Frank Beach Award Winner: The centrality of the hypothalamic-pituitary-adrenal axis in dealing with environmental change across temporal scales

Early development of the glucocorticoid stress response in poison frog tadpoles

In vertebrates, the glucocorticoid response through the hypothalamic-pituitary-adrenal (HPA) axis controls many essential functions, including behavior, metabolism, and ontogenetic transitions. However, there are tradeoffs associated with high levels of glucocorticoids, including reduced growth rate and lowered immunity. These tradeoffs drive variation in the timing of the development of the HPA axis across taxa. In anurans (frogs and toads), corticosterone has critical roles in development and behavior, and concentrations can fluctuate in response to environmental stressors. Given the role of corticosterone in ontogenetic changes and behaviors, we hypothesized that species with immediate habitat transitions and challenges would develop an HPA axis early in development. To test this hypothesis, we studied tadpoles of the dyeing poison frog ( Dendrobates tinctorius ), a species in which tadpoles hatch terrestrially and are transported to pools of water by their parent. We measured the excretion rate and whole-body concentration of corticosterone and the corticosterone response to adrenocorticotropic hormone (ACTH). We found no significant differences in excretion rates and whole-body concentration of corticosterone, nor physiological response to ACTH injection across tadpole development. These findings indicate that the glucocorticoid response is developed early in ontogeny. These findings generally differ from those found in other species of tadpoles, which may suggest the unique ecological pressures of D. tinctorius has shaped the development of its HPA axis. More broadly, this study illustrates how life history strategies and tradeoffs of glucocorticoids impact the timing of the development of the HPA axis.

Highlights

The timing of HPA axis development differs across species. We studied the HPA axis across tadpole development in Dendrobates tinctorius . No difference in corticosterone concentration across development.No difference in corticosterone response to ACTH across development.Results suggest an early developed HPA axis is essential for their life history.

Early life nutrient restriction affects hypothalamic-pituitary-interrenal axis gene expression in a diet type-specific manner

Stressful experiences in early life can alter phenotypic expression later in life. For instance, in vertebrates, early life nutrient restriction can modify later life activity of the hypothalamic-pituitary-adrenal/interrenal axis (the HPI in amphibians), including the up- and downstream regulatory components of glucocorticoid signaling. Early life nutrient restriction can also influence later life behavior and metabolism (e.g., fat accumulation). Yet, less is known about whether nutrient stress-induced carryover effects on HPA/HPI axis regulation can vary across environmental contexts, such as the type of diet on which nutrient restriction occurs. Here, we experimentally address this question using the plains spadefoot toad (Spea bombifrons), whose larvae develop in ephemeral habitats that impose intense competition over access to two qualitatively distinct diet types: detritus and live shrimp prey. Consistent with diet type-specific carryover effects of early life nutrient restriction on later life HPI axis regulation, we found that temporary nutrient restriction at the larval stage reduced juvenile (i.e., post-metamorphic) brain gene expression of an upstream glucocorticoid regulator (corticotropin-releasing hormone) and two downstream regulators (glucocorticoid and mineralocorticoid receptors) only on the shrimp diet. These patterns are consistent with known diet type-specific effects of larval nutrient restriction on juvenile corticosterone and behavior. Additionally, larval nutrient restriction increased juvenile body fat levels. Our study indicates that HPA/HPI axis regulatory responses to nutrient restriction can vary remarkably across diet types. Such diet type-specific regulation of the HPA/HPI axis might provide a basis for developmental or evolutionary decoupling of stress-induced carryover effects.

Stress biomarkers and child development in young children in Bangladesh

BACKGROUND

Hundreds of millions of children in low- and middle-income countries are exposed to chronic stressors, such as poverty, poor sanitation and hygiene, and sub-optimal nutrition. These stressors can have physiological consequences for children and may ultimately have detrimental effects on child development. This study explores associations between biological measures of chronic stress in early life and developmental outcomes in a large cohort of young children living in rural Bangladesh.

METHODS

We assessed physiologic measures of stress in the first two years of life using measures of the hypothalamic-pituitary-adrenal (HPA) axis (salivary cortisol and glucocorticoid receptor gene methylation), the sympathetic-adrenal-medullary (SAM) system (salivary alpha-amylase, heart rate, and blood pressure), and oxidative status (F2-isoprostanes). We assessed child development in the first two years of life with the MacArthur-Bates Communicative Development Inventories (CDI), the WHO gross motor milestones, and the Extended Ages and Stages Questionnaire (EASQ). We compared development outcomes of children at the 75th and 25th percentiles of stress biomarker distributions while adjusting for potential confounders using generalized additive models, which are statistical models where the outcome is predicted by a potentially non-linear function of predictor variables.

RESULTS

We analyzed data from 684 children (49% female) at both 14 and 28 months of age; we included an additional 765 children at 28 months of age. We detected a significant relationship between HPA axis activity and child development, where increased HPA axis activity was associated with poor development outcomes. Specifically, we found that cortisol reactivity (coefficient -0.15, 95% CI (-0.29, -0.01)) and post-stressor levels (coefficient -0.12, 95% CI (-0.24, -0.01)) were associated with CDI comprehension score, post-stressor cortisol was associated with combined EASQ score (coefficient -0.22, 95% CI (-0.41, -0.04), and overall glucocorticoid receptor methylation was associated with CDI expression score (coefficient -0.09, 95% CI (-0.17, -0.01)). We did not detect a significant relationship between SAM activity or oxidative status and child development.

CONCLUSIONS

Our observations reveal associations between the physiological evidence of stress in the HPA axis with developmental status in early childhood. These findings add to the existing evidence exploring the developmental consequences of early life stress.

Associations between glucocorticoids and habitat selection reflect daily and seasonal energy requirements

BACKGROUND

Glucocorticoids are often associated with stressful environments, but they are also thought to drive the best strategies to improve fitness in stressful environments. Glucocorticoids improve fitness in part by regulating foraging behaviours in response to daily and seasonal energy requirements. However, many studies demonstrating relationships between foraging behaviour and glucocorticoids are experimental, and few observational studies conducted under natural conditions have tested whether changing glucocorticoid levels are related to daily and seasonal changes in energy requirements.

METHODS

We integrated glucocorticoids into habitat selection models to test for relationships between foraging behaviour and glucocorticoid levels in elk (Cervus canadensis) as their daily and seasonal energy requirements changed. Using integrated step selection analysis, we tested whether elevated glucocorticoid levels were related to foraging habitat selection on a daily scale and whether that relationship became stronger during lactation, one of the greatest seasonal periods of energy requirement for female mammals.

RESULTS

We found stronger selection of foraging habitat by female elk with elevated glucocorticoids (eß = 1.44 95% CI 1.01, 2.04). We found no difference in overall glucocorticoid levels after calving, nor a significant change in the relationship between glucocorticoids and foraging habitat selection at the time of calving. However, we found a gradual increase in the relationship between glucocorticoids and habitat selection by female elk as their calves grew over the next few months (eß = 1.01, 95% CI 1.00, 1.02), suggesting a potentially stronger physiological effect of glucocorticoids for elk with increasing energy requirements.

CONCLUSIONS

We suggest glucocorticoid-integrated habitat selection models demonstrate the role of glucocorticoids in regulating foraging responses to daily and seasonal energy requirements. Ultimately, this integration will help elucidate the implications of elevated glucocorticoids under natural conditions.

Physiological stress response to urbanisation differs between native and invasive squirrel species

Novel pressures derived from urbanisation can alter native habitats and ultimately impact wildlife. Coping with such human-driven changes might induce shifts in species phenotypic traits, such as physiological responses to anthropogenic stressors. Preadaptation to face those challenges has been suggested to favour settlement and spread of invasive alien species in urbanised areas which, consequently, might respond differently than ecologically similar native species to stressors posed by urbanisation. The activation of the hypothalamic-pituitary-adrenal (HPA) axis and the subsequent release of glucocorticoids (GCs) has been suggested to mediate responses to anthropogenic disturbance in vertebrates. Furthermore, intraspecific competition, in conjunction with stressors related to urbanisation, might affect invasive and native species physiological stress responses differently. Using a parallel pseudo-experimental study system we measured faecal glucocorticoid metabolite (FGM) concentrations of the native Eurasian red squirrel and the invasive alien Eastern grey squirrel along a rural-urban gradient and in relation to conspecific density. The two species responded differently to challenges posed by the synergic effect of urbanisation and intraspecific competition. Association of FGMs and conspecific density in native red squirrels varied between rural and suburban sites, potentially depending on differential HPA axis responses. In urban sites, this relationship did not differ significantly from that in rural and suburban ones. Conversely, invasive grey squirrels' FGMs did not vary in relation to conspecific density, nor differed along the rural-urban gradient. Improving knowledge about native and competing invasive species' physiological responses to anthropogenic stressors can support conservation strategies in habitats altered by man. Our findings suggested that the invasive squirrels might be preadapted to cope with these challenges in urbanised areas, potentially increasing their success under the future global change scenario.

Endocrine flexibility can facilitate or constrain the ability to cope with global change

Global climate change has increased average environmental temperatures world-wide, simultaneously intensifying temperature variability and extremes. Growing numbers of studies have documented phenological, behavioural and morphological responses to climate change in wild populations. As systemic signals, hormones can contribute to orchestrating many of these phenotypic changes. Yet little is known about whether mechanisms like hormonal flexibility (reversible changes in hormone concentrations) facilitate or limit the ability of individuals, populations and species to cope with a changing climate. In this perspective, we discuss different mechanisms by which hormonal flexibility, primarily in glucocorticoids, could promote versus hinder evolutionary adaptation to changing temperature regimes. We focus on temperature because it is a key gradient influenced by climate change, it is easy to quantify, and its links to hormones are well established. We argue that reaction norm studies that connect individual responses to population-level and species-wide patterns will be critical for making progress in this field. We also develop a case study on urban heat islands, where several key questions regarding hormonal flexibility and adaptation to climate change can be addressed. Understanding the mechanisms that allow animals to cope when conditions become more challenging will help in predicting which populations are vulnerable to ongoing climate change. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Predation risk-mediated indirect effects promote submerged plant growth: Implications for lake restoration

Biological manipulation, involving fish stockings, is commonly used to counteract the deterioration of submerged vegetation in eutrophic lakes. Nevertheless, the non-consumptive effects (NCEs) of stocked carnivorous fish are often overlooked. Using a controlled experimental system, we investigated the NCEs of a native carnivorous fish, snakehead (Channa argus), on two key biological factors, herbivore-dominated grass carp (Ctenopharyngodon idella) and disturbance-dominated loach (Misgurnus anguillicaudatus), influencing submerged plants growth. Additionally, we conducted a meta-analysis on predation risk and primary productivity. The results reveal that predation risk induces oxidative stress damage and affects grass carp growth. Non-significant changes in cortisol and glucose may be linked to predation risk prediction. Simultaneously, predation risk reduces fish feeding and disturbance behavior, relieving pressure on submerged plants to be grazed and disturbed, thereby supporting plant development. The presence of submerged plants, in turn, enhances loach activity and influences water body characteristics through negative feedback. Furthermore, the meta-analysis results indicate the facilitative effect of predation risk on primary producers. Our findings contribute to the understanding of biological manipulation theory. We demonstrate that the predation risk associated with introducing carnivorous fish can promote the growth of submerged plants through behaviorally mediated indirect effects. This highlights the potential utility of predation risk in lake restoration efforts.