Glucocorticoids (corticosterone/cortisol; cort) are frequently used in conservation as biomarkers of disturbance in wild populations. However, the context‐dependent nature of cort means that it may not always accurately reflect disturbance. For example, there is growing evidence that wildlife populations can evolve or acclimate to human‐induced environmental change (i.e. contaminants) by expressing higher levels of tolerance. Mechanisms that allow for populations to achieve higher contaminant tolerance can be related to cort and thereby impact the reliability of cort as an indicator of disturbance. This study asks: (1) do wildlife populations that differ in tolerance to contaminants differentially express baseline and stress‐induced cort and (2) is cort a viable indicator of disturbance across populations that differ in tolerance to contaminants? Toward this goal, we identified three wood frog
The Cort-Fitness Hypothesis has generated much interest from investigators integrating field endocrinology with evolutionary biology, ecology, and conservation. The hypothesis was developed to test the assumption that if glucocorticoid levels increase with environmental challenges and fitness decreases with environmental challenges, then there should be a negative relationship between baseline glucocorticoid levels and fitness. Indeed, studies across diverse taxa have found that the relationship between baseline glucocorticoid levels and fitness are not consistent: some studies show a positive relationship, others negative, and some show no correlation. Hence, a deeper understanding of the mechanisms underlying the relationship between baseline glucocorticoid levels, environmental challenges, and fitness is needed. We propose a mathematical model representing the links between baseline glucocorticoid levels, environmental challenges, and fitness. Our model describes how variation in the predictability and intensity of environmental challenges, reproductive strategies, and fitness metrics can all contribute to the variability observed in empirical tests of the Cort-Fitness Hypothesis. We provide qualitative results showing that much of the inconsistency in previous studies can be explained and we discuss how the model can be used to inform future Cort-Fitness studies.
more » « less- NSF-PAR ID:
- 10120706
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Integrative Organismal Biology
- Volume:
- 1
- Issue:
- 1
- ISSN:
- 2517-4843
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Rana sylvatica populations with relatively high NaCl tolerance and three populations with relatively low NaCl tolerance. Tadpoles from these populations were reared to metamorphosis in either an environmentally relevant concentration of NaCl (0.5 g L−1NaCl) or a control (0 g L−1NaCl). At metamorphosis we used a non‐invasive waterborne assay to measure baseline and stress‐induced cort release rates and measured fitness‐related metrics. We found that contaminant tolerance influences cort levels. More tolerant populations had lower baseline cort and higher fitness compared to less tolerant populations. However, despite variation in cort across populations with different levels of tolerance, cort still represents a viable indicator of condition as our results show a consistent negative relationship between cort and fitness. Lastly, we found that levels of cort were consistent regardless of whether amphibians were reared in NaCl contaminated or non‐contaminated environments. Overall, we emphasize the importance of recognizing population‐level variation in cort due to contaminant tolerance when using cort as a biomarker for conservation purposes. -
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Using the van Noordwijk and de Jong Resource Framework to Evaluate Glucocorticoid-Fitness HypothesesAbstract 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.more » « less
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Abstract Vertebrates respond to a diversity of stressors by rapidly elevating glucocorticoid (GC) levels. The changes in physiology and behavior triggered by this response can be crucial for surviving a variety of challenges. Yet the same process that is invaluable in coping with immediate threats can also impose substantial damage over time. In addition to the pathological effects of long-term exposure to stress hormones, even relatively brief elevations can impair the expression of a variety of behaviors and physiological processes central to fitness, including sexual behavior, parental behavior, and immune function. Therefore, the ability to rapidly and effectively terminate the short-term response to stress may be fundamental to surviving and reproducing in dynamic environments. Here we review the evidence that variation in the ability to terminate the stress response through negative feedback is an important component of stress coping capacity. We suggest that coping capacity may also be influenced by variation in the dynamic regulation of GCs—specifically, the ability to rapidly turn on and off the stress response. Most tests of the fitness effects of these traits to date have focused on organisms experiencing severe or prolonged stressors. Here we use data collected from a long-term study of tree swallows (Tachycineta bicolor) to test whether variation in negative feedback, or other measures of GC regulation, predict components of fitness in non-chronically stressed populations. We find relatively consistent, but generally weak relationships between different fitness components and the strength of negative feedback. Reproductive success was highest in individuals that both mounted a robust stress response and had strong negative feedback. We did not see consistent evidence of a relationship between negative feedback and adult or nestling survival: negative feedback was retained in the best supported models of nestling and adult survival, but in two of three survival-related analyses the intercept-only model received only slightly less support. Both negative feedback and stress-induced GC levels—but not baseline GCs—were individually repeatable. These measures of GC activity did not consistently covary across ages and life history stages, indicating that they are independently regulated. Overall, the patterns seen here are consistent with the predictions that negative feedback—and the dynamic regulation of GCs—are important components of stress coping capacity, but that the fitness benefits of having strong negative feedback during the reproductive period are likely to manifest primarily in individuals exposed to chronic or repeated stressors.
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