Seasonal changes of the adrenocortical responses to stress in redpolls,Acanthis flammea, in Alaska

Evaluating variation in acute stress response in captive Wood Ducks (Aix sponsa)

Acute stress response has been linked to body condition and associated with the allocation of finite energy resources in wild, free-living birds. However, the relationship between the body condition of individuals and the acute stress response is less clear for birds kept in captive settings, where energy resources are abundant and readily available. We evaluated how individual variation in body condition, reflected as body mass, relates to the acute stress response in adult captive Wood Ducks (Aix sponsa) hatched from eggs collected in the wild and reared in captivity while birds were fed ad libitum, in and out of the breeding season. We determined plasma corticosterone (CORT) levels from blood collected within 3 min of capture and at 10, 30, and 60 min post-capture from 28 ducks during spring and fall, in and out of breeding season. Body condition measurements were recorded once for each bird in early spring. The effects of body mass, age, and time of year (in vs. out of breeding season) on total and maximum CORT secreted were analyzed in Generalized Linear Mixed Models (GLMM). No relationships between age, body mass, and total or maximum CORT were found. There was a significant difference in response in and out of the breeding season for total CORT (p < 0.01) and maximum CORT (p < 0.001), with a higher response during the breeding season. Measures of individual responses for total (p = 0.001) and maximum CORT (p < 0.01) were consistent across seasons. Baseline CORT levels were consistently low, with 53.6 % and 78.6 % of the samples below the detection limit during and outside of breeding season, respectively. These results highlight differences in baseline CORT maintenance compared to studies on wild Wood Ducks, potentially relating to environmental differences such as unlimited food supply and protection from predators in captivity. Additionally, we found that captive birds maintain seasonal variation in acute stress response, which reflects some trends found in wild birds, although more research across multiple seasons is warranted. We also found that some captive Wood Ducks continue to increase CORT secretion at the end of the 60 min handling period, indicating that longer handling times may reveal differences in total CORT secreted. Further investigation should be done to assess the costs and benefits of variation in body condition in terms of coping ability across life stages.

How does maternal age influence reproductive performance and offspring phenotype in the snow petrel (Pagodroma nivea)?

In wild vertebrates, the increase of breeding success with advancing age has been extensively studied through laying date, clutch size, hatching success, and fledging success. However, to better evaluate the influence of age on reproductive performance in species with high reproductive success, assessing not only reproductive success but also other proxies of reproductive performance appear crucial. For example, the quality of developmental conditions and offspring phenotype can provide robust and complementary information on reproductive performance. In long-lived vertebrate species, several proxies of developmental conditions can be used to estimate the quality of the produced offspring (i.e., body size, body condition, corticosterone levels, and telomere length), and therefore, their probability to survive. By sampling chicks reared by known-aged mothers, we investigated the influence of maternal age on reproductive performance and offspring quality in a long-lived bird species, the snow petrel (Pagodroma nivea). Older females bred and left their chick alone earlier. Moreover, older females had larger chicks that grew faster, and ultimately, those chicks had a higher survival probability at the nest. In addition, older mothers produced chicks with a higher sensitivity to stress, as shown by moderately higher stress-induced corticosterone levels. Overall, our study demonstrated that maternal age is correlated to reproductive performance (hatching date, duration of the guarding period and survival) and offspring quality (body size, growth rate and sensitivity to stress), suggesting that older individuals provide better parental cares to their offspring. These results also demonstrate that maternal age can affect the offspring phenotype with potential long-term consequences.

Physiological but not morphological adjustments along latitudinal gradients in a human commensal species, the Eurasian tree sparrow

Human commensal species take advantage of anthropogenic conditions that are less likely to be challenged by the selective pressures of natural environments. Their morphological and physiological phenotypes can therefore dissociate from habitat characteristics. Understanding how these species adjust their morphological and physiological traits across latitudinal gradients is fundamental to uncovering the eco-physiological strategies underlying coping mechanisms. Here, we studied morphological traits in breeding Eurasian tree sparrows (ETSs; Passer montanus) among low-latitude (Yunnan and Hunan) and middle-latitude (Hebei) localities in China. We then compared body mass; lengths of bill, tarsometatarsus, wing, total body, and tail feather; and baseline and capture stress-induced levels of plasma corticosterone (CORT) and the metabolites including glucose (Glu), total triglyceride (TG), free fatty acid (FFA), total protein, and uric acid (UA). None of the measured morphological parameters varied with latitude except in the Hunan population, which demonstrated longer bills than those in other populations. Stress-induced CORT levels significantly exceeded baseline levels and decreased with increasing latitude, but total integrated CORT levels did not vary with latitude. Capture stress-induced significantly increased Glu levels and decreased TG levels, independent of site. However, the Hunan population had significantly higher baseline CORT, baseline and stress-induced FFA levels, but lower UA levels, which differed from other populations. Our results suggest that rather than morphological adjustments, physiological adjustments are mainly involved in coping mechanisms for middle-latitude adaptation in ETSs. It is worth investigating whether other avian species also exhibit such dissociation from external morphological designs while depending on physiological adjustments.

Among-species variation in hormone concentrations is associated with urban tolerance in birds

As cities expand across the globe, understanding factors that underlie variation in urban tolerance is vital for predicting changes in patterns of biodiversity. Endocrine traits, like circulating hormone concentrations and regulation of endocrine responses, might contribute to variation in species' ability to cope with urban challenges. For example, variation in glucocorticoid and androgen concentrations has been linked to life-history and behavioural traits that are associated with urban tolerance. However, we lack an understanding of the degree to which evolved differences in endocrine traits predict variation in urban tolerance across species. We analysed 1391 estimates of circulating baseline corticosterone, stress-induced corticosterone, and testosterone concentrations paired with citizen-science-derived urban occurrence scores in a broad comparative analysis of endocrine phenotypes across 71 bird species that differ in their occurrence in urban habitats. Our results reveal context-dependent links between baseline corticosterone and urban tolerance, as well as testosterone and urban tolerance. Stress-induced corticosterone was not related to urban tolerance. These findings suggest that some endocrine phenotypes contribute to a species' tolerance of urban habitats, but also indicate that other aspects of the endocrine phenotype, such as the ability to appropriately attenuate responses to urban challenges, might be important for success in cities.

Eye Region Surface Temperature and Corticosterone Response to Acute Stress in a High-Arctic Seabird, the Little Auk

Measuring changes in surface body temperature (specifically in eye-region) in vertebrates using infrared thermography is increasingly applied for detection of the stress reaction. Here we investigated the relationship between the eye-region temperature (TEYE; measured with infrared thermography), the corticosterone level in blood (CORT; stress indicator in birds), and some covariates (ambient temperature, humidity, and sex/body size) in a High-Arctic seabird, the Little Auk Alle alle. The birds responded to the capture-restrain protocol (blood sampling at the moment of capturing, and after 30 min of restrain) by a significant TEYE and CORT increase. However, the strength of the TEYE and CORT response to acute stress were not correlated. It confirms the results of a recent study on other species and all together indicates that infrared thermography is a useful, non-invasive measure of hypothalamic-pituitary-adrenal (HPA) axis reactivity under acute activation, but it might not be a suitable proxy for natural variation of circulating glucocorticoid levels.

Prenatal yolk corticosterone exposure promotes skeletal growth and induces oxidative imbalance in yellow-legged gull embryos

Maternally derived hormones induce variation in offspring phenotype, with consequences that can carry over into post-natal life and even into adulthood. In birds, maternal egg corticosterone (CORT) is known to exert contrasting effects on offspring morphology, physiology and behaviour after hatching. However, information on the effects of CORT exposure on pre-hatching embryonic development is limited. We experimentally increased yolk CORT levels in yellow-legged gull (Larus michahellis) eggs, and assessed the effects on embryo pre-hatching development and oxidative status of brain and liver. CORT-supplemented embryos reached a larger skeletal size and liver mass compared with controls. Embryos from CORT-injected last-laid eggs showed decreased activity of the hepatic antioxidant enzymes superoxide dismutase and catalase, while intermediate-laid eggs showed increased levels of lipid peroxidation. However, elevated yolk CORT did not affect oxidative stress endpoints in the brain. Our results indicate that elevated yolk CORT levels affect prenatal embryo development by promoting skeletal growth, and induce laying sequence- and organ-specific oxidative imbalance, with potential adverse consequences during postnatal life, especially for late-hatched offspring.

Glucocorticoid levels are linked to lifetime reproductive success and survival of adult barn owls

Glucocorticoid hormones, such as corticosterone, are crucial in regulating daily life metabolism and energy expenditure, as well as promoting short-term physiological and behavioural responses to unpredictable environmental challenges. Therefore, glucocorticoids are considered to mediate trade-offs between survival and reproduction. Relatively little is known about how selection has shaped glucocorticoid levels. We used 15 years of capture-recapture and dead recovery data combined with 13 years of corticosterone and breeding success data taken on breeding barn owls (Tyto alba) to investigate such trade-offs. We found that survival was positively correlated with stress-induced corticosterone levels in both sexes, whereas annual and lifetime reproductive success (i.e. the sum of young successfully fledged during the entire reproductive career) was positively correlated with both baseline and stress-induced corticosterone levels in females only. Our results suggest that, in the barn owl, the stress-induced corticosterone response is a good proxy for adult survival and lifetime reproductive success. However, selection pressure appears to act differently on corticosterone levels of males and females.

Physiological and morphological correlates of blood parasite infection in urban and non-urban house sparrow populations

In the last decade, house sparrow populations have shown a general decline, especially in cities. Avian malaria has been recently suggested as one of the potential causes of this decline, and its detrimental effects could be exacerbated in urban habitats. It was initially thought that avian malaria parasites would not have large negative effects on wild birds because of their long co-evolution with their hosts. However, it is now well-documented that they can have detrimental effects at both the primo- and chronical infection stages. In this study, we examined avian malaria infection and its physiological and morphological consequences in four populations of wild house sparrows (2 urban and 2 rural). We did not find any relationship between the proportions of infected individuals and the urbanisation score calculated for our populations. However, we observed that the proportion of infected individuals increased during the course of the season, and that juveniles were less infected than adults. We did not detect a strong effect of malaria infection on physiological, morphological and condition indexes. Complex parasite dynamics and the presence of confounding factors could have masked the potential effects of infection. Thus, longitudinal and experimental studies are needed to understand the evolutionary ecology of this very common, but still poorly understood, wild bird parasite.

Across time and space: Hormonal variation across temporal and spatial scales in relation to nesting success

There is a renewed interest in investigating individual variation in hormone levels in relation to fitness metrics, as hormones act as mediators of life-history trade-offs. Hormone concentrations, however, are labile, responding to both internal and external stimuli, so the relationship between hormones and fitness can be non-consistent. One explanation of this inconsistent relationship is that a single hormone sample may not be representative of individual phenotypes in a free-living species. We addressed this issue by repeatedly sampling a free-living population of mountain white-crowned sparrows, Zonotrichia leucophrys oriantha, for baseline and stress-induced corticosterone (cort) and testosterone (T) across different stages of the breeding season. We measured (co)variation using three different methods, taking into account inter- and intra-individual variances, to determine whether hormone levels and the stress response are repeatable. We documented the temporal (over 3 months) and spatial (home-range) variation of individual hormone phenotypes and investigated how these components related to nesting success. At the population level, we found significant repeatability in male stress-induced cort concentrations but no repeatability in male or female baseline cort or male T concentrations. Using a new metric of intra-individual variance focusing on the stress response (profile repeatability), we found a wide range of variance scores, with most individuals showing high variation in their stress response. Similarly, we found a low level of repeatability of the reaction norm intercept and slope for the stress response across different life-history stages. Males with higher concentrations of stress-induced cort had more central home-ranges. Males with higher body condition had larger home-ranges; however, home-range size did not relate to male hormone concentrations or nesting success. We also did not find any significant relationship between variation in hormone levels and nesting success. We recommend that future studies combine both physiological and environmental components to better understand the relationship between hormones and fitness.

Coping with extremes: Remarkably blunt adrenocortical responses to acute stress in two sympatric snow finches on the Qinghai-Tibet Plateau during winter relative to other seasons

The extreme climatic conditions (ECCs) of the Qinghai-Tibet Plateau impose strong selective pressures on the evolution of phenotypic traits in free-living animals. It is not well understood how animals on the Qinghai-Tibet Plateau modify their adrenocortical functions in response to both predictable and unpredictable events of ECCs, especially when the available resources are lowest during the wintering life-history stage. To uncover potential physiological mechanisms, we studied the life history stage dependent features of morphology, the plasma corticosterone response to acute stress and brain glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) mRNA expression in two sympatric snow finches: the white-rumped snow finch (Onychostruthus taczanowskii, WRSF); and the rufous-necked snow finch, Pyrgilauda ruficollis, RNSF) in Qinghai Province, China. Our results showed that (a) baseline corticosterone and stressor-induced corticosterone levels significantly varied with life history stage, but not between the species; (b) in WRSF, GR mRNA expression in the paraventricular nucleus was higher in the wintering stage compared to the pre-basic molt stage. There were no differences in hippocampus MR mRNA expression between stages in either species; (c) in the wintering stage, the suppression of corticosterone secretion in both species was an unexpected strategy in free-living animals. Both convergent and divergent phenotypic traits of adrenocortical responses to acute stress in two sympatric snow finches contribute to our understanding of the coping mechanisms of closely related species in the severe winter on the Qinghai-Tibet Plateau.

Phenotypic flexibility of glucocorticoid signaling in skeletal muscles of a songbird preparing to migrate

Glucocorticoids are commonly associated with responses to stress, but other important functions include homeostatic regulation, energy metabolism and tissue remodeling. At low circulating levels, glucocorticoids bind to high-affinity mineralocorticoid receptors (MR) to activate tissue repair and homeostasis (anabolic pathways), whereas at elevated levels, glucocorticoids bind to glucocorticoid receptors (GR) to activate catabolic pathways. Long distance migrations, such as those performed by Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), require modification of anatomy, physiology and behavior. Plasma corticosterone (CORT) increases in association with impending departure and flight and may promote muscle-specific anabolic states. To test this idea, we explored glucocorticoid signaling in the pectoralis (flight) and gastrocnemius (leg) muscles of male sparrows on the wintering grounds at three stages leading up to spring departure: winter (February), pre-nuptial molt (March), and pre-departure (April). CORT was detected in plasma and in both muscles, but measures of CORT signaling differed across muscles and stages. Expression of 11β-hydroxysteroid dehydrogenase (11β-HSD) Type 2 (inactivates CORT) increased in the pectoralis at pre-departure, whereas 11β-HSD Type 1 (regenerates CORT) did not change. Neither of the two 11β-HSD isoforms was detectable in the gastrocnemius. Expression of MR, but not GR, was elevated in the pectoralis at pre-departure, while only GR expression was elevated at pre-nuptial molt in gastrocnemius. These data suggest that anabolic functions predominate in the pectoralis only while catabolic activity is undetected in either muscle at pre-departure.

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.

Sex- and breeding stage-specific hormonal stress response of seabird parents

Changes in corticosterone (CORT) and prolactin (PRL) levels are thought to provide complementary information on parental decisions in birds in the context of stressful situations. However, these endocrine mechanisms have yet to be fully elucidated, appearing to vary among avian species without any clear pattern. Here, we examined CORT and PRL stress responses in a small Arctic seabird, the little auk (Alle alle). We analysed the levels of these hormones (baseline, and stress response, i.e. the change in the baseline in response to stress) with respect to the breeding phase (mid incubation and mid chick rearing) and the sex of the birds. Baseline CORT concentrations were similar during both breeding phases but baseline PRL levels were higher during incubation than chick rearing. The CORT and PRL stress responses were stronger during incubation than chick rearing (although with respect to CORT the effect was only marginally significant). There were also some sex-specific baseline levels and stress responses for both hormones (during the incubation period males compared to females exhibited higher CORT stress response and lower baseline PRL; during the chick rearing period males exhibited higher PRL stress response). Our results suggest that in the case of the little auk, both the incubation and the chick rearing periods may represent similar levels of physiological stress. However, the birds may be more sensitive to stress during incubation than during chick rearing, possibly because of inter-phase differences in predation pressure. The sex differences suggest differential exposure of males and females to stressors.

Diel pattern of corticosterone metabolites in Arctic barnacle goslings (Branta leucopsis) under continuous natural light

Here we describe the excretion pattern of corticosterone metabolites collected from droppings in barnacle goslings (Branta leucopsis) raised under 24 hours of continuous natural light in the Arctic. In lower latitudes, circulating corticosterone peaks around waking and shows a nadir between midnight and 4:00, whereas the peak and nadir are time-delayed slightly when measuring corticosterone metabolites from droppings. Photoperiod, along with other environmental factors, helps to entrain an animal's endogenous rhythm to that of the natural world. North of the Arctic Circle, photoperiod may not be a reliable cue as light is continuously absent during the winter and continuously present during the summer. Here, for the first time, we used droppings to describe a 24-hour excretion pattern of corticosterone metabolites (CORTm). By applying circular statistics for dependent data, we found a diel rhythmic pattern even under continuous natural light. We discuss potential alternative 'Zeitgeber' that may function even in the polar regions, focusing on melatonin. We propose a line of research to measure melatonin non-invasively from droppings. We also provide a validation of the adopted enzyme immunoassay (EIA) that was originally developed for greylag geese.

Patterns of stress responses shift during seasonal life-history transitions: An analysis comparing baseline, maximal and integrated corticosterone in female red-sided garter snakes (Thamnophis sirtalis parietalis)

Glucocorticoids often rise and fall with a variety of external and internal cues and frequently vary among life-history stages. This suggests that changing glucocorticoids may coordinate life-history transitions. To explore this hypothesis, we asked if the time-course of stress-induced glucocorticoid levels differ between two life-history transitions (i.e., spring and fall migration) in female red-sided garter snakes (Thamnophis sirtalis parietalis). We collected non-migratory females from a communal den and migratory females from a road along the migration route and treated them with 4h of capture stress; plasma corticosterone was measured before, during and after capture stress. During the spring, den-collected females exhibited a stress-induced peak in corticosterone at an earlier sampling time than migrating, road-collected females. Because the pattern of corticosterone responses varied with migratory state, negative feedback on and/or sensitivity of the hypothalamus-pituitaryadrenal (HPA) axis may be linked to spring migration. During the fall, capture stress elicited an increase in corticosterone in den-collected females but not in migrating, road-collected females. Baseline corticosterone was higher and both maximal and integrated corticosterone responses were lower during the fall compared to spring, indicating that stress responses are smaller when baseline corticosterone is elevated, perhaps due to a "ceiling effect". These data suggest that HPA axis regulation changes during seasonal migration, possibly via altering negative feedback, HPA axis sensitivity, or some other mechanism. This study supports the hypothesis that glucocorticoids coordinate life-history events and suggests that examining a suite of stress response characteristics is most informative for understanding the function of HPA modulation.

The stress response is attenuated during inclement weather in parental, but not in pre-parental, Lapland longspurs (Calcarius lapponicus) breeding in the Low Arctic

Birds breeding at high latitudes can be faced with extreme weather events throughout the breeding season. In response to environmental perturbations, vertebrates activate the hypothalamic-pituitary-adrenal (HPA) axis and synthesize corticosterone, which promotes changes in behavior and physiology to help the animal survive. The parental care hypothesis suggests that the HPA axis activity should be downregulated during the parental stage of breeding to prevent nest abandonment. However, it is unknown what happens to HPA axis activity in response to severe weather at the transition from the pre-parental to parental stages of breeding. We sampled baseline corticosterone levels and the time course of corticosterone elevation over 60min of restraint stress and assessed body condition and fat stores in Lapland longspurs (Calcarius lapponicus) breeding in the Low Arctic in the presence and absence of snowstorms. The results showed that during the pre-parental stage, HPA axis activity was up-regulated in response to snowstorms, with corticosterone levels continuing to increase through 60min of restraint. However, once birds were parental, HPA axis activity was unaffected by snowstorms and levels peaked at 10min. Fat levels and body condition did not change in response to snowstorms but fat levels declined in males during the pre-parental stage. These data suggest that the parental care hypothesis can be applied to severe storm events; parental birds restrained the activity of the HPA axis, likely to focus on the reproductive effort that is already underway, while pre-parental birds greatly upregulated HPA axis activity in response to snowstorms to maximize self-preservation.

Differential response of GnIH in the brain and gonads following acute stress in a songbird

Gonadotropin-inhibitory hormone (GnIH) acts to inhibit reproduction at all levels of the hypothalamo-pituitary-gonad axis. GnIH expression and/or immunoreactivity in the hypothalamus increase with acute stress in some birds and mammals, and thus may be involved in stress-induced reproductive inhibition. Much is known about GnIH and stress in seasonal and continuous breeders, but far less is known about these interactions in opportunistic breeders. For opportunistically breeding animals, reproductive readiness is closely associated with unpredictable environmental cues, and thus the GnIH system may be more sensitive to stress. To test this, we collected tissues from zebra finches immediately following capture or after 60 min of restraint. Restraint significantly increased plasma corticosterone in males and females but, contrary to studies on other species, restrained birds had significantly fewer GnIH immunoreactive (GnIH-ir) cell bodies than control birds. GnIH-ir cell number did not differ between the sexes. Stressed females had lower mRNA expression of the beta subunit of follicle stimulating hormone (FSHβ) in the pituitary, suggesting that the reduction in observed GnIH immunoreactivity in females may have been due to increased GnIH release in response to acute stress. GnIH expression increased in the testes, but not the ovaries, of restrained animals. Our data suggest that although GnIH responsiveness to stress appears to be conserved across species, specific tissue response and direction of GnIH regulation is not. Variation in the GnIH response to stress between species might be the result of ecological adaptations or other species differences in the response of the GnIH system to stress.

Offspring Hormones Reflect the Maternal Prenatal Social Environment: Potential for Foetal Programming?

Females of many species adaptively program their offspring to predictable environmental conditions, a process that is often mediated by hormones. Laboratory studies have shown, for instance, that social density affects levels of maternal cortisol and testosterone, leading to fitness-relevant changes in offspring physiology and behaviour. However, the effects of social density remain poorly understood in natural populations due to the difficulty of disentangling confounding influences such as climatic variation and food availability. Colonially breeding marine mammals offer a unique opportunity to study maternal effects in response to variable colony densities under similar ecological conditions. We therefore quantified maternal and offspring hormone levels in 84 Antarctic fur seals (Arctocephalus gazella) from two closely neighbouring colonies of contrasting density. Hair samples were used as they integrate hormone levels over several weeks or months and therefore represent in utero conditions during foetal development. We found significantly higher levels of cortisol and testosterone (both P < 0.001) in mothers from the high density colony, reflecting a more stressful and competitive environment. In addition, offspring testosterone showed a significant positive correlation with maternal cortisol (P < 0.05). Although further work is needed to elucidate the potential consequences for offspring fitness, these findings raise the intriguing possibility that adaptive foetal programming might occur in fur seals in response to the maternal social environment. They also lend support to the idea that hormonally mediated maternal effects may depend more strongly on the maternal regulation of androgen rather than cortisol levels.

Seasonal and sex differences in responsiveness to adrenocorticotropic hormone contribute to stress response plasticity in red-sided garter snakes (Thamnophis sirtalis parietalis)

Like many vertebrates, hormonal responses to stress vary seasonally in red-sided garter snakes (Thamnophis sirtalis parietalis). For example, males generally exhibit reduced glucocorticoid responses to a standard stressor during the spring mating season. We asked whether variation in adrenal sensitivity to adrenocorticotropic hormone (ACTH) explains why glucocorticoid responses to capture stress vary with sex, season, and body condition in red-sided garter snakes. We measured glucocorticoids at 0, 1, and 4 hours after injection with ACTH (0.1 IU/g body mass) or vehicle in males and females during the spring mating season and fall pre-hibernation period. Because elevated glucocorticoids can influence sex steroids, we also examined androgen and estradiol responses to ACTH. ACTH treatment increased glucocorticoids in both sexes and seasons. Spring-collected males had a smaller integrated glucocorticoid response to ACTH than fall-collected males. The integrated glucocorticoid response to ACTH differed with sex during the spring, with males having a smaller glucocorticoid response than females. Although integrated glucocorticoid responses to ACTH did not vary with body condition, we observed an interaction among season, sex and body condition. In males, ACTH treatment did not alter androgens in either season, but androgens decreased during the sampling period. Similar to previous studies, plasma estradiol was low or undetectable during the spring and fall and therefore any effect of ACTH treatment on estradiol could not be determined. These data provide support for a mechanism that partly explains how the HPA axis integrates information about season, sex, and body condition: namely, variation in adrenal responsiveness to ACTH.

An experimental analysis of the heritability of variation in glucocorticoid concentrations in a wild avian population

Glucocorticoid hormones (CORT) are predicted to promote adaptation to variable environments, yet little is known about the potential for CORT secretion patterns to respond to selection in free-living populations. We assessed the heritable variation underlying differences in hormonal phenotypes using a cross-foster experimental design with nestling North American barn swallows (Hirundo rustica erythrogaster). Using a bivariate animal model, we partitioned variance in baseline and stress-induced CORT concentrations into their additive genetic and rearing environment components and estimated their genetic correlation. Both baseline and stress-induced CORT were heritable with heritability of 0.152 and 0.343, respectively. We found that the variation in baseline CORT was best explained by rearing environment, whereas the variation in stress-induced CORT was contributed to by a combination of genetic and environmental factors. Further, we did not detect a genetic correlation between these two hormonal traits. Although rearing environment appears to play an important role in the secretion of both types of CORT, our results suggest that stress-induced CORT levels are underlain by greater additive genetic variance compared with baseline CORT levels. Accordingly, we infer that the glucocorticoid response to stress has a greater potential for evolutionary change in response to selection compared with baseline glucocorticoid secretion patterns.

Breeding on the extreme edge: modulation of the adrenocortical response to acute stress in two High Arctic passerines

Arctic weather in spring is unpredictable and can also be extreme, so Arctic‐breeding birds must be flexible in their breeding to deal with such variability. Unpredictability in weather conditions will only intensify with climate change and this in turn could affect reproductive capability of migratory birds. Adjustments to coping strategies are therefore crucial, so here we examined the plasticity of the adrenocorticotropic stress response in two Arctic songbird species—the snow bunting (Plectrophenax nivalis) and Lapland longspur (Calcarius lapponicus)—breeding in northwest Greenland. Across the breeding season, the stress response was strongest at arrival and least robust during molt in male snow buntings. Snow bunting females had higher baseline but similar stress‐induced corticosterone levels compared to males. Modification of the stress response was not due to adrenal insensitivity, but likely regulated at the anterior pituitary gland. Compared to independent nestlings and adult snow buntings, parental‐dependent chicks had a more robust stress response. For Lapland longspurs, baseline corticosterone was highest at arrival in both male and females, and arriving males displayed a higher stress response compared to arriving females. Comparison of male corticosterone profiles collected at arrival in Greenland (76°N) and Alaska (67–71°N;) reveal that both species have higher stress responses at the more northern location. Flexibility in the stress response may be typical for birds nesting at the leading edges of their range and this ability will become more relevant as global climate change results in major shifts of breeding habitat and phenology for migratory birds. J. Exp. Zool. 323A: 266–275, 2015. © 2015 The Authors. J. Exp. Zool. published by Wiley Periodicals, Inc.

Modulation of corticosterone secretion in tuatara (Sphenodon punctatus): Evidence of a dampened stress response in gravid females

Baseline and stress response glucocorticoid (GC) secretion can be modulated by individuals to support activities and physiological functions connected with reproduction (migration, mating, oviposition and/or parturition, care of young). Corticosterone (CORT) is the primary GC in reptiles and, in accordance with other vertebrates, an adrenocortical stress response is observed. Modulation of CORT secretion occurs in several reptile species, such that elevated baseline CORT concentration and/or a dampened CORT response are common during reproductive life-history events. We investigated CORT secretion after 24 h capture-restraint in the oviparous tuatara (Sphenodon punctatus), the last living rhynchocephalian, and tested whether gravid females have a dampened CORT response compared with non-gravid females. We also included males as a comparison. We confirmed that gravid females have significantly higher baseline plasma CORT concentrations than non-gravid females, suggesting increased CORT secretion during nesting. Furthermore, we found that gravid females exhibit a dampened CORT response compared to non-gravid females and males. Our results demonstrate that female reproductive condition influences CORT secretion in tuatara, and suggest that gravid females modulate CORT secretion during nesting to maintain homeostasis, effectively increasing chances of reproductive success and promoting overall fitness.

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.

Energetic constraints and parental care: is corticosterone indicative of energetic costs of incubation in a precocial bird?

Suppression of the adrenocortical response (e.g., corticosterone release) to an acute stressor is a physiological adjustment thought to decrease the likelihood of avian parents abandoning their nests. However, some periods of parental care, like incubation, are energetically costly, thus corticosterone could increase during these stages to allow incubating parents to utilize energy reserves. Wood ducks (Aix sponsa) have ~30 day incubation periods and only the female incubates the eggs. We hypothesized that corticosterone would be important in regulating energy availability during incubation in this species. Because resources invested in reproduction increase with clutch size, we also hypothesized that clutch size would influence plasma corticosterone during incubation. We measured baseline and stress-induced corticosterone in incubating females during early and late stages of incubation. At both stages of incubation all hens had low baseline corticosterone levels. However, we found that stress-induced corticosterone was 105% greater late in incubation than early in incubation. We also detected a significant negative correlation between female body mass and stress-induced corticosterone late in incubation, but not during the early stages of incubation. Furthermore, we found a significant positive relationship between stress-induced corticosterone and clutch size. These lines of evidence support the hypothesis that incubation in wood ducks is energetically costly and corticosterone is important in supporting the energetic demands of incubating hens. Our findings suggest that corticosterone's role in supporting parental care behaviors are dynamic and are influenced by several factors and that there is a greater physiological cost associated with incubating larger clutches.

Individual variation in glucocorticoid stress responses in animals

When stimuli from the environment are perceived to be a threat or potential threat then animals initiate stress responses, with activation of the hypothalamo-pituitary-adrenal axis and secretion of glucocorticoid hormones (cortisol and corticosterone). Whilst standard deviation or standard error values are always reported, it is only when graphs of individual responses are shown that the extensive variation between animals is apparent. Some animals have little or no response to a stressor that evokes a relatively large response in others. Glucocorticoid responses of fish, amphibian, reptiles, birds, and mammals are considered in this review. Comparisons of responses between animals and groups of animals focused on responses to restraint or confinement as relatively standard stressors. Individual graphs could not be found in the literature for glucocorticoid responses to capture or restraint in fish or reptiles, with just one graph in mammals with the first sample was collected when animals were initially restrained. Coefficients of variation (CVs) calculated for parameters of glucocorticoid stress responses showed that the relative magnitudes of variation were similar in different vertebrate groups. The overall mean CV for glucocorticoid concentrations in initial (0 min) samples was 74.5%, and CVs for samples collected over various times up to 4 h were consistently between 50% and 60%. The factors that lead to the observed individual variation and the extent to which this variation is adaptive or non-adaptive are little known in most animals, and future studies of glucocorticoid responses in animals can focus on individual responses and their origins and significance.

Validation of an enzyme immunoassay for the measurement of faecal glucocorticoid metabolites in spotted hyenas (Crocuta crocuta)

The use of enzyme immunoassays (EIAs) to measure faecal glucocorticoid metabolites (fGCM) is a useful non-invasive technique to monitor adrenocortical activity in vertebrates. The first objective of this study was to validate an 'in-house' EIA (cortisol-3-CMO) for the measurement of fGCM concentrations in spotted hyenas. High-performance liquid chromatography (HPLC) was used to characterise fGCM in samples from a captive hyena that received an i.v. injection of [(3)H] cortisol. All HPLC fractions were analysed with the EIA for the presence and quantities of radiolabelled fGCM. Radiolabelled fGCM consisted of substances with a higher polarity than cortisol and substances of lower polarity that eluted between cortisol and corticosterone. Authentic radiolabelled cortisol was not detected. The EIA measured substantial amounts of immunoreactivity corresponding to the radioactive peaks. It also detected a significant increase in fGCMs after an adrenocorticotropic hormone (ACTH) challenge in two other captive animals and a significant increase in fGCMs in a fourth captive animal after anaesthesia. The second objective was to investigate an age effect on fGCM: we conducted pairwise comparisons of fGCM concentrations in individual free-ranging juvenile spotted hyenas when less than 6 months of age and when between 6 and 24 months of age. We expected juveniles to experience a more unpredictable and therefore more stressful environment when younger than when older. When younger, juveniles had significantly higher fGCM concentrations than when they were older. Our results demonstrate that our assay can be used to assess adrenocortical activity in spotted hyenas.

Rearing conditions have long-term consequences for stress responsiveness in free-living great tits

In captivity, the adrenocortical stress response can be permanently altered by events that occur during early life. Free-living animals have rarely been examined in this regard. To examine whether early-life events impact the hypothalamo-pituitary-adrenal (HPA) axis in the natural setting, we evaluated the stress response of free-living interspecifically cross-fostered great tits (Parus major). Cross-fostered birds may show a long-term potentiation of the adrenocortical stress response because species-specific nutritional requirements may not be met in the nest and/or cross-fostered birds may experience psychosocial stress while being raised by heterospecifics. Nevertheless, we hypothesized that in the natural setting, programmed changes in HPA function would be eclipsed by reactive responses to the immediate environment. Thus, we predicted that adult cross-fostered great tits and controls would show no differences in their adrenocortical stress response. Contrary to predictions, we found that stress responsiveness (i.e., the rate of the corticosterone increase associated with capture and handling) was significantly higher in cross-fostered great tits than in controls. Further, stress responsiveness was not significantly different between mature adults and first-year juveniles. Thus, data indicate significant effects of early rearing conditions on adrenocortical reactivity in the natural setting and also suggest that effects of rearing conditions in free-living animals can last into adulthood.

Effects of breeding season, testosterone and ACTH on the corticosterone response of free-ranging male fence lizards (Sceloporus undulatus)

An attenuated stress response during the breeding season has been reported for several vertebrate species, but the underlying physiological mechanism has received little attention, particularly in reptiles. Modulation could involve changes in the capacity of the adrenal gland to secrete glucocorticoids in addition to upstream changes in the pituitary or hypothalamus. In this study the magnitude of the corticosterone response to capture and confinement was compared between the breeding and postbreeding season in adult male eastern fence lizards, Sceloporus undulatus. Males were captured in both seasons and subjected to the identical stressor of 4h of confinement. Plasma corticosterone levels in response to confinement were significantly lower in the breeding than the postbreeding season. The effect of testosterone on the stress response was tested by experimentally elevating plasma testosterone levels via silastic implants in free-living males during the postbreeding season. Males with experimentally elevated testosterone exhibited significantly weaker corticosterone responses to 1h of confinement than sham-implanted males. Finally the capacity of the adrenal glands to secrete corticosterone during the breeding season was tested by challenging males with adrenocorticotropin (ACTH) injections. In spite of naturally suppressed corticosterone responses during the breeding season, males nonetheless responded robustly to ACTH. Altogether these results suggest that modulation resides upstream of the adrenal gland, as has been shown in some arctic-breeding avian species, and likely involves seasonal changes in testosterone levels.

Corticosterone stress response and plasma metabolite levels during breeding and molt in a free-living migratory songbird, the wood thrush (Hylocichla mustelina)

Many birds face energetic trade-offs between different life history stages, such as reproductive effort, feather molt and the non-breeding period. Little is known about how physiological measures of condition (corticosterone, plasma metabolites) in free-living birds change from nesting stages to the post-breeding molt period or whether this is influenced by prior reproductive effort. We evaluated whether corticosterone (CORT) and plasma metabolite levels vary with date, nest stage and sex in a free-living migratory songbird, the wood thrush (Hylocichla mustelina). We also tested whether (1) baseline CORT levels early in the season were predictive of subsequent reproductive success and (2) whether prior reproductive effort influenced CORT levels and blood metabolites during molt. Baseline CORT levels decreased with date during both the incubation stage and nestling stage, but did not vary significantly across stage of breeding season. Stress-induced CORT declined with date during incubation and varied significantly across breeding stage, with lower levels during feather molt. Profiles of the metabolites of β-hydroxybutyrate, glycerol, and triglyceride did not vary significantly with date or breeding stage. Only triglycerides varied significantly with sex, with females having higher levels than males. Reproductive output was highly variable (0-10 fledglings per season) but baseline CORT levels in females during the first incubation period of the season was not related to subsequent reproductive output. Prior reproductive effort, measured as the cumulative number of young hatched during the breeding season, was positively related to stress-induced CORT during molt. High reproductive effort in wood thrush appears to have physiological carry-over effects into the molt period which could potentially affect rate of molt and preparation for fall migration.

Context-specific territorial behavior in urban birds: no evidence for involvement of testosterone or corticosterone

Testosterone produced by the gonads is a primary mediator of seasonal patterns of territoriality and may directly facilitate territorial behavior during an encounter with a potential intruder. Costs and benefits associated with territoriality can vary as a function of habitat, for example through differences in resource distribution between areas occupied by different individuals. We investigated behaviors in response to simulated territorial intrusions (hereafter territorial behaviors) in urban (Phoenix, Arizona) and nearby desert populations of two Sonoran Desert birds (Curve-billed Thrasher and Abert's Towhee). We also examined the degree to which these behaviors are mediated by testosterone (T) and the adrenal steroid, corticosterone (CORT), which can interact with T in territorial contexts. In both species, urban birds displayed more territorial behaviors than their desert conspecifics, but this difference was not associated with variation in either plasma total or in plasma free (i.e., unbound to binding globulins) T or CORT. In addition, neither plasma T nor plasma CORT changed as a function of duration of the simulated territorial intrusion. Urban Abert's Towhees displayed more territorial behaviors in areas where their population densities were high than in areas of low population densities. Urban Curve-billed Thrashers displayed more territorial behaviors in areas with a high proportion of desert-type vegetation, particularly in areas that differed in vegetation composition from nearby randomly sampled areas, than in areas with a high proportion of exotic or non-desert type vegetation. Associations between territorial behavior and habitat characteristics were not related to plasma T or CORT. Understanding the hormonal processes underlying these associations between behavior and habitat may provide insight into how free-ranging animals assess territorial quality and alter their defensive behavior accordingly.

Food, stress, and reproduction: short-term fasting alters endocrine physiology and reproductive behavior in the zebra finch

Stress is thought to be a potent suppressor of reproduction. However, the vast majority of studies focus on the relationship between chronic stress and reproductive suppression, despite the fact that chronic stress is rare in the wild. We investigated the role of fasting in altering acute stress physiology, reproductive physiology, and reproductive behavior of male zebra finches (Taeniopygia guttata) with several goals in mind. First, we wanted to determine if acute fasting could stimulate an increase in plasma corticosterone and a decrease in corticosteroid binding globulin (CBG) and testosterone. We then investigated whether fasting could alter expression of undirected song and courtship behavior. After subjecting males to fasting periods ranging from 1 to 10h, we collected plasma to measure corticosterone, CBG, and testosterone. We found that plasma corticosterone was elevated, and testosterone was decreased after 4, 6, and 10h of fasting periods compared with samples collected from the same males during nonfasted (control) periods. CBG was lower than control levels only after 10h of fasting. We also found that, coincident with these endocrine changes, males sang less and courted females less vigorously following short-term fasting relative to control conditions. Our data demonstrate that acute fasting resulted in rapid changes in endocrine physiology consistent with hypothalamo-pituitary-adrenal axis activation and hypothalamo-pituitary-gonadal axis deactivation. Fasting also inhibited reproductive behavior. We suggest that zebra finches exhibit physiological and behavioral flexibility that makes them an excellent model system for studying interactions of acute stress and reproduction.