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1. Glucocorticoids hormones (GCs) are intuitively important for mediation of age-dependent vertebrate life-history transitions through their effects on ontogeny alongside underpinning variation in life-history traits and trade-offs in vertebrates. These concepts largely derive from the ability of GCs to alter energy allocation, physiology and behaviour that influences key life-history traits involving age-specific life-history transitions, reproduction and survival. 2. Studies across vertebrates have shown that the neuroendocrine stress axis plays a role in the developmental processes that lead up to age-specific early life-history transitions. While environmental sensitivity of the stress axis allows for it to modulate the timing of these transitions within species, little is known as to how variation in stress axis function has been adapted to produce interspecific variation in the timing of life-history transitions. 3. Our assessment of the literature confirms that of previous reviews that there is only equivocal evidence for correlative or direct functional relationships between GCs and variation in reproduction and survival. We conclude that the relationships between GCs and life-history traits are complex and general patterns cannot be easily discerned with current research approaches and experimental designs. 4. We identify several future research directions including: (i) integration of proximate and ultimate measures, including longitudinal studies that measure effects of GCs on more than one life-history trait or in multiple environmental contexts, to test explicit hypotheses about how GCs and life-history variation are related and (ii) the measurement of additional factors that modulate the effects of GCs on life-history traits (e.g. GC receptors and binding protein levels) to better infer neurendocrine stress axis actions. 5. Conceptual models of HPA/I axis actions, such as allostatic load and reactive scope, to some extent explicitly predict the role of GCs in a life-history context, but are descriptive in nature. We propose that GC effects on life-history transitions, survival probabilities and fecundity can be modelled in existing quantitative demographic frameworks to improve our understanding of how GC variation influences life-history evolution and GC-mediated effects on population dynamics

Background Psychological stress and coping experienced during pregnancy can have important effects on maternal and infant health, which can also vary by race, ethnicity, and socioeconomic status. Therefore, we assessed stressors, coping behaviors, and resources needed in relation to the COVID-19 pandemic in a sample of 162 perinatal (125 pregnant and 37 postpartum) women in the United States. Methods A mixed-methods study captured quantitative responses regarding stressors and coping, along with qualitative responses to open-ended questions regarding stress and resources needed during the COVID-19 pandemic. Logistic and linear regression models were used to analyze differences between pregnant and postpartum participants, as well as differences across key demographic variables. Qualitative content analysis was used to analyze open-ended questions. Results During the COVID-pandemic, food scarcity and shelter-in-place restrictions made it difficult for pregnant women to find healthy foods. Participants also reported missing prenatal appointments, though many reported using telemedicine to obtain these services. Financial issues were prevalent in our sample and participants had difficulty obtaining childcare. After controlling for demographic variables, pregnant women were less likely to engage in healthy stress-coping behaviors than postpartum women. Lastly, we were able to detect signals of increased stressors induced by the COVID-19 pandemic, and less social support, in perinatal women of racial and ethnic minority, and lower-income status. Qualitative results support our survey findings as participants expressed concerns about their baby contracting COVID-19 while in the hospital, significant others missing the delivery or key obstetric appointments, and wanting support from friends, family, and birthing classes. Financial resources, COVID-19 information and research as it relates to maternal-infant health outcomes, access to safe healthcare, and access to baby supplies (formula, diapers, etc.) emerged as the primary resources needed by participants. Conclusions To better support perinatal women’s mental health during the COVID-19 pandemic, healthcare providers should engage in conversations regarding access to resources needed to care for newborns, refer patients to counseling services (which can be delivered online/via telephone) and virtual support groups, and consistently screen pregnant women for stressors.

Exposure to adverse environmental conditions during early development can shape life-history traits and have lasting effects on physiological function in later life. Although findings within the biomedical literature have shown that environmentally induced elevations in glucocorticoids (GCs) during critical developmental windows can cause persistent carry-over effects (i.e., developmental programming), little is known about whether such effects of GCs can be generalized to wildlife species. Using wood frogs as a study species, we conducted an experiment with a split-plot design to assess the short-term and the long-term physiological consequences of availability of food, hydroperiod length (i.e., pond drying), and the interaction between these two environmental conditions. In outdoor experimental ponds, we reared tadpoles in chronically high or low-food conditions, and tadpoles from each pond experienced either high water until metamorphosis or a reduction in water volume during late developmental stages (after Gosner stage 38). After metamorphosis, animals were housed individually and fed ad libitum for 10 weeks, and growth rate, fat content, and resting and acute stressinduced GC levels were measured. We found that tadpoles experiencing low availability of food and reduced water volume had elevated GC levels, reduced mass, and body condition as they approached metamorphosis. At 10 weeks after metamorphosis, we found that these two conditions also had persistent interactive effects on post-metamorphic allocation of resources to growth, energy storage, and responsiveness of GCs to a novel stressor. Of individuals that experienced reduced water volume, only those that experienced high food as tadpoles were able to catch up to individuals that did not experience reduced water volume in terms of body mass, femur length, and body condition, and they allocated more resources to fat storage. By contrast, 10-week old frogs with low-food and that experienced low water volume and low-food levels as tadpoles allocated fewer resources to mass-specific growth, stored less fat, and exhibited blunted GC response to a novel stressor relative to those that did not experience water-reduction. Our findings demonstrate that environmental conditions experienced prior to and during important developmental transitions shape resource allocation and the ability to physiologically respond to future stressors in juvenile and potentially adult animals. These results suggest that chronic and acute environmental stressors experienced during early life stages can have cumulative and interactive effects that need to be considered when modeling the ecological and evolutionary consequences of environmental change on populations.

Population size is an important metric to inform the conservation and management of species. For aquatic species, environmental DNA (eDNA) concentration has been suggested for non-invasively estimating population size. However, many biotic and abiotic factors simultaneously influence the production and degradation of eDNA which can alter the relationship between population size and eDNA concentration. We investigated the influence of temperature, salinity, and ranavirus infection on eDNA concentrations using tadpole mesocosms. Using linear regression models, we tested the influence of each experimental treatment on eDNA concentrations at three time points before and during epidemics. Prior to infection, elevated temperatures lowered eDNA concentrations, indicating that degradation was the driving force influencing eDNA concentrations. During early epidemics, no treatments strongly influenced eDNA concentrations and in late epidemics, productive forces dominated as ranavirus intensity and dead organisms increased eDNA concentrations. Finally, population size was only an important predictor of eDNA concentration in late epidemics and we observed high levels of variation between samples of replicate mesocosms. We demonstrate the complexities of several interacting factors influencing productive and degradative forces, variation in influences on eDNA concentration over short time spans, and examine the limitations of estimating population sizes from eDNA with precision in semi-natural conditions.

Socially regulated sex change in teleost fishes is a striking example of social status information regulating biological function in the service of reproductive success. The establishment of social dominance in sex changing species is translated into a cascade of changes in behavior, physiology, neuroendocrine function, and morphology that transforms a female into a male, or vice versa. The hypothalamic-pituitary-interrenal axis (HPI, homologous to HP-adrenal axis in mammals and birds) has been hypothesized to play a mechanistic role linking status to sex change. The HPA/I axis responds to environmental stressors by integrating relevant external and internal cues and coordinating biological responses including changes in behavior, energetics, physiology, and morphology (i.e., metamorphosis). Through actions of both corticotropin-releasing factor and glucocorticoids, the HPA/I axis has been implicated in processes central to sex change, including the regulation of agonistic behavior, social status, energetic investment, and life history transitions. In this paper, we review the hypothesized roles of the HPA/I axis in the regulation of sex change and how those hypotheses have been tested to date. We include original data on sex change in the bluebanded goby (Lythyrpnus dalli), a highly social fish capable of bidirectional sex change. We then propose a model for HPA/I involvement in sex change and discuss how these ideas might be tested in the future. Understanding the regulation of sex change has the potential to elucidate evolutionarily conserved mechanisms responsible for translating pertinent information about the environment into coordinated biological changes along multiple body axes.

Leptin, the protein product of the obese (ob) gene, is a type-I cytokine hormone secreted by fat that is integral to food intake regulation and influences almost every physiological system in juvenile and adult mammals. Since the identification of leptin in the mouse in 1994, biologists have searched for orthologous genes in other species with limited success. In this article, we report the identification and functional characterization of leptin and leptin receptor (LR) in Xenopus. Despite low amino acid sequence similarity to mammalian leptins ( approximately 35%) the frog protein has a nearly identical predicted tertiary structure and can activate the frog and mouse LRs in vitro. We showed that recombinant frog leptin (rxLeptin) is a potent anorexigen in frogs, as it is in mammals, but this response does not develop until midprometamorphosis. However, during early prometamorphosis, exogenous rxLeptin induced growth and development of the hind limb, where LR mRNA is expressed. The rxLeptin also stimulated cell proliferation in cultured hind limbs from early prometamorphic tadpoles, as measured by [(3)H]thymidine uptake. These findings are evidence that leptin can influence limb growth and differentiation during early development. Furthermore, the isolation and characterization of leptin and its receptor in a nonamniote provides an essential foundation for elucidating the structural and functional evolution of this important hormone.

Chronic exposure to physical, chemical, and biotic stressors can make animals more susceptible to infections. Such stress‐induced susceptibility is widely expected to make disease—and epidemics—more likely and more severe. However, whether the impacts of stressors on individuals scale up to population‐level outcomes is uncertain, both theoretically and empirically. We initiated ranavirus epidemics in replicate mesocosm populations of larval wood frogs (Lithobates sylvaticus) exposed to conditions known to impact their individual susceptibility to ranavirus infections: low and high salinity crossed factorially with ambient or elevated temperatures. Contrary to expectations, epidemics were no more likely or more severe in stressful conditions, although increased temperatures did speed their progression. We discuss several potential reasons why the effects of stressors did not scale up to epidemics, but our results suggest caution in assuming the individual‐level effects of even well‐studied stressors necessarily scale simply to population‐level outcomes. Experimental ranavirus epidemics in populations of wood frog tadpoles were used to the hypothesis that individual‐level effects of stress and temperature predict population‐level outcomes. In contrast to conventional wisdom, environmental stressors did not appreciably change the outcome or severity of 96 replicate experimental epidemics in mesocosms.

When experiencing resource competition or abrupt environmental change, animals often must transition rapidly from an ancestral diet to a novel, derived diet. Yet, little is known about the proximate mechanisms that mediate such rapid evolutionary transitions. Here, we investigated the role of diet-induced, cryptic genetic variation in facilitating the evolution of novel resource-use traits that are associated with a new feeding strategy—carnivory—in tadpoles of spadefoot toads (genus Spea). We specifically asked whether such variation in trophic morphology and fitness is present in Scaphiopus couchii, a species that serves as a proxy for ancestral Spea. We also asked whether corticosterone, a vertebrate hormone produced in response to environmental signals, mediates the expression of this variation. Specifically, we compared broad-sense heritabilities of tadpoles fed different diets or treated with exogenous corticosterone, and found that novel diets can expose cryptic genetic variation to selection, and that diet-induced hormones may play a role in revealing this variation. Our results therefore suggest that cryptic genetic variation may have enabled the evolutionary transition to carnivory in Spea tadpoles, and that such variation might generally facilitate rapid evolutionary transitions to novel diets.

Environmental DNA (eDNA) is increasingly used to infer species abundance, but uncertainty remains about how well individual water samples reflect system‐wide eDNA concentrations. Currently, high levels of variation in estimated eDNA concentration among samples, even under similar conditions, limit the utility of eDNA estimates of abundance, especially in conservation and management scenarios. To investigate the sources and magnitude of variation in eDNA estimates, we analyzed eDNA from replicate water samples taken from mesocosms housing Lithobates sylvaticus tadpoles. We partitioned the variance of observed eDNA concentrations between biological replicates (distinct water samples) and technical replicates. We further tested whether stochastic variation in extraction efficiency (e.g., silica‐column DNA binding and elution) introduced variance using internal control DNA. We then trialed modifications of sampling methods that could be applied to improve precision, including increased water volume, spatially pooled subsamples, and modified filtration. The observed variance in eDNA concentrations was substantial, ~75% of which was attributable to variation among replicate samples from the same mesocosm and < 5% related to technical replicates; extraction‐related variance was negligible. Sampling modifications, like increasing sample volume and taking combined samples of multiple scoops, improved precision, while others highlighted potential trade‐offs between precision and accuracy. Our results suggest that sample‐to‐sample variation, even from seemingly homogeneous, controlled environments, can be substantial even under highly controlled conditions. This inherent variability imposes limits on the precision of abundance estimates derived from eDNA and underscores the importance of replication and protocol optimization in study design. Quantifying eDNA has the potential to expand applications, such as estimating species abundance, but current measurements of eDNA quantification show high variability. We found that ~75% of observed variation among replicate samples is the result of sample‐to‐sample variation, but that levels of variation in eDNA quantification could be reduced by adjusting sampling methods.

Size variance among similarly aged individuals within populations is a pattern common to many organisms that is a result of interactions between intrinsic and extrinsic traits of individuals. While genetic and maternal effects, as well as physiological and behavioral traits have been shown to contribute to size variation in animal populations, teasing apart the influence of such factors on individual growth rates remain a challenge. Furthermore, tracing the effects of these interactions across life stages and in shaping adult phenotypes also requires further exploration. In this study we investigated the relationship between genetics, hatching patterns, behaviors, neuroendocrine stress axis activity and variance in growth and metamorphosis among same-aged larval amphibians. Through parallel experiments we found that in the absence of conspecific interactions, hatch time and to a lesser extent egg clutch identity (i.e. genetics and maternal effects) influenced the propensity for growth and development in individual tadpoles and determined metamorphic traits. Within experimental groups we found that variance in growth rates was associated with size-dependent foraging behaviors and responses to food restriction. We also found an inverse relationship between glucocorticoid (GC) hormone levels and body mass and developmental stage among group-reared tadpoles, which suggests that GC expression plays a role in regulating differing within-population growth trajectories in response to density-dependent conditions. Taken together these findings suggest that factors that influence hatching conditions can have long-term effects on growth and development. These results also raise compelling questions regarding the extent to which maternal and genetic factors influence physiological and behavioral profiles in amphibians.