Integrating Ecological and Evolutionary Context in the Study of Maternal Stress

Predation risk affects egg mass but not egg steroid hormone concentrations in yellow-legged gulls

Predators have both direct, consumptive effects on their prey and non-lethal effects on physiology and behavior, including reproductive decisions, with cascading effects on prey ecology and evolution. Here, we experimentally tested such non-lethal effects of exposure to increased predation risk on clutch size, egg mass, and the concentration of yolk steroid hormones in the yellow-legged gull Larus michahellis. We simulated increased predation risk by displaying stuffed predators (adult fox Vulpes vulpes, and adult buzzard Buteo buteo) to breeding adults before egg laying. The concentration of corticosterone, which has been shown to increase under exposure to maternal predation risk in other species, and of testosterone did not differ between eggs from mothers exposed to the predators and eggs from control mothers (i.e., eggs exposed to a novel object of similar size and position to the stuffed predators). The concentration of the two hormones negatively covaried. Clutch size did not vary according to experimental treatment, whereas egg mass was markedly larger in clutches from nests exposed to predators than in clutches from control nests. By increasing egg mass, mothers may reduce the risk of cooling of the eggs when incubation is impeded by predators, boost energy reserves, reduce post-natal detectability caused by food solicitation, and/or enhance development at hatching, thus increasing the chances of offspring survival. In general, our results are inconsistent with most of the few previous studies on similar non-lethal predator effects and suggest that such effects may vary among species according to ecological conditions, social behavior, and developmental mode.

The development of individual differences in cooperative behaviour: maternal glucocorticoid hormones alter helping behaviour of offspring in wild meerkats

The phenotype of parents can have long-lasting effects on the development of offspring as well as on their behaviour, physiology and morphology as adults. In some cases, these changes may increase offspring fitness but, in others, they can elevate parental fitness at a cost to the fitness of their offspring. We show that in Kalahari meerkats ( Suricata suricatta), the circulating glucocorticoid (GC) hormones of pregnant females affect the growth and cooperative behaviour of their offspring. We performed a 3-year experiment in wild meerkats to test the hypothesis that GC-mediated maternal effects reduce the potential for offspring to reproduce directly and therefore cause them to exhibit more cooperative behaviour. Daughters (but not sons) born to mothers treated with cortisol during pregnancy grew more slowly early in life and exhibited significantly more of two types of cooperative behaviour (pup rearing and feeding) once they were adults compared to offspring from control mothers. They also had lower measures of GCs as they aged, which could explain the observed increases in cooperative behaviour. Because early life growth is a crucial determinant of fitness in female meerkats, our results indicate that GC-mediated maternal effects may reduce the fitness of offspring, but may elevate parental fitness as a consequence of increasing the cooperative behaviour of their daughters. This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

Challenges to the parental brain: Neuroethological and translational considerations

Extending from research documenting adaptive parental responses in nonthreatening contexts, the influences of various neuroethological and physiological challenges on effective parenting responses are considered in the current review. In natural habitats, rodent family units are exposed to predators, compromised resources, and other environmental stressors that disrupt HPA axis functions. With the additional physiological demands associated with caring for offspring, alterations in stress-related neuroendocrine responsiveness contribute to adaptive responses in many challenging contexts. Some environmental contexts, however, such as restricted nesting resources, result in disrupted maternal responses that have a negative impact on offspring wellbeing. Additionally, parental dysregulation associated with exposure to environmental chemicals or pharmacological substances, also compromise maternal responses with effects that often extend to future generations. Continued preclinical and clinical research elucidating parental responses to various stressors and physiological disruptors is necessary to provide valuable translational information identifying threats to effective parenting outcomes.

Sibling sex, but not androgens, shapes phenotypes in perinatal common marmosets (Callithrix jacchus)

When offspring share a womb, interactions among fetuses can impart lasting impressions on phenotypic outcomes. Such intrauterine interactions often are mediated by sex steroids (estrogens and androgens) produced by the developing fetuses. In many mammals, intrauterine interactions between brothers and sisters lead to masculinization of females, which can induce fitness consequences. Many litter-bearing primates, though, seem to escape androgen-mediated litter effects, begging why? Here, we investigated how the sex composition (i.e., same- or mixed-sex) of litters influences perinatal outcomes in the common marmoset monkey (Callithrix jacchus), using a combination of physiological, morphological, and behavioural assays. We hypothesized that androgens from male fetuses would mediate developmental differences across litter types. We found that newborns (24-36 hours old) from same- and mixed-sex litters were indistinguishable by urinary androgen profiles, birth weights, morphometrics, and behaviour. However, monkeys born into same- and mixed-sex litters exhibited subtle morphological and neurobehavioral differences later in the perinatal period, independent of their androgen profiles. Our findings suggest that while androgens from male fetuses likely do not organize their siblings' phenotypes, perinatal stimuli may initiate divergent developmental trajectories among siblings, which, in turn, promotes inter-individual variability within families.

Parental habituation to human disturbance over time reduces fear of humans in coyote offspring

A fundamental tenet of maternal effects assumes that maternal variance over time should have discordant consequences for offspring traits across litters. Yet, seldom are parents observed across multiple reproductive bouts, with few studies considering anthropogenic disturbances as an ecological driver of maternal effects. We observed captive coyote (Canis latrans) pairs over two successive litters to determine whether among‐litter differences in behavior (i.e., risk‐taking) and hormones (i.e., cortisol and testosterone) corresponded with parental plasticity in habituation. Thus, we explicitly test the hypothesis that accumulating experiences of anthropogenic disturbance reduces parental fear across reproductive bouts, which should have disparate phenotypic consequences for first‐ and second‐litter offspring. To quantify risk‐taking behavior, we used foraging assays from 5–15 weeks of age with a human observer present as a proxy for human disturbance. At 5, 10, and 15 weeks of age, we collected shaved hair to quantify pup hormone levels. We then used a quantitative genetic approach to estimate heritability, repeatability, and between‐trait correlations. We found that parents were riskier (i.e., foraged more frequently) with their second versus first litters, supporting our prediction that parents become increasingly habituated over time. Second‐litter pups were also less risk‐averse than their first‐litter siblings. Heritability for all traits did not differ from zero (0.001–0.018); however, we found moderate support for repeatability in all observed traits (r = 0.085–0.421). Lastly, we found evidence of positive phenotypic and cohort correlations among pup traits, implying that cohort identity (i.e., common environment) contributes to the development of phenotypic syndromes in coyote pups. Our results suggest that parental habituation may be an ecological cue for offspring to reduce their fear response, thus emphasizing the role of parental plasticity in shaping their pups’ behavioral and hormonal responses toward humans.

Evidence of embryonic regulation of maternally derived yolk corticosterone

In recent years, the potential for maternal stress effects to adaptively alter offspring phenotype has received considerable attention. This research has identified offspring traits that are labile in response to maternal stress; however, an understanding of the mechanisms underlying these effects is lagging and is crucial to appreciating the significance of this maternal effect. In the present study, we sought to better understand maternal stress effects by examining the potential for embryonic regulation of corticosterone exposure, determining the phenotypic consequences of elevated corticosterone during development, and characterizing the levels of maternally transferred corticosterone in unmanipulated eggs using Trachemys scripta By dosing eggs with tritiated corticosterone and tracking the steroid throughout development, we found that most corticosterone is metabolized, and less than 1% of the corticosterone dose reaches the embryo as free corticosterone. We also found that exogenous dosing of corticosterone, in concentrations sufficient to overwhelm embryonic metabolism, reduces embryonic survival and negatively impacts hatchling traits important to fitness. Our results demonstrate that concentrations of maternal corticosterone in the yolks of unmanipulated eggs are low and are significantly lower than the doses of corticosterone required to elicit phenotypic effects in hatchlings. Taken together, these results provide evidence that both the embryo and the female may minimize corticosterone accumulation in the embryo to avoid reductions in embryonic survival and negative impacts on offspring phenotype and fitness.

Survival and reproductive costs of repeated acute glucocorticoid elevations in a captive, wild animal

Organisms are continuously encountering both predictable and unpredictable ecological stressors within their environment. The activation of the hypothalamic-pituitaryadrenal (stress) axis is a fundamental process allowing animals to cope with and respond to such encounters. A main consequence of HPA axis activation is the release of glucocorticoid hormones. Although short-term glucocorticoid elevations lead to changes in physiological and behavioral processes that are often adaptive, our understanding of fitness consequences of repeated acute elevations in glucocorticoid hormones over a longer time period is largely lacking. This is of particular current importance as animals are facing a significant increase in exposure to stressors including those associated with human-induced rapid environmental change. Here, we test fitness-relevant consequences of repeated exposure to glucocorticoids in the absence of natural challenges, by treating wild-caught gravid female eastern fence lizards (Sceloporus undulatus) with a daily transdermal dose of a glucocorticoid hormone until laying. This treatment causes an increase in plasma glucocorticoids that mimics the natural response lizards have when they encounter a stressor in the wild, without confounding effects associated with the encounter itself. This treatment reduced females' reproductive success (hatching success) and survival. Further, glucocorticoid-induced reductions in reproductive success were greater when females had experienced higher temperatures the previous winter. This demonstrates the potential significant consequences of repeated exposure to acute elevations in glucocorticoid hormones. Additionally, the costs of repeated glucocorticoid elevation may be further exaggerated by an individual's previous experience, such as the potential compounding effects of winter warming increasing animals' vulnerability to increased glucocorticoid levels during spring breeding.

Pregnancy stage determines the effect of chronic stress on ovarian progesterone synthesis

Although stress-induced glucocorticoid release is thought to be a primary driver by which maternal stress negatively impacts pregnancy outcomes, the downstream neuroendocrine targets mediating these adverse outcomes are less well understood. We hypothesized that stress-induced glucocorticoid secretion inhibits pituitary hormone secretion, resulting in decreased ovarian progesterone synthesis. Using a chronic restraint model of stress in mice, we quantified steroid hormone production, pituitary hormones, and expression of ovarian genes that support progesterone production at both early ( day 5) and midpregnancy ( day 10). Females subjected to daily restraint had elevated baseline glucocorticoids during both early and midpregnancy; however, lower circulating progesterone was observed only during early pregnancy. Lower progesterone production was associated with lower expression of steroidogenic enzymes in the ovary of restrained females during early pregnancy. There were no stress-related changes to luteinizing hormone (LH) or prolactin (PRL). By midpregnancy, circulating LH decreased regardless of treatment, and this was associated with downregulation of ovarian steroidogenic gene expression. Our results are consistent with a role for LH in maintaining steroidogenic enzyme expression in the ovary, but neither circulating PRL nor LH were associated with the stress-induced inhibition of ovarian progesterone production during early pregnancy. We conclude that chronic stress impacts endocrine networks differently in pregnant and nonpregnant mammals. These findings underscore the need for further studies exploring dynamic changes in endocrine networks participating in pregnancy initiation and progression to elucidate the physiological mechanisms that connect stress exposure to adverse pregnancy outcomes.

Maternal stress alters the phenotype of the mother, her eggs and her offspring in a wild-caught lizard

While biomedical researchers have long appreciated the influence of maternally derived glucocorticoids (GCs) on offspring phenotype, ecologists have only recently begun exploring its impact in wild animals. Interpreting biomedical findings within an ecological context has posited that maternal stress, mediated by elevations of maternal GCs, may play an adaptive role preparing offspring for a stressful or rigorous environment. Yet, the influence of maternal stress on offspring phenotype has been little studied in wild animals. We experimentally elevated GCs to ecologically relevant levels (mimicking increases in maternal stress hormones following a nonlethal predator encounter, a heat challenge, or a chasing or confinement stressor) in female eastern fence lizards Sceloporus undulatus during gestation. We tested the hypothesis that maternally derived stress hormones themselves are sufficient to alter offspring phenotype. Specifically, we examined the effects of experimentally elevated maternal GCs on fitness-relevant traits of the mother, her eggs and her subsequent offspring. We found that daily maternal GC elevation: (a) increased maternal antipredator behaviours and postlaying glucose levels; (b) had no effect on egg morphology or caloric value, but altered yolk hormone (elevated GC) and nutrient content; and (c) altered offspring phenotype including stress-relevant physiology, morphology and behaviour. These findings reveal that maternally derived GCs alone can alter offspring phenotype in a wild animal, changes that may be mediated via maternal behaviour, and egg hormone and nutrient content. Understanding the ecological consequences of these effects under different environmental conditions will be critical for determining the adaptive significance of elevated maternal GCs for offspring.

Pre- and postnatal effects of experimentally manipulated maternal corticosterone on growth, stress reactivity and survival of nestling house wrens

Corticosterone plays a central role in maintaining homeostasis, promoting energy acquisition, and regulating the stress response in birds. Exposure to elevated levels of corticosterone during development can profoundly alter offspring behaviour and physiology, but the effects of elevated maternal corticosterone on offspring development remain poorly understood.We tested two competing hypotheses concerning the effect of maternally derived corticosterone on growth and development of free-living house wrens: (i) elevated maternal corticosterone causes damaging effects on nestling phenotype and fitness (collateral damage hypothesis) and (ii) increased maternal corticosterone enhances offspring fitness by preparing nestlings for the environment experienced by their mother (environmental/maternal-matching hypothesis).We used a non-invasive means to increase maternal corticosterone by providing females with corticosterone-injected mealworms prior to and during egg production in the absence of any overt pre-natal maternal stress. To disentangle pre- and post-natal effects of this elevation in maternal corticosterone, we cross-fostered young in two experiments: (i) nestlings of control and experimental females were reared by unmanipulated, natural females in a uniform maternal environment; (ii) a split-brood design that enabled us to assess the interaction between the mother's corticosterone treatment and that of the nestlings.There were significant pre-natal effects of increased maternal corticosterone on nestling growth and survival. Offspring of females experiencing experimentally increased corticosterone were heavier and larger than offspring of control females. There also was a significant interaction between maternal corticosterone treatment and the corticosterone treatment to which young were exposed within the egg in their effect on nestling survival while in the nest; experimental young exhibited greater survival than control young, but only when reared by control mothers. There was also a significant effect of maternal corticosterone treatment on nestling stress reactivity and, in both experiments, on the eventual recruitment of offspring as breeding adults in the local population.These patterns are broadly consistent with the environmental/maternal-matching hypothesis, and highlight the importance of disentangling pre- and post-natal effects of manipulations of maternal hormone levels on offspring phenotype.

Error management theory and the adaptive significance of transgenerational maternal-stress effects on offspring phenotype

It is well established that circulating maternal stress hormones (glucocorticoids, GCs) can alter offspring phenotype. There is also a growing body of empirical work, within ecology and evolution, indicating that maternal GCs link the environment experienced by the mother during gestation with changes in offspring phenotype. These changes are considered to be adaptive if the maternal environment matches the offspring's environment and maladaptive if it does not. While these ideas are conceptually sound, we lack a testable framework that can be used to investigate the fitness costs and benefits of altered offspring phenotypes across relevant future environments. We present error management theory as the foundation for a framework that can be used to assess the adaptive potential of maternal stress hormones on offspring phenotype across relevant postnatal scenarios. To encourage rigorous testing of our framework, we provide field-testable hypotheses regarding the potential adaptive role of maternal stress across a diverse array of taxa and life histories, as well as suggestions regarding how our framework might provide insight into past, present, and future research. This perspective provides an informed lens through which to design and interpret experiments on the effects of maternal stress, provides a framework for predicting and testing variation in maternal stress across and within taxa, and also highlights how rapid environmental change that induces maternal stress may lead to evolutionary traps.

Glucocorticoids and CBG during pregnancy in mammals: diversity, pattern, and function

Pregnancy is one of the defining characteristics of placental mammals. Key in the growth and development of the fetus during pregnancy are the dynamics of glucocorticoids (GCs) and their binding protein,corticosteroid-binding globulin (CBG), which determines how much of the GCs are free and biologically active. Out of more than 5000 species of placental mammals in 19 different orders, our understanding of the dynamics of maternal GCs and CBG during pregnancy is largely limited to the detailed study of 3 groups - sheep, laboratory rodents, and humans. The assumption is often made that what we see in these few species applies to the rest. To examine this generality, we compared patterns of maternal GCs over pregnancy from all placental mammals where data is available: in the blood of 13 species from 5 different orders and in metabolites in excreta in an additional 20 species from 9 orders. We found that maternal free GCs increase by late pregnancy in most taxa. This increase is achieved by either an increase in total GC secretion or a decrease in CBG. A major exception is found in the even-toed ungulates (sheep, cows, etc.) where maternal GCs and CBG remain stable, but where the fetal adrenals mature in late pregnancy and produce the majority of their own GCs. We conclude that patterns of change in maternal GCs and CBG during pregnancy are species-specific but are alternative means to the same end: increased fetal exposure to GCs in late pregnancy, which is essential for development.

Impact of prenatal stress on offspring glucocorticoid levels: A phylogenetic meta-analysis across 14 vertebrate species

Prenatal exposure to maternal stress is commonly associated with variation in Hypothalamic Pituitary Adrenal (HPA)-axis functioning in offspring. However, the strength or consistency of this response has never been empirically evaluated across vertebrate species. Here we meta-analyzed 114 results from 39 studies across 14 vertebrate species using Bayesian phylogenetic mixed-effects models. We found a positive overall effect of prenatal stress on offspring glucocorticoids (d' = 0.43) though the 95% Highest Posterior Density Interval overlapped with 0 (-0.16-0.95). Meta-regressions of potential moderators highlighted that phylogeny and life history variables predicted relatively little variation in effect size. Experimental studies (d' = 0.64) produced stronger effects than observational ones (d' = -0.01), while prenatal stress affected glucocorticoid recovery following offspring stress exposure more strongly (d' = 0.75) than baseline levels (d' = 0.48) or glucocorticoid peak response (d' = 0.36). These findings are consistent with the argument that HPA-axis sensitivity to prenatal stress is evolutionarily ancient and occurs regardless of a species' overall life history strategy. These effects may therefore be especially important for mediating intra-specific life-history variation. In addition, these findings suggest that animal models of prenatal HPA-axis programming may be appropriate for studying similar effects in humans.

Why and how the early-life environment affects development of coping behaviours

Understanding the ways in which individuals cope with threats, respond to challenges, make use of opportunities and mediate the harmful effects of their surroundings is important for predicting their ability to function in a rapidly changing world. Perhaps one of the most essential drivers of coping behaviour of adults is the environment experienced during their early-life development. Although the study of coping, defined as behaviours displayed in response to environmental challenges, has a long and rich research history in biology, recent literature has repeatedly pointed out that the processes through which coping behaviours develop in individuals are still largely unknown. In this review, we make a move towards integrating ultimate and proximate lines of coping behaviour research. After broadly defining coping behaviours (1), we review why, from an evolutionary perspective, the development of coping has become tightly linked to the early-life environment (2), which relevant developmental processes are most important in creating coping behaviours adjusted to the early-life environment (3), which influences have been shown to impact those developmental processes (4) and what the adaptive significance of intergenerational transmission of coping behaviours is, in the context of behavioural adaptations to a fast changing world (5). Important concepts such as effects of parents, habitat, nutrition, social group and stress are discussed using examples from empirical studies on mammals, fish, birds and other animals. In the discussion, we address important problems that arise when studying the development of coping behaviours and suggest solutions.

Coping with differences in snow cover: the impact on the condition, physiology and fitness of an arctic hibernator

The Earth's climate is changing at an unprecedented rate and, as ecologists, we are challenged with the difficult task of predicting how individuals and populations will respond to climate-induced changes to local and global ecosystems. Although we are beginning to understand some of the responses to changing seasonality, the physiological mechanisms that may drive these responses remain unknown. Using long-term data comparing two nearby populations (<20 km apart) of free-living arctic ground squirrels in northern Alaska, we have previously shown that the timing of spring snowmelt greatly influences their phenology of hibernation and reproduction in a population and site-specific manner. Here, we integrate these site-specific phenologies with body condition, stress physiology, reproductive success and juvenile recruitment to understand phenotypic selection in the two populations. We found that at the site with relatively late spring snowmelt and early autumn snow cover: (i) adult females were larger and in better body condition but had significantly higher stress hormone levels; (ii) females had similar numbers of comparably sized offspring, but offspring had higher stress hormone levels; and (iii) offspring density was lower just prior to hibernation. Thus, adult females at the two sites appear to use different coping strategies that allow them to maintain reproductive fitness; however, marked shortening of the active season because of later snowmelt in spring and earlier snow cover in autumn may compromise juvenile recruitment. We discuss the significance of these findings within the broader context of changing animal-environment relationships.

Prenatal stress accelerates offspring growth to compensate for reduced maternal investment across mammals

Across mammals, prenatal maternal stress (PREMS) affects many aspects of offspring development, including offspring growth. However, how PREMS translates to offspring growth is inconsistent, even within species. To explain the full range of reported effects of prenatal adversity on offspring growth, we propose an integrative hypothesis: developmental constraints and a counteracting adaptive growth plasticity work in opposition to drive PREMS effects on growth. Mothers experiencing adversity reduce maternal investment leading to stunted growth (developmental constraints). Concomitantly, the pace of offspring life history is recalibrated to partly compensate for these developmental constraints (adaptive growth plasticity). Moreover, the relative importance of each process changes across ontogeny with increasing offspring independence. Thus, offspring exposed to PREMS may grow at the same rate as controls during gestation and lactation, but faster after weaning when direct maternal investment has ceased. We tested these predictions with a comparative analysis on the outcomes of 719 studies across 21 mammal species. First, the observed growth changes in response to PREMS varied across offspring developmental periods as predicted. We argue that the observed growth acceleration after weaning is not "catch-up growth," because offspring that were small for age grew slower. Second, only PREMS exposure early during gestation produced adaptive growth plasticity. Our results suggest that PREMS effects benefit the mother's future reproduction and at the same time accelerate offspring growth and possibly maturation and reproductive rate. In this sense, PREMS effects on offspring growth allow mother and offspring to make the best of a bad start.