higher human population density, particular human activities promoting encounters, or clear habitat differences in the area of this subpopulation, we propose that a plausible explanation for the unbalanced geographical attack pattern is that this subpopulation, separated a century earlier from the western subpopulation, may harbour a higher proportion of bolder bears. In the absence of genetic analyses this explanation remains speculative, but supports the hypothesis that genetic variation on the shy–bold continuum may influence attacks of large carnivores on people.

When personality matters: personality and social structure in wild bottlenose dolphins, Tursiops truncatus

There is increasing evidence that animal personality can affect many aspects of an individual's behaviour, life history and fitness. However, there have been few studies about the link between personality and social organization in the context of wild mammals in their own natural environments. This article reports on ecologically relevant data, linking experimental data from the wild to long-term social association data in a socially and cognitively complex mammal species (bottlenose dolphin, Tursiops truncatus). Here, I used behavioural data to describe personality differences between bottlenose dolphins and social network analysis to assess the relationship between personality and social structure. First, I measured the reaction of photo-identified individuals over time and across contexts as a trade-off between a novelty-seeking behaviour (boldness) and a novelty-averse behaviour (shyness). Second, I applied social network analysis to understand the link between the observed shy–bold continuum and social organization, while controlling for other factors that could contribute to affiliation. This study presents for the first time consistent individual differences in behavioural response to novelty, as a proxy for the shy–bold continuum, in wild bottlenose dolphins. Bold individuals had a central role in the social network with stronger associations than shy individuals, suggesting that bold individuals may play an important role in group cohesion, group stability and the spread of information through the network. Together, these findings provide insights into how a social network is structured by personality in wild bottlenose dolphins, with potential fitness consequences. Furthermore, this study provides additional evidence of the existence of social personalities in nonhuman animals and contributes to the understanding of the role of personality in determining the extent to which mammals associate with others.

Towards a comparative approach to the structure of animal personality variation.

Latent personality traits underpinning observed behavioral variation have been studied in a great many species. However, a lack of standardized behavioral assays, coupled to a common reliance on inferring personality from a single, observed, behavioral trait makes it difficult to determine if, when, and how conclusions can be directly compared across taxa. Here, we estimate the among-individual (co)variance structure (ID) for a set of four behaviors expressed in an open field trial, putatively indicative of boldness, in seven species of small freshwater fish. We show that the ID matrices differ in terms of the total amount of variation present, and crucially the orientation, and as a consequence, biological interpretation of the first eigenvector. Specifically, loading of observed traits on the main axis of variation in ID matched a priori expectations for a shy-bold continuum in only three of the seven cases. Nonetheless, when the “shape” of the matrices was compared in higher dimensions, there was a high level of similarity among species, and weak evidence of phylogenetic signal. Our study highlights the present difficulty of trying to compare empirical inferences about specific personality traits across studies. However, it also shows how multivariate data collection and analysis allows the structure of behavioral variation to be quantitatively compared across populations or species without reliance on ambiguous verbal labels. This suggests that the field may have much to gain from greater uptake of phylogenetically informed comparative approaches when seeking to test evolutionary hypotheses about the origin and maintenance of personality variation.

Hypoxia but not shy-bold phenotype mediates thermal preferences in a threatened freshwater fish, Notropis percobromus.

For ectothermic animals, ambient temperature strongly influences developmental growth rate and individual fitness. While many ectotherms live in environments that are spatially hetero-thermal, the coupling between behavioural phenotypes (e.g., shy or bold behaviour) and thermal preferences remains uncertain. Relative to shy counterparts, bolder phenotypes may exert higher preference for ambient temperatures that are closer to their thermal optimum, thereby accelerating development. In addition, ectotherms should select colder temperatures in low oxygen conditions (hypoxia) according to the oxygen- and capacity-limited thermal tolerance (OCLTT) hypothesis. Using wild caught carmine shiner (Notropis percobromus), this study examined thermoregulatory behaviour in individuals exhibiting consistent behavioural phenotypes along the shy-bold continuum and between ecologically relevant normal oxygen concentration (normoxic) and hypoxic treatments. Furthermore, the behaviour observed in the laboratory was compared to environmental data from the natal stream. Results demonstrated that individual shy-bold behavioural phenotype was consistent before and after a simulated aerial predator attack, indicating consistency of behaviour across situations. Individual preferred and avoidance temperatures varied substantially, but were unrelated to shy-bold behavioural phenotypes. In contrast, individual preferred and maximum avoidance temperatures were significantly reduced in hypoxia, consistent with the OCLTT hypothesis. These findings might indicate suppressed development rates in hypoxia, not only by the limited oxygen for aerobic metabolism, but also by the preference for colder water in hypoxia. Furthermore, the tolerated thermal ranges were reduced in hypoxia. Using test conditions confirmed by field data, our study demonstrates the strong influence of oxygen availability on thermoregulatory behaviours and preferences in aquatic environments.

Evaluating the Behavior and Temperament of African Penguins in a Non-Contact Animal Encounter Program.

Simple SummaryAs animal ambassador programs increase across zoos, it is important to assess the impact they may have on animal welfare. We investigated possible behavioral impacts of a penguin encounter program at the Lincoln Park Zoo in Chicago, Illinois, on a colony of 15 African penguins. We also investigated whether individual characteristics, including temperament, related to penguins’ voluntary participation in the program. We collected 16 weeks of behavioral data, during which 43 Penguin Encounters occurred, which allowed zoo guests to enter part of the penguins’ enclosure. We found no significant differences in colony affiliative or aggressive behavior between days with or without encounter programs, suggesting that offering encounters did not disrupt social behavior in the colony. We also measured penguin temperament on a shy-bold continuum by recording birds’ responses to novel objects and found that temperament, as well as age and sex were predictive of their voluntary participation. We concluded that this program had a neutral or positive impact on penguin welfare, possibly due to aspects of the ambassador program that provided penguins with control over their involvement.Animal ambassador programs are increasingly prevalent in zoos, yet few studies have investigated their impact on animal welfare. We assessed the effects of an ambassador program on the behavior of a colony (N = 15) of zoo-housed African penguins (Spheniscus demersus) and evaluated whether individual characteristics were predictive of participation. Behavioral data were collected for 16 weeks and included 43 “penguin encounters”, during which zoo visitors entered a designated portion of the penguins’ enclosure. When comparing colony behavior following encounters to behavior during a matched control period lacking an encounter, we found no significant difference between affiliative or aggressive behaviors, suggesting that the encounters did not disrupt interactions in the colony. The same was true when comparing behavior preceding the encounter to a matched control period, indicating that any anticipatory period was similarly non-disruptive. Space use during encounters suggested comfort near visitors. We also measured penguin temperament on the shy-bold continuum by recording the birds’ response to novel objects and found that penguins’ temperament, sex, and age were predictive of participation. We concluded that this program had a neutral or positive impact on penguin welfare and considered the findings in relation to aspects of the ambassador program that provided penguins with control over their involvement.

Nutritional condition determines behavioral response of nestling Black-browed albatrosses to a shy-bold continuum test

We used biochemical analyses of blood to quantify nutritional status and a standardized test to measure the response to a novel object (bold-shy test). Our aim was to examine the relationship between nutritional status and behavioral responses in nestlings of Black-browed albatrosses (Thalassarche melanophrys). A total of 66 nestlings from Saunders Island (Falkland) were analyzed. Around 41% of the differences in the intensity of responses (number of peaks per 1 min) were explained by the age and physiological conditions of the individual. Blood urea levels alone explained more than 28% of differences in number of pecks per minute. Nutritional condition can be a confounding factor when we study behavioral responses using a typical bold-shy test. Our results encourage other authors to be more cautious when designing experiments and to take into account the potential effect of nutritional level and age as potential partial explanations of individual behavioral responses.

Repeatability and reproductive consequences of boldness in female gray seals

Wild animals show consistent individual variation in behavior across time and/or contexts, now referred to as animal personality. While this variability may have important ecological and evolutionary implications, how and why variation in animal personality is maintained in a natural population remains unclear. In this study, we assessed the influence of environmental and biological sources of variation on behavioral responses measured along the shy-bold continuum in a long-lived, iteroparous marine mammal, the gray seal (Halichoerus grypus). Between 2008 and 2016, 469 females from the Sable Island, Nova Scotia breeding colony of gray seals were given a boldness score in response to a human approach, designed to stimulate maternal defense of offspring. Using generalized linear mixed-effects models (GLMM) in a Bayesian framework, we show that boldness is highly repeatable between and within years. There were age differences in boldness, with younger females being less bold than older, more experienced females providing some support for the life history trade-off hypothesis. We further used GLMMs to assess sources of variation on offspring weaning mass. We found that young females that were bolder produced heavier pups than shyer counterparts, and that pups produced by bolder females were on average ~ 2 kg heavier than pups of shy females. These results provide further evidence that personality influences life history strategies, and illustrates the evolutionary potential of animal personality in response to selection. Consistent individual differences in behavior influence various aspects of ecology including species interactions, species distributions, and life history strategies. However, how and why this individual variation is maintained in a natural population remains uncertain. In this study, we assessed the influence of boldness, specifically maternal defense of offspring, on a component of reproductive success in a long-lived marine mammal. We showed highly repeatable behavioral differences, and found that boldness varied with age, with younger individuals being less bold than older individuals. Younger individuals that were bolder produced heavier offspring than shyer counterparts. Our study contributes to an under-represented group of animals, wild marine mammals, in the personality literature, and further prompts the investigation into the proximate and ultimate factors influencing personality in an ecologically important marine predator.

Important tools for Amazon Parrot reintroduction programs

Anti-predator behavior and personality have important consequences for the survival of captive-bred animals after reintroduction in the wild. The personality of an animal can be defined as low within-individual variation in behavior relative to between-individual variation in behavior. Mortality caused by predation is the main reason for reintroduction failure, and training captive-born animals to avoid predators can solve this problem. However, how anti-predator training affects the personality of animals is uncertain. The objective of this study was to test the behavioral responses of captive Amazon Parrots (Amazona aestiva, Psittacidae) submitted to an anti-predator conditioning protocol, and to evaluate if anti-predator training affects parrot personalities. Twenty-six parrots were trained against predators using taxidermized models; their personalities were evaluated by calculating boldness scores before and after anti-predator training sessions. Parrots increased the expression of anti-predator behaviors when tested with all models; control groups behaved in a more relaxed way. The anti-predator responses persisted for 60 days after the end of the training sessions. Boldness scores increased in 50% of cases after anti-predator training sessions, and in only 33% of cases did parrots become shyer after anti-predator training. The tendency of parrots to exhibit bold behaviors in the personality tests, even after the training sessions, may be explained by their early experiences, low behavioral plasticity or high cognitive ability. Training naive parrots was an effective tool to enhance behavioral responses against predators before reintroduction. The study of personality is of great importance in reintroduction and translocation programs to determine the position of each individual in the shy-bold continuum and to help select individuals more suited for reintroduction.

Investigating movement behavior of invasive Burmese pythons on a shy–bold continuum using individual-based modeling

Burmese pythons are established in the Everglades and are expanding their population in Florida to more urbanized and fragmented habitats. Understanding movement and dispersal behavior contributes to our knowledge of how landscape factors influence the persistence of pythons in Florida's habitat. Our goal was to examine personality-dependent dispersal in juvenile Burmese pythons by creating behavioral scenarios of risk-taking behavior on a shy–bold continuum using an individual-based model. We observed that a behaviorally plastic strategy best resembled empirically derived patterns of the Burmese python's expansion from the Everglades into the increasingly urbanized landscapes of Homestead and Miami, Florida. This result is consistent with the notion that animal personalities can be flexible in different situations and that animals must make decisions based on trade-offs while dispersing.

The shy prefer familiar congeners

The shy–bold continuum is both a fundamental aspect of human behavior and a relatively stable behavioral trait for many other species. Here we assessed whether shy individuals prefer familiar congeners, taking the European sea bass, a recently domesticated fish showing similar behavioral responses to wild fish, as a model to better understand the inter-individual variability in social behavior previously observed in this species. In the wild, the link between familiarity i.e., the preference of fish for familiar congeners and boldness could be part of the mechanism underlying shoaling formation in fish. Thirty fish were individually tested in a device designed to assess the preference for a familiar vs. an unfamiliar congener on the basis of visual cues only. An open field test (OFT) with shelter was performed on the same fish 32 days later to assess the boldness of each individual. Variables of interest included the proportion of time spent in the shelter, border and center zone of the arena and variables of activity. Variables measured in OFT were collapsed into first principal component scores using Principal Components Analysis (PCA) which allowed characterizing a shy-bold continuum. Time spent near the familiar congener was negatively correlated with boldness i.e., shy individuals spent most of the time near the familiar congener. We discuss the relevance of these findings to the understanding of the behavior of European sea bass and suggest that the link between familiarity and shyness is a general aspect of both animal and human behavior.

Differences in boldness are repeatable and heritable in a long‐lived marine predator

Animal personalities, composed of axes of consistent individual behaviors, are widely reported and can have important fitness consequences. However, despite theoretical predictions that life-history trade-offs may cause and maintain personality differences, our understanding of the evolutionary ecology of personality remains poor, especially in long-lived species where trade-offs and senescence have been shown to be stronger. Furthermore, although much theoretical and empirical work assumes selection shapes variation in personalities, studies exploring the genetic underpinnings of personality traits are rare. Here we study one standard axis of personality, the shy–bold continuum, in a long-lived marine species, the wandering albatross from Possession Island, Crozet, by measuring the behavioral response to a human approach. Using generalized linear mixed models in a Bayesian framework, we show that boldness is highly repeatable and heritable. We also find strong differences in boldness between breeding colonies, which vary in size and density, suggesting birds are shyer in more dense colonies. These results demonstrate that in this seabird population, boldness is both heritable and repeatable and highlights the potential for ecological and evolutionary processes to shape personality traits in species with varying life-history strategies.

“Boldness” in the domestic dog differs among breeds and breed groups

“Boldness” in dogs is believed to be one end of the shy–bold axis, representing a super-trait. Several personality traits fall under the influence of this super-trait. Previous studies on boldness in dogs have found differences among breeds, but grouping breeds on the basis of behavioural similarities has been elusive. This study investigated differences in the expression of boldness among dog breeds, kennel club breed groups, and sub-groups of kennel club breed groups by way of a survey on dog personality circulated among Australian dog-training clubs and internet forums and lists. Breed had a significant effect on boldness (F=1.63, numDF=111, denDF=272, p<0.001), as did breed group (F=10.66, numDF=8, denDF=772, p<0.001). Herding and gundog groups were broken into sub-groups based on historic breed purpose. Retrievers were significantly bolder than flushing and pointing breeds (Reg. Coef.=2.148; S.E.=0.593; p<0.001), and tending and loose-eyed herding breeds were bolder than heading and cattle-herding breeds (Reg. Coef.=1.744; S.E.=0.866; p=0.045 and Reg. Coef.=1.842; S.E.=0.693; p=0.0084, respectively). This study supports the existence of the shy–bold continuum in dogs. Differences in boldness among groups and sub-groups suggest that behavioural tendencies may be influenced by historical purpose regardless of whether that purpose still factors in selective breeding.

Individual personality and the spatial distribution of groups of grazing animals: An example with sheep

Impacts of individual personality on group distribution were investigated using sheep (Ovis aries) as a model. In an indoor exploration test, individuals who visited <4 (out of 6) objects in a novel environment were classified as 'shy' (n=10), and those who visited 5 or 6 objects were classified as 'bold' (n=10). Nine weeks later, using a series of groups (n=40) of either 5 shy or 5 bold sheep, we measured distribution at pasture and responses to disturbance and the approach of a human handler. When grazing undisturbed, the mean nearest neighbour distance and spread (minimum convex hull area) of shy groups were less than those of bold groups, with shy individuals moving towards one another more often. Shy groups explored a smaller area than bold groups. When disturbed, shy sheep were more likely to stop grazing and move closer together. Shy sheep kept further away from the handler and moved faster when driven. The results demonstrate a link between personality and group distribution, suggesting that our 'shy' and 'bold' individuals may occupy different positions on the shy-bold continuum documented for other species. We discuss implications for diet composition and impacts on vegetation grazed by animals with different personalities.

Variation along the shy–bold continuum in extremophile fishes (Poecilia mexicana, Poecilia sulphuraria)

One potential trade-off that bold individuals face is between increased predation risks and gains in resources. Individuals experiencing high predation and hungry individuals (or individuals with low body condition) are predicted to show increased boldness. We examined one behavioral trait previously reported to be associated with boldness (the time individual fish needed to emerge from shelter) in various populations of mollies (Poecilia spp.). Our study system included several southern Mexican surface streams with high piscine predation and high food availability, sulfidic surface streams with high avian predation, in which the inhabiting fish show reduced body condition, and a sulfidic cave, where predation and body condition are low. Our comparison revealed very short times to emerge from the start box in populations from non-sulfidic streams. In sulfidic habitats (whether surface or cave), it took individual Poecilia mexicana considerably longer to emerge from the start box, and the same difference was also found in an independent comparison between P. mexicana and the closely related, highly sulfide-adapted Poecilia sulphuraria. Fish reared under common garden conditions (in the absence of predators and hydrogen sulfide) showed intermediate boldness scores to the extremes observed in the field. Our data thus indicate that (a) boldness is shaped by environmental conditions/experiential effects, but is not heritable, (b) predation affects boldness in the predicted direction, but (c) low body condition leads to reduced boldness. Extremophile Poecilia spp. spend most of their time surfacing to survive under sulfidic and hypoxic conditions, which exposes them to increased levels of predations, but the fish forage on the bottom. Hence, in this system, increased boldness does not increase foraging success. We argue that energy limitation favors reducing energetically costly behaviors, and exploring novel environments may be just one of them.

Different neural responses to stranger and personally familiar faces in shy and bold adults.

The shy-bold continuum is a fundamental behavioral trait conserved across human and nonhuman animals. Individual differences along the shy-bold continuum are presumed to arise from, and are maintained by, differences in the excitability of forebrain limbic areas involved in the evaluation of stimulus saliency. To test this hypothesis, the authors conducted an event-related functional MRI (fMRI) study in which brain scans were acquired on shy and bold adults during the presentation of neutral stranger and personally familiar faces. Shy adults exhibited greater bilateral amygdala activation during the presentation of stranger faces and greater left amygdala activation during personally familiar faces than their bold counterparts. Bold adults exhibited greater bilateral nucleus accumbens activation in response to stranger and personally familiar faces than shy adults. Findings suggest that there are distinct neural substrates underlying and maintaining individual differences along a shy-bold continuum in humans.

Timidez e coragem em emas Rhea americana Linnaeus (Rheiformes, Rheidae): os efeitos do treinamento anti-predação na personalidade das aves

The shy-bold continuum is an axis of behavioural variation for some species, but the consequences of shyness and boldness in antipredatory behaviour is unknown. Bold animals have the tendency to be predated first after release in comparison to shy animals, who naturally avoid the predators. Antipredatory training has been used to enhance the defence behaviours of naive animals by various researchers around the world. For greater rheas, Rhea americana (Linnaeus, 1758), this kind of study is pioneer. In this study we have investigated if there are relationships between personality and performance of greater rheas in antipredatory training. We also investigated if the training procedures influenced the behaviour of the birds when presented to novel objects. Fifteen zoo-borne greater rheas were studied and 16 personality tests were run, being eight before the application of antipredator training, and eight after the training. We presented to the birds four novel objects (ball, box, bag and person) and recorded their behaviour and the distance of the birds in relation to the objects. Results showed that the birds behave boldly before training and shyly after it. The antipredator training modified significantly the behaviour of the rheas, making them more careful about novel situations. Personalities affected the behaviour of the birds during antipredator training. The study of the animal personalities can be an useful tool in reintroduction programs since it helps to choose the animals with the highest chance of survival to reintroduce.

Individual variation in preferred body temperature covaries with social behaviours and colour in male lizards

1. I tested if individual variation in preferred body temperature (PBT) covaried with behaviours and male phenotypic traits (size and colouration) in Pseudemoia entrecasteauxii. 2. Individuals varied in their PBT and this variation was repeatable across days. PBT was not related to body size but males with orange ventral colour had higher PBT. 3. Males with orange ventral colour were more aggressive and dominated males with white venters. 4. Male behaviours were correlated and they covaried with PBT. Correlated behaviours such as these may be related to a shy–bold continuum.

Shyness and boldness in greater rheas Rhea americana Linnaeus (Rheiformes, Rheidae): the effects of antipredator training on the personality of the birds

The shy-bold continuum is an axis of behavioural variation for some species, but the consequences of shyness and boldness in antipredatory behaviour is unknown. Bold animals have the tendency to be predated first after release in comparison to shy animals, who naturally avoid the predators. Antipredatory training has been used to enhance the defence behaviours of naive animals by various researchers around the world. For greater rheas, Rhea americana (Linnaeus, 1758), this kind of study is pioneer. In this study we have investigated if there are relationships between personality and performance of greater rheas in antipredatory training. We also investigated if the training procedures influenced the behaviour of the birds when presented to novel objects. Fifteen zoo-borne greater rheas were studied and 16 personality tests were run, being eight before the application of antipredator training, and eight after the training. We presented to the birds four novel objects (ball, box, bag and person) and recorded their behaviour and the distance of the birds in relation to the objects. Results showed that the birds behave boldly before training and shyly after it. The antipredator training modified significantly the behaviour of the rheas, making them more careful about novel situations. Personalities affected the behaviour of the birds during antipredator training. The study of the animal personalities can be an useful tool in reintroduction programs since it helps to choose the animals with the highest chance of survival to reintroduce.

Sources of individual shy–bold variations in antipredator behaviour of male Iberian rock lizards

Individual animals from the same population, sex, age and reproductive status often respond differently and consistently to predators. One important dimension of this behavioural variation is the shy–bold continuum. Innate differences in boldness might explain why individuals differ in their antipredator behaviour. In a laboratory experiment, we examined the sources of individual variation in antipredator behaviour of adult male lizards Lacerta monticola. We simulated in the laboratory repeated predatory attacks of low or high risk and analysed activity levels and refuge use in both situations. Multivariate analyses suggested the existence of two consistent and independent shy–bold continua. The first described a gradient from bold lizards that spent shorter times in the refuge after predatory approaches to shy lizards with longer emergence times, whereas the other described a gradient from bold lizards with a low propensity to hide when the predator was close but risk was low to shy lizards that hid more often. We analysed whether morphological characteristics, body condition and health (estimated from their T cell immunocompetence) of individuals might account for the differences observed. Bold individuals had smaller absolute body size, but relatively larger heads, better body condition and better health. Bold individuals with a low propensity to hide when risk was low had larger absolute body sizes, whereas relative head size, and body condition and health were not important. We suggest that the position of an individual in the shy–bold continua might reflect its optimal antipredator behaviour, which would be a function of its health, general quality and ability to evade predators.