The adaptive value of stress-induced phenotypes: effects of maternally derived corticosterone on sex-biased investment, cost of reproduction, and maternal fitness

Prenatal Corticosterone Impacts Nestling Condition and Immunity in Eastern Bluebirds

Exposure of avian mothers to stressful conditions permanently alters offspring behavior and physiology. Yet, the effects of maternal stress on the development of offspring immunity in birds remain unclear, particularly in wild species. We injected Eastern bluebird (Sialia sialis) eggs with either a corticosterone or control solution, then measured the impacts on nestling morphology and two measures of immunity, bactericidal capacity and swelling responses to phytohemagglutinin. Nestlings from corticosterone-treated eggs had lower condition indices at hatch but quickly caught up to their control counterparts by Day 5 posthatch and until fledging. Corticosterone-exposed nestlings also mounted smaller swelling responses to phytohemagglutinin, whereas there were no effects on bactericidal capacity. These results indicate that maternal stress can impact offspring immunocompetence, fitness prospects, and potentially their ability to fend off parasites and pathogens.

Forecasting the future? Differential allocation of maternal hormones under different social contexts in the blue tit Cyanistes caeruleus

Sociability, i.e. the tendency to interact with other individuals, varies significantly within populations, with some individuals being consistently more sociable than others. Variation may be maintained because the balance between costs (e.g. increase in aggressive disputes, infection risk) and benefits (e.g. information exchange, cooperation) of sociability varies with the environmental context. At the proximate level, apart from genes, mothers transfer non-genetic compounds to their offspring that can influence the development of social skills. In this context, they may adjust their offspring's sociability to match the social environment they will experience after birth, for example, via prenatal hormones. To test this, we experimentally manipulated the social density as perceived by blue tit females before egg laying. We subsequently measured yolk testosterone concentrations and social interactions among family members post-hatching. Females that were exposed to a simulated high social density transferred less testosterone to their eggs than control females. Network average degree (i.e. the number of social interactions of the brood) was not affected by the social density treatment, but broods with lower yolk testosterone concentrations showed a higher network average degree. This suggests that mothers experiencing an environment with high social density (but not increased resource competition) deposit less yolk testosterone to produce offspring that are probably less aggressive but more sociable.

The effects of pre-hatch elevated corticosterone and post-hatch restrictive food availability on the HPA axis development of mallard ducks (Anas platyrhynchos)

Environmental changes can be stressors (altered habitat and food supply, climate change, etc.) to wild animals. Stressors trigger the hypothalamic pituitary adrenal (HPA) axis to release corticosterone (CORT) which modifies energy homeostasis. During nesting, stressed females can deposit increased concentrations of CORT into eggs, altering egg viability and offspring characteristics, constituting a significant mechanism regulating population productivity in subsequent generations. In this study, increased maternal disposition of CORT was mimicked through a 15 ng/g in ovo injection of CORT into mallard duck eggs. Growth and HPA axis function were measured during post-hatch development. For growth, changes in mass were compared at hatch, 7 weeks and 11 weeks. The HPA axis was assessed at seven weeks by measuring CORT at baseline, followed by restraint stress, dexamethasone (negative feedback) and ACTH (maximal adrenal capacity) challenges. At eleven weeks of age, ducks were subjected to a 6-day 25% feed reduction to simulate a poor quality environment to evaluate response to a chronic stressor by comparing CORT at baseline and after restraint stress. Growth and CORT concentration did not differ between treatments at seven weeks or after feed restriction (11 weeks). The CORT dosage administered did not appear to affect HPA axis development in ducklings. Mallards are a highly adaptable species and may have overcome any early alterations to their phenotype. Further research is needed to determine the effects of increased maternal CORT on growth and the development of the HPA axis in ducks. SUMMARY STATEMENT: This study examines how maternal stress (simulated through elevated corticosterone in ovo) and post-hatch chronic stressors (food restriction) affect the development of the HPA axis in a precocial bird.

Stress in the social context: a behavioural and eco-evolutionary perspective

The social environment is one of the primary sources of challenging stimuli that can induce a stress response in animals. It comprises both short-term and stable interactions among conspecifics (including unrelated individuals, mates, potential mates and kin). Social stress is of unique interest in the field of stress research because (1) the social domain is arguably the most complex and fluctuating component of an animal's environment; (2) stress is socially transmissible; and (3) stress can be buffered by social partners. Thus, social interactions can be both the cause and cure of stress. Here, we review the history of social stress research, and discuss social stressors and their effects on organisms across early life and adulthood. We also consider cross-generational effects. We discuss the physiological mechanisms underpinning social stressors and stress responses, as well as the potential adaptive value of responses to social stressors. Finally, we identify outstanding challenges in social stress research, and propose a framework for addressing these in future work.

Maternal provisioning interacts with incubation temperature to affect hatchling mercury exposure in an oviparous reptile

The thermal environment experienced by developing embryos can influence the utilization of maternally provisioned resources. Despite being particularly consequential for oviparous ectotherms, these dynamics are largely unexplored within ecotoxicological frameworks. Here, we test if incubation temperature interacts with maternally transferred mercury to affect subsequent body burdens and tissue distributions of mercury in hatchling American alligators (Alligator mississippiensis). Nine clutches of alligator eggs were collected from a mercury-contaminated reservoir and incubated at either female- or male-promoting temperatures. Total mercury (THg) concentration was measured in egg yolk collected during incubation and in a suite of tissues collected from hatchlings. THg concentrations in residual yolk and blood were higher in hatchlings incubated at cooler, female-promoting temperatures compared to the warmer, male-promoting temperatures. THg concentrations in most tissues were positively correlated with THg concentrations in blood and dermis, and egg yolk THg concentration was the best predictor of THg concentration in many resultant tissues. Our results highlight a hereto unknown role of the developmental environment in mediating tissue specific uptake of contaminants in an oviparous reptile.

The response to stressors in adulthood depends on the interaction between prenatal exposure to glucocorticoids and environmental context

Maternal stress during reproduction can influence how offspring respond to stress later in life. Greater lifetime exposure to glucocorticoid hormones released during stress is linked to greater risks of behavioral disorders, disease susceptibility, and mortality. The immense variation in individual's stress responses is explained, in part, by prenatal glucocorticoid exposure. To explore the long-term effects of embryonic glucocorticoid exposure, we injected Japanese quail (Coturnix japonica) eggs with corticosterone. We characterized the endocrine stress response in offspring and measured experienced aggression at three different ages. We found that prenatal glucocorticoid exposure affected (1) the speed at which the stress response was terminated suggesting dysregulated negative feedback, (2) baseline corticosterone levels in a manner dependent on current environmental conditions with higher levels of experienced aggression associated with higher levels of baseline corticosterone, (3) the magnitude of an acute stress response based on baseline concentrations. We finish by proposing a framework that can be used to test these findings in future work. Overall, our findings suggest that the potential adaptive nature of prenatal glucocorticoid exposure is likely dependent on environmental context and may also be tempered by the negative effects of longer exposure to glucocorticoids each time an animal faces a stressor.

Changes in the rearing environment cause reorganization of molecular networks associated with DNA methylation

Disentangling the interaction between the genetic basis and environmental context underlying phenotypic variation is critical for understanding organismal evolution. Environmental change, such as increased rates of urbanization, can induce shifts in phenotypic plasticity with some individuals adapting to city life while others are displaced. A key trait that can facilitate adaptation is the degree at which animals respond to stressors. This stress response, which includes elevation of baseline circulating concentrations of glucocorticoids, has a heritable component and exhibits intra- and inter-individual variation. However, the mechanisms behind this variability and whether they might be responsible for adaptation to different environments are not known. Variation in DNA methylation can be a potential mechanism that mediates environmental effects on the stress response, as early-life stressors increase glucocorticoid concentrations and change adult phenotype. We used an inter- and intra-environmental cross-foster experiment to analyse the contribution of DNA methylation to early-life phenotypic variation. We found that at hatching, urban house wren (Troglodytes aedon) offspring had higher methylation frequencies compared with their rural counterparts. We also observed age-related patterns in offspring methylation, indicating the developmental effects of the rearing environment on methylation. At fledgling, differential methylation analyses showed that cellular respiration genes were differentially methylated in broods of different origins and behavioural and metabolism genes were differentially methylated in broods of different rearing environments. Lastly, hyper-methylation of a single gene (CNTNAP2) is associated with decreased glucocorticoid levels and the rearing environment. These differential methylation patterns linked to a specific physiological phenotype suggest that DNA methylation may be a mechanism by which individuals adjust to novel environments during their lifespan. Characterizing genetic and environmental influences on methylation is critical for understanding the role of epigenetic mechanisms in evolutionary adaptation.

Urbanization and maternal hormone transfer: Endocrine and morphological phenotypes across ontogenetic stages

The phenotypes observed in urban and rural environments are often distinct; however, it remains unclear how these novel urban phenotypes arise. Hormone-mediated maternal effects likely play a key role in shaping developmental trajectories of offspring in different environments. Thus, we measured corticosterone (Cort) and testosterone (T) concentrations in eggs across the laying sequence in addition to Cort concentrations in nestling and adult female house wrens (Troglodytes aedon) at one urban and one rural site. We found that egg T concentrations were not different between birds from urban and rural sites. However, across all life stages (egg, nestling, and adult female), Cort concentrations were higher at the urban site. Additionally, urban nestling Cort concentrations, but not rural, correlated with fine-scale urban density scores. Furthermore, rural egg volume increased over the laying sequence, but urban egg volume leveled off mid-sequence, suggesting either that urban mothers are resource limited or that they are employing a different brood development strategy than rural mothers. Our study is one of the first to show that egg hormone concentrations differ in an urban environment with differences persisting in chick development and adult life stages. We suggest that maternal endocrine programing may shape offspring phenotypes in urban environments and are an overlooked yet important aspect underlying mechanisms of urban evolution.

The maternal energetic environment affects both egg and offspring phenotypes in green anole lizards (Anolis carolinensis)

Animals exist in dynamic environments that may affect both their own fitness and that of their offspring. Maternal effects might allow mothers to prepare their offspring for the environment in which they will be born via several mechanisms, not all of which are well understood. Resource scarcity and forced resource allocation are two scenarios that could affect maternal investment by altering the amount and type of resources available for investment in offspring, albeit in potentially different ways. We tested the hypothesis that maternal dietary restriction and sprint training have different consequences for the offspring phenotype in an oviparous lizard (Anolis carolinensis). To do this, we collected and reared eggs from adult diet-manipulated females (low-diet [LD] or high-diet [HD]) and sprint-trained females (sprint trained [ST] or untrained [UT]) and measured both egg characteristics and hatchling morphology. ST and LD mothers laid both the fewest and heaviest eggs, and ST, UT, and LD eggs also had significantly longer incubation periods than the HD group. Hatchlings from the diet experiment (LD and HD offspring) were the heaviest overall. Furthermore, both body mass of the mother at oviposition and change in maternal body mass over the course of the experiment had significant and sometimes different effects on egg and offspring phenotypes, highlighting the importance of maternal energetic state to the allocation of maternal resources.

Egg corticosterone can stimulate telomerase activity and promote longer telomeres during embryo development

It is often assumed that the transfer of maternal glucocorticoids (GCs; e.g., corticosterone or cortisol) to offspring is an inevitable cost associated with adverse or stressful conditions experienced by mothers. However, recent evidence indicates that maternal GCs may adaptively programme particular physiological and molecular pathways during development to enhance offspring fitness. In this context, an important mechanism through which maternal GCs may lastingly affect offspring phenotypic quality and survival is via effects on embryo telomerase activity and so on offspring postnatal telomere length. Here, using a field experimental design for which we manipulated the corticosterone content in yellow-legged gull (Larus michahellis) eggs, we show that embryos from corticosterone-injected eggs not only had a higher telomerase activity but also longer telomeres just after hatching. A complementary analysis further revealed that gull hatchlings with longer telomeres had a higher survival probability during the period when most of the chick mortality occurs. Given the important role that telomere length and its restoring mechanisms have on ageing trajectories and disease risk, our findings provide a new mechanistic link by which mothers may presumably shape offspring life-history trajectories and phenotype.

Experimental evidence that chronic outgroup conflict reduces reproductive success in a cooperatively breeding fish

Conflicts with conspecific outsiders are common in group-living species, from ants to primates, and are argued to be an important selective force in social evolution. However, whilst an extensive empirical literature exists on the behaviour exhibited during and immediately after interactions with rivals, only very few observational studies have considered the cumulative fitness consequences of outgroup conflict. Using a cooperatively breeding fish, the daffodil cichlid (Neolamprologus pulcher), we conducted the first experimental test of the effects of chronic outgroup conflict on reproductive investment and output. ‘Intruded’ groups received long-term simulated territorial intrusions by neighbours that generated consistent group-defence behaviour; matched ‘Control’ groups (each the same size and with the same neighbours as an Intruded group) received no intrusions in the same period. Intruded groups had longer inter-clutch intervals and produced eggs with increasingly less protein than Control groups. Despite the lower egg investment, Intruded groups provided more parental care and achieved similar hatching success to Control groups. Ultimately, however, Intruded groups had fewer and smaller surviving offspring than Control groups at 1-month post-hatching. We therefore provide experimental evidence that outgroup conflict can decrease fitness via cumulative effects on reproductive success, confirming the selective potential of this empirically neglected aspect of sociality.

Sex ratios and the city: Secondary offspring sex ratios, parental corticosterone, and parental body condition in an urban-adapted bird

The Trivers–Willard hypothesis states that mothers should adjust their offspring sex ratio according to their own condition and the environment they face during breeding. Past tests of this hypothesis have focused on how natural variation in weather, food availability, or predation pressure shapes sex allocation trade-offs. However, anthropogenic activities, such as urbanization, can alter all of the above characteristics presenting animals with novel challenges in optimizing their brood sex ratio. Previous research has examined how urban living influences individual body condition in several bird taxa, but few have explored subsequent impacts on secondary offspring sex ratio. One likely mediator of the link between environmental conditions, parental condition, and sex ratios is corticosterone (CORT), the primary glucocorticoid in birds. Research on CORT’s influence on sex ratios has focused solely on maternal CORT. However, for species with biparental care, paternal CORT or the similarity of maternal and paternal phenotypes may also help ensure that offspring demand matches parental care quality. To test these hypotheses, we explore offspring secondary sex ratios in European starlings (Sturnus vulgaris). We did not find an effect of site or parental body condition on the production of the more costly sex (males). Instead, we found preliminary evidence suggesting that the similarity of maternal and paternal CORT levels within a breeding pair may increase the likelihood of successfully fledging sons. Maternal and paternal CORT were not significant predictors of secondary sex ratio, suggesting that parental similarity, rather than parental CORT alone, could play a role in shaping secondary offspring sex ratios, but additional work is needed to support this pattern. Starlings are considered an urban-adapted species, making them a compelling model for future studies of the relationship between urbanization, parental body condition, and sex ratios.

Glucocorticoids in a warming world: Do they help birds to cope with high environmental temperatures?

Climate change is threatening biodiversity world-wide. One of its most prominent manifestations are rising global temperatures and higher frequencies of heat waves. High environmental temperatures may be particularly challenging for endotherms, which expend considerable parts of their energy budget and water resources on thermoregulation. Thermoregulation involves phenotypic plasticity in behavioral and physiological traits. Information on causal mechanisms that support plastic thermoregulatory strategies is key to understand how environmental information is transmitted and whether they impose trade-offs or constraints that determine how endotherms cope with climate warming. In this review, we focus on glucocorticoids, metabolic hormones that orchestrate plastic responses to various environmental stimuli including temperature. To evaluate how they may mediate behavioral and physiological responses to high environmental temperatures, we 1) briefly review the major thermoregulatory strategies in birds; 2) summarize the functions of baseline and stress-induced glucocorticoid concentrations; 3) synthesize the current knowledge of the relationship between circulating glucocorticoids and high environmental temperatures in birds; 4) generate hypotheses for how glucocorticoids may support plastic thermoregulatory responses to high environmental temperatures that occur over different time-frames (i.e., acute, short- and longer-term); and 5) discuss open questions on how glucocorticoids, and their relationship with thermoregulation, may evolve. Throughout this review we highlight that our knowledge, particularly on free-living populations, is really limited and outline promising avenues for future research. As evolutionary endocrinologists we now need to step up and identify the costs, benefits, and evolution of glucocorticoid plasticity to elucidate how they may help birds cope with a warming world.

Sex-specific effects of hatching order on nestling baseline corticosterone in a wild songbird

Variation in nestling growth and survival is often influenced by hatching order, with first-hatched offspring having an advantage over later-hatched younger siblings. In house wrens (Troglodytes aedon), this effect of hatching order is especially evident in asynchronously hatched broods and can lead to sex-specific differences in the size and condition of nestlings. Females appear to allocate the sex of their offspring across the laying order to capitalize on these differences. We hypothesized that levels of circulating corticosterone, the primary metabolic hormone in birds, mediates these sex-specific effects in nestlings. We predicted that: i) baseline levels of corticosterone in nestlings should vary along the hatching order, ii) effects of hatching order on baseline corticosterone should be sex specific, and iii) any sex-specificity of hatching order on baseline corticosterone could be contingent on the degree of hatching synchrony. We tested these predictions in a study in which we measured baseline corticosterone in first- and last-hatched nestlings in synchronously and asynchronously hatching broods. To assess whether any differences in nestling baseline corticosterone levels could be attributed to pre-natal maternal effects, the post-natal environment, or both, we conducted two additional studies in which we measured i) yolk corticosterone in first- and last-laid eggs and ii) baseline corticosterone in nestlings that were cross-fostered to create simulated 'asynchronously' hatched broods. There was a significant interaction between sex and relative hatching order in their effects on nestling baseline corticosterone, but no effect of hatching synchrony. Corticosterone levels remained relatively constant across the hatching order in males but decreased in females. There was a significant effect of laying order on yolk corticosterone, with first-laid eggs containing significantly higher levels of yolk corticosterone than last-laid eggs. Cross-fostering of nestlings at different points of development had no significant effect on nestling corticosterone levels. These results indicate that sex-dependent differences in corticosterone levels across the hatching order may arise, at least in part, from embryonic exposure to maternally derived corticosterone, whereas the post-natal rearing environment plays, at best, a minimal role in determining nestling baseline corticosterone levels.

Pre- and Postnatal Predator Cues Shape Offspring Anti-predatory Behavior Similarly in the Bank Vole

Prey animals can assess the risks predators present in different ways. For example, direct cues produced by predators can be used, but also signals produced by prey conspecifics that have engaged in non-lethal predator-prey interactions. These non-lethal interactions can thereby affect the physiology, behavior, and survival of prey individuals, and may affect offspring performance through maternal effects. We investigated how timing of exposure to predation-related cues during early development affects offspring behavior after weaning. Females in the laboratory were exposed during pregnancy or lactation to one of three odor treatments: (1) predator odor (PO) originating from their most common predator, the least weasel, (2) odor produced by predator-exposed conspecifics, which we call conspecific alarm cue (CAC), or (3) control odor (C). We monitored postnatal pup growth, and we quantified foraging and exploratory behaviors of 4-week-old pups following exposure of their mothers to each of the three odour treatments. Exposure to odors associated with predation risk during development affected the offspring behavior, but the timing of exposure, i.e., pre- vs. postnatally, had only a weak effect. The two non-control odors led to different behavioral changes: an attraction to CAC and an avoidance of PO. Additionally, pup growth was affected by an interaction between litter size and maternal treatment, again regardless of timing. Pups from the CAC maternal treatment grew faster in larger litters; pups from the PO maternal treatment tended to grow faster in smaller litters. Thus, in rodents, offspring growth and behavior are seemingly influenced differently by the type of predation risk perceived by their mothers.

Maternal age and maternal environment affect stress reactivity and measures of social behaviour in laying hens

Maternal effects can shape the phenotypes of offspring, but the extent to which a layer breeder's experience can affect commercial laying hens remains unclear. We aimed to investigate the effects of maternal age and maternal environment on laying hens' behaviour and stress response. In our first experiment (E1), commercial hybrid hens were reared either in aviary or barren brooding cages, then housed in aviary, conventional cages or furnished (enriched) cages, thus forming different maternal housing treatments. Hens from each treatment were inseminated at three ages, and measures of response to manual restraint and social stress were assessed in offspring. In experiment 2 (E2), maternal age effects on offsprings' stress response were further investigated using fertile eggs from commercial breeder flocks at three ages. In E1, maternal age affected struggling and corticosterone during manual restraint, feather pecking and pulling and comb wounds. Additionally, maternal rearing and housing in aviary systems showed positive effects on measures of behaviour and stress response in offspring. Effects of maternal age were not replicated in E2, possibly due to methodological differences or higher tolerance to maternal effects in commercial breeders. Overall, we recommend researchers report parent stock age to increase comparison across studies and thus our understanding of maternal age effects.

Landscape of stress: Tree mortality influences physiological stress and survival in a native mesocarnivore

Climate change and anthropogenic modifications to the landscape can have both positive and negative effects on an animal. Linking landscape change to physiological stress and fitness of an animal is a fundamental tenet to be examined in applied ecology. Cortisol is a glucocorticoid hormone that can be used to indicate an animal's physiological stress response. In the Sierra Nevada Mountains of California, fishers (Pekania pennanti) are a threatened mesocarnivore that have been subjected to rapid landscape changes due to anthropogenic modifications and tree mortality related to a 4-year drought. We measured cortisol concentrations in the hair of 64 fishers (41 females, 23 males) captured and radio-collared in the Sierra National Forest, California. We addressed two main questions: (1) Is the physiological stress response of fishers influenced by anthropogenic factors, habitat type, canopy cover, and tree mortality due to drought in their home range? (2) Does the physiological stress response influence survival, reproduction, or body condition? We examined these factors within a fisher home range at 3 scales (30, 60, 95% isopleths). Using model selection, we found that tree mortality was the principle driver influencing stress levels among individual fishers with female and male fishers having increasing cortisol levels in home ranges with increasing tree mortality. Most importantly, we also found a link between physiological stress and demography where female fishers with low cortisol levels had the highest annual survival rate (0.94), whereas females with medium and high cortisol had lower annual survival rates, 0.78 and 0.81, respectively. We found no significant relationships between cortisol levels and body condition, male survival, or litter size. We concluded that tree mortality related to a 4-year drought has created a "landscape of stress" for this small, isolated fisher population.

Poor prey quality is compensated by higher provisioning effort in passerine birds

In altricial avian species, nutrition can significantly impact nestling fitness by increasing their survival and recruitment chances after fledging. Therefore, the effort invested by parents towards provisioning nestlings is crucial and represents a critical link between habitat resources and reproductive success. Recent studies suggest that the provisioning rate has little or no effect on the nestling growth rate. However, these studies do not consider prey quality, which may force breeding pairs to adjust provisioning rates to account for variation in prey nutritional value. In this 8-year study using black-capped (Poecile atricapillus) and boreal (Poecile hudsonicus) chickadees, we hypothesized that provisioning rates would negatively correlate with prey quality (i.e., energy content) across years if parents adjust their effort to maintain nestling growth rates. The mean daily growth rate was consistent across years in both species. However, prey energy content differed among years, and our results showed that parents brought more food to the nest and fed at a higher rate in years of low prey quality. This compensatory effect likely explains the lack of relationship between provisioning rate and growth rate reported in this and other studies. Therefore, our data support the hypothesis that parents increase provisioning efforts to compensate for poor prey quality and maintain offspring growth rates.

Maternal and personal information mediates the use of social cues about predation risk

Organisms can gain information about predation risks from their parents, their own personal experience, and their conspecifics and adjust their behavior to alleviate these risks. These different sources of information can, however, provide conflicting information due to spatial and temporal variation of the environment. This raises the question of how these cues are integrated to produce adaptive antipredator behavior. We investigated how common lizards (Zootoca vivipara) adjust the use of conspecific cues about predation risk depending on whether the information is maternally or personally acquired. We experimentally manipulated the presence of predator scent in gestating mothers and their offspring in a full-crossed design. We then tested the consequences for social information use by monitoring offspring social response to conspecifics previously exposed to predator cues or not. Lizards were more attracted to the scent of conspecifics having experienced predation cues when they had themselves no personal information about predation risk. In contrast, they were more repulsed by conspecific scent when they had personally obtained information about predation risk. However, the addition of maternal information about predation risk canceled out this interactive effect between personal and social information: lizards were slightly more attracted to conspecific scent when these two sources of information about predation risk were in agreement. A chemical analysis of lizard scent revealed that exposure to predator cues modified the chemical composition of lizard scents, a change that might underlie lizards’ use of social information. Our results highlight the importance of considering multiple sources of information while studying antipredator defenses.

Genetic Variation in Parental Effects Contributes to the Evolutionary Potential of Prey Responses to Predation Risk

AbstractDespite the ubiquity of parental effects and their potential effect on evolutionary dynamics, their contribution to the evolution of predator-prey interactions remains poorly understood. Using quantitative genetics, here we demonstrate that parental effects substantially contribute to the evolutionary potential of larval antipredator responses in a leaf beetle (Leptinotarsa decemlineata). Previous research showed that larger L. decemlineata larvae elicit stronger antipredator responses, and mothers perceiving predators improved offspring responses by increasing intraclutch cannibalism-an extreme form of offspring provisioning. We now report substantial additive genetic variation underlying maternal ability to induce intraclutch cannibalism, indicating the potential of this adaptive maternal effect to evolve by natural selection. We also show that paternal size, a heritable trait, affected larval responses to predation risk but that larval responses themselves had little additive genetic variation. Together, these results demonstrate how larval responses to predation risk can evolve via two types of parental effects, both of which provide indirect sources of genetic variation for offspring traits.

Sex differences in foetal origins of child emotional symptoms: a test of evolutionary hypotheses in a large, general population cohort

BACKGROUND

Based on previous findings from the Wirral Child Health and Development Study (WCHADS), and on evolutionary hypotheses, we preregistered analyses of data from a large epidemiological sample (https://osf.io/fn5g9/register/564d31db8c5e4a7c9694b2be), to test for sex-dependent moderation by prenatal maternal depressive symptoms of the association between postnatal maternal depressive symptoms and child emotional problems.

METHODS

A total of 8,354 mothers and children were followed from pregnancy to 3.5 years in the Avon Longitudinal Study of Parents and Children (ALSPAC). Self-report measures of prenatal and postnatal maternal depressive symptoms, and maternal report of child emotional symptoms were administered.

RESULTS

There was a three-way interaction between maternal prenatal and postnatal depression, and child sex (Coeff .042 95% CI 0.015 to 0.068, p = .002). This arose from moderation by prenatal depression, in opposite directions in boys and in girls. In boys, the association between postnatal depression and child emotional symptoms was weaker following lower prenatal depressive symptoms (interaction term coeff = .030, p = .001), and in girls, to a lesser extent, the association was stronger following lower prenatal depressive symptoms (interaction term coeff = -.012, p = .221).

CONCLUSIONS

We replicated the finding from the WCHADS that prenatal depression modifies the association between postnatal depression and children's emotional problems in a sex-dependent fashion. In ALSPAC, the sex difference was explained mainly by a protective effect of low prenatal depression in boys, while in WCHADS, it arose from greater vulnerability of girls to postnatal depression following low prenatal depression. In the light of these findings, in evaluating and implementing early interventions, there is need to consider that risks associated with postnatal depression may vary depending on maternal mood during pregnancy and may differ between boys and girls.

Maternal glucocorticoid levels during incubation predict breeding success, but not reproductive investment, in a free-ranging bird

The hormone corticosterone (CORT) has been hypothesized to be linked with fitness, but the directionality of the relationship is unclear. The ‘CORT-fitness hypothesis’ proposes that high levels of CORT arise from challenging environmental conditions, resulting in lower reproductive success (a negative relationship). In contrast, the CORT-adaptation hypothesis suggests that, during energetically demanding periods, CORT will mediate physiological or behavioral changes that result in increased reproductive investment and success (a positive relationship). During two breeding seasons, we experimentally manipulated circulating CORT levels in female tree swallows (Tachycineta bicolor) prior to egg laying, and measured subsequent reproductive effort, breeding success, and maternal survival. When females were recaptured during egg incubation and again during the nestling stage, the CORT levels were similar among individuals in each treatment group, and maternal treatment had no effect on indices of fitness. By considering variation among females, we found support for the CORT-adaptation hypothesis; there was a significant positive relationship between CORT levels during incubation and hatching and fledging success. During the nestling stage CORT levels were unrelated to any measure of investment or success. Within the environmental context of our study, relationships between maternal glucocorticoid levels and indices of fitness vary across reproductive stages.

Summary: Levels of the stress biomarker corticosterone predict breeding success in female tree swallows. However, correlations between hormone levels and fitness differ between life-history stages.

Early-life stress influences ion balance in developing zebrafish (Danio rerio)

As a key endocrine axis involved in responding to stress, the hypothalamic-pituitary-interrenal axis plays dual roles in mobilizing energy and maintaining ionic/osmotic balance in fishes. Although these roles have been examined independently in detail in adult fishes, less attention has been paid to the effects of an endogenous stress response during early life, particularly with respect to its potential effects on ionic/osmotic balance. The present study tested the hypothesis that exposure of zebrafish to stress during early development would alter ion balance later in life. Zebrafish at three developmental stages (4, 7, or 15 days post-fertilization, dpf) were subjected to an air-exposure stressor twice a day for 2 days, causing elevation of whole-body cortisol levels. Individuals stressed early in life exhibited decreased survival and growth, altered cortisol responses to a subsequent air-exposure stressor, and increased whole-body Na⁺ and Ca²⁺ concentrations. Changes in whole-body Ca²⁺ concentrations were accompanied by increased ionocyte abundance at 7 dpf and increased rates of Ca²⁺ uptake from the environment. Differences in whole-body ion concentrations at 15 and 35 dpf were not accompanied by altered ion uptake rates. Across all ages examined, air-exposure stress experienced at 7 dpf was particularly effective at eliciting phenotypic changes, suggesting a critical window at this age for a stress response to influence development. These findings demonstrate that early-life stress in zebrafish triggers developmental plasticity, with age-dependent effects on both the cortisol stress axis and ion balance.

Effect of early thermal environment on the morphology and performance of a lizard species with bimodal reproduction

Early developmental environments influence life-history traits and survival of reptiles. In fact, rising global temperatures have already caused widespread extinctions among lizards. Viviparous species might be more susceptible to increasing temperatures because of their inability to meet their energetic demands following rapid environmental changes. Nevertheless, we do not yet fully understand how lizards with different reproductive modes can respond to climate change. We investigated the effect of both maternal thermal environment during pregnancy and incubation temperature on hatchling morphology and physiological performance of two populations of the lizard Saiphos equalis differing in their mode of reproduction, to test whether reproductive mode affects the ability to buffer against rising temperatures. Gravid females from both populations were subjected to current or projected end-of-century (future) thermal environments, to evaluate differences in the body size, growth rate, thermal preference, and locomotor performance of their offspring. Our results show that independently of the mode of reproduction, high temperatures accelerated gestation periods. Thermal environments did not affect hatchling thermal preference, but viviparous hatchlings consistently preferred lower temperatures. Unlike viviparous lizards, oviparous hatchlings incubated under future temperatures were smaller and had a lower growth rate compared to current-incubated hatchlings. Finally, thermal environments did not affect hatchling endurance and speed when controlling for body size. Our results show that global warming is likely to have a negative impact on S. equalis, but suggest that some of its effects may be ameliorated by maternal responses during pregnancy, particularly in viviparous populations.

Testing for context-dependent effects of prenatal thyroid hormones on offspring survival and physiology: an experimental temperature manipulation

Maternal effects via hormonal transfer from the mother to the offspring provide a tool to translate environmental cues to the offspring. Experimental manipulations of maternally transferred hormones have yielded increasingly contradictory results, which may be explained by differential effects of hormones under different environmental contexts. Yet context-dependent effects have rarely been experimentally tested. We therefore studied whether maternally transferred thyroid hormones (THs) exert context-dependent effects on offspring survival and physiology by manipulating both egg TH levels and post-hatching nest temperature in wild pied flycatchers (Ficedula hypoleuca) using a full factorial design. We found no clear evidence for context-dependent effects of prenatal THs related to postnatal temperature on growth, survival and potential underlying physiological responses (plasma TH levels, oxidative stress and mitochondrial density). We conclude that future studies should test for other key environmental conditions, such as food availability, to understand potential context-dependent effects of maternally transmitted hormones on offspring, and their role in adapting to changing environments.

The development of flight behaviours in birds

Flight is a unique adaptation at the core of many behaviours in most bird species, whether it be foraging, migration or breeding. Birds have developed a wide diversity of flight modes (e.g. flapping, gliding, soaring, hovering) which involves very specialized behaviours. A key issue when studying flight behaviours is to understand how they develop through all the ontogenetic stages of birds, from the embryo to the flying adult. This question typically involves classical debates on animal behaviour about the importance of maturation and experience. Here, we review the literature available on the development of flight behaviours in birds. First, we focus on the early period when young birds are not yet capable of flight. We discuss examples and show how endogenous processes (e.g. wing flapping in the nest, flight development timing) and environmental factors (e.g. maternal stress, nutritional stress) can influence the development of flight behaviours. Then, we review several examples showing the different processes involved in the development of flight in flight-capable juveniles (e.g. practice, trial and error learning, social learning). Despite the lack of experimental studies investigating this specific question at different developmental stages, we show that several patterns can be identified, and we anticipate that the development of new tracking techniques will allow us to study this question more thoroughly in more bird species.

The relation between liver damage and reproduction in female Japanese quail (Coturnix japonica) exposed to high ambient temperature

Reproductive failure associated with heat stress is a well-known phenomenon in poultry. High temperatures also induce various metabolic disturbances in many animals. Because the liver plays a central role in metabolism, the present study aimed to clarify the relationship between liver and reproduction in Japanese quails exposed to high temperatures. In the consecutive 20-D experimental period, quails were treated with 25°C (control) or 34°C (heat) from 12:00 to 16:00. Eggs were collected for hatching. On completion of the experimental period, quails were humanely euthanized for hormone analyses (e.g., serum and ovarian follicles). Serum metabolites were analyzed using GC/MS. Liver and ovary samples were collected for mRNA levels, histomorphology, and metabolic analysis. Ovary and oviduct weights significantly decreased after daily heat exposure. The number and weight of hierarchical follicles also decreased. Consequently, egg weight decreased. Although there was no difference in fertilization rate, chick birth weight significantly decreased in the heated group. Corticosterone and 17β-estradiol in the serum significantly increased in the heated group. Yolk corticosterone and 17β-estradiol concentration and content were higher in the heated group. Ovary sterologenic enzymes gene P450scc and estrogen receptor expression level increased. The FSH receptor decreased in heat-stressed quails. MetaboAnalyst analysis indicated that high temperature affects propanoate metabolism, beta-alanine metabolism, aspartate metabolism, and histidine metabolism. Triglyceride and cholesterol levels in the liver increased in the heated group. The heated group also had an increased mRNA expression of AGPAT5, apoptosis gene caspase3, and the immunocytokine genes IL-6 and TLR4. However, NF-κB gene expression decreased. These results suggest that high temperatures affect lipid metabolism and apoptosis and cause inflammation in the liver. High temperature induced ovarian dysfunction, which resulted in the decline of hierarchical follicle number and weight, egg weight, and chick birth weight. The increased level of 17β-estradiol suggests liver damage. Protecting liver function from damage may assist quails cope in summer.

Post-natal corticosterone exposure downregulates the HPA axis through adulthood in a common passerine

The hypothalamic-pituitaryadrenal (HPA) axis is one of the most important physiological mechanisms for mediating life-history trade-offs by reallocating resources to immediate survival from other life-history components during a perturbation. Early-life stressor experience and associated upregulation of glucocorticoids can induce short- and long-term changes to the HPA axis in ways that may optimize survival and/or reproduction for the expected adult environment. Although short-term changes to the HPA axis following perinatal stress are well documented, we know less about the long-term effects of early-life stress especially for non-mammalian wild species. Here, we determined long-term effects of experimental post-natal increases in a circulating glucocorticoid on the HPA axis in a common passerine bird, the house sparrow (Passer domesticus). We manipulated circulating corticosterone in wild, free-living nestlings, transferred fledglings to captivity and assessed corticosterone response to a standardized capture-restraint protocol at the pre-fledging, juvenile, and adult stages. Early-life corticosterone manipulation was associated with depressed baseline and stress-induced concentrations of corticosterone at all stages of life, through adulthood. These results provide rare evidence for the effects of early-life stressor experiences through adulthood, with important implications for understanding developmental programming of an endocrine mediator of life history trade-offs.

Developmental plasticity as adaptation: adjusting to the external environment under the imprint of maternal capital

Plasticity is assumed to enable beneficial adjustment to the environment. In this context, developmental plasticity is generally approached within a two-stage framework, whereby adjustments to ecological cues in stage 1 are exposed to selection in stage 2. This conceptual approach may have limitations, because in species providing parental investment, particularly placental mammals such as humans, initial adjustments are not to the environment directly, but rather to the niche generated by parental phenotype (in mammals, primarily that of the mother). Only as maternal investment is withdrawn is the developing organism exposed directly to prevailing ecological conditions. A three-stage model may therefore be preferable, where developmental trajectory first adjusts to maternal investment, then to the external environment. Each offspring experiences a trade-off, benefitting from maternal investment during the most vulnerable stages of development, at the cost of exposure to investment strategies that maximize maternal fitness. Maternal life-history trade-offs impact the magnitude and schedule of her investment in her offspring, generating lifelong effects on traits related to health outcomes. Understanding the imprint of maternal capital on offspring is particularly important in species demonstrating social hierarchy. Interventions targeting maternal capital might offer new opportunities to improve health outcomes of both mother and offspring. This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

Exposure to exogenous egg cortisol does not rescue juvenile Chinook salmon body size, condition, or survival from the effects of elevated water temperatures

Climate change is leading to altered temperature regimes which are impacting aquatic life, particularly for ectothermic fish. The impacts of environmental stress can be translated across generations through maternally derived glucocorticoids, leading to altered offspring phenotypes. Although these maternal stress effects are often considered negative, recent studies suggest this maternal stress signal may prepare offspring for a similarly stressful environment (environmental match). We applied the environmental match hypothesis to examine whether a prenatal stress signal can dampen the effects of elevated water temperatures on body size, condition, and survival during early development in Chinook salmon Oncorhynchus tshawytscha from Lake Ontario, Canada. We exposed fertilized eggs to prenatal exogenous egg cortisol (1,000 ng/ml cortisol or 0 ng/ml control) and then reared these dosed groups at temperatures indicative of current (+0°C) and future (+3°C) temperature conditions. Offspring reared in elevated temperatures were smaller and had a lower survival at the hatchling developmental stage. Overall, we found that our exogenous cortisol dose did not dampen effects of elevated rearing temperatures (environmental match) on body size or early survival. Instead, our eyed stage survival indicates that our prenatal cortisol dose may be detrimental, as cortisol-dosed offspring raised in elevated temperatures had lower survival than cortisol-dosed and control reared in current temperatures. Our results suggest that a maternal stress signal may not be able to ameliorate the effects of thermal stress during early development. However, we highlight the importance of interpreting the fitness impacts of maternal stress within an environmentally relevant context.

Stress hypothesis overload: 131 hypotheses exploring the role of stress in tradeoffs, transitions, and health

Stress is ubiquitous and thus, not surprisingly, many hypotheses and models have been created to better study the role stress plays in life. Stress spans fields and is found in the literature of biology, psychology, psychophysiology, sociology, economics, and medicine, just to name a few. Stress, and the hypothalamic-pituitaryadrenal/interrenal (HPA/I) axis and sympathetic nervous system (SNS), are involved in a multitude of behaviors and physiological processes, including life-history and ecological tradeoffs, developmental transitions, health, and survival. The goal of this review is to highlight and summarize the large number of available hypotheses and models, to aid in comparative and interdisciplinary thinking, and to increase reproducibility by a) discouraging hypothesizing after results are known (HARKing) and b) encouraging a priori hypothesis testing. For this review I collected 214 published hypotheses or models dealing broadly with stress. In the main paper, I summarized and categorized 131 of those hypotheses and models which made direct connections among stress and/or HPA/I and SNS, tradeoffs, transitions, and health. Of those 131, the majority made predictions about reproduction (n = 43), the transition from health to disease (n = 38), development (n = 23), and stress coping (n = 18). Additional hypotheses were classified as stage-spanning or models (n = 37). The additional 83 hypotheses found during searches were tangentially related, or pertained to immune function or oxidative stress, and these are listed separately. Many of the hypotheses share underlying rationale and suggest similar, if not identical, predictions, and are thus not mutually exclusive; some hypotheses spanned classification categories. Some of the hypotheses have been tested multiple times, whereas others have only been examined a few times. It is the hope that multi-disciplinary stress researchers will begin to harmonize their naming of hypotheses in the literature so as to build a clearer picture of how stress impacts various outcomes across fields. The paper concludes with some considerations and recommendations for robust testing of stress hypotheses.

In utero behavioral imprinting to predation risk in pups of the bank vole

Abstract

In the predator–prey arms race, survival-enhancing adaptive behaviors are essential. Prey can perceive predator presence directly from visual, auditory, or chemical cues. Non-lethal encounters with a predator may trigger prey to produce special body odors, alarm pheromones, informing conspecifics about predation risks. Recent studies suggest that parental exposure to predation risk during reproduction affects offspring behavior cross-generationally. We compared behaviors of bank vole (Myodes glareolus) pups produced by parents exposed to one of three treatments: predator scent from the least weasel (Mustela nivalis nivalis); scent from weasel-exposed voles, i.e., alarm pheromones; or a control treatment without added scents. Parents were treated in semi-natural field enclosures, but pups were born in the lab and assayed in an open-field arena. Before each behavioral test, one of the three scent treatments was spread throughout the test arena. The tests followed a full factorial design (3 parental treatments × 3 area treatments). Regardless of the parents’ treatment, pups exposed to predator odor in the arena moved more. Additionally, pups spend more time in the center of the arena when presented with predator odor or alarm pheromone compared with the control. Pups from predator odor–exposed parents avoided the center of the arena under control conditions, but they spent more time in the center when either predator odor or alarm pheromone was present. Our experiment shows that cross-generational effects are context-sensitive, depending on the perceived risk. Future studies should examine cross-generational behavioral effects in ecologically meaningful environments instead of only neutral ones.

Significance statement

We exposed bank voles to odors signaling predation risk to assess the effects parental predation exposure on the behavior of their offspring. Besides predator odor, we also assessed the role of a conspecific alarm cue as a novel way of spreading the predation risk information. Pup behaviors were assessed in the open-field arena, a standard way of assessing animal behavior in a wide range of contexts. We found that also alarm pheromone increased the time pups spend in the center of the arena similarly to predator odor. While previous studies suggested that offspring would be more fearful, our results indicate that the cross-generational effects are very context-dependent; i.e., they differ significantly depending on which scent cue is presented in the open-field arena. This shows the need for better tools or measurements to translate laboratory results into ecologically meaningful frameworks.

Lactating Coquerel's sifaka (Propithecus coquereli) exhibit reduced stress responses in comparison to males and nonlactating females

Cortisol is a glucocorticoid hormone that plays a principal role in metabolic function and stress responses in wild primates. Stressors are ubiquitous in environments and elicit a variety of physiological and behavioral responses. While stress responses are adaptive in the short-term, they can have negative effects when experienced over longer durations. As a physiological stressor, the process of lactation is an energetically expensive activity for mammals. Milk production increases water loss and increased hydration demands are amplified in mammalian species inhabiting xeric habitats, including lemur species living in northwestern Madagascar-the region for this research work. Here, sifakas give birth during the dry season (May-October) and wean infants during the subsequent wet season (November-April). The author collected fecal samples during the 24 weeks following infant births in 10 groups of Coquerel's sifaka in Ankarafantsika Park, Madagascar. The author analyzed the samples by comparing the first 12-week time block to the second 12-week time block, which corresponded to the dry and the beginning of the wet seasons, respectively. Analyses were based on 375 samples collected over two birth seasons (2010 and 2011). A linear mixed model determined the relationships between reproductive class and temporal cortisol variation. The three reproductive classes had significantly different cortisol concentrations. Lactating females had lower cortisol than adult males and nonlactating females in all weeks postnatal. Males had significantly higher cortisol in weeks 13-24 relative to weeks 1-12. Examining seasonal changes in cortisol concentrations demonstrates how lemurs respond physiologically to the energetic constraints of lactation during the critical life history stage of infant development.

Posthatching Parental Care and Offspring Growth Vary with Maternal Corticosterone Level in a Wild Bird Population

Corticosterone is the primary metabolic steroid in birds and is vital for maintaining homeostasis. However, the relationship between baseline corticosterone and reproduction is unclear, and we lack an understanding of how differences in baseline corticosterone at one stage of the breeding cycle influence reproductive effort at later stages. In a wild population of house wrens, we quantified the concentration of corticosterone in yolks of freshly laid eggs as an integrated measure of maternal physiology and related this to a behavioral measure of stress reactivity made during the nestling period, namely, the latency with which females resumed parental activities following a standardized disturbance at their nest (setting up a camera to record provisioning). Females that recently produced eggs containing higher corticosterone concentrations, which were significantly repeatable within females, took longer to resume activity related to parental care (i.e., feeding and brooding young) following the disturbance. Moreover, a female's latency to resume parental activities negatively predicted her provisioning of nestlings with food and the condition of these young at fledging but did not predict the number fledged. We cross-fostered offspring prior to hatching so these effects on maternal behavior are independent of any prenatal maternal effects on nestlings via the egg. These results are consistent with earlier findings, suggesting that females with higher baseline corticosterone during egg laying or early incubation tend to prioritize self-maintenance over reproduction compared with females with lower baseline corticosterone and suggest that a female's latency to return to her nest and resume parental care following a disturbance might represent a simple, functional measure of maternal stress reactivity.

Effects of prenatal stress and genetics on embryonic survival and offspring growth of laying hens

Early-life exposure to stressors can shape the phenotype of the offspring resulting in changes that may affect their prehatch and posthatch development. This can be modeled indirectly through maternal exposure to stressors (natural model) or by offspring exposure to stress hormones (pharmacological model). In this study, both models were used to investigate the effects of genetic line on hatchability, late embryonic mortality, sex ratio, and body weight until 17 wk of age. To form the parent stock, fertilized eggs of 4 commercial genetic lines — two brown (brown 1 and 2), two white (white 1 and 2), and a pure line White Leghorn — were incubated, hatched, and housed identically in 4 flocks of 27 birds (24 females and 3 males) per strain. Each strain was equally separated into 2 groups: “maternal stress,” where hens were subjected to a series of acute psychological stressors (e.g., physical restraint, transportation) for 8 D before egg collection, and “control,” where hens received routine husbandry. At 3 maternal ages, fertile eggs from both treatments were collected, and additional eggs from the control group were injected with corticosterone (10 ng/mL egg content) (“CORT”). A “vehicle” treatment was included to account for effects of egg manipulation. Each maternal age comprised a replicate over time. Eggs were incubated and hatched, and the offspring (N = 1,919) were brooded until 17 wk under identical conditions. The results show that prenatal stress interacted with strain to decrease embryonic survival and growth. Among all strains, brown 2 was consistently the most affected line in both prehatch and posthatch development. Our study shows that embryonic survival and offspring growth are mostly affected by the pharmacological model and that strain differences may increase susceptibility to prenatal stress. Moreover, it suggests that the natural stressor model may be useful for quantifying the response of the mother to stressors, whereas the pharmacological model may be useful for quantifying the response of the embryo to increased levels of corticosterone.

Do hormone manipulations reduce fitness? A meta-analytic test of the Optimal Endocrine Phenotype Hypothesis

Endocrine traits (e.g., circulating hormone concentrations, receptor expression) can vary considerably among individuals within populations. Here, we develop two evolutionary hypotheses to explain this variation. Under the Optimal Endocrine Phenotype Hypothesis, adaptive plastic responses to environmental variation generate individual variation in endocrine traits and allow individuals to express near-optimal endocrine phenotypes. In contrast, under the Ongoing Selection Hypothesis, individual variation in endocrine traits reflects varying adaptive value, with some individuals expressing suboptimal phenotypes that are selected against. These two hypotheses generate distinct predictions for the effects of hormone manipulations on fitness. Under the Optimal Endocrine Phenotype Hypothesis, all hormone manipulations should incur fitness costs, whereas under the Ongoing Selection Hypothesis, manipulating endocrine phenotypes toward a putative optimum should increase fitness. Using a meta-analysis of findings from experimental field studies that involved manipulation of circulating glucocorticoids or androgens and measurement of fitness effects, we test and find some support for the Optimal Endocrine Phenotype Hypothesis. On average, fitness was reduced across 97 estimates of the effects of experimental hormone manipulations on fitness. However, the fitness effects of glucocorticoid manipulations varied with the sex of the individuals being studied. Fitness was more uniformly reduced by glucocorticoid manipulations in males and when both sexes were considered together. In females, effects on fitness varied from highly positive to highly negative. The effects of androgen manipulations varied across males and females, and depending upon whether fitness was estimated using measures of reproductive success or survival. Reproductive success was consistently decreased by androgen manipulation in females, but was increased almost as often as it was decreased across experiments in males. When survival was estimated as a component of fitness, it was fairly uniformly compromised by exogenous androgens in males. This variation in fitness effects of hormone manipulations across sexes and fitness metrics is consistent with the expectation that hormones differentially regulate life-history investment and that optimal endocrine phenotypes differ between males and females. Overall, our meta-analysis provides some support for the Optimal Endocrine Phenotype Hypothesis, but we await direct tests of the Ongoing Selection Hypothesis to determine the degree to which individual variation in endocrine traits continues to be shaped by natural selection.

Maternal glucocorticoids promote offspring growth without inducing oxidative stress or shortening telomeres in wild red squirrels

Elevations in glucocorticoid (GC) levels in breeding females may induce adaptive shifts in offspring life histories. Offspring produced by mothers with elevated GCs may be better prepared to face harsh environments, where a faster pace of life is beneficial. We examined how experimentally elevated GCs in pregnant or lactating North American red squirrels (Tamiasciurus hudsonicus) affected offspring postnatal growth, structural size and oxidative stress levels (two antioxidants and oxidative protein damage) in three different tissues (blood, heart and liver) and liver telomere lengths. We predicted that offspring from mothers treated with GCs would grow faster but would also have higher levels of oxidative stress and shorter telomeres, which may predict reduced longevity. Offspring from mothers treated with GCs during pregnancy were 8.3% lighter around birth but grew (in body mass) 17.0% faster than those from controls, whereas offspring from mothers treated with GCs during lactation grew 34.8% slower than those from controls and did not differ in body mass around birth. Treating mothers with GCs during pregnancy or lactation did not alter the oxidative stress levels or telomere lengths of their offspring. Fast-growing offspring from any of the treatment groups did not have higher oxidative stress levels or shorter telomere lengths, indicating that offspring that grew faster early in life did not exhibit oxidative costs after this period of growth. Our results indicate that elevations in maternal GCs may induce plasticity in offspring growth without long-term oxidative costs to the offspring that might result in a shortened lifespan.

Parasitism by an invasive nest fly reduces future reproduction in Galápagos mockingbirds

Organisms allocate limited resources to competing activities such as reproduction, growth, and defense against parasites and predators. The introduction of a novel parasite may create new life history trade-offs. As hosts increase their investment in self-maintenance or defense, the cost of parasitism may carry over to other aspects of host biology. Here, in an experimental field study, we document delayed effects of an introduced nest parasite, Philornis downsi, on reproduction of Galápagos mockingbirds (Mimus parvulus). Parasitism of first nests reduced both the number and size of chicks that parents hatched when they re-nested several weeks later. The delayed effect of P. downsi on future reproduction may have been mediated by behavioral shifts by the parents to avoid or resist parasitism. Our results demonstrate that effects of parasitism can persist even after immediate exposure ends. We draw attention to the potential implications that introduced parasites have for host reproductive strategies.

In ovo metabolism of progesterone to 5β-pregnanedione in chicken eggs: Implications for how yolk progesterone influences embryonic development

Progesterone has received substantial attention for the essential role it plays in establishing and maintaining pregnancy in placental vertebrates. Despite the prevalence of progesterone during development, relatively little is known about how embryos respond to progesterone. This is true of placental vertebrates as well as egg-laying vertebrates where levels of progesterone in the yolk tend to be higher than most other steroids in the yolk. Bird eggs provide an opportunity to investigate the effects of progesterone on embryonic development because progesterone can be easily manipulated without any confounding effects on maternal physiology. To understand how progesterone might influence embryonic development, it is important to characterize the metabolic fate of progesterone given its potential to be converted to a wide range of steroids. We investigated the metabolic fate of tritiated progesterone over the first four days of development using chicken eggs (Gallus gallus) and identified 5β-pregnanedione as the primary metabolite during this period. After only one day of development, 5β-pregnanedione could be detected within the yolk. Levels of 5β-pregnanedione in both the yolk and albumen tended to rise early in development but conjugated metabolites began to accumulate towards the end of our sampling period. Additionally, in vitro assays using embryo homogenates collected after 72 h of development demonstrated that embryos were capable of carrying out the conversion of progesterone to 5β-pregnanedione. Overall these results have important implications for deciphering the mechanisms through which yolk progesterone might influence embryonic development. Effects could arise via progesterone receptors or receptors capable of binding 5β-pregnanedione but we found no evidence that progesterone is serving as a precursor for androgen or estrogen production.

Developmental responses to early-life adversity: Evolutionary and mechanistic perspectives

Adverse ecological and social conditions during early life are known to influence development, with rippling effects that may explain variation in adult health and fitness. The adaptive function of such developmental plasticity, however, remains relatively untested in long-lived animals, resulting in much debate over which evolutionary models are most applicable. Furthermore, despite the promise of clinical interventions that might alleviate the health consequences of early-life adversity, research on the proximate mechanisms governing phenotypic responses to adversity have been largely limited to studies on glucocorticoids. Here, we synthesize the current state of research on developmental plasticity, discussing both ultimate and proximate mechanisms. First, we evaluate the utility of adaptive models proposed to explain developmental responses to early-life adversity, particularly for long-lived mammals such as humans. In doing so, we highlight how parent-offspring conflict complicates our understanding of whether mothers or offspring benefit from these responses. Second, we discuss the role of glucocorticoids and a second physiological system-the gut microbiome-that has emerged as an additional, clinically relevant mechanism by which early-life adversity can influence development. Finally, we suggest ways in which nonhuman primates can serve as models to study the effects of early-life adversity, both from evolutionary and clinical perspectives.

Mismatched Prenatal and Postnatal Maternal Depressive Symptoms and Child Behaviours: A Sex-Dependent Role for NR3C1 DNA Methylation in the Wirral Child Health and Development Study

Evolutionary hypotheses predict that male fetuses are more vulnerable to poor maternal conditions (Sex-biased Maternal Investment), but female fetuses are at greater risk of glucocorticoid-mediated disorders where there is a mismatch between fetal and postnatal environments (Predictive Adaptive Response). Self-reported prenatal and postnatal depression and maternal report of child anxious-depressed symptoms at 2.5, 3.5 and 5.0 years were obtained from an ‘extensive’ sample of first-time mothers (N = 794). Salivary NR3C1 1-F promoter methylation was assayed at 14 months in an ‘intensive’ subsample (n = 176) and stratified by psychosocial risk. Generalised structural equation models were fitted and estimated by maximum likelihood to allow the inclusion of participants from both intensive and extensive samples. Postnatal depression was associated with NR3C1 methylation and anxious-depressed symptoms in daughters of mothers with low prenatal depression (prenatal-postnatal depression interaction for methylation, p < 0.001; for child symptoms, p = 0.011). In girls, NR3C1 methylation mediated the association between maternal depression and child anxious-depressed symptoms. The effects were greater in girls than boys: the test of sex differences in the effect of the prenatal-postnatal depression interaction on both outcomes gave X² (2) = 5.95 (p = 0.051). This was the first human study to show that epigenetic and early behavioural outcomes may arise through different mechanisms in males and females.

Between- and Within-Individual Variation of Maternal Thyroid Hormone Deposition in Wild Great Tits (Parus major)

Maternal hormones are often considered a mediator of anticipatory maternal effects; namely, mothers adjust maternal hormone transfer to prepare the offspring for the anticipated environment. The flexibility for mothers to adjust hormone transfer is therefore a prerequisite for such anticipatory maternal effects. Nevertheless, previous studies have focused only on the average differences of maternal hormone transfer between groups and neglected the substantial individual variation, despite the fact that individual plasticity in maternal hormone transfer is actually the central assumption. In this study, we studied the between- and within-individual variation of maternal thyroid hormones (THs) in egg yolk of wild great tits (Parus major) and estimated the individual plasticity of maternal yolk THs across environmental temperature, clutch initiation dates, and egg laying order using linear mixed effects models. Interestingly, our models provide statistical evidence that the two main THs-the main biologically active hormone T3 and T4, which is mostly considered a prohormone-exhibited different variation patterns. Yolk T3 showed significant between-individual variation on the average levels, in line with its previously reported moderate heritability. Yolk T4, however, showed significant between-clutch variation in the pattern over the laying sequence, suggesting a great within-individual plasticity. Our findings suggest that the role and function of the hormone within the endocrine axis likely influences its flexibility to respond to environmental change. Whether the flexibility of T4 deposition brings a fitness advantage should be examined along with its potential effects on offspring, which remain to be further investigated.

Yolk immunoreactive corticosterone in hierarchical follicles of Japanese quail (Coturnix japonica) exposed to heat challenge

High temperature decreases the egg number, ovarian weight, and hierarchical follicle number. In the present study, we investigated the effect of high temperature on the quality of eggs of adult female quails. Laying quail were raised under a standard thermal condition of 25 °C until exposed to an elevated temperature of 34 °C (experimental) or maintained at 25 °C (control) from 12:00 to 16:00 for 10 consecutive days. Weight and number of eggs were measured; serum and the largest follicles were collected and used for hormone measurement. Ovaries and adrenals were collected for expression analysis of 3β- and 17β-HSD, genes encoding steroidogenic enzymes. Egg weight slightly decreased with an increase in the treatment time in the heat-challenged group; the egg weight significantly decreased in the heat treatment group than in the control group during the last 2 days of experiment (P < 0.05). The laying rate showed no difference during the experimental period but significantly decreased on the last day in the heat treatment group. In the experimental group the ovaries and oviducts were lighter (P < 0.05) and the hierarchical follicle number and ovarian weight decreased (P < 0.05) compared to the control group. Although serum corticosterone level significantly increased after heat challenge (P < 0.05) and immediately recovered to the normal level, yolk immunoreactive corticosterone in the hierarchical follicle (F1, F2, F3) significantly increased (P < 0.05). The expression level of 17β-HSD showed no changes in the ovary but significantly increased in adrenals (P < 0.05). Our findings indicate that the effects of heat challenge on the maternal ovary in the quail are mediated through the adrenal function.

Senescence and costs of reproduction in the life history of a small precocial species

Species following a fast life history are expected to express fitness costs mainly as increased mortality, while slow-lived species should suffer fertility costs. Because observational studies have limited power to disentangle intrinsic and extrinsic factors influencing senescence, we manipulated reproductive effort experimentally in the cavy (Cavia aperea) which produces extremely precocial young. We created two experimental groups: One was allowed continuous reproduction (CR) and the other intermittent reproduction (IR) by removing males at regular intervals. We predicted that the CR females should senesce (and die) earlier and produce either fewer and/or smaller, slower growing offspring per litter than those of the IR group. CR females had 16% more litters during three years than IR females. CR females increased mass and body condition more steeply and both remained higher until the experiment ended. Female survival showed no group difference. Reproductive senescence in litter size, litter mass, and reproductive effort (litter mass/maternal mass) began after about 600 days and was slightly stronger in CR than IR females. Litter size, litter mass, and offspring survival declined with maternal age and were influenced by seasonality. IR females decreased reproductive effort less during cold seasons and only at higher age than CR females. Nevertheless, offspring winter mortality was higher in IR females. Our results show small costs of reproduction despite high reproductive effort, suggesting that under ad libitum food conditions costs depend largely on internal regulation of allocation decisions.