Effects of corticosterone on territorial behavior of free-living male song sparrows Melospiza melodia

Corticosterone treatment results in fat deposition and body mass maintenance without effects on feeding behaviour or immunity in female lizards (Tropidurus catalanensis)

Throughout life, animals must maintain homeostasis while coping with challenging events. The period after reproduction can be challenging for oviparous females to maintain homeostasis since they direct most of their energy stores to vitellogenesis, possibly increasing the vulnerability to stressors. Changes in glucocorticoids' (GC) secretion promote various behavioural and physiological adjustments daily and to restore balance after facing stressors. However, when GC are elevated for extended periods, which usually occurs in response to chronic exposure to stressors, they can affect feeding behaviour and suppress the immune function. We aim to elucidate the effects of chronic corticosterone (CORT) exposure on feeding behaviour, body condition and immune function in female lizards, Tropidurus catalanensis, in the post-reproductive period. Thirty animals were divided into three groups: 1. Control (no experimental procedure performed); 2. Empty Implant (animals implanted with empty silastic tube); and 3. CORT Implant (animals implanted with silastic tube filled with CORT, with a chronic continuous release for at least a week). CORT plasma levels feeding behaviour, body condition (body index [BI] and fat index [FI]), leukocyte count, and several immune function variables (bacterial killing ability [BKA], hemagglutination titer, phytohemagglutinin [PHA] immune challenge and leukocyte count) were evaluated. After implantation, CORT treated animals maintained stable body mass through the experiment, while Control and Empty Implant groups displayed weight loss. In the CORT treated animals, there was also a positive relation between BI and FI, and higher FI when compared to groups 1 and 2. No effects of CORT were observed on feeding behaviour nor on the immune function.

Experimentally elevated corticosterone increases song output and complexity in common mynas

Vocalization is an important communication tool that can reflect many aspects of an individual's internal and external condition. This is especially true for birds. Previous research has shown that bird calls and songs change in response to a variety of potential stressors, although the extent and direction of the changes depend on the nature of the stressor and the environment. Circulating glucocorticoids, such as corticosterone, often increase in response to stressors and mediate some of the observed changes via alterations of the individual's physiological state. Acute elevations of corticosterone often occur as a physiological response to short-term stressors; however, the effects of this elevation on adult vocalizations have not been well documented. Here, we experimentally elevated corticosterone at two different levels using a noninvasive method and examined the effects on the vocal communication of male and female adult common mynas (Acridotheres tristis). Corticosterone elevation temporarily increased song output and some measures of song complexity, while call output decreased. These effects were dosage dependent (higher corticosterone levels had a stronger effect), most evident 40 min after ingestion, and some vocal changes were sex-specific. Future studies should investigate whether the changes in vocal performance due to elevated glucocorticoids have consequences for the birds' behavior, reproductive success, and survival.

Effect of a Territorial Challenge on the Steroid Profile of a Juvenile Songbird

Aggression is a social behavior that is critical for survival and reproduction. In adults, circulating gonadal hormones, such as androgens, act on neural circuits to modulate aggressive interactions, especially in reproductive contexts. In many species, individuals also demonstrate aggression before reaching gonadal maturation. Adult male song sparrows, Melospiza melodia, breed seasonally but maintain territories year-round. Juvenile (hatch-year) males aggressively compete for territory ownership during their first winter when circulating testosterone is low. Here, we characterized the relationship between the steroid milieu and aggressive behavior in free-living juvenile male song sparrows in winter. We investigated the effect of a 10 min simulated territorial intrusion (STI) on behavior and steroid levels in blood, 10 microdissected brain regions, and four peripheral tissues (liver, pectoral muscle, adrenal glands, and testes). Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we quantified 12 steroids: pregnenolone, progesterone, corticosterone, 11-dehydrocorticosterone, dehydroepiandrosterone, androstenedione, testosterone, 5α-dihydrotestosterone, 17β-estradiol, 17α-estradiol, estrone, and estriol. We found that juvenile males are robustly aggressive, like adult males. An STI increases progesterone and corticosterone levels in blood and brain and increases 11-dehydrocorticosterone levels in blood only. Pregnenolone, androgens, and estrogens are generally non-detectable and are not affected by an STI. In peripheral tissues, steroid concentrations are very high in the adrenals. These data suggest that adrenal steroids, such as progesterone and corticosterone, might promote juvenile aggression and that juvenile and adult songbirds might rely on distinct neuroendocrine mechanisms to support similar aggressive behaviors.

Circulating hormones and dominance status predict female behavior during courtship in a lekking species

Female competitive behaviors during courtship can have substantial fitness consequences yet we know little about the physiological and social mechanisms underlying these behaviors - particularly for females of polygynous lek mating species. We explored the hormonal and social drivers of female intersexual and intrasexual behavior during courtship by males in a captive population of Indian peafowl. We investigated whether (1) female non-stress induced circulating estradiol (E2) and corticosterone (CORT) levels or (2) female dominance status in a dyad predict female solicitation behavior. We also tested whether female circulating E2 and CORT predict dominant females' aggressive behaviors toward subordinate females in the courtship context. Our findings demonstrate that females with higher levels of circulating E2 as well as higher levels of circulating CORT solicit more courtships from males. Dominant females also solicit more courtships from males than subordinate females. Female intrasexual aggressive behaviors during courtship, however, were not associated with circulating levels of E2 or CORT. Overall, we conclude that circulating steroid hormones in conjunction with social dominance might play a role in mediating female behaviors associated with competition for mates. Experimental manipulation and measures of hormonal flexibility throughout the breeding season in relation to competitive and sexual behaviors will be necessary to further examine the link between hormonal mechanisms and female behavior in polygynous lekking systems.

Chronic stress reverses enhanced neophobia following an acute stressor in European starlings

Neophobia is an animal's avoidance of novelty. Animals tend to respond to novel objects by increasing their latency to approach the objects, and they eventually habituate after repeated exposure by attenuating this increased approach latency. Interestingly, the physiological stress response does not appear to have a causal link to neophobia, although acute stress can prevent animals from habituating to novel objects, possibly through a permissive effect. Chronic stress can induce an anxiety-like state in animals, while often disrupting the ability to respond to acute stress. We thus hypothesized that chronic stress may increase neophobia and tested this by inducing chronic stress in wild-caught European starlings (Sturnus vulgaris). Four distinct anthropogenic stressors were administered daily for 30 min each in a randomized order for 21 days. We then evaluated whether exposure to chronic stress altered the latency to approach a novel object placed on or near a food dish presented after overnight fasting. Chronically stressed birds and nonstressed controls exhibited similar initial neophobic responses to novel objects and showed similar habituation in response to repeated exposure. However, when birds were exposed to 15 min of restraint before repeated exposure to the same object, habituation was eliminated in control birds (i.e., they continued to respond with neophobia), whereas chronically stressed birds continued to show habituation as measured by attenuated approach latencies. These results demonstrate that an acute stress response (restraint) has a different impact on neophobia depending upon whether the bird is or is not concurrently exposed to chronic stress.

Using the van Noordwijk and de Jong Resource Framework to Evaluate Glucocorticoid-Fitness Hypotheses

Ten years ago, two reviews clarified the need to tie glucocorticoid (GC) levels directly to survival and reproductive measures. Three primary hypotheses emerged from that work: the CORT-Fitness hypothesis, the CORT-Adaptation hypothesis, and the CORT-Tradeoff hypothesis. The two reviews have since been cited nearly 900 times, but no clear consensus has emerged supporting one hypothesis over another. We propose that resource availability may be a major confound across studies. Life-history investment is determined by both allocation and acquisition, but current literature testing among the three GC-fitness hypotheses rarely incorporate metrics of resource availability. In 1986, van Noordwijk and de Jong (vN and dJ) proposed the acquisition/allocation Y-model to explain positive versus negative correlations between reproduction and survival across individuals. Their model elevated resources as critical to evaluating individual allocation strategies (favoring reproduction vs. survival), and therefore provides the ideal framework for testing across the three CORT hypotheses. Here, we review the three hypotheses in light of the last 10 years of data, introduce the vN and dJ framework as a model for fitness/GC hypothesis testing, and discuss best practices for using this framework. We believe incorporation of resource availability will reduce unexplained variability in GC-fitness tests, clarify support among the three hypotheses, and allow for greater power in testing across other context dependencies (e.g., life-history strategy) that likely regulate differential allocation to reproduction versus survival as GCs increase.

Testosterone, cortisol, and status-striving personality features: A review and empirical evaluation of the Dual Hormone hypothesis

Decades of research in behavioral endocrinology has implicated the gonadal hormone testosterone in the regulation of mating effort, often expressed in primates in the form of aggressive and/or status-striving behavior. Based on the idea that neuroendocrine axes influence each other, recent work among humans has proposed that links between testosterone and indices of status-striving are rendered conditional by the effects of glucocorticoids. The Dual Hormone hypothesis is one particular instance of this argument, predicting that cortisol blocks the effects of testosterone on dominance, aggression, and risk-taking in humans. Support for the Dual Hormone hypothesis is wide-ranging, but considerations of theoretical ambiguity, null findings, and low statistical power pose problems for interpreting the published literature. Here, we contribute to the development of the Dual Hormone hypothesis by (1) critically reviewing the extant literature-including p-curve analyses of published findings; and, (2) "opening the file drawer" and examining relationships between testosterone, cortisol, and status-striving personality features in seven previously published studies from our laboratories (total N = 718; median N per feature = 318) that examined unrelated predictions. Results from p-curve suggest that published studies have only 16% power to detect effects, while our own data show no robust interactions between testosterone and cortisol in predicting status-striving personality features. We discuss the implications of these results for the Dual Hormone hypothesis, limitations of our analyses, and the development of future research.

Corticosterone profiles in northern cardinals (Cardinalis cardinalis): Do levels vary through life history stages?

As animals move through life history stages, energy requirements for each stage will vary. Both daily and annual variation in the glucocoriticoid hormones (specifically corticosterone, or CORT, in birds) helps provide the variable energy needed through life history stages. In many bird species, CORT is higher in the breeding season when energy demands can be quite high and is often lower in the non-breeding season. Additionally, CORT has a role to play in the response to stressful stimuli and the level to which CORT is elevated following stressful events can vary through the annual cycle as well. Here we report on baseline and stress-induced CORT levels in both sexes of northern cardinals, Cardinalis cardinalis, a non-migrating, year-round territorial species across life history stages. Corticosterone is overall higher in the non-breeding season than the breeding season in both sexes. Males tend to have higher levels of stress-induced CORT than females, although the observed patterns are complex. Our findings differ from one of the more common profile reported in songbirds where breeding season CORT tends to be higher than non-breeding levels. A strong influence may be the prolonged breeding season seen in cardinals; lower levels of CORT during breeding may guard against adverse maternal effects, interruptions in breeding behaviors, or egg production. Additional investigation of species with similar ecologies to northern cardinals, and more populations of cardinals, may show that annual glucocorticoid profiles are more labile than previously appreciated.

Roles and Mechanistic Bases of Glucocorticoid Regulation of Avian Reproduction

To maximize fitness, organisms must invest energetic and nutritional resources into developing, activating, and maintaining reproductive physiology and behavior. Corticosterone (CORT), the primary avian glucocorticoid, regulates energetic reserves to meet metabolic demands. At low (baseline) plasma levels, CORT activates avian mineralocorticoid receptors and may stimulate lipid mobilization, foraging activity, and feeding behavior. During stress in birds, elevated plasma CORT also stimulates glucocorticoid receptors and may promote glycemia, lipolysis, and proteolysis. Furthermore, CORT orchestrates physiological and behavioral adjustments to perceived threats. While many avian studies demonstrate effects of CORT on reproduction, few studies have elucidated the mechanisms, including receptor activation and site(s) of action, which underlie these effects. Even fewer studies have investigated how low and elevated plasma CORT regulates energetic reserves to meet the metabolic demands of reproduction. Here, we propose several hypotheses to clarify the direct and indirect effects of CORT on avian reproductive physiology and behavior. In addition, we emphasize the need for new manipulative studies involving alterations of endogenous plasma CORT levels and/or food availability to elucidate how CORT regulates the energetic demands of reproduction.

Effects of acute corticosterone treatment on male prairie voles (Microtus ochrogaster): Territorial aggression does not accompany induced social preference

Corticosterone (CORT) is a stress-related steroid hormone found in vertebrates, and is known to interact with behavior. In the socially monogamous prairie vole (Microtus ochrogaster), acute stress and specifically acute CORT administration have been shown to facilitate male social preference for a familiar female, and this effect has been described as facilitation of the monogamous pair bond. It is possible, however, that the effects of stress on social preference may initially represent a short-term coping strategy. Here we test whether the effect of acute CORT administration extends to territoriality, a defining component of the prairie vole monogamous suite of behaviors. Onset of territoriality would provide further support for an induced pair bond, whereas no increase in aggression would suggest an initial coping response. Using acute exogenous CORT injections followed by behavioral trials, we found a facilitation of social preference, but we did not find increased aggression. This result suggests that the social preference that develops in response to CORT is at least in part a coping response rather than facilitation of comprehensive monogamous pair bond behavior. Our results are consistent with previous studies both within prairie voles and across other taxa that suggest that social contact may be involved in the regulation of stress responses. (PsycINFO Database Record

Baseline and stress-induced blood properties of male and female Darwin's small ground finch (Geospiza fuliginosa) of the Galapagos Islands

Birds are renowned for exhibiting marked sex-specific differences in activity levels and reproductive investment during the breeding season, potentially impacting circulating blood parameters associated with stress and energetics. Males of many passerines often do not incubate, but they experience direct exposure to intruder threat and exhibit aggressive behaviour during the nesting phase in order to defend territories against competing males and predators. Nesting females often have long bouts of inactivity during incubation, but they must remain vigilant of the risks posed by predators and conspecific intruders approaching the nest. Here, we use 33 free-living male (n = 16) and female (n = 17) Darwin's small ground finches (Geospiza fuliginosa) on Floreana Island (Galapagos Archipelago) to better understand how sex-specific roles during the reproductive period impact baseline and stress-induced levels of plasma corticosterone (CORT), blood glucose and haematocrit. Specifically, we hypothesise that males are characterised by higher baseline values given their direct and relatively frequent exposure to intruder threat, but that a standardised stress event (capture and holding) overrides any sex-specific differences. In contrast with expectations, baseline levels of all blood parameters were similar between sexes (13.4 ± 1.9 ng ml^-1 for CORT, 13.7 ± 0.4 mmol l^-1 for glucose, 58.3 ± 0.8% for haematocrit). Interestingly, females with higher body condition had lower baseline haematocrit. All blood parameters changed with time since capture (range 1.2-41.3 min) in both sexes, whereby CORT increased linearly, haematocrit decreased linearly, and glucose increased to a peak at ∼20 min post-capture and declined to baseline levels thereafter. Our results do not support the hypothesis that sex-specific roles during the reproductive period translate to differences in blood parameters associated with stress and energetics, but we found some evidence that blood oxygen transport capacity may decline as finches increase in body condition.

Understanding immune function as a pace of life trait requires environmental context

This article provides a brief historical perspective on the integration of physiology into the concept of the pace of life of birds, evaluates the fit of immune function into this framework, and asks what it will take to fruitfully understand immune functioning of birds in pace of life studies in the future. In the late 1970s, physiology started to seriously enter avian life history ecology, with energy as the main currency of interest, inspired by David Lack's work in the preceding decades emphasizing how food availability explained life history variation. In an effort to understand the trade-off between survival and reproduction, and specifically the mortality costs associated with hard work, in the 1980s and 1990s, other physiological phenomena entered the realm of animal ecologists, including endocrinology, oxidative stress, and immunology. Reviewing studies thus far to evaluate the role of immune function in a life history context and particularly to address the questions whether immune function (1) consistently varies with life history variation among free-living bird species and (2) mediates life history trade-offs in experiments with free-living bird species; I conclude that, unlike energy metabolism, the immune system does not closely covary with life history among species nor mediates the classical trade-offs within individuals. Instead, I propose that understanding the tremendous immunological variation uncovered among free-living birds over the past 25 years requires a paradigm shift. The paradigm should shift from viewing immune function as a costly trait involved in life history trade-offs to explicitly including the benefits of the immune system and placing it firmly in an environmental and ecological context. A first step forward will be to quantify the immunobiotic pressures presented by diverse environmental circumstances that both shape and challenge the immune system of free-living animals. Current developments in the fields of infectious wildlife diseases and host-microbe interactions provide promising steps in this direction.

Stress hormones predict a host superspreader phenotype in the West Nile virus system

Glucocorticoid stress hormones, such as corticosterone (CORT), have profound effects on the behaviour and physiology of organisms, and thus have the potential to alter host competence and the contributions of individuals to population- and community-level pathogen dynamics. For example, CORT could alter the rate of contacts among hosts, pathogens and vectors through its widespread effects on host metabolism and activity levels. CORT could also affect the intensity and duration of pathogen shedding and risk of host mortality during infection. We experimentally manipulated songbird CORT, asking how CORT affected behavioural and physiological responses to a standardized West Nile virus (WNV) challenge. Although all birds became infected after exposure to the virus, only birds with elevated CORT had viral loads at or above the infectious threshold. Moreover, though the rate of mortality was faster in birds with elevated CORT compared with controls, most hosts with elevated CORT survived past the day of peak infectiousness. CORT concentrations just prior to inoculation with WNV and anti-inflammatory cytokine concentrations following viral exposure were predictive of individual duration of infectiousness and the ability to maintain physical performance during infection (i.e. tolerance), revealing putative biomarkers of competence. Collectively, our results suggest that glucocorticoid stress hormones could directly and indirectly mediate the spread of pathogens.

Diet and condition of mesopredators on coral reefs in relation to shark abundance

Reef sharks may influence the foraging behaviour of mesopredatory teleosts on coral reefs via both risk effects and competitive exclusion. We used a "natural experiment" to test the hypothesis that the loss of sharks on coral reefs can influence the diet and body condition of mesopredatory fishes by comparing two remote, atoll-like reef systems, the Rowley Shoals and the Scott Reefs, in northwestern Australia. The Rowley Shoals are a marine reserve where sharks are abundant, whereas at the Scott Reefs numbers of sharks have been reduced by centuries of targeted fishing. On reefs where sharks were rare, the gut contents of five species of mesopredatory teleosts largely contained fish while on reefs with abundant sharks, the same mesopredatory species consumed a larger proportion of benthic invertebrates. These measures of diet were correlated with changes in body condition, such that the condition of mesopredatory teleosts was significantly poorer on reefs with higher shark abundance. Condition was defined as body weight, height and width for a given length and also estimated via several indices of condition. Due to the nature of natural experiments, alternative explanations cannot be discounted. However, the results were consistent with the hypothesis that loss of sharks may influence the diet and condition of mesopredators and by association, their fecundity and trophic role. Regardless of the mechanism (risk effects, competitive release, or other), our findings suggest that overfishing of sharks has the potential to trigger trophic cascades on coral reefs and that further declines in shark populations globally should be prevented to protect ecosystem health.

The Ecology of Stress: linking life-history traits with physiological control mechanisms in free-living guanacos

BACKGROUND

Providing the context for the evolution of life-history traits, habitat features constrain successful ecological and physiological strategies. In vertebrates, a key response to life's challenges is the activation of the Stress (HPA) and Gonadal (HPG) axes. Much of the interest in stress ecology is motivated by the desire to understand the physiological mechanisms in which the environment affects fitness. As reported in the literature, several intrinsic and extrinsic factors affect variability in hormone levels. In both social and non-social animals, the frequency and type of interaction with conspecifics, as well as the status in social species, can affect HPA axis activity, resulting in changes in the reproductive success of animals. We predicted that a social environment can affect both guanaco axes by increasing the secretion of testosterone (T) and Glucocorticoid (GCs) in response to individual social interactions and the energetic demands of breeding. Assuming that prolonged elevated levels of GCs over time can be harmful to individuals, it is predicted that the HPA axis suppresses the HPG axis and causes T levels to decrease, as GCs increase.

METHODS

All of the data for individuals were collected by non-invasive methods (fecal samples) to address hormonal activities. This is a novel approach in physiological ecology because feces are easily obtained through non-invasive sampling in animal populations.

RESULTS

As expected, there was a marked adrenal (p-value = .3.4e-12) and gonadal (p-value = 0.002656) response due to seasonal variation in Lama guanicoe. No significant differences were found in fecal GCs metabolites between males/females*season for the entire study period (p-value = 0.2839). Despite the seasonal activity variation in the hormonal profiles, our results show a positive correlation (p-value = 1.952e-11, COR = 0.50) between the adrenal and gonadal system. The marked endocrine (r2 = 0.806) and gonad (r2 = 0.7231) response due to seasonal variation in male guanaco individuals highlights the individual's energetic demands according to life-history strategies. This is a remarkable result because no inhibition was found between the axes as theory suggests. Finally, the dataset was used to build a reactive scope model for guanacos.

DISCUSSION

Guanacos cope with the trade-off between sociability and reproductive benefits and costs, by regulating their GCs and T levels on a seasonal basis, suggesting an adaptive role of both axes to different habitat pressures. The results presented here highlight the functional role of stress and gonad axes on a critical phase of a male mammal's life-the mating period-when all of the resources are at the disposal of the male and must be used to maximize the chances for reproductive success.

Beeswax corticosterone implants produce long-term elevation of plasma corticosterone and influence condition

Glucocorticoids can play a critical role in modulating life-history trade-offs. However, studying the effects of glucocorticoids on life-history often requires experimentally elevating plasma glucocorticoid concentrations for several weeks within normal physiological limits and without repeated handling of the animal. Recently, implants made of beeswax and testosterone (T) were shown to have release dynamics superior to some currently available T implants, and these beeswax implants dissolved, eliminating the need to recapture the animal. We evaluated the utility of beeswax implants containing four different dosages of corticosterone (CORT; the primary glucocorticoid in birds) and their effect on several condition indices in a captive colony of zebra finches (Taeniopygia guttata). The three implants with the greatest CORT doses (0.05, 0.1, and 0.5mg) produced spikes in plasma CORT concentrations 20h after treatment, but were within the limits that zebra finches may normally experience. The 0.5mg CORT implant elevated plasma CORT between typical baseline and restraint stress levels reported in other studies of zebra finches for the entire 35day experiment. Birds in the 0.5mg implant group were heavier, had greater furcular fat scores, and had lower hematocrit than birds in the control and other CORT implant groups. Beeswax CORT implants are a low cost method of elevating plasma CORT for a prolonged time. Furthermore, because there is no need to remove these implants at the end of a study, this method may be amenable to studies of free-ranging animals.

Does corticosterone regulate the onset of breeding in free-living birds?: The CORT-Flexibility Hypothesis and six potential mechanisms for priming corticosteroid function

For many avian species, the decision to initiate breeding is based on information from a variety of environmental cues, including photoperiod, temperature, food availability, and social interactions. There is evidence that the hormone corticosterone may be involved in delaying the onset of breeding in cases where supplemental cues, such as low food availability and inclement weather, indicate that the environment is not suitable. However, not all studies have found the expected relationships between breeding delays and corticosterone titers. In this review, we present the hypothesis that corticosterone physiology mediates flexibility in breeding initiation (the "CORT-Flexibility Hypothesis"), and propose six possible corticosterone-driven mechanisms in pre-breeding birds that may delay breeding initiation: altering hormone titers, negative feedback regulation, plasma binding globulin concentrations, intracellular receptor concentrations, enzyme activity and interacting hormone systems. Based on the length of the breeding season and species-specific natural history, we also predict variation in corticosterone-regulated pre-breeding flexibility. Although few studies thus far have examined mechanisms beyond plasma hormone titers, the CORT-Flexibility Hypothesis is grounded on a solid foundation of research showing seasonal variation in the physiological stress response and knowledge of physiological mechanisms modulating corticosteroid effects. We propose six possible mechanisms as testable and falsifiable predictions to help clarify the extent of HPA axis regulation of the initiation of breeding.

Determinants and significance of corticosterone regulation in the songbird brain

Songbirds exhibit significant adult neuroplasticity that, together with other neural specializations, makes them an important model system for neurobiological studies. A large body of work also points to the songbird brain as a significant target of steroid hormones, including corticosterone (CORT), the primary avian glucocorticoid. Whereas CORT positively signals the brain for many functions, excess CORT may interfere with natural neuroplasticity. Consequently, mechanisms may exist to locally regulate CORT levels in brain to ensure optimal concentrations. However, most studies in songbirds measure plasma CORT as a proxy for levels at target tissues. In this paper, we review literature concerning circulating CORT and its effects on behavior in songbirds, and discuss recent work suggesting that brain CORT levels are regulated independently of changes in adrenal secretion. We review possible mechanisms for CORT regulation in the avian brain, including corticosteroid-binding globulins, p-glycoprotein activity in the blood-brain barrier and CORT metabolism by the 11ß hydroxysteroid dehydrogenases. Data supporting a role for CORT regulation within the songbird brain have only recently begun to emerge, suggesting that this is an avenue for important future research.

Prior exposure to capture heightens the corticosterone and behavioural responses of little penguins (Eudyptula minor) to acute stress

Studies of physiology can provide important insight into how animals are coping with challenges in their environment and can signal the potential effects of exposure to human activity in both the short and long term. In this study, we measured the physiological and behavioural response of little penguins (Eudyptula minor) that were naïve to human activity over 30 min of capture and handling. We assessed relationships between corticosterone secretion, behaviour, sex and time of day in order to characterize the determinants of the natural stress response. We then compared the response of these naïve penguins with the responses of female little penguins that had been exposed to research activity (bimonthly nest check and weighing) and to both research activity (monthly nest check and weighing) and evening viewing by tourists. We found that corticosterone concentrations increased significantly over 30 min of capture, with naïve penguins demonstrating a more acute stress response during the day than at night. Penguins that had previously been exposed to handling at the research and research/visitor sites showed elevated corticosterone concentrations and consistently more aggressive behaviour after 30 min compared with naïve birds, although there were no significant differences in baseline corticosterone concentrations. Our findings demonstrate that these little penguins have not habituated to routine capture, but rather mount a heightened physiological and behavioural response to handling by humans. Less invasive research monitoring techniques, such as individual identification with PIT tags and automatic recording and weighing, and a reduction in handling during the day should be considered to mitigate some of the potentially negative effects of disturbance. Given the paucity of data on the long-term consequences of heightened stress on animal physiology, our study highlights the need for further investigation of the relationship between the corticosterone stress response and fitness outcomes, such as breeding success and survival.

Effects of human disturbance on cave-nesting seabirds: the case of the storm petrel

Human disturbance is an important stress factor with potentially strong impact on breeding activity in animals. The consequences can be extinction of the breeding population, because disturbed animals might desert their breeding area and find no suitable substitute area. In this study, we investigated the effects of anthropogenic disturbance on a breeding population of Mediterranean storm petrels. Seabirds are increasingly used as bio-indicators for sea environmental parameters, because they are very sensitive to changing conditions. Burrowing or cave-nesting species may be particularly susceptible to human disturbance because their direct contact with humans is usually minimal or absent. First, we compared two different populations (exposed or not exposed to human disturbance) for their individual stress response to a standardized stressor (handling and keeping in a cloth bag). Second, we compared the two sub-colonies for their population-level stress response. Third, we tested experimentally whether sub-colonies of storm petrels exposed to tourism have physiological adaptations to anthropogenic disturbances. Our results indicate that storm petrels may be habituated to moderate disturbance associated with boat traffic close to the colony.

Disturbance of wildlife by outdoor winter recreation: allostatic stress response and altered activity-energy budgets

Anthropogenic disturbance of wildlife is of growing conservation concern, but we lack comprehensive approaches of its multiple negative effects. We investigated several effects of disturbance by winter outdoor sports on free-ranging alpine Black Grouse by simultaneously measuring their physiological and behavioral responses. We experimentally flushed radio-tagged Black Grouse from their snow burrows, once a day, during several successive days, and quantified their stress hormone levels (corticosterone metabolites in feces [FCM] collected. from individual snow burrows). We also measured feeding time allocation (activity budgets reconstructed from radio-emitted signals) in response to anthropogenic disturbance. Finally, we estimated the related extra energy expenditure that may be incurred: based on activity budgets, energy expenditure was modeled from measures of metabolism obtained from captive birds subjected to different ambient temperatures. The pattern of FCM excretion indicated the existence of a funneling effect as predicted by the allostatic theory of stress: initial stress hormone concentrations showed a wide inter-individual variation, which decreased during experimental flushing. Individuals with low initial pre-flushing FCM values augmented their concentration, while individuals with high initial FCM values lowered it. Experimental disturbance resulted in an extension of feeding duration during the following evening foraging bout, confirming the prediction that Black Grouse must compensate for the extra energy expenditure elicited by human disturbance. Birds with low initial baseline FCM concentrations were those that spent more time foraging. These FCM excretion and foraging patterns suggest that birds with high initial FCM concentrations might have been experiencing a situation of allostatic overload. The energetic model provides quantitative estimates of extra energy expenditure. A longer exposure to ambient temperatures outside the shelter of snow burrows, following disturbance, could increase the daily energy expenditure by > 10%, depending principally on ambient temperature and duration of exposure. This study confirms the predictions of allostatic theory and, to the best of our knowledge, constitutes the first demonstration of a funneling effect. It further establishes that winter recreation activities incur costly allostatic behavioral and energetic adjustments, which call for the creation of winter refuge areas together with the implementation of visitor-steering measures for sensitive wildlife.

Seasonal variation in glucocorticoid and mineralocorticoid receptors in metabolic tissues of the house sparrow (Passer domesticus)

Glucocorticoid hormones like corticosterone (CORT) play essential metabolic roles at both baseline and stress-induced concentrations, and CORT titers vary seasonally in patterns occurring across many different vertebrate species. It has been hypothesized that CORT may vary seasonally due to changing energy requirements at different times of year. However, hormone effects are dependent on binding to receptors in target tissues, and receptors might also vary seasonally. CORT alters metabolism primarily through binding to two receptors, the high-affinity mineralocorticoid receptor (MR) and low-affinity glucocorticoid receptor (GR). We quantified GR and MR in metabolic tissues (liver, kidney, omental and subcutaneous fat, and gastrocnemius and pectoralis muscle) of wild-caught house sparrows (Passer domesticus) to assess these tissues' capacity to respond to CORT-mediated metabolic demands. We quantified receptors using radioligand binding assays in early and late winter, pre-egg-laying, breeding, late breeding and molt (n=12 at each stage). MR binding did not vary significantly in any tissue over the course of the year. Because MR is associated with baseline CORT effects, this suggests that changing hormone titers may primarily regulate baseline CORT effects on metabolism. Seasonal modulation of GR binding occurred in every tissue but omental fat, though peak receptor density did not coincide with peak stress-induced CORT concentrations measured previously. Because GR is associated with stress-induced CORT effects, these data demonstrate seasonal patterns in stress-induced CORT are not driven by metabolic needs alone, although at different times of year sparrows may vary which tissue types respond to increased energy demands resulting from exposure to stressors.

Does short-term fasting lead to stressed-out parents? A study of incubation commitment and the hormonal stress responses and recoveries in snow petrels

The hormonal stress response is flexible and can be modulated by individuals according to its costs and benefits. Therefore, it is predicted that parents in poor body condition should modify their hormonal stress response, and thus, redirect energy allocation processes from parental care to self-maintenance when stressors occur. To test this prediction, most studies on free-living vertebrates have only focused on the stress response while the stress recovery - how quickly hormonal levels return to baseline values - has been neglected. Moreover, most studies have only focused on corticosterone - the primary mediator of allostasis - without paying attention to prolactin despite its major role in mediating parental behaviors. Here, we examined the effect of a short-term fasting event on the corticosterone and prolactin stress responses and recoveries, and we subsequently explored their relationships with parental decision in the snow petrel (Pagodroma nivea). By comparing the hormonal profiles of fasting and non-fasting snow petrels, we showed that parents modulate their corticosterone (but not prolactin) stress response according to their energetic status. We also described for the first time the hormonal stress recoveries in wild birds and found that they did not differ between fasting and non-fasting birds. Importantly, egg neglect was negatively correlated with circulating prolactin but not corticosterone levels in this species, demonstrating therefore a complex link between body condition, parental behavior and circulating corticosterone and prolactin levels. We suggest that both corticosterone and prolactin play a major role in the way parents adjust to stressors. This multiple signaling may allow parents to fine-tune their response to stressors, and especially, to activate specific allostasis-related mechanisms in a timely manner.

Ambient temperature and air pressure modulate hormones and behaviour in Greylag geese (Anser anser) and Northern bald ibis (Geronticus eremita)

Ambient temperature and air pressure are relevant stimuli that can elicit hormonal responses in alignment with adjusting individuals' physiology and behaviour. This study investigated possible changes in corticosterone (C) and testosterone (T) and contingencies with behaviour in response to ambient temperature and air pressure, and it evaluated the temporal response dynamics of these hormones in 12 individual Greylag geese (Anser anser) over 26 and 12 individual Northern bald ibis (Geronticus eremita) over 27 days, during late winter. Immunoreactive metabolites of C and T were analysed non-invasively from 626 fecal samples by means of group-specific antibodies and correlated to behaviour and weather factors. In both species, high C levels correlated with low temperatures 24h before sampling, but low C levels correlated with high air pressure 6-12h before sampling. In both species, C levels and behavioural activity were negatively correlated. In addition, temperature had a positive influence on T levels in both species 12-24h before sampling. The fact that weather conditions influenced changes in levels of C, while social interactions did not, is indicative of a general mechanism of graduated physiological adjustment to environmental variations affecting metabolism, stress responses and behaviour.

Regulation of hypothalamic-pituitary-interrenal axis function in male smallmouth bass (Micropterus dolomieu) during parental care

Male smallmouth bass (Micropterus dolomieu) provide sole parental care until offspring reach independence, a period of several weeks. During the early parental care period when males are guarding fresh eggs (MG-FE), cortisol responsiveness is attenuated; the response is re-established when males reach the end of the parental care period and are guarding free-swimming fry (MG-FSF). It was hypothesized that attenuation of the cortisol response in male smallmouth bass during early parental care reflected modulation of hypothalamic-pituitary-interrenal (HPI) axis function. Male smallmouth bass were sampled at the beginning (MG-FE) and end of the parental care period (MG-FSF), before and/or 25 min after exposure to a standardized stressor consisting of 3 min of air exposure. Repeated sampling of stressed fish for analysis of plasma cortisol and adrenocorticotropic hormone (ACTH) levels was carried out. Males significantly elevated both plasma cortisol and ACTH levels when guarding free-swimming fry but not during early parental care. Control and stressed fish were terminally sampled for tissue mRNA abundance of preoptic area (POA) and hypothalamic corticotropin-releasing factor (CRF) as well as head kidney melanocortin 2 receptor (MC2R), steroidogenic acute regulatory protein (StAR) and cytochrome P450 side chain cleavage enzyme (P450scc). No significant differences in either hypothalamus CRF or head kidney P450scc mRNA abundance were found across parental care stages or in response to stress. However, POA CRF mRNA abundance and interrenal cell MC2R and StAR mRNA abundances failed to increase in response to stress in MG-FE. Thus, the attenuated cortisol response in males guarding fresh eggs may be explained by hypoactive HPI axis function in response to stress. The present is one of few studies, and the first teleost study, to address the mechanisms underlying resistance to stress during the reproductive/parental care period.

Parental behavior correlates to baseline corticosterone of mates and offspring in nesting eastern bluebirds (Sialia sialis)

Raising nestlings in a biparental species involves a complex and dynamic interaction of physiology and behavior among a group of organisms. Parents may be predicted to vary their behaviors based not only upon their own state, but also in relation to the states of both offspring and the other parent. In this study we explore the relationships between parental feeding behaviors and family member condition in eastern bluebirds, with a special emphasis on baseline corticosterone, a hormone associated with energy mediation and stress. We found that the overall number of feeding trips made by both male and female parents were positively correlated to the corticosterone levels of nestlings. Maternal, but not paternal, baseline corticosterone levels were positively correlated to nestling baseline corticosterone levels. Additionally, adult males' feeding behavior was positively correlated to adult females' baseline corticosterone levels. These findings suggest a complex interplay between parental behavior and physiological state not only within a given organism, but also across organisms operating within a family unit. In addition, these results provide evidence that paternal and maternal efforts are influenced by related but distinct pressures, and that male and female parenting may be governed differentially even in species with relatively equitable biparental care.

Short- and long-term effects of unpredictable repeated negative stimuli on Japanese quail's fear of humans

Numerous aversive events occur in poultry production, and if repeated and unpredictable, can result in an impaired welfare. Some events such as handling can be perceived negatively and it is of interest to understand how humans' behaviour could affect poultry's behaviours and especially its avoidance of humans. Our aim was to evaluate short- and long-lasting effects of a 3-week procedure involving unpredictable repeated negative stimuli (URNS) applied during the post-juvenile period on quail's reactivity to humans. We compared the reactions of two sets of quail: URNS was applied to one set (treated quail) and the other set was left undisturbed (control quail). When two weeks old, treated quail were exposed to a variety of negative stimuli, either applied automatically or involving human presence. One and seven weeks after the termination of the procedure, the reactivity of control and treated quail to a passive human being was evaluated. Furthermore, the experimenter with her hand on a trough containing a mealworm assessed the propensity of quail of both groups to habituate to feed close to a human being. In the presence of a seated observer, treated quail were more inhibited and more alert than control quail. Likewise, seven weeks after the end of the URNS procedure, more treated than control quail adopted a fear posture. Moreover, whereas control quail spent as much time in the different areas of their cages, treated quail spent more time in the rear part of their cages. Finally, whereas control quail habituated gradually to feed near the experimenter's hand, treated quail did not. All these tests evidence negative short- and long-term effects on treated quail's reactivity to a passive human being and on their habituation to a human being when her presence is positively reinforced. This highlights the importance of young poultry's experience with humans in production.

Baseline corticosterone and stress response in the Thorn-tailed Rayadito (Aphrastura spinicauda) along a latitudinal gradient

Glucocorticoids are essential for life and their secretion is regulated by the hypothalamic-pituitary-adrenal axis (HPA). The HPA axis is often divided into two components: baseline glucocorticoids levels and stress response glucocorticoids levels, which are affected by changes in ambient temperature and productivity among others factors. An approximation to evaluate how a species copes with these changes is to evaluate differences of this hormone amongst populations of the same species that inhabit places ideally presenting all the possible combinations of temperature and productivity. We aimed to evaluate whether environmental temperature or productivity, represent challenges in terms of stress in the Thorn-tailed Rayadito (Aphrastura spinicauda). We examined circulating baseline levels of CORT and stress responses from three populations, covering the whole geographic distribution of the species across large gradients in weather conditions. If low temperature influences baseline CORT levels, we expect higher levels of this hormone in the southernmost population (higher latitude). However, if productivity is the factor that influences baseline CORT levels, we expect the contrary pattern, that is, lower values of this hormone in the southernmost population (more productive environment). We observed that baseline CORT levels presented lower values in the southernmost population, supporting the environmental productivity hypothesis. Secondly, we tested the hypothesis that individuals breeding at higher latitudes should have a lower stress response than individuals breeding at lower latitudes. Contrary to our expectations, we found that stress response did not vary among populations in any of the three years. We concluded that low environmental temperatures did not represent a stress situation for the Thorn-tailed Rayadito if food abundance was sufficient to support energetic demands.

Developmental programming of the HPA and HPG axes by early-life stress in male and female song sparrows

Variation in early environmental conditions can have long-term effects on physiology and behavior, a process referred to as developmental programming. In particular, exposure to early-life stressors can have long-term effects on regulation of the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes. Although these effects have been well documented in mammals, less is known about how early-life stress affects regulation of these endocrine systems in non-mammalian species. In the current study, we determined the long-term effects of early-life food restriction or corticosterone (CORT) treatment on the HPA axis of song sparrows (Melospiza melodia), including the responses to restraint stress, dexamethasone challenge, and ACTH challenge. In addition, we assessed long-term effects on the HPG axis by measuring sex steroid levels (testosterone in males and 17β-estradiol in females) before and after a gonadotropin-releasing hormone (GnRH) challenge. Subjects treated with CORT during development had larger increases in CORT in response to ACTH challenge than food-restricted or control subjects. Neither treatment affected the responses of CORT to restraint or dexamethasone. CORT-treated males also had higher initial testosterone levels, but neither treatment affected testosterone levels post-GnRH. Lastly, although GnRH challenge failed to increase circulating estradiol levels in females, females exposed to food restriction or CORT treatment had lower estradiol levels than control females. These results show that exposure to stress can developmentally program the endocrine system of songbirds and illustrate the importance of considering developmental conditions when determining the factors responsible for inter-individual variation in endocrine regulation.

The importance of novelty: male-female interactions among blue-black grassquits in captivity

Mate choice is a primary mechanism driving the evolution of sexually selected traits such as elaborate displays and ornaments. In a majority of taxa studied to date, females are seen to actively sample and evaluate multiple males, presumably to optimize mating opportunities. During this process females may encounter males both familiar and novel, a distinction that might influence how mate choice proceeds. Using a socially monogamous passerine, the blue-black grassquit (Volatinia jacarina), we studied how females respond to novel versus familiar ("paired") males, and how encounters with novel males influence subsequent interactions with their paired males. Additionally, we measured the hormonal response of males after visualizing their paired females interacting with novel males. We found that females were attentive to novel males irrespective of these males' phenotypic attributes, suggesting that in these interactions novelty is highly relevant. After exposure to novel males, females tended to respond aggressively towards their paired males; by contrast, the behaviour of males towards their paired females did not change. Moreover, we did not detect any hormonal responses of males to viewing their paired females interacting with novel males. Together these results suggest that the distinction between familiarity and novelty may hold special relevance for females in mate choice, a finding that bears upon our understanding of the evolution of extra-pair paternity and reproductive behaviour.

Chronic variable stress in fathers alters paternal and social behavior but not pup development in the biparental California mouse (Peromyscus californicus)

Stress and chronically elevated glucocorticoid levels have been shown to disrupt parental behavior in mothers; however, almost no studies have investigated corresponding effects in fathers. The present experiment tested the hypothesis that chronic variable stress inhibits paternal behavior and consequently alters pup development in the monogamous, biparental California mouse (Peromyscus californicus). First-time fathers were assigned to one of three experimental groups: chronic variable stress (CVS, n=8), separation control (SC, n=7), or unmanipulated control (UC, n=8). The CVS paradigm (3 stressors per day for 7 days) successfully stressed mice, as evidenced by increased baseline plasma corticosterone concentrations, increased adrenal mass, decreased thymus mass, and a decrease in body mass over time. CVS altered paternal and social behavior of fathers, but major differences were observed only on day 6 of the 7-day paradigm. At that time point, CVS fathers spent less time with their pairmate and pups, and more time autogrooming, as compared to UC fathers; SC fathers spent more time behaving paternally and grooming the female mate than CVS and UC fathers. Thus, CVS blocked the separation-induced increase in social behaviors observed in the SC fathers. Nonetheless, chronic stress in fathers did not appear to alter survival or development of their offspring: pups from the three experimental conditions did not differ in body mass gain over time, in the day of eye opening, or in basal or post-stress corticosterone levels. These results demonstrate that chronic stress can transiently disrupt paternal and social behavior in P. californicus fathers, but does not alter pup development or survival under controlled, non-challenging laboratory conditions.

Experimental test of the effect of introduced hematophagous flies on corticosterone levels of breeding Darwin's finches

Parasites can negatively affect the evolutionary fitness of their hosts by eliciting physiological stress responses. Parasite-induced stress can be monitored by measuring changes in the adrenal steroid hormone corticosterone. We examined the effect of an invasive parasite on the corticosterone concentrations of a common species of Darwin's finch, the medium ground finch (Geospiza fortis). Philornis downsi (Diptera: Muscidae) is a parasitic nest fly recently introduced to the Galapagos Islands, where it feeds on the blood of nestlings and breeding adult female finches. Previous work shows that P. downsi significantly reduces the reproductive success of several species of finches. We predicted that the effect of P. downsi on host reproductive success is mediated by stress responses in breeding female finches. High stress levels could reduce the ability of females to invest in offspring, thus decreasing their reproductive success. To test this hypothesis, we experimentally manipulated the abundance of P. downsi in nests, then measured baseline and acute stress-induced corticosterone levels, body condition, and hematocrit (red blood cell content). Acute stress-induced corticosterone levels increased over baseline levels, but this response did not differ significantly with parasite treatment. There was also no significant difference in the body condition or hematocrit of females from parasitized versus non-parasitized nests. Our results suggest that the lower reproductive success of females from parasitized nests is not mediated by a physiological stress response.

Obligate versus rich patch opportunism: evolution and endocrine mechanisms

Opportunistic breeding has been hypothesized to evolve in response to rare or unpredictable resource pulses. In this traditional view of opportunism, individuals invest heavily in reproduction whenever conditions are permissive for breeding, perhaps at the expense of investment in survival. We term this strategy 'obligate opportunism' (OBO). We also present an additional strategy that could account for the evolution of opportunism. High mobility may allow individuals to move between rich patches of resources that are spatially or temporally unpredictable, reducing exposure to food scarcity and taking advantage of breeding opportunities. This strategy, which we term 'rich patch exploiter' (RPE), predicts that investment in survival-enhancing processes may occur at the expense of reproduction despite high resource availability. We review examples to determine which opportunists better match predictions from the OBO strategy or the RPE strategy and then review endocrine profiles in the context of the two strategies.

Seasonal stress physiology and body condition differ among co-occurring tropical finch species

Seasonal changes in avian hormonal stress responses and condition are well known for common species found at temperate and arctic latitudes, but declining and tropical species are poorly studied. This study compares stress and condition measures of co-occurring declining and non-declining tropical grass finch species in Australia. We monitored declining Gouldian finches (Erythrura gouldiae) and non-declining long-tailed and masked finches (Poepila acuticauda and P. personata) during two seasons that are potentially stressful: peak breeding (early dry season when food is plentiful) and moult (late dry to early wet season when food may be scarce). We measured body condition (muscle and fat), haematocrit, and stress response to capture using plasma corticosterone and binding globulin concentrations. All species had higher muscle and lower fat indices during breeding than moult. Haematocrit did not consistently differ between seasons. Long-tailed finches had higher stress responses during breeding than moult, similar to other passerines studied. Masked finches showed no seasonal changes in stress response. Gouldian finches had stress response patterns opposite to those of long-tailed finches, with higher stress responses during moult. However, seasonal trends in Gouldian and long-tailed finch stress responses sometimes differed between years or sites. The differences in stress response patterns between species suggest that the declining Gouldian finch is more sensitive to recent environmental changes which are thought to further reduce grass seed food resources during the late dry to early wet season. Retention of stress responsiveness during a protracted moult could increase the survival potential of Gouldian finches. This study highlights the utility of stress and condition indices to determine the sensitivity of co-occurring species to environmental conditions.

Glucocorticoid and androgen signaling pathways diverge between advertisement calling and non-calling fish

Behavioral and neuroendocrine mechanisms of social vocalization in teleost fish are influenced by the glucocorticoid cortisol and the androgen 11-ketotestosterone (11kT). The relative abundance of both 11kT, which binds to androgen receptors (ARα, ARβ), and cortisol, which binds to glucocorticoid receptors (GR-1, GR-2), is regulated by 11β-hydroxylase (11βH) that converts 11-deoxycortisol to cortisol and testosterone to 11β-OH-testosterone, and 11β-hydroxysteroid dehydrogenase (11βHSD) that converts cortisol to the inactive metabolite cortisone and 11β-OH-testosterone to 11kT. In midshipman fish, we tested the hypothesis that plasma steroid levels, mRNA abundance for 11βH and 11βHSD in the vocal muscle and testis (known site of 11kT synthesis), and mRNA abundances for ARs and GRs in vocal muscle, would differ between males that did or did not recently produce 'hum' advertisement calls. Quantitative real-time PCR demonstrated that non-calling male vocal muscle had significantly higher mRNA levels for all receptors except ARα, and a strong trend for higher 11βHSD; 11βH was similar to that in calling males. Calling males had higher plasma and testis 11kT, but lower plasma cortisol, levels. Testis enzyme levels did not differ between male groups, although calling males showed a positive linear correlation between plasma 11kT and testis 11βHSD mRNA levels, consistent with testis being the main source of plasma 11kT. We propose that higher vocal muscle 11βHSD levels in non-calling males reflect increased local conversion of elevated cortisol to cortisone, providing protection from cortisol-related toxicity, while increased receptor expression in non-calling males functions as a preparatory mechanism for meeting the physiological demands of future vocalization.