Conserved features of chronic stress across phyla: the effects of long-term stress on behavior and the concentration of the neurohormone octopamine in the cricket, Gryllus texensis

Many of the deleterious effects of chronic stress in vertebrates are caused by the long-term elevation of stress hormones. These negative effects are thought to be unavoidable by-products of sustained activation of the stress response, but the details remain unclear. A comparative perspective may help in understanding chronic stress. We exposed crickets (Gryllus texensis) to a mock predator. A single exposure to a mock predator induced a transient increase in the hemolymph (blood) concentration of the insect stress neurohormone, octopamine. Repeated exposure to the mock predator increased basal levels of octopamine, similar to the effects of chronic stress on the basal levels of vertebrate stress hormones. This study is the first to report an increase in the basal levels of an invertebrate stress hormone in response to repeated flight-or-fight stress. Chronic stress reduced weight gain, and decreased feeding and enhanced weight loss after food deprivation in adult female crickets. However, chronic stress also increased the tendency of crickets to produce sustained flight. Therefore, this study supports the hypothesis that increasing basal levels of stress hormones may be a phylogenetically common response to chronically stressful conditions. It also demonstrates that chronic stress has both positive and negative effects in insects.

Older mothers follow conservative strategies under predator pressure: the adaptive role of maternal glucocorticoids in yellow-bellied marmots

When the maternal environment is a good predictor of the offspring environment, maternal glucocorticoid (GC) levels might serve to pre-program offspring to express certain phenotypes or life-history characteristics that will increase their fitness. We conducted a field study to assess the effects of naturally occurring maternal GC levels on their offspring in yellow-bellied marmots (Marmota flaviventris) subjected to different predator pressures. Maternal fecal corticosteroid metabolites (FCM) were positively correlated with predator pressure. Predators had both direct and indirect effects on pups. We found that older mothers with higher FCM levels had smaller and female-biased litters. Moreover, sons from older mothers with high FCM levels dispersed significantly more than those from older mothers with low FCM levels, whereas the opposite pattern was found in pups from younger mothers. These age-related effects may permit females to make adaptive decisions that increase their pups' fitness according to their current situation.

Population dynamics of red-backed voles (Myodes) in North America

We review the population dynamics of red-backed voles (Myodes species) in North America, the main deciduous and coniferous forest-dwelling microtines on this continent, and compare and contrast their pattern with that of the same or similar species in Eurasia. We identify 7 long-term studies of population changes in Myodes in North America. Using autoregressive and spectral analysis, we found that only 2 of the 7 show 3- to 5-year cycles like those found in some Eurasian populations. There was no relationship between latitude and cycling. The general lack of cyclicity is associated with two key aspects of their demography that act in tandem: first, poor overwinter survival in most years; second, chronically low densities, with irregular outbreak years. Eight factors might explain why some Myodes populations fluctuate in cycles and others fluctuate irregularly, and we review the evidence for each factor: food supplies, nutrients, predation, interspecific competition, disease, weather, spacing behavior and interactive effects. Of these eight, only food supplies appear to be sufficient to explain the differences between cyclic and non-cyclic populations. Irregular fluctuations are the result of pulsed food supplies in the form of berry crops (M. rutilus) or tree seeds (M. gapperi) linked to weather patterns. We argue that, to understand the cause for the patterns in the respective hemispheres, we must know the mechanism(s) driving population change and this must be linked to rigorous field tests. We suggest that a large-scale, year-round feeding experiment should improve overwintering survival, increase standing densities, and flip non-cyclic Myodes populations into cyclic dynamics that would mimic the patterns seen in the cyclic populations found in parts of Eurasia.

Discriminatory mass de-housing and low-weight births: scales of geography, time, and level

Struening et al.1 demonstrated a widening disparity of low birthweight (LOB) rates among New York City health areas from 1980-1986, clearly a dynamic process. In contrast, the New York City Department of Health reported static citywide LOB rate in 1988-2008.2 Struening et al.1 is extended here at the health district level with mapping and regression analyses. Additionally, birthweight data are reported for babies born in 1998-2001 to a group of African-American and Dominican women in Upper Manhattan. The data reported in this paper indicate that both fetal programming of the mother herself (life course model) and stress during or shortly before pregnancy may play a role in LOB. Current stress may arise from past events. Intergenerational effects, thus, could arise from stresses on the grandmother and their residual impacts on the mother as well as new stresses on the mother as an adult. The average weight of babies born to the Upper Manhattan mothers who were born in 1970-1974 was 3,466 g, with 1.6% below 2,500 g; that of babies of mothers born in 1975-1979, 3,320 g, with 6% below 2,500 g. The latter group was born during the 1975-1979 housing destruction. Intergenerational impacts of that event may be reflected in this elevated rate of LOB. Health district maps of LOB incidence ranges show improvement from 1990-2000 and then deterioration in 2005 and 2008. Bivariate regressions of socioeconomic (SE) factors and LOB incidence showed many strong associations in 1990; but by 2000, the number and strength of these associations declined. In 1990, 2000, and 2008, black segregation was the SE factor most strongly associated with LOB. Black segregation and murder rate explained about 85% of the pattern of 1990 LOB. Regressing the 1970-1980 percent population change against the SE factors showed effects even in 2000. The 1990 murder rate and 1989 percentage of public assistance explained over half the 2008 LOB incidence pattern. The housing destruction of the 1970s continued to influence LOB incidence indirectly in 2008. The ability of community and individual to cope with current stressors may hinge on resilience status, which is shaped by past events and circumstances. The present interacts with the past in many ways. Serial displacement exemplifies this interaction of immense importance to public health.

Maternal imprints and the origins of variation

The non-genomic transmission of maternal behavior from one generation to the next illustrates the pervasive influence of maternal care on offspring development and the high degree of plasticity within the developing maternal brain. Investigations of the mechanisms through which these maternal effects are achieved have demonstrated environmentally-induced changes in gene expression associated with epigenetic modifications within the promoter region of target genes. These findings raise challenging questions regarding the pathways linking experience to behavioral variation and the broader ecological/evolutionary implications of the dynamic changes in neuroendocrine function that emerge. This review will highlight studies in laboratory rodents which demonstrate plasticity in the maternal brain and the role of maternally-induced changes in DNA methylation in establishing the link between variations in maternal care and consequent developmental outcomes. The persistence of maternal effects across generations and the trade-offs in reproduction that are evident in female offspring who experience high vs. low levels of maternal care contribute to our understanding of the divergent strategies that are triggered by the quality of early-life experiences. Evolving concepts of inheritance and the interplay between genes and the environment may advance our understanding of the origins of individual differences in phenotype.

The Neurological Ecology of Fear: Insights Neuroscientists and Ecologists Have to Offer one Another

That the fear and stress of life-threatening experiences can leave an indelible trace on the brain is most clearly exemplified by post-traumatic stress disorder (PTSD). Many researchers studying the animal model of PTSD have adopted utilizing exposure to a predator as a life-threatening psychological stressor, to emulate the experience in humans, and the resulting body of literature has demonstrated numerous long-lasting neurological effects paralleling those in PTSD patients. Even though much more extreme, predator-induced fear and stress in animals in the wild was, until the 1990s, not thought to have any lasting effects, whereas recent experiments have demonstrated that the effects on free-living animals are sufficiently long-lasting to even affect reproduction, though the lasting neurological effects remain unexplored. We suggest neuroscientists and ecologists both have much to gain from collaborating in studying the neurological effects of predator-induced fear and stress in animals in the wild. We outline the approaches taken in the lab that appear most readily translatable to the field, and detail the advantages that studying animals in the wild can offer researchers investigating the "predator model of PTSD."

Measuring stress in wildlife: techniques for quantifying glucocorticoids

Stress responses play a key role in allowing animals to cope with change and challenge in the face of both environmental certainty and uncertainty. Measurement of glucocorticoid levels, key elements in the neuroendocrine stress axis, can give insight into an animal's well-being and can aid understanding ecological and evolutionary processes as well as conservation and management issues. We give an overview of the four main biological samples that have been utilized [blood, saliva, excreta (feces and urine), and integumentary structures (hair and feathers)], their advantages and disadvantages for use with wildlife, and some of the background and pitfalls that users must consider in interpreting their results. The matrix of choice will depend on the nature of the study and of the species, on whether one is examining the impact of acute versus chronic stressors, and on the degree of invasiveness that is possible or desirable. In some cases, more than one matrix can be measured to achieve the same ends. All require a significant degree of expertise, sometimes in obtaining the sample and always in extracting and analyzing the glucocorticoid or its metabolites. Glucocorticoid measurement is proving to be a powerful integrator of environmental stressors and of an animal's condition.

Of lemmings and snowshoe hares: the ecology of northern Canada

Two population oscillations dominate terrestrial community dynamics in northern Canada. In the boreal forest, the snowshoe hare (Lepus americanus) fluctuates in cycles with an 8-10 year periodicity and in tundra regions lemmings typically fluctuate in cycles with a 3-4 year periodicity. I review 60 years of research that has uncovered many of the causes of these population cycles, outline areas of controversy that remain and suggest key questions to address. Lemmings are keystone herbivores in tundra ecosystems because they are a key food resource for many avian and mammalian predators and are a major consumer of plant production. There remains much controversy over the role of predation, food shortage and social interactions in causing lemming cycles. Predation is well documented as a significant mortality factor limiting numbers. Food shortage is less likely to be a major limiting factor on population growth in lemmings. Social interactions might play a critical role in reducing the rate of population growth as lemming density rises. Snowshoe hares across the boreal forest are a key food for many predators and their cycles have been the subject of large-scale field experiments that have pinpointed predation as the key limiting factor causing these fluctuations. Predators kill hares directly and indirectly stress them by unsuccessful pursuits. Stress reduces the reproductive rate of female hares and is transmitted to their offspring who also suffer reduced reproductive rates. The maternal effects produced by predation risk induce a time lag in the response of hare reproductive rate to density, aiding the cyclic dynamics.

From process to pattern: how fluctuating predation risk impacts the stress axis of snowshoe hares during the 10-year cycle

Predation is a central organizing process affecting populations and communities. Traditionally, ecologists have focused on the direct effects of predation--the killing of prey. However, predators also have significant sublethal effects on prey populations. We investigated how fluctuating predation risk affected the stress physiology of a cyclic population of snowshoe hares (Lepus americanus) in the Yukon, finding that they are extremely sensitive to the fluctuating risk of predation. In years of high predator numbers, hares had greater plasma cortisol levels at capture, greater fecal cortisol metabolite levels, a greater plasma cortisol response to a hormone challenge, a greater ability to mobilize energy and poorer body condition. These indices of stress had the same pattern within years, during the winter and over the breeding season when the hare:lynx ratio was lowest and the food availability the worst. Previously we have shown that predator-induced maternal stress lowers reproduction and compromises offspring's stress axis. We propose that predator-induced changes in hare stress physiology affect their demography through negative impacts on reproduction and that the low phase of cyclic populations may be the result of predator-induced maternal stress reducing the fitness of progeny. The hare population cycle has far reaching ramifications on predators, alternate prey, and vegetation. Thus, predation is the predominant organizing process for much of the North American boreal forest community, with its indirect signature--stress in hares--producing a pattern of hormonal changes that provides a sensitive reflection of fluctuating predator pressure that may have long-term demographic consequences.

Developmental plasticity and developmental origins of non-communicable disease: theoretical considerations and epigenetic mechanisms

There is now evidence that developmental influences have lifelong effects on cardiovascular and metabolic function and that elements of the heritable or familial component of susceptibility to cardiovascular disease, obesity and other non-communicable diseases (NCD) can be transmitted across generations by non-genomic means. In animals the developmental environment induces altered phenotypes through genetic, physiological (especially endocrine) and epigenetic mechanisms. The latter include DNA methylation, covalent modifications of histones and non-coding RNAs. Such 'tuning' of phenotype has potential adaptive value and may confer Darwinian fitness advantage because it either adjusts the phenotype to current circumstances and/or attempts to match an individual's responses to the environment predicted to be experienced later. When the phenotype is mismatched to the later environment, e.g. from inaccurate nutritional cues from the mother or placenta before birth, or from rapid environmental change through improved socio-economic conditions, risk of NCD increases. Such mechanisms are also thought to play roles in ageing and early onset of puberty, reinforcing a life-course perspective on such adaptive responses, especially the detrimental later effects of trade-offs. Epigenetic changes induced during development are highly gene-specific and function at the level of individual CpG dinucleotides in both gene promoter and intergenic regions. Evidence is accruing that endocrine or nutritional interventions during early postnatal life can reverse epigenetic and phenotypic changes induced, for example, by unbalanced maternal diet during pregnancy. Elucidation of epigenetic processes may permit perinatal identification of individuals most at risk of later NCD and enable early intervention strategies to reduce such risk.

Female sticklebacks transfer information via eggs: effects of maternal experience with predators on offspring

There is growing evidence that maternal experience influences offspring via non-genetic mechanisms. When female three-spined sticklebacks (Gasterosteus aculeatus) were exposed to the threat of predation, they produced larger eggs with higher cortisol content, which consumed more oxygen shortly after fertilization compared with a control group. As juveniles, the offspring of predator-exposed mothers exhibited tighter shoaling behaviour, an antipredator defence. We did not detect an effect of maternal exposure to predation risk on the somatic growth of fry. Altogether, we found that exposure to an ecologically relevant stressor during egg formation had several long-lasting consequences for offspring, some of which might be mediated by exposure to maternally derived cortisol. These results support the hypothesis that female sticklebacks might influence the development, growth and behaviour of their offspring via eggs to match their future environment.