Stress, the HPA axis, and nonhuman primate well-being: A review

Exploratory Investigation of Infrared Thermography for Measuring Gorilla Emotional Responses to Interactions with Familiar Humans

Simple Summary

Interactions between zoo professionals and animals, such as positive reinforcement training, occur regularly and are thought to be enriching for animals. However, there is little empirical information on how animals perceive these interactions or on the interactions’ effects on animals’ emotional states. Our objective was to assess the effectiveness of infrared thermography for measuring the emotional responses of three western lowland gorillas at the Detroit Zoo to routine interactions (positive reinforcement training and cognitive tasks) with familiar humans. In addition to thermal images, we collected saliva samples for hormone analysis before and after human–animal interactions and a control condition, and we recorded behavioral data during all conditions. Nasal temperatures consistently decreased for two gorillas during interactions, while the third gorilla showed repeated increases. The behavior of all three gorillas suggested that they were engaged in the interactions, without exhibiting behaviors that could indicate negative welfare impacts. Oxytocin and cortisol both decreased following all conditions, including the control, and were thus equivocal for interpreting the meaning of the changes in nasal temperature. As mixed results in previous research show, infrared thermography may detect emotional arousal; however, additional indicators are necessary to determine the valence of the observed changes. The variability in responses we observed do not lend themselves to making firm conclusions about the validity of infrared thermography (IRT) for measuring emotion in this context or about how these gorillas responded to interactions. Challenges and suggestions for future studies using infrared thermography to examine interactions between humans and zoo animals are discussed.

Abstract

Interactions between zoo professionals and animals occur regularly and are believed to be enriching for animals. Little empirical information exists on how animals perceive these interactions, and particularly how the interactions affect the emotional states of animals. Infrared thermography (IRT) has shown some promise in the assessment of emotions in a variety of species, but further research is needed to determine if this method is useful in a zoo setting. We conducted a pilot study to determine if IRT is a valid measure of the emotional responses to routine interactions (positive reinforcement training and cognitive tasks, compared to a control condition) with familiar humans on three western lowland gorillas at the Detroit Zoo. We measured nasal temperatures associated with emotional change using IRT. To examine the validity of the IRT data, we collected saliva samples for hormone analysis before and after each condition, in addition to behavioral data during the interactions and control condition. Decreases in nasal temperatures for two gorillas and an increase in the third indicate that arousal changed consistently within individuals following the interactions but not the control condition. Pre-post cortisol levels and oxytocin concentrations decreased for all conditions, but the decreases seen did not differ among the conditions. The gorillas were highly engaged in the interactions, and two produced more grumble vocalizations during the human-animal interactions (HAIs) compared to the control condition. Additionally, the gorillas performed self-directed behaviors more often during the control condition, also suggesting HAIs were not a negative experience. In summary, we were able to measure changes in arousal using IRT, but we were unable to determine the emotional valence of these changes based on the additional indicators employed. Additionally, the inconsistency across these measures precluded firm conclusions about either the validity of IRT for measuring emotion in this context or how the interactions impacted these gorillas. These findings highlight the challenges of using IRT to measure emotional states in non-human animals, and we discuss further steps necessary to apply this method in future studies.

Nonhuman primate welfare: Can there be a relationship between personality, lateralization and physiological indicators?

Measuring personality is being used to improved nonhuman primate welfare. To expand its use, it is important to identify traits that are shared between species and that measures are reliable, easy to use and less time consuming. Combining personality and other indicators strong validation of the results can be obtained. In the present study, we sought to determine if there is a link between physiological stress response (fecal cortisol metabolites), personality (ratings made by animal keepers and reaction to novel objects) and lateralization of the brain (hand preferences) on eight species of nonhuman primates: Callithrix jacchus, Callithrix geoffroyi, Cebuella Pygmaea, Saguinus imperator, Saguinus oedipus, Leontopithecus rosalia, Pithecia pithecia and Nycticebus pygmaeus. Personality assessments achieved good levels of interrater reliability and revealed three components of personality in our sample: fearfulness, activeness and aggressiveness. More exploratory individuals were more active, aggressive and showed higher cortisol metabolite levels. Right-handed subjects inspected novel objects sooner and the strength of the lateralization was linked with individual stress and the aggressiveness component. Our results highlight that there is a relation between personality, lateralization and physiological indicators in nonhuman primates, but although some aspects can be generalized across species and/or sexes others are species/sex dependent.

Aggression and social support predict long-term cortisol levels in captive tufted capuchin monkeys (Cebus [Sapajus] apella)

Many nonhuman primates live in complex social groups in which they regularly encounter both social stressors such as aggression and social support such as that provided by long-term affiliative relationships. Repeated exposure to social stressors may result in chronically elevated cortisol levels that can have deleterious physical effects such as impaired immune function, cardiovascular disease, and reduced brain function. In contrast, affiliative social relationships may act as a buffer, dampening the release of cortisol in response to acute and chronic stressors. Understanding how social stressors and social support predict cortisol levels is therefore essential to understanding how social situations relate to health and welfare. We studied this relationship in 16 socially housed captive brown capuchin monkeys (Cebus [Sapajus] apella) by comparing long-term hair cortisol levels with behavioral measures of social stress (dominance rank, rank certainty, and amount of aggression received) and social support (amount of affiliation and centrality in the affiliative social network of the group). Dominance rank, rank certainty, amount of affiliation, and age were not significant predictors of long-term cortisol levels in this population. Instead, long-term cortisol levels were positively related to the amount of aggression received and negatively related to centrality in the affiliative social network of the group. This pattern may be attributed to the species' socially tolerant dominance system and to the availability of social support across the dominance hierarchy.

The use of hair cortisol for the assessment of stress in animals

The hair cortisol concentration (HCC) is assumed to be a retrospective marker of integrated cortisol secretion and stress over longer periods of time. Its quantification is increasingly used in psychoneuroendocrinological studies in humans, but also in animal stress and welfare research. The measurement of HCCs for the assessment of stress offers many considerable benefits for use in domesticated and wild animals, especially due to the easy and minimally invasive sampling procedure and the representation of longer time periods in one sample. This review aims to outline the different fields of application and to assess the applicability and validity of HCC as an indicator for chronic stress or long-term activity of the hypothalamic-pituitary-adrenal axis in wild and domesticated animals. Specific hair characteristics are presented and the advantages and limitations of using HCC are discussed. An overview of findings on the impact of stress- and health-related factors on HCCs and of diverse influencing factors causing variation in hair cortisol levels in different species is given. Recommendations for the use of hair cortisol analysis are proposed and potential fields of future research are pointed out. The studies indicate an effect of age and pregnancy on HCCs, and cortisol incorporation into hair was also found to depend on hair colour, body region, sex and season of year, but these results are less consistent. Furthermore, the results in animals show that a wide array of stressors and pathological conditions alters the cortisol concentrations in hair and that HCC thereby provides a reliable and valid reflection of long-term cortisol secretion in many species. However, more research is necessary to investigate the underlying mechanisms of cortisol incorporation into the hair and to explore the hair growth characteristics in the species of interest. To overcome confounding influences, the use of standardized sampling protocols is strongly advised.

Effects of housing conditions on behaviors and biochemical parameters in juvenile cynomolgus monkeys (Macaca fascicularis)

To investigate the effects of environmental enrichment on laboratory monkeys, we studied behavioral and physiological differences following changes in housing conditions. Ten male and female juvenile cynomolgus monkeys were first housed in pairs for 8 weeks after quarantine/acclimatization (singly housed) and subsequently housed alone for the next 8 weeks. Monkeys were subjected to evaluations of body weight gain, stereotypic or affiliative behaviors, cortisol, 4-ethylphenyl sulfate (4EPS) and catecholamine concentrations in biological samples, and blood chemistry tests under both housing conditions. Under paired housing, the stereotypic behavioral score decreased in both sexes, and the affiliative behavioral score increased in males and showed an increasing trend in females. Under single housing, the stereotypic score increased in both sexes, and the affiliative score decreased in males. Paired housing decreased serum calcium and urine cortisol concentrations in both sexes and decreased plasma cortisol in males and plasma 4EPS concentrations in females. The stereotypic score was positively correlated with serum calcium, plasma and urine cortisol, and plasma 4EPS concentration and negatively correlated with the affiliative score. The feces painting score, affiliative score, and plasma cortisol and serum calcium concentrations showed sex differences, suggesting differences in responsiveness to environmental changes between males and females. In conclusion, paired housing improved behavioral abnormalities in juvenile cynomolgus monkeys, suggesting that it may be an effective environmental enrichment paradigm. Calcium, cortisol, and 4EPS concentrations in biological samples may be useful indices for evaluating the effects of environmental enrichment.

Does the Stress of Laboratory Life and Experimentation on Animals Adversely Affect Research Data? A Critical Review

Recurrent acute and/or chronic stress can affect all vertebrate species, and can have serious consequences. It is increasingly and widely appreciated that laboratory animals experience significant and repeated stress, which is unavoidable and is caused by many aspects of laboratory life, such as captivity, transport, noise, handling, restraint and other procedures, as well as the experimental procedures applied to them. Such stress is difficult to mitigate, and lack of significant desensitisation/habituation can result in considerable psychological and physiological welfare problems, which are mediated by the activation of various neuroendocrine networks that have numerous and pervasive effects. Psychological damage can be reflected in stereotypical behaviours, including repetitive pacing and circling, and even self-harm. Physical consequences include adverse effects on immune function, inflammatory responses, metabolism, and disease susceptibility and progression. Further, some of these effects are epigenetic, and are therefore potentially transgenerational: the biology of animals whose parents/grandparents were wild-caught and/or have experienced chronic stress in laboratories could be altered, as compared to free-living individuals. It is argued that these effects must have consequences for the reliability of experimental data and their extrapolation to humans, and this may not be recognised sufficiently among those who use animals in experiments.

Radiolabel validation of cortisol in the hair of rhesus monkeys

Analysis of cortisol in hair has become a widespread tool for assessment of hypothalamic-pituitary-adrenal (HPA) axis activity because of its ease of collection and its ability to provide cumulative data over a period of months. In order to meaningfully interpret hair cortisol, however a direct validation by radio-metabolism is required to understand cortisol incorporation into hair. Tritiated [3H]-cortisol was IV administered to adult rhesus monkeys to determine 1) if [3H] is found in the hair after injection of [3H]-cortisol, 2) the growth rate of hair and 3) the form in which cortisol is found in hair. Samples of hair were collected from newly and previously shaved patches at 14-days and 28-days after [3H]-cortisol injection. Hair was processed by external wash, grinding, and hormone extractions. Samples were separated by high-performance liquid chromatography (HPLC) and fractions were collected and radioactivity assessed. We found [3H] incorporated into the hair by the 14-day hair collection and no new radioactivity was found by the 28-day collection. Individual hair growth rate was highly variable between monkeys, indicating that the between-subject hair growth patterns were not consistent. Importantly, for the first time, we showed that systemically administered [3H]-cortisol was found in the hair as [3H]-cortisol and [3H]-cortisone, as well as other glucocorticoid metabolites.

Single- or Pair-Housed: Which Is Better for Captive Southern Tamanduas?

The captive environment can limit some important behavioral options for nonhuman animals, which often results in decreased welfare. The companion of a conspecific can be a source of complexity in captivity, but this aspect has received little attention for solitary species. This study investigated the effects of two different housing conditions on the behavior of a solitary species, the Southern tamandua (Tamandua tetradactyla). Six individuals housed singly and four housed in pairs in two Brazilian zoological institutions were observed for 10 days from 09:00h to 17:00h during the rainy season. Each individual was observed for 26:40 hours. Compared with singly housed tamanduas, paired animals were less inactive (single = 74.62 ± 5.2%, paired = 28.51 ± 0.04% of the records), foraged and explored more (single = 11.07 ± 3.0%, paired 31.48 ± 0.01% of the records), and exhibited a diversity of behaviors 102.56% greater than single tamanduas. These results suggest that interactions with conspecifics in captivity, even for solitary animals, can promote the performance of desirable behaviors, which in several species has been correlated with improved welfare.

Intermittent pair-housing, pair relationship qualities, and HPA activity in adult female rhesus macaques

Laboratory rhesus macaques are often housed in pairs and may be temporarily or permanently separated for research, health, or management reasons. While both long-term social separations and introductions can stimulate a stress response that impacts inflammation and immune function, the effects of short-term overnight separations and whether qualities of the pair relationship mediate these effects are unknown. In this study, we investigated the effects of overnight separations on the urinary cortisol concentration of 20 differentially paired adult female rhesus macaques (Macaca mulatta) at the California National Primate Research Center. These females were initially kept in either continuous (no overnight separation) or intermittent (with overnight separation) pair-housing and then switched to the alternate pair-housing condition part way through the study. Each study subject was observed for 5 weeks, during which we collected measures of affiliative, aggressive, anxious, abnormal, and activity-state behaviors in both pair-housing conditions. Additionally, up to three urine samples were collected from each subject per week and assayed for urinary free cortisol and creatinine. Lastly, the behavioral observer scored each pair on four relationship quality attributes ("Anxious," "Tense," "Well-meshed," and "Friendly") using a seven-point scale. Data were analyzed using a generalized linear model with gamma distribution and an information theoretic approach to determine the best model set. An interaction between the intermittent pairing condition and tense pair adjective rating was in the top three models of the best model set. Dominance and rates of affiliation were also important for explaining urinary cortisol variation. Our results suggest that to prevent significant changes in HPA-axis activation in rhesus macaque females, which could have unintended effects on research outcomes, pairs with "Tense" relationships and overnight separations preventing tactile contact should be avoided.

Assessment of stress responses in rhesus macaques (Macaca mulatta) to daily routine procedures in system neuroscience based on salivary cortisol concentrations

Non-human primates participating in neurophysiological research are exposed to potentially stressful experimental procedures, such as dietary control protocols, surgical implants and their maintenance, or social separation during training and experimental session. Here, we investigated the effect of controlled access to fluid, surgical implants, implant-related cleaning of skin margins, and behavioral training sessions on salivary cortisol levels of adult male rhesus macaques participating in neurophysiological research. The animals were trained to chew flavored cotton swabs to non-invasively collect saliva samples. Our data show no differences in cortisol levels between animals with and without implants, but both, controlled access to fluid and cleaning of implants individually increased salivary cortisol concentrations, while both together did not further increase the concentration. Specifically, before cleaning, individuals with controlled access to fluid had 55% higher cortisol concentrations than individuals with free access to fluid. Under free access to fluid, cortisol concentrations were 27% higher after cleaning while no effect of cleaning was found for individuals under controlled fluid access. Training sessions under controlled access to fluid also did not affect salivary cortisol concentrations. The observed changes in cortisol concentrations represent mild stress responses, as they are only a fraction of the range of the regular circadian changes in cortisol levels in rhesus monkeys. They also indicate that combinations of procedures do not necessarily lead to cumulative stress responses. Our results indicate that salivary cortisol levels of rhesus monkeys respond to neurophysiological experimental procedures and, hence, may be used to assess further refinements of such experimental methods.

Measuring physiological stress in the common marmoset (Callithrix jacchus): Validation of a salivary cortisol collection and assay technique

Cortisol levels are often used as a physiological measure of the stress response in captive primates, with non-invasive measures of this being an important step in welfare assessment. We report a method of collecting saliva samples voluntarily from unrestrained captive common marmosets (Callithrix jacchus), and validate an enzyme-linked immunosorbent assay (ELISA) technique previously unused in this species. Saliva samples were collected from marmosets housed in pairs in a UK laboratory. The assay showed parallelism, precision, accuracy and sensitivity, meeting the criteria typically used to investigate the effectiveness of new analytical techniques. Use of Salimetrics® Oral Swabs considerably increased the amount of cortisol recovered in comparison with previous studies using cotton buds. However, while use of banana on the swabs can encourage chewing, it may influence results. Although increases in cortisol levels have traditionally been interpreted as an indicator of stress in primates, there are many factors that affect the hypothalamic-pituitary-adrenal axis, with some studies showing decreases in cortisol levels post-stressor. Following a likely stressful event (capture for weighing), we also found cortisol levels significantly decreased, possibly due to social buffering or 'blunting' of the HPA axis. Order of weighing also had an effect. The method therefore provided an effective non-invasive means of assessing acute changes in cortisol level that may be more useful than previous methods, improving our ability to study physiological aspects of welfare in primates. We discuss methodological considerations, as well as implications of using cortisol as a measure of stress.

Hypercortisolemia and Depressive-like Behaviors in a Rhesus Macaque (Macaca mulatta) Involved in Visual Research

A 10-y-old cranially implanted rhesus macaque (Macaca mulatta) involved in visual research was presented for dull mentation and weight loss. Physical examination revealed alopecia and poor body conditioning, and bloodwork revealed marked hypercortisolemia (23 μg/dL). Differential diagnoses for hypercortisolemia, weight loss, and alopecia included Cushing and pseudo-Cushing syndromes. To further evaluate hypercortisolemia, we compared the urine cortisol:creatinine ratio (UCCR) at baseline and after low-dose dexamethasone suppression (LDDS) testing in the presenting animal and healthy naïve and implanted working monkeys. At baseline, UCCR was 10 times higher in the presenting macaque (118.1 ± 7.1) than in naïve animals (12.5 ± 12.8) and 3 times higher than in healthy implanted working macaques (44.4 ± 6.9); however, levels were suppressed similarly by dexamethasone in both the presenting animal and healthy controls. In addition, healthy implanted working macaques had significantly higher baseline UCCR levels than naïve controls, suggesting chronic stress in working animals. Abdominal ultrasonography and radiographs of the presenting animal revealed marked bilateral adrenal mineralization but no overt adrenal tumor or hyperplasia. Overall, these results excluded endogenous Cushing syndrome and prompted us to evaluate different causes of pseudo-Cushing syndrome, including depression. Using videorecordings to evaluate behavior, we used published criteria for macaque models of depression models, including huddling, to make a presumptive diagnosis of depression. The macaque was treated with fluoxetine (2 mg/kg PO daily), provided increased environmental enrichment, and followed over time by regular UCCR assessment and videorecordings. The animal improved clinically and behaviorally, and UCCR returned to levels observed in working implanted macaques (44.4) after 8 wk of treatment. This case highlights the potential effect of research-related work on stress and pathologic behaviors in macaques and demonstrates the utility of UCCR and LDDS for screening behavioral and hypothalamic-pituitary-adrenal abnormalities in these animals.

Environmental Enrichment in the 21st Century

More than a quarter of a century has elapsed since the Animal Welfare Act mandated that research facilities develop and follow a plan to promote the psychological well-being of captive primates. Since passage of this law, considerable effort and resources have been directed to designing environmental enrichment strategies in an effort to improve animal welfare. These plans typically consist of environmental enrichment and socialization efforts. While environmental enhancement has undergone a great deal of improvement in the past 25 years, it should be viewed as a continual work in progress, which takes advantage of emergent and future technologies. In this review, we discuss the objectives of the environmental enhancement plan along with relevant outcome measures, as well as ongoing challenges, costs, and benefits. We then review various enrichment strategies and assess their efficacy in meeting goals and objectives. Finally, we look forward to consider what the future might hold for environmental enrichment of nonhuman primates used in research.

The 24/7 approach to promoting optimal welfare for captive wild animals

We have an ethical responsibility to provide captive animals with environments that allow them to experience good welfare. Husbandry activities are often scheduled for the convenience of care staff working within the constraints of the facility, rather than considering the biological and psychological requirements of the animals themselves. The animal welfare 24/7 across the lifespan concept provides a holistic framework to map features of the animal's life cycle, taking into account their natural history, in relation to variations in the captive environment, across day and night, weekdays, weekends, and seasons. In order for animals to have the opportunity to thrive, we argue the need to consider their lifetime experience, integrated into the environments we provide, and with their perspective in mind. Here, we propose a welfare assessment tool based upon 14 criteria, to allow care staff to determine if their animals' welfare needs are met. We conclude that animal habitat management will be enhanced with the use of integrated technologies that provide the animals with more opportunities to engineer their own environments, providing them with complexity, choice and control.

Assessing the utility of urinary and fecal cortisol as an indicator of stress in golden snub-nosed monkeys (Rhinopithecus roxellana)

Cortisol concentration (CC) is often used as a stress indicator in animals, as high CC is associated with elevated stress levels. During field research, non-invasive methods of measuring CC, such as collection of urine and feces, are superior to using blood samples when monitoring free-ranging animals’ stress levels. However, due to different metabolic pathways, whether CC can be detected in urine and feces to reliably assess stress varies across species. Therefore, it is important to ascertain whether urine and fecal samples are a reliable source for determining CCs and to determine a suitable sampling regime. In this study, we subjected three captive adult golden snub-nosed monkeys (Rhinopithecus roxellana) to a high-stress situation (capture and injection). Urine and feces were collected for four days before and for four days after the manipulations for laboratory analysis. Immunoreactive CC was detected with a commercial enzyme immunoassay (EIA) kit and showed distinct rises. Peak CC values in urine were detected within 5 h, while peak fecal CC ranged between 5 and 24 hours post-interference. These results provide evidence that CC in urine and feces can be used to assess stress levels in the golden snub-nosed monkey. The optimal time frame to collect urinary and fecal samples for CC analysis is within one day of a potential stressful event.

Costs and benefits of group living in primates: an energetic perspective

Group size is a fundamental component of sociality, and has important consequences for an individual's fitness as well as the collective and cooperative behaviours of the group as a whole. This review focuses on how the costs and benefits of group living vary in female primates as a function of group size, with a particular emphasis on how competition within and between groups affects an individual's energetic balance. Because the repercussions of chronic energetic stress can lower an animal's fitness, identifying the predictors of energetic stress has important implications for understanding variation in survivorship and reproductive success within and between populations. Notably, we extend previous literature on this topic by discussing three physiological measures of energetic balance-glucocorticoids, c-peptides and thyroid hormones. Because these hormones can provide clear signals of metabolic states and processes, they present an important complement to field studies of spatial and temporal changes in food availability. We anticipate that their further application will play a crucial role in elucidating the adaptive significance of group size in different social and ecological contexts.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Hair cortisol and dehydroepiandrosterone concentrations in naturally Taenia solium infected pigs in Tanzania

The aim of this study was to measure hair cortisol and dehydroepiandrosterone (DHEA) concentrations in naturally Taenia solium infected and non-infected control pigs and assess the effect of an environmental change on the aforementioned parameters. Three hair patches were obtained from 13 T. solium infected and 15 non-infected controls sows, respectively corresponding to 3 time points (prior to, at and approximately two weeks after arrival at the research facility). Cortisol and DHEA were extracted using methanol and analysed by radio immune assay. Mean hair cortisol concentrations were significantly lower (p<0.001) in T. solium infected (4.7±3.0pg/mg) compared to control pigs (9.0±3.7pg/mg) prior to arrival at the research facility, however no significant difference was observed between the two groups at arrival and after approximately two weeks. Similar patterns were also observed for DHEA concentrations (infected pigs 253.9±82.3pg/mg, control pigs 387.7±116.4pg/mg) (p<0.001). Results showed that lean animals had significantly higher cortisol concentrations in both groups, infected and controls pigs, while DHEA was not significantly different between lean and normal animals. Results of this study have shown that an environmental change could have an effect on pigs' hormonal levels suggesting an undergoing adaptation process. After the pigs were kept under the same conditions, fed and watered ad libitum, no significant differences were observed between the groups, but a drop in DHEA concentrations was observed in all the pigs. Weight however had an effect on cortisol levels as lean animals had significantly higher cortisol concentrations in both groups, compared to normal pigs.

Non-invasive monitoring of physiological markers in primates

The monitoring of endocrine markers that inform about an animal's physiological state has become an invaluable tool for studying the behavioral ecology of primates. While the collection of blood samples usually requires the animal to be caught and immobilized, non-invasively collected samples of saliva, urine, feces or hair can be obtained without any major disturbance of the subject of interest. Such samples enable repeated collection which is required for matching behavioral information over long time periods with detailed information on endocrine markers. We start our review by giving an overview of endocrine and immune markers that have been successfully monitored in relation to topics of interest in primate behavioral ecology. These topics include reproductive, nutritional and health status, changes during ontogeny, social behavior such as rank relationships, aggression and cooperation as well as welfare and conservation issues. We continue by explaining which hormones can be measured in which matrices, and potential problems with measurements. We then describe different methods of hormone measurements and address their advantages and disadvantages. We finally emphasize the importance of thorough validation procedures when measuring a specific hormone in a new species or matrix.

Group size and visitor numbers predict faecal glucocorticoid concentrations in zoo meerkats

Measures of physiological stress in zoo animals can give important insights into how they are affected by aspects of their captive environment. We analysed the factors influencing variation in glucocorticoid metabolites in faeces (fGCs) from zoo meerkats as a proxy for blood cortisol concentration, high levels of which are associated with a stress response. Levels of fGCs in captive meerkats declined with increasing group size. In the wild, very small groups of meerkats are at a higher risk of predation, while in larger groups, there is increased competition for resources. Indeed, group sizes in captivity resemble those seen in unstable coalitions in the wild, which may represent a stressful condition and predispose meerkats to chronic stress, even in the absence of natural predators. Individuals in large enclosures showed lower levels of stress, but meerkat density had no effect on the stress measures. In contrast with data from wild meerkats, neither sex, age nor dominance status predicted stress levels, which may reflect less food stress owing to more equal access to resources in captivity versus wild. The median number of visitors at the enclosure was positively correlated with fGC concentrations on the following day, with variation in the visitor numbers having the opposite effect. Our results are consistent with the hypothesis that there is an optimum group size which minimizes physiological stress in meerkats, and that zoo meerkats at most risk of physiological stress are those kept in small groups and small enclosures and are exposed to consistently high numbers of visitors.

Robust gene expression changes in the ganglia following subclinical reactivation in rhesus macaques infected with simian varicella virus

Varicella zoster virus (VZV) causes varicella during acute infection and establishes latency in the sensory ganglia. Reactivation of VZV results in herpes zoster, a debilitating and painful disease. It is believed that VZV reactivates due to a decline in cell-mediated immunity; however, the roles that CD4 versus CD8 T cells play in the prevention of herpes zoster remain poorly understood. To address this question, we used a well-characterized model of VZV infection where rhesus macaques are intrabronchially infected with the homologous simian varicella virus (SVV). Latently infected rhesus macaques were thymectomized and depleted of either CD4 or CD8 T cells to induce selective senescence of each T cell subset. After T cell depletion, the animals were transferred to a new housing room to induce stress. SVV reactivation (viremia in the absence of rash) was detected in three out of six CD8-depleted and two out of six CD4-depleted animals suggesting that both CD4 and CD8 T cells play a critical role in preventing SVV reactivation. Viral loads in multiple ganglia were higher in reactivated animals compared to non-reactivated animals. In addition, reactivation results in sustained transcriptional changes in the ganglia that enriched to gene ontology and diseases terms associated with neuronal function and inflammation indicative of potential damage as a result of viral reactivation. These studies support the critical role of cellular immunity in preventing varicella virus reactivation and indicate that reactivation results in long-lasting remodeling of the ganglia transcriptome.

A longitudinal study of hair cortisol concentrations in Macaca nemestrina mothers and infants

Cortisol is a well-known glucocorticoid that can be used as a biomarker of hypothalamic-pituitary-adrenocortical activity. To explore basal cortisol physiology during pregnancy and infancy in Macaca nemestrina monkeys, hair was collected from a convenience sample of 22 healthy mother-infant dyads. Adult females were housed in pairs as part of a small breeding colony at the Washington National Primate Research Center and infants were reared in a specialized nursery. Maternal samples were collected from females during a pregnancy-detection ultrasound and immediately following labor and delivery. Infant samples were collected at birth, 20 days, 4, 6, 8, and 10 months of age. Hair cortisol concentrations (HCCs) were determined using an enzyme immunoassay in washed and ground hair samples. Like human mothers, macaque HCCs rose during pregnancy (paired t = 5.8, df = 16, P < 0.001). Maternal HCCs at pregnancy-detection (114.2 ± 12.07 picogram/milligram [pg/mg]) were highly predictive of maternal HCCs at delivery (144.8 ± 13.60 pg/mg), suggesting a trait-like quality (r = 0.90, P < 0.001). When maternal HCCs were viewed on a continuum, the absolute rise in cortisol over the course of pregnancy was significantly related to newborn HCCs (r = 0.55, P = 0.02). Infant birth HCCs (1,027.43 ± 97.95 pg/mg) were seven times higher than maternal HCCs at delivery (paired t = 19.1, df = 16, P < 0.001). Higher birth HCCs were strongly associated with larger decreases in infant hair cortisol until 6 months of postnatal age when infant HCCs converged on values indistinguishable from adults. Overall, study results demonstrate a marked degree of fetal cortisol exposure during the latter part of gestation and suggest that the rise in maternal cortisol over pregnancy may play an influential role on HCCs in the newborn.

Exposure to a High-Fat Diet during Early Development Programs Behavior and Impairs the Central Serotonergic System in Juvenile Non-Human Primates

Perinatal exposure to maternal obesity and high-fat diet (HFD) consumption not only poses metabolic risks to offspring but also impacts brain development and mental health. Using a non-human primate model, we observed a persistent increase in anxiety in juvenile offspring exposed to a maternal HFD. Postweaning HFD consumption also increased anxiety and independently increased stereotypic behaviors. These behavioral changes were associated with modified cortisol stress response and impairments in the development of the central serotonin synthesis, with altered tryptophan hydroxylase-2 mRNA expression in the dorsal and median raphe. Postweaning HFD consumption decreased serotonergic immunoreactivity in area 10 of the prefrontal cortex. These results suggest that perinatal exposure to HFD consumption programs development of the brain and endocrine system, leading to behavioral impairments associated with mental health and neurodevelopmental disorders. Also, an early nutritional intervention (consumption of the control diet at weaning) was not sufficient to ameliorate many of the behavioral changes, such as increased anxiety, that were induced by maternal HFD consumption. Given the level of dietary fat consumption and maternal obesity in developed nations these findings have important implications for the mental health of future generations.

Effect of continuous access to feeding stalls during mixing on behavior, welfare, and performance of group-housed gestating sows in different social ranks

This study was conducted to evaluate a strategy of using feeding stalls to protect low-ranking sows in group-housing systems. Sows (n = 150, parity 1–9) were mixed at weaning in pens of 15 sows. Control pens allowed sows to access feeding stalls for 1 h of feeding daily during the initial 2 d after mixing. Treatment pens allowed sows to access feeding stalls continuously. Social rank was determined based on outcomes of aggressive interactions among sows after mixing. Low-ranking sows used the feeding stalls more frequently (P < 0.01) than high-ranking sows during the initial 6 h after mixing. Continuous stall access reduced frequency of aggressive interactions (P = 0.05) and, consequently, reduced skin lesions (P = 0.05) of sows in the pen. Neither continuous access to stalls nor social rank affected performance of sows. These results suggest that low-ranking sows used feeding stalls as hiding spaces to escape from aggressive interactions during mixing, which reduced skin lesions caused by aggression and improved the welfare of sows in the group-housing system studied.

Pituitary-adrenocortical adjustments to transport stress in horses with previous different handling and transport conditions

Aim:

The changes of the hypothalamic pituitary adrenal (HPA) axis response to a long distance transportation results in increase of adrenocorticotropic hormone (ACTH) and cortisol levels. The purpose of the study was to quantify the level of short-term road transport stress on circulating ACTH and cortisol concentrations, related to the effect of previous handling and transport experience of horses.

Materials and Methods:

The study was performed on 56 healthy horses after short-term road transport of 30 km. The horses were divided into four groups, Groups A, B, C, and D, with respect to the handling quality: Good (Groups A and B), bad (Group D), and minimal handling (Group C) conditions. According to the previous transport, experience horses were divided as follows: Horses of Groups A and D had been experienced long-distance transportation before; horses of Groups B and C had been limited experience of transportation.

Results:

One-way RM-ANOVA showed significant effects of transport on ACTH changes in Groups B and C and on cortisol changes in both Groups A and B. Groups A and B showed lower baseline ACTH and cortisol values than Groups C and D; Groups A and B showed lower post-transport ACTH values than Groups C and D. Groups A, B, and C showed lower post-transport cortisol values than Group D. Only Groups A and B horses have shown an adequate capacity of stress response to transportation.

Conclusion:

The previous transport experience and quality of handling could influence the HPA axis physiological responses of horses after short-term road transport.

Biologic Data of Cynomolgus Monkeys Maintained under Laboratory Conditions

The cynomolgus monkey (Macaca fascicularis) is a well-known non-human primate species commonly used in non-clinical research. It is important to know basal clinical pathology parameters in order to have a reference for evaluating any potential treatment-induced effects, maintaining health status among animals and, if needed, evaluating correct substantiative therapies. In this study, data from 238 untreated cynomolgus monkeys (119 males and 119 females of juvenile age, 2.5 to 3.5 years) kept under laboratory conditions were used to build up a reference database of clinical pathology parameters. Twenty-two hematology markers, 24 clinical chemistry markers and two blood coagulation parameters were analyzed. Gender-related differences were evaluated using statistical analyses. To assess the possible effects of stress induced by housing or handling involved in treatment procedures, 78 animals (35 males and 35 females out of 238 juvenile monkeys and four adult males and four adult females) were used to evaluate cortisol, corticosterone and behavioral assessment over time. Data were analyzed using a non-parametric statistical test and machine learning approaches. Reference clinical pathology data obtained from untreated animals may be extremely useful for investigators employing cynomolgus monkeys as a test system for non-clinical safety studies.

Matrilineal Behavioral and Physiological Changes following the Death of a Non-Alpha Matriarch in Rhesus Macaques (Macaca mulatta)

In many species, the loss of alpha matriarchs is associated with a number of negative outcomes such as troop fission, eviction, wounding, and reduced vitality. However, whether the dramatic consequences of their loss are due to their role as an old experienced figure or to their alpha status remains unclear. We tested the hypothesis that in a semi-free ranging colony of rhesus macaques (Macaca mulatta), the death of a non-alpha matriarch, who had a large set of kin, would lead to changes in behavior and physiological stress within her matriline. Following her death, her matriline increased in aggression, vigilance, and social grooming. Additionally, hierarchical stability, measured by levels of rank changes, decreased within her matriline, and levels of intense aggression by high-ranking animals were more frequent, as well as matrilineal wounding. Although ordinal rank was positively associated with higher chronic hair cortisol concentrations (HCCs) in the months before the matriarch's death, following her death, only those who experienced large increases in rank within her matriline displayed higher HCCs. Changes in matrilineal stability, aggression, behavior, and HCCs within the other two matrilines in the troop were not evident, although caution is needed due to the small sample sizes. We conclude that the death of the non-alpha matriarch led to matrilineal instability, characterized by higher levels of aggression and subsequent vigilance, rank changes, physiological stress, and grooming. We suggest that non-alpha matriarchs with a large number of kin and social support can be integral to the stability of matrilines.

Relocation and Hair Cortisol Concentrations in New Zealand White Rabbits

To investigate how long relocation modified hair cortisol concentrations in New Zealand white rabbits, 19 rabbits were subjected to a change in their breeding facility at the beginning of the trial and then were kept under stable environmental conditions. Hair samples were collected at the time of arrival to the nonhuman animal facility and at 40-day intervals from the same skin area for up to 440 days after the animals' arrival to the facility. A period effect on the hair cortisol concentration was found (p < .01). The transfer of the rabbits to the new facility might have induced an increase in the hypothalamic-pituitary-adrenal axis activity (p < .01). A second increase in hair cortisol concentration (p < .01) occurred at 320 days, after a change of personnel at the facility that occurred at 280 days, which was the only environmental change. The relocation of rabbits to the facility resulted in a stress response leading to elevated cortisol levels. The effect of relocation on mean cortisol concentrations was exhausted within 120 days when all environmental factors were kept stable.

Associations between early life experience, chronic HPA axis activity, and adult social rank in rhesus monkeys

Early life experience and socioeconomic status (SES) are well-established predictors of health outcomes in people. Both factors likely influence health outcomes via hypothalamic-pituitary-adrenal (HPA) axis regulation. However, it is unclear how early experience and HPA axis activity influence adult social status. We studied differentially reared female rhesus monkeys (Macaca mulatta, N = 90) as models to test the hypothesis that chronic HPA axis activity assessed via hair cortisol concentrations (HCCs) mediated the relationship between early life experience and adult social rank. We found that mother-peer-reared (MPR) monkeys acquired higher social ranks than either of the two nursery-reared (NR) groups (peer-reared, PR, or surrogate-peer-reared, SPR monkeys) (β = -0.07, t₍₈₉₎ = -2.16, p = 0.034). We also found that MPR HCCs were lower during the juvenile period at 18 months (F_(2,25) = 3.49, p = 0.047). Furthermore, for MPR but not NR monkeys, changes in HCCs from 18 to 24 months (r_(s) = -0.627, p = 0.039) and adult HCCs (r_(s) = -0.321, p = 0.03) were negatively correlated with adult social rank. These findings suggest that chronic HPA axis regulation in juvenility, and perhaps in adulthood, may influence adult social status for primates that experience typical early rearing. However, early life adversity may result in dissociation between neuroendocrine stress regulation and adult social competence, which may be risk factors for adverse health outcomes.

Tailored Enrichment Strategies and Stereotypic Behavior in Captive Individually Housed Macaques (Macaca spp.)

The welfare of nonhuman animals in captivity is widely dependent on the natural psychological, physical, and behavioral needs of the animals and how adequately these needs are met. Inability to engage in natural behaviors can lead to chronic stress and expression of stereotypic behavior. The majority of research on decreasing stereotypic behavior in captivity addresses problems at the group level and does not account for individual variability in each animal's needs, history, and preferences. This study combined physiological and behavioral measures of well being to comprehensively assess the unique needs of individually housed captive macaques (Macaca spp.) with the aim of developing tailored welfare strategies. Behavioral and hormonal data were collected under 2 conditions: baseline and individualized enrichment. The results showed a significant decrease in stereotypic behavior under the enrichment condition. Additionally, 7 out of 9 individuals showed a decrease in fecal glucocorticoid (stress hormone) levels, indicating a reduction in hypothalamic-pituitary-adrenal activity. Addressing welfare on an individual, rather than group, level allows for a better overall characterization of well being and maximizes the probability of improving the welfare of each animal.

Responses to the Human Intruder Test are related to hair cortisol phenotype and sex in rhesus macaques (Macaca mulatta)

Measurement of cortisol in hair provides a chronic index of hypothalamic-pituitary-adrenal (HPA) axis activity and has been applied to assessments of temperament (stable behavioral differences between individuals). However, the extent to which chronically high HPA axis activity relates to a correspondingly high degree of behavioral reactivity is as yet unknown. Therefore, the goal of the present experiment was to assess the relationship between hair cortisol and a reactive temperament. We administered the Human Intruder Test (HIT) twice to 145 (80 male) rhesus macaques (Macaca mulatta) in order to assess behavioral reactivity. The HIT presents monkeys with an unfamiliar experimenter and is composed of a Baseline phase (no intruder) followed by three experimental phases in which the orientation of the intruder changes (Profile, Stare, Back). Behavioral responses to the test were videotaped and behaviors thought to reflect a reactive response to the intruder were scored for duration. Hair samples collected within ±1 month of the first HIT session were analyzed for cortisol by enzyme immunoassay. Subjects were assigned to three groups based on hair cortisol concentration: high, intermediate, and low cortisol phenotypes. Monkeys with the high cortisol phenotype were more reactive to the presence of the intruder than those with the low cortisol phenotype: they were more aggressive, scratched more, and spent more time in the back half of the cage. Males yawned significantly more while females spent more time immobile and in the back of the cage. Overall, monkeys with higher hair cortisol demonstrated an exaggerated response to the presence of the human intruder, supporting a relationship between high levels of chronic HPA axis activity and a reactive temperament. These results indicate that high levels of HPA axis activity, which may result from either genetic variation or environmental stress, correspond with heightened behavioral responses to a stressful experience. Am. J. Primatol. 79:e22526, 2017. © 2016 Wiley Periodicals, Inc.

What cortisol can tell us about the costs of sociality and reproduction among free-ranging rhesus macaque females on Cayo Santiago

Research with the rhesus macaque population on Cayo Santiago can provide a unique perspective on the costs of sociality and reproduction in primates. Because the Cayo macaques live in unusually large groups and in a predator-free environment, in which their artificial food source lacks seasonal variation in abundance or quality, these monkeys constitute a semi-experimental study of the costs and benefits of group living. Here we review several long- and short-term studies that have focused on female life history and stress physiology. Long-term demographic data have shown that rhesus macaque females of middle- and low-ranking matrilines have lower adult survival probabilities than females of high-ranking matrilines. Costs of reproductive effort are also evident: adult females were more likely to die during the birth than during the mating season and they experienced higher cortisol levels when lactating. Lower-ranking females, in particular, experienced greater relative increase in cortisol production during lactation, in comparison to middle- and high-ranking females. Older high-ranking females had lower plasma cortisol levels than younger ones but cortisol levels were similarly high among young and old middle- and low-ranking females. Higher plasma cortisol levels and/or fecal glucocorticoid concentrations are associated with higher plasma concentrations of some proinflammatory cytokines. High cortisol, in turn, may be associated with chronic inflammation, and perhaps also with immunosuppression. In sum, the studies reviewed here provide multiple lines of evidence that sociality and reproductive effort impose measurable costs on female rhesus macaques. In line with socio-ecological theory, female dominance rank consistently emerges as an important modulator of variation in female life histories and physiology. The Cayo Santiago macaques are therefore a valuable model for elucidating the mechanisms by which within-group competition and reproduction impact health and survival in nonhuman primates and in humans.

Associations between Parity, Hair Hormone Profiles during Pregnancy and Lactation, and Infant Development in Rhesus Monkeys (Macaca mulatta)

Studies examining hormones throughout pregnancy and lactation in women have been limited to single, or a few repeated, short-term measures of endocrine activity. Furthermore, potential differences in chronic hormonal changes across pregnancy/lactation between first-time and experienced mothers are not well understood, especially as they relate to infant development. Hormone concentrations in hair provide long-term assessments of hormone production, and studying these measures in non-human primates allows for repeated sampling under controlled conditions that are difficult to achieve in humans. We studied hormonal profiles in the hair of 26 female rhesus monkeys (Macaca mulatta, n=12 primiparous), to determine the influences of parity on chronic levels of cortisol (hair cortisol concentration, HCC) and progesterone (hair progesterone concentration, HPC) during early- to mid-pregnancy (PREG1), in late pregnancy/early lactation (PREG2/LACT1), and in peak lactation (LACT2). We also assessed infants' neurobehavioral development across the first month of life. After controlling for age and stage of pregnancy at the first hair sampling period, we found that HCCs overall peaked in PREG2/LACT1 (p=0.02), but only in primiparous monkeys (p<0.001). HPCs declined across pregnancy and lactation for all monkeys (p<0.01), and primiparous monkeys had higher HPCs overall than multiparous monkeys (p=0.02). Infants of primiparous mothers had lower sensorimotor reflex scores (p=0.02) and tended to be more irritable (p=0.05) and less consolable (p=0.08) in the first month of life. Moreover, across all subjects, HCCs in PREG2/LACT1 were positively correlated with irritability (r(s)=0.43, p=0.03) and negatively correlated with sensorimotor scores (r(s)=-0.41, p=0.04). Together, the present results indicate that primiparity influences both chronic maternal hormonal profiles and infant development. These effects may, in part, reflect differential reproductive and maternal effort in mothers with varied caretaking experience. In addition, infant exposure to relatively higher levels of maternal cortisol during the late fetal and early postnatal periods is predictive of poorer developmental outcomes.

Why primate models matter

Research involving nonhuman primates (NHPs) has played a vital role in many of the medical and scientific advances of the past century. NHPs are used because of their similarity to humans in physiology, neuroanatomy, reproduction, development, cognition, and social complexity-yet it is these very similarities that make the use of NHPs in biomedical research a considered decision. As primate researchers, we feel an obligation and responsibility to present the facts concerning why primates are used in various areas of biomedical research. Recent decisions in the United States, including the phasing out of chimpanzees in research by the National Institutes of Health and the pending closure of the New England Primate Research Center, illustrate to us the critical importance of conveying why continued research with primates is needed. Here, we review key areas in biomedicine where primate models have been, and continue to be, essential for advancing fundamental knowledge in biomedical and biological research.

Use of an aquarium as a novel enrichment item for singly housed rhesus macaques (Macaca mulatta)

Locomotor stereotypies are behaviors often seen in singly housed rhesus macaques (Macaca mulatta) and are considered to represent a maladaptive response to captive environments. Active and passive enrichment items are commonly used to mitigate these and other abnormal behaviors. Active enrichment items allow physical manipulation and may be temporarily successful in reducing stereotypies, but their beneficial effects usually are confined to relatively short periods of active use. Passive enrichment items that do not involve physical manipulation are less well studied, and the results are mixed. This study evaluated an aquarium with live fish for use as a novel passive enrichment item in a common facility setting as a means to decrease locomotor stereotypy. We hypothesized that the introduction of the aquarium would decrease the frequency of locomotor stereotypy in a group of singly housed rhesus macaques (n = 11) with a known history of abnormal behaviors. Unexpectedly, locomotor stereotypy increased with the introduction of the aquarium and then decreased over time. Furthermore, when the aquarium was removed, the frequency of stereotypy decreased to below baseline levels. These unexpected results are best explained by neophobia, a common phenomenon documented in many animal species. The increase in abnormal behavior is likely to result from the addition of a novel object within the environment. This study demonstrates that, in the context of reducing abnormal behavior, presumably innocuous enrichment items may have unexpected effects and should be evaluated critically after their introduction to a captive population.

Population density-dependent hair cortisol concentrations in rhesus monkeys (Macaca mulatta)

Population density is known to influence acute measures of hypothalamic-pituitary-adrenal (HPA) axis activity in a variety of species, including fish, deer, birds, and humans. However, the effects of population density on levels of chronic stress are unknown. Given the fact that exposure to chronically elevated levels of circulating glucocorticoids results in a host of health disparities in animals and humans alike, it is important to understand how population density may impact chronic stress. We assessed hair cortisol concentrations (HCCs), which are reliable indicators of chronic HPA axis activity, in rhesus monkeys (Macaca mulatta) to determine the influence of population density on these values. In Experiment 1, we compared HCCs of monkeys living in high-density (HD; 1 monkey/0.87m(2)) and low-density (LD; 1 monkey/63.37m(2)) environments (N=236 hair samples) and found that HD monkeys exhibited higher hair cortisol across all age categories (infant, juvenile, young adult, adult, and aged) except infancy and aged (F(5)=4.240, p=0.001), for which differences were nearly significant. HD monkeys also received more severe fight wounds than LD monkeys (χ(2)=26.053, p<0.001), though no effects of dominance status emerged. In Experiment 2, we examined how HCCs change with fluctuating population levels across 5 years in the adult LD monkeys (N=155 hair samples) and found that increased population density was significantly positively correlated with HCCs in this semi-naturalistic population (r(s)=0.975, p=0.005). These are the first findings to demonstrate that increased population density is associated with increased chronic, endogenous glucocorticoid exposure in a nonhuman primate species. We discuss the implications of these findings with respect to laboratory research, population ecology, and human epidemiology.

Self-injurious behaviour in autistic children: a neuro-developmental theory of social and environmental isolation

RATIONALE

Self-injurious behaviour is not one of the three core symptoms that define autism. However, children on the autism spectrum appear to be particularly vulnerable. Afflicted children typically slap their faces, punch or bang their heads, and bite or pinch themselves. These behaviours can be extremely destructive, and they interfere with normal social and educational activities. However, the neurobiological mechanisms that confer vulnerability in children with autism have not been adequately described.

OBJECTIVES

This review explores behavioural and neurobiological characteristics of children with autism that may be relevant for an increased understanding of their vulnerability for self-injurious behaviour.

METHODS

Behavioural characteristics that are co-morbid for self-injurious behaviour in children with autism are examined. In addition, the contributions of social and environmental deprivation in self-injurious institutionalized orphans, isolated rhesus macaques, and additional animal models are reviewed.

RESULTS

There is extensive evidence that social and environmental deprivation promotes self-injurious behaviour in both humans (including children with autism) and animal models. Moreover, there are multiple lines of convergent neuroanatomical, neurophysiological, and neurochemical data that draw parallels between self-injurious children with autism and environmentally deprived humans and animals.

CONCLUSIONS

A hypothesis is presented that describes how the core symptoms of autism make these children particularly vulnerable for self-injurious behaviour. Relevant neurodevelopmental pathology is described in cortical, limbic, and basal ganglia brain regions, and additional research is suggested.

Interindividual Variability in Stress Susceptibility: A Role for Epigenetic Mechanisms in PTSD

Post-traumatic stress disorder (PTSD) is a psychiatric condition characterized by intrusive and persistent memories of a psychologically traumatic event that leads to significant functional and social impairment in affected individuals. The molecular bases underlying persistent outcomes of a transient traumatic event have remained elusive for many years, but recent studies in rodents have implicated epigenetic modifications of chromatin structure and DNA methylation as fundamental mechanisms for the induction and stabilization of fear memory. In addition to mediating adaptations to traumatic events that ultimately cause PTSD, epigenetic mechanisms are also involved in establishing individual differences in PTSD risk and resilience by mediating long-lasting effects of genes and early environment on adult function and behavior. In this review, we discuss the current evidence for epigenetic regulation of PTSD in human studies and in animal models and comment on ways in which these models can be expanded. In addition, we identify key outstanding questions in the study of epigenetic mechanisms of PTSD in the context of rapidly evolving technologies that are constantly updating and adjusting our understanding of epigenetic modifications and their functional roles. Finally, we discuss the potential application of epigenetic approaches in identifying markers of risk and resilience that can be utilized to promote early intervention and develop therapeutic strategies to combat PTSD after symptom onset.