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1. On again, off again: Acute stress response and negative feedback together predict resilience to experimental challenges

   https://doi.org/10.1111/1365-2435.13281  

   Zimmer, Cedric ; Taff, Conor C. ; Ardia, Daniel R. ; Ryan, Thomas A. ; Winkler, David W. ; Vitousek, Maren N. ; Angelier, ed., Frédéric ( January 2019 , Functional Ecology)

   Individuals often vary markedly in their ability to cope with stressors, but the drivers of this variation remain poorly understood. Many studies have tested relationships among individual variation in glucocorticoid levels and the response to challenges—often finding inconsistent patterns; however, few have addressed whether variation in the capacity to terminate the stress response through negative feedback is associated with stress resilience.

   While conceptual models predict that interactions among different components of hypothalamic–pituitary–adrenal (HPA) axis regulation may be important predictors of the phenotypic and fitness effects of stress, we are aware of no previous experimental tests of this hypothesis.

   We investigate whether individual variation in HPA axis regulation is related to resilience to experimental challenges in free‐living tree swallows (Tachycineta bicolor). We mimicked salient natural challenges by temporarily reducing flight efficiency or increasing perceived predation risk during incubation, and determined whether HPA axis responsiveness prior to treatments predicted resilience.

   Females that exhibited both a robust HPA axis activation and strong negative feedback were less likely to abandon nests during incubation.

   Our results suggest that exhibiting a strong HPA axis activation coupled with effective negative feedback may predict stress resilience. Therefore, the ability to turn on and then off the HPA axis efficiently may be important for fitness.

   Our results also suggest that the interactions between different components of the HPA axis may provide greater insight into differences in stress coping capacity.

   Aplain language summaryis available for this article.

    

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2. Prior parental experience attenuates hormonal stress responses and alters hippocampal glucocorticoid receptors in biparental rock doves

   https://doi.org/10.1242/jeb.244820  

   Farrar, Victoria S. ; Morales Gallardo, Jaime ; Calisi, Rebecca M. ( December 2022 , Journal of Experimental Biology)

   In the face of challenges, animals must balance investments in reproductive effort versus their own survival. Physiologically, this trade-off may be mediated by glucocorticoid release by the hypothalamic–pituitary–adrenal axis and prolactin release from the pituitary to maintain parental care. The degree to which animals react to and recover from stressors likely affects maintenance of parental behavior and, ultimately, fitness. However, less is known about how gaining parental experience may alter hormonal stress responses and their underlying neuroendocrine mechanisms. To address this gap, we measured the corticosterone (CORT) and prolactin (PRL) stress response in individuals of both sexes of the biparental rock dove (Columba livia) that had never raised chicks versus birds that had fledged at least one chick. We measured both CORT and PRL at baseline and after an acute stressor (30 min restraint). We also measured negative feedback ability by administering dexamethasone, a synthetic glucocorticoid that suppresses CORT release, and measured CORT and PRL after 60 min. All hormones were measured when birds were not actively nesting to assess whether effects of parental experience extend beyond the breeding bout. Experienced birds had lower stress-induced and negative-feedback CORT, and higher stress-induced PRL than inexperienced birds. In a separate experiment, we measured glucocorticoid receptor subtype expression in the hippocampus, a key site of negative feedback regulation. Experienced birds showed higher glucocorticoid receptor expression than inexperienced controls, which may mediate their ability to attenuate CORT release. Together, these results shed light on potential mechanisms by which gaining experience may improve parental performance and fitness.

    

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3. Genome‐wide variation in DNA methylation is associated with stress resilience and plumage brightness in a wild bird

   https://doi.org/10.1111/mec.15186  

   Taff, Conor C. ; Campagna, Leonardo ; Vitousek, Maren N. ( August 2019 , Molecular Ecology)

   Individuals often differ in their ability to cope with challenging environmental and social conditions. Evidence from model systems suggests that patterns of DNA methylation are associated with variation in coping ability. These associations could arise directly if methylation plays a role in controlling the physiological response to stressors by, among other things, regulating the release of glucocorticoids in response to challenges. Alternatively, the association could arise indirectly if methylation and resilience have a common cause, such as early‐life conditions. In either case, methylation might act as a biomarker for coping ability. At present, however, relatively little is known about whether variation in methylation is associated with organismal performance and resilience under natural conditions. We studied genome‐wide patterns of DNA methylation in free‐living female tree swallows (Tachycineta bicolor) using methylated DNA immunoprecipitation (MeDIP) and a tree swallow genome that was assembled for this study. We identified areas of the genome that were differentially methylated with respect to social signal expression (breast brightness) and physiological traits (ability to terminate the glucocorticoid stress response through negative feedback). We also asked whether methylation predicted resilience to a subsequent experimentally imposed challenge. Individuals with brighter breast plumage and higher stress resilience had lower methylation at differentially methylated regions across the genome. Thus, widespread differences in methylation predicted both social signal expression and the response to future challenges under natural conditions. These results have implications for predicting individual differences in resilience, and for understanding the mechanistic basis of resilience and its environmental and social mediators.

    

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4. Physiological and behavioral responses of house sparrows to repeated stressors

   https://doi.org/10.7717/peerj.4961  

   Gormally, Brenna M.G. ; Wright-Lichter, Jessica ; Reed, J. Michael ; Romero, L. Michael ( January 2018 , PeerJ)

   Despite decades of research, we still lack a complete understanding of what factors influence the transition of the necessary and adaptive acute stress response to what has become known as chronic stress. This gap in knowledge has illuminated the necessity for studies that examine the thresholds between these two sides of the stress response. Here, we determine how repeated exposure to acute stressors influences physiological and behavioral responses. In this repeated measures study, house sparrows ( Passer domesticus ) were exposed to a chronic stress protocol. We took physiological and behavioral measurements before, during, and after the protocol. Blood samples were used to assess four aspects of hypothalamic-pituitary-adrenal (HPA) axis function: baseline corticosterone, stress-induced corticosterone, negative feedback, and the maximal capacity to secrete corticosterone. We also assessed bacterial killing capacity and changes in uric acid concentration. Neophobia trials were used to assess behavioral changes throughout the protocol. We found no significant changes in HPA axis regulation in any of the four aspects we tested. However, we found that uric acid concentrations and neophobia significantly decreased after only four days of the chronic stress protocol, while bacterial killing capacity did not decrease until after eight days of exposure. These results indicate that different components of the stress response can be impacted by chronic stress on different timescales. Our results further indicate the importance of assessing multiple aspects of both physiology and behavior in order to understand how exposure to chronic stress may influence ability to cope with future challenges. 

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5. Social and ecological factors alter stress physiology of Virunga mountain gorillas ( Gorilla beringei beringei )

   https://doi.org/10.1002/ece3.5115  

   Eckardt, Winnie ; Stoinski, Tara S. ; Rosenbaum, Stacy ; Santymire, Rachel ( April 2019 , Ecology and Evolution)

   Living in a rapidly changing environment can alter stress physiology at the population level, with negative impacts on health, reproductive rates, and mortality that may ultimately result in species decline. Small, isolated animal populations where genetic diversity is low are at particular risks, such as endangered Virunga mountain gorillas (Gorilla beringei beringei). Along with climate change‐associated environmental shifts that are affecting the entire population, subpopulations of the Virunga gorillas have recently experienced extreme changes in their social environment. As the growing population moves closer to the forest's carrying capacity, the gorillas are coping with rising population density, increased frequencies of interactions between social units, and changing habitat use (e.g., more overlapping home ranges and routine ranging at higher elevations). Using noninvasive monitoring of fecal glucocorticoid metabolites (FGM) on 115 habituated Virunga gorillas, we investigated how social and ecological variation are related to baseline FGM levels, to better understand the adaptive capacity of mountain gorillas and monitor potential physiological indicators of population decline risks. Generalized linear mixed models revealed elevated mean monthly baseline FGM levels in months with higher rainfall and higher mean maximum and minimum temperature, suggesting that Virunga gorillas might be sensitive to predicted warming and rainfall trends involving longer, warmer dry seasons and more concentrated and extreme rainfall occurrences. Exclusive use of smaller home range areas was linked to elevated baseline FGM levels, which may reflect reduced feeding efficiency and increased travel efforts to actively avoid neighboring groups. The potential for additive effects of stress‐inducing factors could have short‐ and long‐term impacts on the reproduction, health, and ultimately survival of the Virunga gorilla population. The ongoing effects of environmental changes and population dynamics must be closely monitored and used to develop effective long‐term conservation strategies that can help address these risk factors.

    

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