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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. 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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3. 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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4. Short-term elevations in glucocorticoids do not alter telomere lengths: A systematic review and meta-analysis of non-primate vertebrate studies

   https://doi.org/10.1371/journal.pone.0257370  

   Zane, Lauren ; Ensminger, David C. ; Vázquez-Medina, José Pablo ( October 2021 , PLOS ONE)

   Saretzki, Gabriele (Ed.)

   Background The neuroendocrine stress response allows vertebrates to cope with stressors via the activation of the Hypothalamic-Pituitary-Adrenal (HPA) axis, which ultimately results in the secretion of glucocorticoids (GCs). Glucocorticoids have pleiotropic effects on behavior and physiology, and might influence telomere length dynamics. During a stress event, GCs mobilize energy towards survival mechanisms rather than to telomere maintenance. Additionally, reactive oxygen species produced in response to increased GC levels can damage telomeres, also leading to telomere shortening. In our systematic review and meta-analysis, we tested whether GC levels impact telomere length and if this relationship differs among time frame, life history stage, or stressor type. We hypothesized that elevated GC levels are linked to a decrease in telomere length. Methods We conducted a literature search for studies investigating the relationship between telomere length and GCs in non-human vertebrates using four search engines: Web of Science, Google Scholar, Pubmed and Scopus, last searched on September 27th, 2020. This review identified 31 studies examining the relationship between GCs and telomere length. We pooled the data using Fisher’s Z for 15 of these studies. All quantitative studies underwent a risk of bias assessment. This systematic review study was registered in the Open Science Framework Registry ( https://osf.io/rqve6 ). Results The pooled effect size from fifteen studies and 1066 study organisms shows no relationship between GCs and telomere length (Fisher’s Z = 0.1042, 95% CI = 0.0235; 0.1836). Our meta-analysis synthesizes results from 15 different taxa from the mammalian, avian, amphibian groups. While these results support some previous findings, other studies have found a direct relationship between GCs and telomere dynamics, suggesting underlying mechanisms or concepts that were not taken into account in our analysis. The risk of bias assessment revealed an overall low risk of bias with occasional instances of bias from missing outcome data or bias in the reported result. Conclusion We highlight the need for more targeted experiments to understand how conditions, such as experimental timeframes, stressor(s), and stressor magnitudes can drive a relationship between the neuroendocrine stress response and telomere length. 

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5. Damage-Fitness Model: Evaluation and synthesis

   https://doi.org/10.1093/icb/icz060  

   Wada, Haruka ; Heidinger, Britt ( June 2019 , Integrative and Comparative Biology)

   Decades of research into stress responses have highlighted large variation among individuals, populations, and species, and the sources of this variation have been a center of research across disciplines. The most common measure of the vertebrate stress response is glucocorticoids. However, the predictive power of glucocorticoid responses to fitness is surprisingly low. This is partly because the hormone levels rapidly change in response to stressor exposure and elevated levels at one time point can indicate either that glucocorticoids are helping the organism cope with the stressor or that dysregulation of hormone release is harming the organism. Meaning, the fitness consequences of the stressor depends on how efficient the stress responses are at negating the harmful impacts of stressors to cells and tissues. To encompass the idea of the efficiency of stress responses and to integrate cellular and organismal stress responses, a new theoretical model called the Damage-Fitness Model was developed. The model focuses on the downstream effects of stress responses and predicts that the accumulation of damage in cells and tissues (e.g., persistent damage to proteins, lipids, and DNA) negatively impacts fitness components. In this mini-review, we examine evidence supporting the Damage-Fitness Model and explore new directions forward.

    

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