Effect of exogenous corticosterone on respiration in a reptile

Husbandry Protocols for Juvenile Loggerhead Sea Turtles (Caretta caretta) Based on Stress Response to Stocking Density and Dry-Dock Time

When necessary, sea turtles are held captive for veterinarian care and research purposes. Protocols and basic guidelines have been described for husbandry of sea turtles with veterinarian needs but not considering physiological indicators of animal welfare. Because all sea turtle are imperiled species, monitoring their welfare is important. The aim of this study was to standardize husbandry protocols for loggerhead (Caretta caretta) juveniles held under seminatural conditions, based on circulating concentration of plasma corticosterone (Cort) and behavior. Two experiments were performed to analyze physiological and behavioral responses of the animals facing changes in stocking density and different dry-docking times. Cort analyses suggested that the number of animals per tank can be modified occasionally, without affecting their health and welfare. However, dry-docking time should be < 30 min, as indicated by the significant elevation of circulating Cort at ≥ 30 min, rising from 1.51- ng/ml to 5.28-ng/ml. Protocols tested did not affect behavioral responses, except for the breaths per move, which increased while Cort increased, despite differences exhibited by experimental animals in behavioral responses according to daily times (morning vs afternoon) and the sex of the animals.

Metabolic consequences of sex reversal in two lizard species: a test of the like-genotype and like-phenotype hypotheses

Vertebrate sex is typically determined genetically, but in many ectotherms sex can be determined by genes (genetic sex determination, GSD), temperature (temperature-dependent sex determination, TSD), or interactions between genes and temperature during development. TSD may involve GSD systems with either male or female heterogamety (XX/XY or ZZ/ZW) where temperature overrides chromosomal sex determination to cause a mismatch between genetic sex and phenotypic sex (sex reversal). In these temperature-sensitive lineages, phylogenetic investigations point to recurrent evolutionary shifts between genotypic and temperature-dependent sex determination. These evolutionary transitions in sex determination can occur rapidly if selection favours the reversed sex over the concordant phenotypic sex. To investigate the consequences of sex reversal on offspring phenotypes, we measured two energy-driven traits (metabolism and growth) and 6 month survival in two species of reptile with different patterns of temperature-induced sex reversal. Male sex reversal occurs in Bassiana duperreyi when chromosomal females (female XX) develop male phenotypes (maleSR XX), while female sex reversal occurs in Pogona vitticeps when chromosomal males (male ZZ) develop female phenotypes (femaleSR ZZ). We show metabolism in maleSR XX was like that of male XY; that is, reflective of phenotypic sex and lower than genotypic sex. In contrast, for Pogona vitticeps, femaleSR ZZ metabolism was intermediate between male ZZ and female ZW metabolic rate. For both species, our data indicate that differences in metabolism become more apparent as individuals become larger. Our findings provide some evidence for an energetic advantage from sex reversal in both species but do not exclude energetic processes as a constraint on the distribution of sex reversal in nature.

Blood chemistry and biliverdin differ according to reproduction and tourism in a free-living lizard

Changes in the physiological health of species are an essential indicator of changing conditions and environmental challenges. Reponses to environmental challenges can often induce stress, influence physiology, and change metabolism in organisms. Here we tested blood chemistry parameters indicative of stress and metabolic activity using an i-STAT point-of-care blood analyzer in seven populations of free-ranging rock iguanas exposed to varying levels of tourism and supplemental feeding. We found significant differences in blood chemistry (glucose, oxygen, carbon dioxide, hematocrit, hemoglobin, calcium, potassium, and biliverdin levels) among populations exposed to varying levels of tourism, and some variation between sexes and reproductive states. However, different variables are not directly related to one another, suggesting that the causal physiological pathways driving tourism-induced differences are influenced by mechanisms that are not detected by common analyses of blood chemistry. Future work should investigate upstream regulators of these factors affected by tourism. Regardless, these blood metrics are known to be both stress sensitive and related to metabolic activity, suggesting that exposure to tourism and associated supplemental feeding by tourists are generally driven by stress-related changes in blood chemistry, biliverdin, and metabolism.

The Impacts of Transdermal Application of Corticosterone on Toad (Rhinella icterica) Immunity

Recent studies have shown that acute physiological increases in endogenous glucocorticoid levels have immunostimulatory effects. Although post-acute stress immunosuppressive effects have also been described, the difference between enhancing and suppressing the immune response seems mediated by the stressor's duration, intensity, and the immune component under analysis. To elicit physiologically relevant corticosterone levels that can be found in Rhinella icterica toads after stressful events (e.g., restraint or captivity) and understand how acute increased glucocorticoid levels of different intensities affect corticosterone and testosterone plasma levels and immune parameters (in vitro plasma bacterial killing ability, neutrophil-to-lymphocyte ratio, and in vivo phagocytosis of peritoneal leukocytes), we submitted toads to the transdermal application of two corticosterone doses (1 and 10 μg). Corticosterone transdermal application increased corticosterone plasma levels with different intensities: 3 times for 1 μg and fourteen times for 10 μg, compared to the vehicle, and the neutrophil-to-lymphocyte ratio increased regardless of the corticosterone dose. However, there was no effect on testosterone levels and bacterial killing ability. Interestingly, both corticosterone doses promoted immunosuppression, decreasing peritoneal leukocytes' phagocytosis activity by 60% for toads receiving the dose of 1µg and 40% for those receiving 10 μg. Our results show the complexity of the relationship between increased corticosterone levels and immunomodulation. The different corticosterone doses promoted increases of distinct magnitudes in corticosterone plasma levels, with the less intense increase in corticosterone levels generating greater cell-mediated immunosuppression. Future studies using different corticosterone doses to achieve and compare physiological vs. pharmacological hormone levels are imperative to understanding these interrelationships between corticosterone and immune response.

Mitochondrial oxidative phosphorylation response overrides glucocorticoid-induced stress in a reptile

Stress hormones and their impacts on whole organism metabolic rates are usually considered as appropriate proxies for animal energy budget that is the foundation of numerous concepts and models aiming at predicting individual and population responses to environmental stress. However, the dynamics of energy re-allocation under stress make the link between metabolism and corticosterone complex and still unclear. Using ectopic application of corticosterone for 3, 11 and 21 days, we estimated a time effect of stress in a lizard (Zootoca vivipara). We then investigated whole organism metabolism, muscle cellular O₂ consumption and liver mitochondrial oxidative phosphorylation processes (O₂ consumption and ATP production) and ROS production. The data showed that while skeletal muscle is not impacted, stress regulates the liver mitochondrial functionality in a time-dependent manner with opposing pictures between the different time expositions to corticosterone. While 3 days exposition is characterized by lower ATP synthesis rate and high H₂O₂ release with no change in the rate of oxygen consumption, the 11 days exposition reduced all three fluxes of about 50%. Oxidative phosphorylation capacities in liver mitochondria of lizard treated with corticosterone for 21 days was similar to the hepatic mitochondrial capacities in lizards that received no corticosterone treatment but with 40% decrease in H₂O₂ production. This new mitochondrial functioning allows a better capacity to respond to the energetic demands imposed by the environment but do not influence whole organism metabolism. In conclusion, global mitochondrial functioning has to be considered to better understand the proximal causes of the energy budget under stressful periods.

Two stressors are worse than one: combined heatwave and drought affect hydration state and glucocorticoid levels in a temperate ectotherm

Heatwaves and droughts are becoming more intense and frequent with climate change. These extreme weather events often occur simultaneously and may alter organismal physiology, yet their combined impacts remain largely unknown. Here, we experimentally investigated physiological responses of a temperate ectotherm, the asp viper (Vipera aspis), to a simulated heatwave and drought. We applied a two-by-two factorial design by manipulating the daily temperature cycle (control vs. heatwave) and the water availability (water available vs. water-deprived) over a month followed by exposure to standard thermal conditions with ad libium access to water. Simulated heatwave and water deprivation additively increased mass loss, while water deprivation led to greater plasma osmolality (dehydration). Mass gain from drinking after the treatment period was higher in vipers from the heatwave and water-deprived group suggesting that thirst was synergistically influenced by thermal and water constraints. Heatwave conditions and water deprivation also additively increased baseline corticosterone levels but did not influence basal metabolic rates and plasma markers of oxidative stress. Our results demonstrate that a short-term exposure to combined heatwave and drought can exacerbate physiological stress through additive effects, and interactively impact behavioral responses to dehydration. Considering combined effects of temperature and water availability is thus crucial to assess organismal responses to climate change.

Compensating for a stressful pregnancy? Glucocorticoid treatment during gravidity reduces metabolic rate in female fence lizards post-parturition

Reproduction is a critical part of an animal's life history, but one which incurs significant costs to survival and future reproductive potential. These physiological consequences are likely to be influenced by context - for example, if an individual is subject to environmental stressors, physiological and behavioral changes associated with reproduction may be altered. Glucocorticoids, hormones produced as part of the physiological response to stressors, may alter how reproduction affects female physiology and behavior, and therefore the outcomes of reproductive trade-offs. Glucocorticoids prioritize immediate survival over reproduction, for example through changes in immune function, metabolic rate, and foraging, which may reduce energy expenditure or increase energy gain. However, we previously found that female eastern fence lizards (Sceloporus undulatus) experiencing elevated glucocorticoid levels during gestation were nevertheless able to maintain reproductive output and body condition. Here we investigate compensatory mechanisms by which eastern fence lizard females may maintain reproduction under experimental increases in a glucocorticoid, corticosterone (CORT). We found that, although CORT-treated females had similar immune function and behavior, they had reduced metabolic rates 3-5 days post-parturition compared to control females. Given that CORT-treated females spent a similar time basking and had equal food intake compared to control females, we suggest that the reduced metabolic rate is a mechanism by which CORT-treated females maintain their energy balance and reduce the energetic costs of gestation during periods of stress. This study suggests that physiological responses to reproduction may be context-dependent and could act to minimize costs of reproduction in situations where CORT is elevated (such as during periods of environmental stress).

Commensal microbiota and host metabolic divergence are associated with the adaptation of Diploderma vela to spatially heterogeneous environments

Heterogeneous environment adaptation is critical to understand the species evolution and response to climate change. However, how narrow-range species adapt to micro-geographic heterogeneity has been overlooked, and there is a lack of insights from metabolism and commensal microbiota. Here, we studied the environmental adaptation for 3 geographic populations (>40 km apart) of Diploderma vela, a lizard endemic to dry-hot valleys of the Hengduan Mountain Region. The climatic boundary caused a cooler, droughtier, and barren environment for northernmost population (RM) than the middle (QZK) and southernmost populations (FS). Correspondingly, significant divergences in liver and muscle metabolism and commensal microbiota were detected between RM and QZK or FS individuals, but not between QZK and FS individuals. Phospholipid composition, coenzyme level (i.e. pyridoxal and NAD⁺ ), and cholesterol metabolism (e.g. androgen and estriol synthesis) constituted the major metabolic difference between RM and QZK/FS groups. FS and QZK individuals kept abundant Proteobacteria and antifungal strains, while RM individuals maintained more Firmicutes and Bacteroidota. Strong associations existed between varied host metabolite and gut microbes. How were these interpopulation variations associated to the environment adaptation were discussed. These results provided some novel insights into the environmental adaptation and implicated the consequence of climate change on narrow-range species.

Surviving winter: Physiological regulation of energy balance in a temperate ectotherm entering and exiting brumation

Characterizing the physiological response to prolonged cold exposure is essential for understanding the maintenance of long-term energy balance. As part of their natural life cycle, temperate ectotherms are often exposed to seasonal variation in temperatures, including extended periods of cold well below their activity range. Relatively little is known about variation in physiological responses as vertebrate ectotherms enter and exit brumation in response to sustained cold temperatures. We tested the influence of temperature on physiology before, during, and after a simulated brumation in the checkered garter snake (Thamnophis marcianus), a widespread ectothermic vertebrate. We tested for the relative effect of immediate temperature and physiological context (entering or exiting brumation) on hormones regulating energy balance, indicators of energy availability, and resting metabolic rate (V̇_O2). Plasma corticosterone, glucose, and insulin, as well as immune cell heterophil: lymphocyte ratios responded to temperature, though they did so with different thermal response curves. Thermal sensitivity varied both among and within physiological measures depending on whether animals were going into or coming out of brumation. Additionally, V̇_O2 was regulated beyond simple temperature-dependence, whereby post-brumation measures were depressed relative to pre-brumation measures at the same temperature. This pattern was characterized by a change in the temperature coefficient (Q₁₀), with a larger pre-brumation Q_10, suggesting reduced thermal sensitivity of metabolic rate following a period of extended cold exposure. The integrated physiological response presented here demonstrates not only temperature dependence across physiological axes, but seasonal variation in thermal responsiveness. Our results suggest that energy allocation decisions and hormonal regulation of underlying processes promote differing levels of thermal sensitivity when entering or exiting brumation.

Hormonal correlates of the annual cycle of activity and body temperature in the South-American tegu lizard (Salvator merianae)

Life history transitions and hormones are known to interact and influence many aspects of animal physiology and behavior. The South-American tegu lizard (Salvator merianae) exhibits a profound seasonal shift in metabolism and body temperature, characterized by high daily activity during warmer months, including reproductive endothermy in spring, and metabolic suppression during hibernation in winter. This makes S. merianae an interesting subject for studies of interrelationships between endocrinology and seasonal changes in physiology/behavior. We investigated how plasma concentrations of hormones involved in regulation of energy metabolism (thyroid hormones T₄ and T₃; corticosterone) and reproduction (testosterone in males and estrogen/progesterone in females) correlate with activity and body temperature (Tb) across the annual cycle of captive held S. merianae in semi-natural conditions. In our initial model, thyroid hormones and corticosterone showed a positive relationship with activity and Tb with independent of sex: T₃ positively correlated with activity and Tb, while T₄ and corticosterone correlated positively with changes in Tb only. This suggests that thyroid hormones and glucocorticoids may be involved in metabolic transitions of annual cycle events. When accounting for sex-steroid hormones, our sex separated models showed a positive relationship between testosterone and Tb in males and progesterone and activity in females. Coupling seasonal endocrine measures with activity and Tb may expand our understanding of the relationship between animal's physiology and its environment. Manipulative experiments are required in order to unveil the directionality of influences existing among abiotic factors and the hormonal signaling of annual cyclicity in physiology/behavior.

Thermal regime effects on the resting metabolic rate of rattlesnakes depend on temperature range

While ectothermic organisms often experience considerable circadian variation in body temperature under natural conditions, the study of the effects of temperature on metabolic rates are traditionally based on subjecting animals to constant temperature regimes. Whether data resulting from constant-temperature experiments accurately predicts temperature effects under more natural fluctuating temperature regimes remains uncertain. To address such possibility, we measured the resting metabolic rates of the South American rattlesnakes (Crotalus durissus) under constant and circadian fluctuating thermal regimes in a range of temperatures. Metabolic rates measured at constant 20 °C and 25 °C did not differ from the rates measured at fluctuating regimes with corresponding mean temperatures. However, the difference between thermal regimes increased with temperature, with the metabolic rate measured at constant 30 °C being greater than that measured at the fluctuating thermal regime with corresponding mean temperature. Therefore, our results indicate that thermal regime effects on rattlesnakes' metabolism is dependent on temperature range. Broadly, our results highlight the importance of considering multi-factorial attributes of temperature variation in the exam of its effects over functional traits. Such approach provides a more solid support for inferences about temperature effects on the life history, ecology and conservation of ectothermic organisms.

Repeated stimulation of the pituitary-adrenal axis alters offspring phenotype of a wild passerine

Prolonged stress can have long-lasting effects on an individual's physiology and growth. However, the impact of chronically elevated glucocorticoids on the expression of early antipredator responses is still poorly documented. In this study, I simulated the effect of repeated acute stress on offspring phenotype in free-living pied flycatchers (Ficedula hypoleuca) by administering adrenocorticotropic hormone (ACTH) to nestlings for 6 days. The results showed that frequent induction of stress responses by ACTH injections, independent of parental care, adversely affected offspring final body size, wing length and baseline corticosterone levels. Nestling behavioural activity did not differ between ACTH- and saline-treated groups during exposure to control sounds, whereas behavioural activity during exposure to alarm calls was reduced in manipulated offspring only. I conclude that prolonged physiological stress may have short-term benefits to nest-bound offspring, such as more effective antipredator behaviour, but at the expense of negative effects on body size and developmental speed.

Acute stress, steroid plasma levels, and innate immunity in Brazilian toads

Stress from habitat fragmentation has been shown to impact amphibian declines. Studies from a variety of vertebrates indicate that stressed animals exhibit an acute increase in circulating plasma glucocorticoid (GC) levels and consequent immunomodulation. To further explore the relationship between GCs and immunity, we subjected three species of newly captured Brazilian toads, Rhinella ornata, R. icterica and R. schneideri to restraint with or without movement restriction (maintenance in a moistened cloth bag vs. maintenance in a bin) for 24 h. We compared various parameters from baseline (field conditions) with values after restraint, including those associated with stress (corticosterone [CORT] plasma levels), and the neutrophil/lymphocyte ratio [N:L ratio]), potential reproduction (testosterone [T] plasma levels), and innate immunity (bacterial killing ability [BKA]). General responses to the restraint challenge (baseline vs. restraint) included increased CORT levels and N:L ratio, and decreased T levels and BKA. Additionally, CORT levels and N:L ratio tended to increase more from restraint with movement restriction than to restraint without movement restriction, indicating toads showed increased stress response to the more intense stressor. All variables showed interspecific variation at baseline conditions: R. ornata had higher CORT levels when compared to the other two species, while R. icterica had the highest BKA values. After restraint (with or without movement restriction), R. ornata displayed higher values for T and N:L ratio, and showed higher CORT values after restraint without movement restriction; however, the CORT values were similar among species after restraint with movement restriction. In terms of immunity, in response to restraint, BKA was different among species only after restraint with movement restriction, with R. schneideri showing the lowest BKA values. Our results show that restraint increases common markers of the stress response, and could reduce potential reproduction and innate immune responses in toads from all studied species. Our results also showed variation at the interspecific level, with the amplitude of change in the studied variables being consistent and more pronounced following restraint with movement restriction for the three-studied species.

Exposure to fluctuating temperatures leads to reduced immunity and to stress response in rattlesnakes

Ectothermic organisms often experience considerable variation in their body temperature throughout the circadian cycle. However, studies focusing on the measurement of physiological traits are usually performed under constant temperature regimes. This mismatch between thermal exposure in the field and experimental conditions could act as a stressor agent, since physiological functions are strongly influenced by temperature. Herein, we asked the question whether constant thermal regimes would cause a stress response and impact the immunity of the South American rattlesnake (Crotalus durissus) when compared to a fluctuating thermal regime. We addressed this question by determining heterophil:lymphocyte ratio (H:L), plasma bacteria killing ability (BKA) and corticosterone levels (CORT) in snakes kept under a constant temperature regime at 30°C, and under a fluctuating regime that oscillated between 25°C at nighttime to 35°C at daytime. The experiments had a mirrored design, in which half of the snakes were subjected to a fluctuating-to-constant treatment, while the other half was exposed to a constant-to-fluctuating treatment. The shift from constant to fluctuating thermal regime was accompanied by an increase in plasma CORT levels indicating the activation of a stress response. Exposure to a fluctuating thermal regime at the onset of the experiments induced a decrease in the BKA of rattlesnakes. H:L was not affected by treatments and, therefore, the shift between thermal regimes seems to have acted as a low intensity stressor. Our results suggest that the removal from temperatures close to the snakés preferred body temperature triggers a stress response in rattlesnakes.

Interactions between corticosterone phenotype, environmental stressor pervasiveness and irruptive movement-related survival in the cane toad

Animals use irruptive movement to avoid exposure to stochastic and pervasive environmental stressors that impact fitness. Beneficial irruptive movements transfer individuals from high-stress areas (conferring low fitness) to alternative localities that may improve survival or reproduction. However, being stochastic, environmental stressors can limit an animal's preparatory capacity to enhance irruptive movement performance. Thus individuals must rely on pre-existing, or rapidly induced, physiological and behavioural responses. Rapid elevation of glucocorticoid hormones in response to environmental stressors are widely implicated in adjusting physiological and behaviour processes that could influence irruptive movement capacity. However, there remains little direct evidence demonstrating that corticosterone-regulated movement performance or interaction with pervasiveness of environmental stress, confers adaptive movement outcomes. Here, we compared how movement-related survival of cane toads (Rhinella marina) varied with three different experimental corticosterone phenotypes across four increments of increasing environmental stressor pervasiveness (i.e. distance from water in a semi-arid landscape). Our results indicated that toads with phenotypically increased corticosterone levels attained higher movement-related survival compared with individuals with control or lowered corticosterone phenotypes. However, the effects of corticosterone phenotypes on movement-related survival to some extent co-varied with stressor pervasiveness. Thus, our study demonstrates how the interplay between an individual's corticosterone phenotype and movement capacity alongside the arising costs of movement and the pervasiveness of the environmental stressor can affect survival outcomes.

Influence of temperature, size and confinement on testosterone and corticosterone levels in breeding male spined toads (Bufo spinosus)

Winter breeding amphibians opportunistically engage in reproductive behaviour when environmental conditions become favourable. In such explosive breeding systems, males strongly compete for reproductive females. Although most research has been oriented on species in which males use mating calls to attract females, many high-density explosive breeding amphibians do not rely on mating calls. In such systems, larger and stronger males are thought to have significant advantages to access reproductive females. Testosterone (T) is expected to increase with the physical attributes that facilitate access to females, while increased corticosterone levels (CORT) are needed to sustain the energetic requirements associated with mating behaviour. In this study, we investigated how environmental temperature, and body size influence testosterone and corticosterone in males of an explosive winter-breeding species with low investment in mating call, the spined toad (Bufo spinosus). We found that both baseline CORT and T were positively correlated with environmental temperature. Interestingly, despite a remarkable range of variation in CORT and T, there was no evidence that either hormone was correlated with body size. Finally, we found no effect of confinement (13 h of captivity) on circulating CORT and T levels. This suggests that breeding male toads may be relatively insensitive to stress in order to maintain reproductive effort during their short mating period. Future studies should investigate both the influence of the phenology of breeding and the social interactions on these hormonal levels.

Survival and reproductive costs of repeated acute glucocorticoid elevations in a captive, wild animal

Organisms are continuously encountering both predictable and unpredictable ecological stressors within their environment. The activation of the hypothalamic-pituitaryadrenal (stress) axis is a fundamental process allowing animals to cope with and respond to such encounters. A main consequence of HPA axis activation is the release of glucocorticoid hormones. Although short-term glucocorticoid elevations lead to changes in physiological and behavioral processes that are often adaptive, our understanding of fitness consequences of repeated acute elevations in glucocorticoid hormones over a longer time period is largely lacking. This is of particular current importance as animals are facing a significant increase in exposure to stressors including those associated with human-induced rapid environmental change. Here, we test fitness-relevant consequences of repeated exposure to glucocorticoids in the absence of natural challenges, by treating wild-caught gravid female eastern fence lizards (Sceloporus undulatus) with a daily transdermal dose of a glucocorticoid hormone until laying. This treatment causes an increase in plasma glucocorticoids that mimics the natural response lizards have when they encounter a stressor in the wild, without confounding effects associated with the encounter itself. This treatment reduced females' reproductive success (hatching success) and survival. Further, glucocorticoid-induced reductions in reproductive success were greater when females had experienced higher temperatures the previous winter. This demonstrates the potential significant consequences of repeated exposure to acute elevations in glucocorticoid hormones. Additionally, the costs of repeated glucocorticoid elevation may be further exaggerated by an individual's previous experience, such as the potential compounding effects of winter warming increasing animals' vulnerability to increased glucocorticoid levels during spring breeding.

Chronic stress, energy transduction, and free-radical production in a reptile

Stress hormones, such as corticosterone, play a crucial role in orchestrating physiological reaction patterns shaping adapted responses to stressful environments. Concepts aiming at predicting individual and population responses to environmental stress typically consider that stress hormones and their effects on metabolic rate provide appropriate proxies for the energy budget. However, uncoupling between the biochemical processes of respiration, ATP production, and free-radical production in mitochondria may play a fundamental role in the stress response and associated life histories. In this study, we aim at dissecting sub-cellular mechanisms that link these three processes by investigating both whole-organism metabolism, liver mitochondrial oxidative phosphorylation processes (O₂ consumption and ATP production) and ROS emission in Zootoca vivipara individuals exposed 21 days to corticosterone relative to a placebo. Corticosterone enhancement had no effect on mitochondrial activity and efficiency. In parallel, the corticosterone treatment increased liver mass and mitochondrial protein content suggesting a higher liver ATP production. We also found a negative correlation between mitochondrial ROS emission and plasma corticosterone level. These results provide a proximal explanation for enhanced survival after chronic exposure to corticosterone in this species. Importantly, none of these modifications affected resting whole-body metabolic rate. Oxygen consumption, ATP, and ROS emission were thus independently affected in responses to corticosterone increase suggesting that concepts and models aiming at linking environmental stress and individual responses may misestimate energy allocation possibilities.

Development of behavioural profile in the Northern common boa (Boa imperator): Repeatable independent traits or personality?

Recent studies of animal personality have focused on its proximate causation and ecological and evolutionary significance in particular, but the question of its development was largely overlooked. The attributes of personality are defined as between-individual differences in behaviour, which are consistent over time (differential consistency) and contexts (contextual generality) and both can be affected by development. We assessed several candidates for personality variables measured in various tests with different contexts over several life-stages (juveniles, older juveniles, subadults and adults) in the Northern common boa. Variables describing foraging/feeding decision and some of the defensive behaviours expressed as individual average values are highly repeatable and consistent. We found two main personality axes-one associated with foraging/feeding and the speed of decision, the other reflecting agonistic behaviour. Intensity of behaviour in the feeding context changes during development, but the level of agonistic behaviour remains the same. The juveniles and adults have a similar personality structure, but there is a period of structural change of behaviour during the second year of life (subadults). These results require a new theoretical model to explain the selection pressures resulting in this developmental pattern of personality. We also studied the proximate factors and their relationship to behavioural characteristics. Physiological parameters (heart and breath rate stress response) measured in adults clustered with variables concerning the agonistic behavioural profile, while no relationship between the juvenile/adult body size and personality concerning feeding/foraging and the agonistic behavioural profile was found. Our study suggests that it is important for studies of personality development to focus on both the structural and differential consistency, because even though behaviour is differentially consistent, the structure can change.

Acute embryonic exposure to corticosterone alters physiology, behaviour and growth in nestlings of a wild passerine

Maternally-derived glucocorticoids can modify the normal development of young animals. To date, little is known about maternal effects that are mediated by acute embryonic exposure to glucocorticoids. In birds, elevated maternal transmission of corticosterone (CORT) to egg albumen is mainly dependent on acute stress. In this study, we increased CORT levels in the egg albumen of a wild passerine, the great tit (Parus major), breeding in favourable deciduous and less suitable coniferous habitat. Subsequently we measured the somatic growth, baseline and acute glucocorticoid responses, immunity and behaviour of prenatally manipulated offspring with respect to control siblings. We found that prenatally CORT-exposed nestlings had lower baseline CORT levels, a more rapid decline in CORT during recovery from a standardized stressor, and a reduced heterophil/lymphocyte ratio compared with controls. Although stress-induced total CORT levels remained unchanged, free CORT levels were significantly lower and the levels of corticosteroid binding globulins (CBG) significantly higher in experimental offspring. Prenatally CORT-exposed offspring begged longer after hatching than controls. Stress-induced behavioural activity of fledglings did not differ between treatments, while its association with baseline CORT levels was significant in the control group only. The body mass and tarsus length of fledglings was positively affected by manipulation in unfavourable coniferous habitat only. We conclude that maternal effects related to elevated levels of albumen CORT modify diverse aspects of offspring phenotype and potentially increase offspring performance in resource poor environments. Moreover, our results indicate that maternal glucocorticoids may suppress the effect of hormones on behavioural responses.

Wound-healing ability is conserved during periods of chronic stress and costly life history events in a wild-caught bird

Chronic stress, potentially through the actions of corticosterone, is thought to directly impair the function of immune cells. However, chronic stress may also have an indirect effect by influencing allocation of energy, ultimately shifting resources away from the immune system. If so, the effects of chronic stress on immune responses may be greater during energetically-costly life history events. To test whether the effects of chronic stress on immune responses differ during expensive life history events we measured wound healing rate in molting and non-molting European starlings (Sturnus vulgaris) exposed to control or chronic stress conditions. To determine whether corticosterone correlated with wound healing rates before starting chronic stress, we measured baseline and stress-induced corticosterone and two estimates of corticosterone release and regulation, negative feedback (using dexamethasone injection), and maximal capacity of the adrenals to secrete corticosterone (using adrenocorticotropin hormone [ACTH] injection). After 8days of exposure to chronic stress, we wounded both control and chronically stressed birds and monitored healing daily. We monitored nighttime heart rate, which strongly correlates with energy expenditure, and body mass throughout the study. Measures of corticosterone did not differ with molt status. Contrary to work on lizards and small mammals, all birds, regardless of stress or molt status, fully-healed wounds at similar rates. Although chronic stress did not influence healing rates, individuals with low baseline corticosterone or strong negative feedback had faster healing rates than individuals with high baseline corticosterone or weak negative feedback. In addition, wound healing does appear to be linked to energy expenditure and body mass. Non-molting, chronically stressed birds decreased nighttime heart rate during healing, but this pattern did not exist in molting birds. Additionally, birds of heavier body mass at the start of the experiment healed wounds more rapidly than lighter birds. Finally, chronically stressed birds lost body mass at the start of chronic stress, but after wounding all birds regardless of stress or molt status started gaining weight, which continued for the remainder of the study. Increased body mass could suggest compensatory feeding to offset energetic or resource demands (e.g., proteins) of wound healing. Although chronic stress did not inhibit healing, our data suggest that corticosterone may play an important role in mediating healing processes and that molt could influence energy saving tactics during periods of chronic stress. Although the experiment was designed to test allostasis, interpretation of data through reactive scope appears to be a better fit.

Endocrine consequences of an acute stress under different thermal conditions: A study of corticosterone, prolactin, and thyroid hormones in the pigeon (Columbia livia)

In the context of global change, the physiological and hormonal stress responses have received much attention because of their implications in terms of allostasis. However, most studies have focused on glucocorticoids only as the "common" response to stressors while neglecting other endocrine axes and hormones (e.g. prolactin, thyroid hormones) that play a crucial role in metabolic adjustments. Interestingly, the responsiveness of all these endocrine axes to stress may depend on the energetic context and this context-dependent stress response has been overlooked so far. In the wild, temperature can vary to a large extent within a short time window and ambient temperature may affect these metabolic-related endocrine axes, and potentially, their responsiveness to an acute stressor. Here, we explicitly tested this hypothesis by examining the effect of a standardized stress protocol on multiple hormonal responses in the rock pigeon (Columbia livia). We tested the effect of an acute restraint stress on (1) corticosterone levels, (2) prolactin levels, and (3) thyroid hormone levels (triiodothyronine, thyroxine) in pigeons that were held either at cool temperature (experimental birds) or at room temperature (control birds) during the stress protocol. Although we found a significant influence of restraint stress on most hormone levels (corticosterone, prolactin, and thyroxine), triiodothyronine levels were not affected by the restraint stress. This demonstrates that stressors can have significant impact on multiple endocrine mechanisms. Importantly, all of these hormonal responses to stress were not affected by temperature, demonstrating that the exposure to cold temperature does not affect the way these hormone levels change in response to handling stress. This suggests that some endocrine responses to temperature decreases may be overridden by the endocrine responses to an acute restraint stress.

Hormonal and metabolic responses to upper temperature extremes in divergent life-history ecotypes of a garter snake

Extreme temperatures constrain organismal physiology and impose both acute and chronic effects. Additionally, temperature-induced hormone-mediated stress response pathways and energetic trade-offs are important drivers of life-history variation. This study employs an integrative approach to quantify acute physiological responses to high temperatures in divergent life-history ecotypes of the western terrestrial garter snake (Thamnophis elegans). Using wild-caught animals, we measured oxygen consumption rate and physiological markers of hormonal stress response, energy availability, and anaerobic respiration in blood plasma across five ecologically relevant temperatures (24, 28, 32, 35, and 38° C; 3-hour exposure). Corticosterone, insulin, and glucose concentrations all increased with temperature, but with different thermal response curves, suggesting that high temperatures differently affect energy-regulation pathways. Additionally, oxygen consumption rate increased without plateau and lactate concentration did not increase with temperature, challenging the recent hypothesis that oxygen limitation sets upper thermal tolerance limits. Finally, animals had similar physiological thermal responses to high-temperature exposure regardless of genetic background, suggesting that local adaptation has not resulted in fixed differences between ecotypes. Together, these results identify some of the mechanisms by which higher temperatures alter hormonal-mediated energy balance in reptiles and potential limits to the flexibility of this response.

Elevated Corticosterone Levels and Changes in Amphibian Behavior Are Associated with Batrachochytrium dendrobatidis (Bd) Infection and Bd Lineage

Few studies have examined the role hormones play in mediating clinical changes associated with infection by the parasite Batrachochytrium dendrobatidis (Bd). Glucocorticoid (GC) hormones such as corticosteroids (CORT) regulate homeostasis and likely play a key role in response to infection in amphibians. We explore the relationship between CORT release rates and Bd infection in tadpoles of the common midwife toad, Alytes obstetricians, using a non-invasive water-borne hormone collection method across seven populations. We further examined whether tadpoles of A. muletensis infected with a hypervirulent lineage of Bd, BdGPL, had greater CORT release rates than those infected with a hypovirulent lineage, BdCAPE. Finally, we examined the relationship between righting reflex and CORT release rates in infected metamorphic toads of A. obstetricans. We found an interaction between elevation and Bd infection status confirming that altitude is associated with the overall severity of infection. In tandem, increasing elevation was associated with increasing CORT release rates. Tadpoles infected with the hypervirulent BdGPL had significantly higher CORT release rates than tadpoles infected with BdCAPE showing that more aggressive infections lead to increased CORT release rates. Infected metamorphs with higher CORT levels had an impaired righting reflex, our defined experimental endpoint. These results provide evidence that CORT is associated with an amphibian’s vulnerability to Bd infection, and that CORT is also affected by the aggressiveness of infection by Bd. Together these results indicate that CORT is a viable biomarker of amphibian stress.

Interplay among nocturnal activity, melatonin, corticosterone and performance in the invasive cane toad (Rhinella marinus)

Most animals conduct daily activities exclusively either during the day or at night. Here, hormones such as melatonin and corticosterone, greatly influence the synchronization or regulation of physiological and behavioral cycles needed for daily activity. How then do species that exhibit more flexible daily activity patterns, responses to ecological, environmental or life-history processes, regulate daily hormone profiles important to daily performance? This study examined the consequences of (1) nocturnal activity on diel profiles of melatonin and corticosterone and (2) the effects of experimentally increased acute melatonin levels on physiological and metabolic performance in the cane toad (Rhinella marinus). Unlike inactive captive toads that had a distinct nocturnal melatonin profile, nocturnally active toads sampled under field and captive conditions, exhibited decreased nocturnal melatonin profiles with no evidence for any phase shift. Nocturnal corticosterone levels were significantly higher in field active toads than captive toads. In toads with experimentally increased melatonin levels, plasma lactate and glucose responses following recovery post exercise were significantly different from control toads. However, exogenously increased melatonin did not affect resting metabolism in toads. These results suggest that toads could adjust daily hormone profiles to match nocturnal activity requirements, thereby avoiding performance costs induced by high nocturnal melatonin levels. The ability of toads to exhibit plasticity in daily hormone cycles, could have broad implications for how they and other animals utilize behavioral flexibility to optimize daily activities in response to natural and increasingly human mediated environmental variation.

Environmental acidification is not associated with altered plasma corticosterone levels in the stream-side salamander, Desmognathus ochrophaeus

As environments become increasingly altered due to anthropogenic factors, interest is growing in how endocrine systems respond to pollution and environmental degradation. Glucocorticoid hormones (GCs) are a type of stress hormones that are released upon activation of the hypothalamic-pituitary-adrenal axis and have widespread effects throughout the body. We tested the hypothesis that exposure to environmental acidification is associated with altered levels of plasma GCs in adult, stream-side Allegheny Mountain dusky salamanders (Desmognathus ochrophaeus). We compared plasma corticosterone (CORT) in salamanders living in 9 streams that differed in pH. Although capture and handling induced a robust increase in plasma CORT in all populations of salamanders, we discerned no significant effect of environmental pH on baseline CORT or handling-induced CORT levels. In a laboratory study, low pH decreased salamander locomotory activity compared to acid-neutral controls, but there was no effect of pH on plasma CORT. Decreased locomotory activity is a common amphibian response to stress, indicating that low pH has adverse effects on Allegheny Mountain dusky salamanders. Overall, we conclude that the effects of environmental pH on salamander behavior and other potential responses are not mediated by changes in plasma CORT levels. We discuss alternative explanations for our results and describe difficulties involved in searching for relationships between plasma GCs and environmental degradation.

Influence of temperature on the corticosterone stress-response: an experiment in the Children's python (Antaresia childreni)

To cope with environmental challenges, organisms have to adjust their behaviours and their physiology to the environmental conditions they face (i.e. allostasis). In vertebrates, such adjustments are often mediated through the secretion of glucocorticoids (GCs) that are well-known to activate and/or inhibit specific physiological and behavioural traits. In ectothermic species, most processes are temperature-dependent and according to previous studies, low external temperatures should be associated with low GC concentrations (both baseline and stress-induced concentrations). In this study, we experimentally tested this hypothesis by investigating the short term influence of temperature on the GC stress response in a squamate reptile, the Children's python (Antaresia childreni). Snakes were maintained in contrasting conditions (warm and cold groups), and their corticosterone (CORT) stress response was measured (baseline and stress-induced CORT concentrations), within 48h of treatment. Contrary to our prediction, baseline and stress-induced CORT concentrations were higher in the cold versus the warm treatment. In addition, we found a strong negative relationship between CORT concentrations (baseline and stress-induced) and temperature within the cold treatment. Although it remains unclear how cold temperatures can mechanistically result in increased CORT concentrations, we suggest that, at suboptimal temperature, high CORT concentrations may help the organism to maintain an alert state.

Adrenocortical stress responses influence an invasive vertebrate's fitness in an extreme environment

Continued range expansion into physiologically challenging environments requires invasive species to maintain adaptive phenotypic performance. The adrenocortical stress response, governed in part by glucocorticoid hormones, influences physiological and behavioural responses of vertebrates to environmental stressors. However, any adaptive role of this response in invasive populations that are expanding into extreme environments is currently unclear. We experimentally manipulated the adrenocortical stress response of invasive cane toads (Rhinella marina) to investigate its effect on phenotypic performance and fitness at the species' range front in the Tanami Desert, Australia. Here, toads are vulnerable to overheating and dehydration during the annual hot-dry season and display elevated plasma corticosterone levels indicative of severe environmental stress. By comparing unmanipulated control toads with toads whose adrenocortical stress response was manipulated to increase acute physiological stress responsiveness, we found that control toads had significantly reduced daily evaporative water loss and higher survival relative to the experimental animals. The adrenocortical stress response hence appears essential in facilitating complex phenotypic performance and setting fitness trajectories of individuals from invasive species during range expansion.

Host stress response is important for the pathogenesis of the deadly amphibian disease, Chytridiomycosis, in Litoria caerulea

Chytridiomycosis, a disease caused by Batrachochytrium dendrobatidis, has contributed to worldwide amphibian population declines; however, the pathogenesis of this disease is still somewhat unclear. Previous studies suggest that infection disrupts cutaneous sodium transport, which leads to hyponatremia and cardiac failure. However, infection is also correlated with unexplained effects on appetite, skin shedding, and white blood cell profiles. Glucocorticoid hormones may be the biochemical connection between these disparate effects, because they regulate ion homeostasis and can also influence appetite, skin shedding, and white blood cells. During a laboratory outbreak of B. dendrobatidis in Australian Green Tree Frogs, Litoria caerulea, we compared frogs showing clinical signs of chytridiomycosis to infected frogs showing no signs of disease and determined that diseased frogs had elevated baseline corticosterone, decreased plasma sodium and potassium, and altered WBC profiles. Diseased frogs also showed evidence of poorer body condition and elevated metabolic rates compared with frogs showing no signs of disease. Prior to displaying signs of disease, we also observed changes in appetite, body mass, and the presence of shed skin associated with infected but not yet diseased frogs. Collectively, these results suggest that elevated baseline corticosterone is associated with chytridiomycosis and correlates with some of the deleterious effects observed during disease development.

Nestling corticosterone response to microclimate in an altricial bird

Although altricial young are dependent on their parents during early life, they must respond to environmental variation to maintain homeostasis. The hormone corticosterone (CORT) may be an important link between environment and phenotype during early life; however, no previous study has experimentally assessed the sensitivity of CORT to nest microclimate in altricial birds beginning to thermoregulate. We tested the hypothesis that microclimate influences CORT by cross-fostering nestling Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) between thicker-walled “aspen” nest boxes and thinner-walled “plywood” nest boxes. Quantification of CORT in nestling feathers allowed us to consider hormone secreted over days, rather than instantaneously from blood samples. In agreement with our hypothesis, we detected significant positive relationships between feather CORT and increased temperature variability and maximum, but not minimum, nest box temperatures. This could reflect the energetic challenge of warmer temperatures or positive developmental effects on the nestling hypothalamic–pituitary–adrenal axis. Feather CORT was significantly lower in chicks hatched in aspen nest boxes compared with plywood ones, but cross-fostering did not influence nestling CORT. This suggests that the influence of natal nest box environment on feather CORT was likely greater than the influence of the foster nest box environment. The relationships we detected highlight the sensitivity of feather CORT to environmental variation and contribute insight into nestling responses to environmental change.

Effects of temperature on urinary corticosterone metabolite responses to short-term capture and handling stress in the cane toad (Rhinella marina)

Extreme temperature can cause metabolic, immune and behavioural changes in amphibians. Short-term stress hormonal response via increased secretion of corticosterone enables amphibians to make necessary physiological and behavioural adjustments for coping with stressors. The effect of temperature on short-term corticosterone responses has not been studied in amphibians. In this study, this relationship was evaluated in adult male cane toads (Rhinella marina). We acclimated male toads (n=24 toads per group) at low, medium and high temperature (15, 25 or 35°C) under controlled laboratory conditions for a 14 day period. After thermal acclimation, short-term corticosterone responses were evaluated in the toads subjected to a standard capture and handling stress protocol over a 24h period. Corticosterone metabolites in toad urine were measured via enzyme-immunoassay. During acclimation, mean baseline urinary corticosterone level increased after transfer of the toads from wild into captivity and returned to baseline on day 14 of acclimation for each of the three temperatures. At the end of the 14 days of thermal acclimation period, baseline corticosterone level were highest for toad group at 35°C and lowest at 15°C. All toads generated urinary corticosterone responses to the standard capture and handling stressor for each temperature. Both individual and mean short-term corticosterone responses of the toads were highest at 35°C and lowest at 15°C. Furthermore, Q(10) values (the factor by which the reaction rate increases when the temperature is raised by 10°) were calculated for mean corrected integrated corticosterone responses as follows; (15-35°C) Q(10)=1.51, (15-25°C) Q(10)=1.60; (25-35°C) Q(10)=1.43. Both total and corrected integrated corticosterone responses were highest for toads at 35°C followed by 25°C and lowest for the 15°C toad group. Overall, the results have demonstrated the thermodynamic response of corticosterone secretion to short-term capture and handling stress in an amphibian species.

Using operant conditioning and desensitization to facilitate veterinary care with captive reptiles

In addition to being a large component of most zoological collections, reptile species are becoming more popular as family pets. Reptiles have the cognitive ability to be trained to facilitate daily husbandry and veterinary care. Desensitization and operant conditioning can alleviate some of the behavioral and physiological challenges of treating these species. A survey of reptile training programs at zoos in the United States and worldwide reveals that there are many successful training programs to facilitate veterinary care and minimize stress to the animal. Many of the techniques being used to train reptiles in zoological settings are transferable to the exotic pet clinician.

Elevated plasma corticosterone increases metabolic rate in a terrestrial salamander

Plasma glucocorticoid hormones (GCs) increase intermediary metabolism, which may be reflected in whole-animal metabolic rate. Studies in fish, birds, and reptiles have shown that GCs may alter whole-animal energy expenditure, but results are conflicting and often involve GC levels that are not physiologically relevant. A previous study in red-legged salamanders found that male courtship pheromone increased plasma corticosterone (CORT; the primary GC in amphibians) concentrations in males, which could elevate metabolic processes to sustain courtship behaviors. To understand the possible metabolic effect of elevated plasma CORT, we measured the effects of male courtship pheromone and exogenous application of CORT on oxygen consumption in male red-legged salamanders (Plethodon shermani). Exogenous application of CORT elevated plasma CORT to physiologically relevant levels. Compared to treatment with male courtship pheromone and vehicle, treatment with CORT increased oxygen consumption rates for several hours after treatment, resulting in 12% more oxygen consumed (equivalent to 0.33 J) during our first 2h sampling period. Contrary to our previous work, treatment with pheromone did not increase plasma CORT, perhaps because subjects used in this study were not in breeding condition. Pheromone application did not affect respiration rates. Our study is one of the few to evaluate the influence of physiologically relevant elevations in CORT on whole-animal metabolism in vertebrates, and the first to show that elevated plasma CORT increases metabolism in an amphibian.

Development of the adrenocortical response to stress in Eurasian kestrel nestlings: defence ability, age, brood hierarchy and condition

The developmental hypothesis proposes that the adrenocortical response to stress during postnatal development in birds should not develop when the benefits of elevated corticosterone do not outweigh the deleterious effects on growth and development. We tested three predictions developed from this hypothesis in free-living, semi-altricial Eurasian kestrel Falco tinnunculus nestlings. We measured baseline and handling-induced corticosterone levels and the binding capacity of corticosteroid binding globulins CBG on day 10 and 21 of age and related these to age, the development of the defence behaviour, hatching asynchrony and fat stores (a measure of body condition). First, the adrenocortical response to handling (total plasma corticosterone) increased with age and thus during the time when nestlings developed the ability to defend themselves, but free corticosterone did not, because of a concomitant increase of CBG with age. Second, nestlings with adequate fat stores mounted a stronger adrenocortical stress response to an acute stressor, while nestlings with low fat stores avoided additional energy expenses. While baseline corticosterone levels were negatively related to fat stores, increase in corticosterone to handling was positively related. Third, both baseline corticosterone levels and the adrenocortical response to handling were not related to hatching order, but predominantly determined by body condition. The pattern of decreasing corticosterone levels with hatching order found in the lab seems to be neutralized by opposite effects of varying body condition on corticosterone levels in free-living birds. We argue that the postnatal adrenocortical response to stress is adaptively modulated by both variations in the release of corticosterone and in CBG, which is particularly important because elevated corticosterone may adversely affect the phenotype.

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.

Information from the geomagnetic field triggers a reduced adrenocortical response in a migratory bird

Long-distance migrants regularly pass ecological barriers, like the Sahara desert, where extensive fuel loads are necessary for a successful crossing. A central question is how inexperienced migrants know when to put on extensive fuel loads. Beside the endogenous rhythm, external cues have been suggested to be important. Geomagnetic information has been shown to trigger changes in foraging behaviour and fuel deposition rate in migratory birds. The underlying mechanism for these adjustments, however, is not well understood. As the glucocorticoid hormone corticosterone is known to correlate with behaviour and physiology related to energy regulation in birds, we here investigated the effect of geomagnetic cues on circulating corticosterone levels in a long-distance migrant. Just as in earlier studies, juvenile thrush nightingales (Luscinia luscinia) caught during autumn migration and exposed to the simulated geomagnetic field of northern Egypt increased food intake and attained higher fuel loads than control birds experiencing the ambient magnetic field of southeast Sweden. Our results further show that experimental birds faced a reduced adrenocortical response compared with control birds, thus for the first time implying that geomagnetic cues trigger changes in hormonal secretion enabling appropriate behaviour along the migratory route.

Interactions and trade-offs among physiological determinants of performance and reproductive success

How an animal performs in its natural environment ultimately plays a key role in its reproductive success. While a number of studies have investigated how selection acts on performance-related traits, far fewer studies have examined the mechanisms responsible for variation in performance. Among mechanisms, variable morphology has received the most attention. Although physiological traits have received less attention, they are intrinsically related to performance and ultimately to reproductive success. We present a framework whereby investigators can link some basic physiological functions with organismal performance and ultimately with reproductive success. We propose that performance and ultimately reproductive success are strongly influenced by hormones, immune functions, and energetics. We further argue that no physiological function can be considered in isolation and thus our model emphasizes interactions and trade-offs both within each physiological function as well as among them. Some of the most commonly studied trade-offs are between reproduction and immune functions, with energetics as one of the key common currencies for these trade-offs. From an evolutionary perspective, the largest gaps in our knowledge lie in how these interactions and trade-offs influence reproductive success. We believe that a full understanding of how hormones, immune functions, and energetics influence performance traits related to reproduction and, ultimately, lifetime reproductive success requires recognition of the complex relationships, interactions, and trade-offs among these processes.

Effects of a short period of elevated circulating corticosterone on postnatal growth in free-living Eurasian kestrels Falco tinnunculus

Environmental conditions affect growth and development and, through developmental plasticity, create phenotypic variation. In suboptimal conditions current survival is traded-off against development. Corticosterone, the main glucocorticoid in birds, may be involved in the reallocation of energy from growth to maintenance, but its effect on growth has rarely been investigated in altricial birds under natural conditions in the wild. In free-living Eurasian kestrel Falco tinnunculus nestlings, we artificially elevated corticosterone to stress-induced levels over 2-3 days in the middle of the nestling stage by implanting biodegradable implants, controlling the treatment with a placebo group. We measured the length of primary feather 8, hand length, tarsus length, body mass and subcutaneous fat stores from day 10 to 25. During corticosterone elevation, primary growth of cort-nestlings was significantly reduced to 71% of placebo-nestlings, hand and tarsus growth were significantly reduced to 14% and 26% of placebo-nestlings, respectively, and body mass increase stopped, while subcutaneous fat-store growth was not affected. Over the following 5 days, primary growth was still significantly suppressed to 84% of placebo-nestlings, while hand, tarsus and body mass growth were back to normal. During the subsequent 4 days, cort-nestlings partly compensated for the lag in body mass by significantly accelerating the body mass increase compared with placebo-nestlings. Before fledging, primary length was 10% shorter, hand and tarsus 5% and 4% shorter and body mass 8.5% lower in cort-nestlings than in placebo-nestlings, while fat score did not differ significantly between the two groups. Thus, we have shown that in free-living, altricial nestlings a few days of elevated plasma corticosterone levels alone, without food restriction, suppressed growth and this could only partly be compensated for afterwards. Feather, bone and body mass growth were reduced to different degrees, indicating that corticosterone had a differential effect on different structures. This demonstrates that corticosterone is probably involved in the control of developmental plasticity.