Rank-related differences in cardiovascular function among wild baboons: Role of sensitivity to glucocorticoids

Two Routes to Status, One Route to Health: Trait Dominance and Prestige Differentially Associate with Self-reported Stress and Health in Two US University Populations

Objective

Social status has been extensively linked to stress and health outcomes. However, two routes by which status can be earned - dominance and prestige - may not uniformly relate to lower stress and better health because of inherent behavioral and stress-exposure differences in these two routes.

Methods

In one exploratory and two preregistered studies, participants (total N = 978) self-reported their trait dominance and prestige and self-reported several stress and health outcomes.

Results

The meta-effects evident across the three studies indicate that higher trait dominance was associated with worse outcomes - higher stress, poorer physical and mental health, poorer behavioral health, poorer life satisfaction, higher negative affect (range of absolute values of non-zero correlations, |r| = [0.074, 0.315], ps < 0.021) - and higher trait prestige was associated with better outcomes - lower stress, better physical and mental health, better behavioral health, better life satisfaction, higher positive and lower negative mood (|r| = [0.134, 0.478], ps < 0.001). These effects remained evident (with few exceptions) after controlling for socioeconomic status, other status-relevant traits, or self-enhancing motives; associations with behavior relevant to the COVID19 pandemic generally were not robust.

Conclusions

This work indicates that evolved traits related to the preferred route by which status is earned likely impact self-reported stress and health outcomes. Future research is necessary to examine physiological and other objective indicators of stress and health in more diverse populations.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40750-022-00199-3.

Measuring dominance certainty and assessing its impact on individual and societal health in a nonhuman primate model: a network approach

The notion of dominance is ubiquitous across the animal kingdom, wherein some species/groups such relationships are strictly hierarchical and others are not. Modern approaches for measuring dominance have emerged in recent years taking advantage of increased computational power. One such technique, named Percolation and Conductance (Perc), uses both direct and indirect information about the flow of dominance relationships to generate hierarchical rank order that makes no assumptions about the linearity of these relationships. It also provides a new metric, known as 'dominance certainty', which is a complimentary measure to dominance rank that assesses the degree of ambiguity of rank relationships at the individual, dyadic and group levels. In this focused review, we will (i) describe how Perc measures dominance rank while accounting for both nonlinear hierarchical structure as well as sparsity in data-here we also provide a metric of dominance certainty estimated by Perc, which can be used to compliment the information dominance rank supplies; (ii) summarize a series of studies by our research team reflecting the importance of 'dominance certainty' on individual and societal health in large captive rhesus macaque breeding groups; and (iii) provide some concluding remarks and suggestions for future directions for dominance hierarchy research. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Mathematical modeling reveals how the speed of endocrine regulation should affect baseline and stress-induced glucocorticoid levels

Unpredictable environmental changes displace individuals from homeostasis and elicit a stress response. In vertebrates, the stress response is mediated mainly by glucocorticoids (GCs) which initiate physiological changes while minimizing allostatic overload. Individuals and species vary consistently in baseline and stress-induced GC levels and the speed with which GC levels can be upregulated or downregulated, but the extent to which variation in hormone regulation influences baseline and stress-induced GC levels is unclear. Using mathematical modeling, we tested how GC regulation rate, frequencies and durations of acute stressors, fitness functions, and allostatic overload affect GC levels during control and acute stress periods. As GC regulation rate slows, baseline and acute stress-induced GC levels become more similar. When the speed of up- and downregulation decreased, hormone levels became more linked to anticipated future conditions to avoid fitness costs of mismatching a new environmental state. More frequent acute stressors caused baseline and acute stress-induced GC levels to converge. When fitness was more tightly linked to hormone levels during acute stress periods than during control states, the speed of upregulation influenced optimal hormone levels more than the downregulation rate. With allostatic overload costs included, predicted GC levels were lower and more dependent on the frequency of past acute stressors. Our results show the value of optimality modeling to study the hormonal response to stressors and suggest GC levels depend on past and anticipated future environmental states as well as individual differences in hormone regulation.

Sex differences in the impact of social status on hair cortisol concentrations in rhesus monkeys (Macaca mulatta)

Social status impacts stress in primates, but the direction of the effect differs depending on species, social style, and group stability. This complicates our ability to identify broadly applicable principles for understanding how social status impacts health and fitness. One reason for this is the fact that social status is often measured as linear dominance rank, yet social status is more complex than simply high or low rank. Additionally, most research on social status and health ignores the effects of sex and sex-specific relationships, despite known differences in disease risk, coping strategies, and opposite-sex dominance interactions between males and females in many species. We examine the influence of social status, sex, and opposite-sex interactions on hair cortisol concentrations (HCC) in a well-studied species, rhesus macaques, where the literature predicts low ranking individuals would experience more chronic stress. Animals in three captive, seminaturalistic social groups (N = 252; 71 male) were observed for 6 weeks to obtain metrics of social status (rank and dominance certainty [DC]). DC is a measure of one's fit within the hierarchy. Hair samples were collected from each subject and analyzed for HCC. Generalized linear mixed models were used to examine (a) whether rank, DC, or sex predicted HCC; (b) whether same- or opposite-sex dominance relationships differentially impacted HCC; and (c) whether aggressive interactions initiated or received could explain any observed relationships. Results indicated that DC, not rank, predicted HCC in a sex-specific manner. For males, high HCC were predicted by receiving aggression from or having high DC with other males as well as having low DC with females. For females, only high DC with males predicted high HCC. These results likely relate to sex-specific life history pattern differences in inherited versus earned rank that are tied to female philopatry and male immigration.

Organic cation transporter 3: A cellular mechanism underlying rapid, non-genomic glucocorticoid regulation of monoaminergic neurotransmission, physiology, and behavior

Contribution to Special Issue on Fast effects of steroids. Corticosteroid hormones act at intracellular glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) to alter gene expression, leading to diverse physiological and behavioral responses. In addition to these classical genomic effects, corticosteroid hormones also exert rapid actions on physiology and behavior through a variety of non-genomic mechanisms, some of which involve GR or MR, and others of which are independent of these receptors. One such GR-independent mechanism involves corticosteroid-induced inhibition of monoamine transport mediated by "uptake₂" transporters, including organic cation transporter 3 (OCT3), a low-affinity, high-capacity transporter for norepinephrine, epinephrine, dopamine, serotonin and histamine. Corticosterone directly and acutely inhibits OCT3-mediated transport. This review describes the studies that initially characterized uptake₂ processes in peripheral tissues, and outlines studies that demonstrated OCT3 expression and corticosterone-sensitive monoamine transport in the brain. Evidence is presented supporting the hypothesis that corticosterone can exert rapid, GR-independent actions on neuronal physiology and behavior by inhibiting OCT3-mediated monoamine clearance. Implications of this mechanism for glucocorticoid-monoamine interactions in the context-dependent regulation of behavior are discussed.

Predictors of insubordinate aggression among captive female rhesus macaques

OBJECTIVES

Cercopithicine primates tend to have nepotistic hierarchies characterized by predictable, kinship-based dominance. Although aggression is typically directed down the hierarchy, insubordinate aggression does occur. Insubordination is important to understand because it can precipitate social upheaval and undermine group stability; however, the factors underlying it are not well understood. We test whether key social and demographic variables predict insubordination among captive female rhesus macaques.

MATERIALS AND METHODS

To identify factors influencing insubordination, multivariate analyses of 10,821 dyadic conflicts among rhesus macaque females were conducted, using data from six captive groups. A segmented regression analysis was used to identify dyads with insubordination. Negative binomial regression analyses and an information theoretic approach were used to assess predictors of insubordination among dyads.

RESULTS

In the best models, weight difference (w = 1.0; IRR = 0.930), age (dominant: w = 1.0, IRR = 0.681; subordinate: w = 1.0, IRR = 1.069), the subordinate's total number of allies (w = 0.727, IRR = 1.060) or non-kin allies (w = 0.273, IRR = 1.165), the interaction of the dominant's kin allies and weight difference (w = 0.938, IRR = 1.046), violation of youngest ascendancy (w = 1.0; IRR = 2.727), and the subordinate's maternal support (w = 1.0; IRR = 2.928), are important predictors of insubordination.

DISCUSSION

These results show that both intrinsic and social factors influence insubordinate behavior. This adds to evidence of the importance of intrinsic factors and flexibility in a social structure thought to be rigid and predetermined by external factors. Further, because insubordination can precipitate social overthrow, determining predictors of insubordination will shed light on mechanisms underlying stability in nepotistic societies.

Decoupling social status and status certainty effects on health in macaques: a network approach

BACKGROUND

Although a wealth of literature points to the importance of social factors on health, a detailed understanding of the complex interplay between social and biological systems is lacking. Social status is one aspect of social life that is made up of multiple structural (humans: income, education; animals: mating system, dominance rank) and relational components (perceived social status, dominance interactions). In a nonhuman primate model we use novel network techniques to decouple two components of social status, dominance rank (a commonly used measure of social status in animal models) and dominance certainty (the relative certainty vs. ambiguity of an individual's status), allowing for a more complex examination of how social status impacts health.

METHODS

Behavioral observations were conducted on three outdoor captive groups of rhesus macaques (N = 252 subjects). Subjects' general physical health (diarrhea) was assessed twice weekly, and blood was drawn once to assess biomarkers of inflammation (interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and C-reactive protein (CRP)).

RESULTS

Dominance rank alone did not fully account for the complex way that social status exerted its effect on health. Instead, dominance certainty modified the impact of rank on biomarkers of inflammation. Specifically, high-ranked animals with more ambiguous status relationships had higher levels of inflammation than low-ranked animals, whereas little effect of rank was seen for animals with more certain status relationships. The impact of status on physical health was more straightforward: individuals with more ambiguous status relationships had more frequent diarrhea; there was marginal evidence that high-ranked animals had less frequent diarrhea.

DISCUSSION

Social status has a complex and multi-faceted impact on individual health. Our work suggests an important role of uncertainty in one's social status in status-health research. This work also suggests that in order to fully explore the mechanisms for how social life influences health, more complex metrics of social systems and their dynamics are needed.

Using individual attributes to predict hierarchical position in a Macaca sylvanus group at 'La forêt des singes', Rocamadour

The Macaca genus is a biological model for animal societies, known for its social structuration by age, sex, and kinship. We managed to establish a mathematical model, through multiple regression tests, that emphasizes the relationship between several individual attributes (sex, age, matrilineal belonging) with hierarchical rank among a Barbary macaque (Macaca sylvanus) group (N=50) observed during 11 months at 'La forêt des singes' in Rocamdour, France. We established two models according to sex, one for males and one for females, explaining 99.1% and 99.4% of the hierarchical rank variance respectively. Our study is the first step in studying the weight of each attribute in hierarchical structuration among this species and consequently the individual hierarchical rank determination. Such a model could be used as the basis for future comparative studies among the Macaca genus.

Puberty and adolescence as a time of vulnerability to stressors that alter neurobehavioral processes

Puberty and adolescence are major life transitions during which an individual's physiology and behavior changes from that of a juvenile to that of an adult. Here we review studies documenting the effects of stressors during pubertal and adolescent development on the adult brain and behavior. The experience of complex or compound stressors during puberty/adolescence generally increases stress reactivity, increases anxiety and depression, and decreases cognitive performance in adulthood. These behavioral changes correlate with decreased hippocampal volumes and alterations in neural plasticity. Moreover, stressful experiences during puberty disrupt behavioral responses to gonadal hormones both in sexual performance and on cognition and emotionality. These behavioral changes correlate with altered estrogen receptor densities in some estrogen-concentrating brain areas, suggesting a remodeling of the brain's response to hormones. A hypothesis is presented that activation of the immune system results in chronic neuroinflammation that may mediate the alterations of hormone-modulated behaviors in adulthood.

ALLOSTATIC LOAD AND SOCIOECONOMIC STATUS IN POLISH ADULT MEN

This study considers the relationship between a cumulative index of biological dysregulation (allostatic load) and several dimensions of socioeconomic status (SES) and lifestyle in adult Polish males. The extent to which lifestyle variables can explain SES variation in allostatic load was also evaluated. Participants were 3887 occupationally active men aged 25–60 years living in cities and villages in the Silesia region of Poland. The allostatic load indicator included eleven markers: % fat (adverse nutritional intake), systolic and diastolic blood pressures (cardiovascular activity), FEV1 (lung function), erythrocyte sedimentation rate (inflammatory processes), glucose and total cholesterol (cardiovascular disease risk), total plasma protein (stress-haemoconcentration), bilirubin, creatinine clearance and alkaline phosphatase activity (hepatic and renal functions). A higher level of completed education, being married and residing in an urban area were associated with lower physiological dysregulation. The association between indicators of SES and allostatic load was not eliminated or attenuated when unhealthy lifestyle variables were included in the model. Smoking status and alcohol consumption played minimal roles in explaining the association between SES and allostatic load; physical activity, however, had a generally protective effect on allostatic load.

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.

Self-structuring properties of dominance hierarchies a new perspective

Using aggressive behavior, animals of many species establish dominance hierarchies in both nature and the laboratory. Rank in these hierarchies influences many aspects of animals' lives including their health, physiology, weight gain, genetic expression, and ability to reproduce and raise viable offspring. In this chapter, we define dominance relationships and dominance hierarchies, discuss several model species used in dominance studies, and consider factors that predict the outcomes of dominance encounters in dyads and small groups of animals. Researchers have shown that individual differences in attributes, as well as in states (recent behavioral experiences), influence the outcomes of dominance encounters in dyads. Attributes include physical, physiological, and genetic characteristics while states include recent experiences such as winning or losing earlier contests. However, surprisingly, we marshal experimental and theoretical evidence to demonstrate that these differences have significantly less or no ability to predict the outcomes of dominance encounters for animals in groups as small as three or four individuals. Given these results, we pose an alternative research question: How do animals of so many species form hierarchies with characteristic linear structures despite the relatively low predictability based upon individual differences? In answer to this question, we review the evidence for an alternative approach suggesting that dominance hierarchies are self-structuring. That is, we suggest that linear forms of organization in hierarchies emerge from several kinds of behavioral processes, or sequences of interaction, that are common across many different species of animals from ants to chickens and fish and even some primates. This new approach inspires a variety of further questions for research.

Assessing stress in animal populations: Do fecal and plasma glucocorticoids tell the same story?

Many studies have recently focused on stress as a marker of an animal's well being. Since animals respond to a stressor by increasing their glucocorticoid (GC) levels there has been much interest in measuring these hormones. Fecal GC analyses have been used in a wide range of studies as they are an easily obtained, non-invasive measure of these stress hormones. However, these analyses rest on two major assumptions. First, they assume that fecal GC metabolites reflect free, biologically active levels of GCs in the plasma. Second, they assume that differences in fecal GC metabolite levels among animals are an accurate reflection of their physiological state and thus of their ability to respond to a stressor. We tested these assumptions in a population of free-ranging snowshoe hares (Lepus americanus) in the southwestern Yukon, from 2006 to 2008. Both assumptions were verified. Plasma free cortisol levels mirrored bile and fecal cortisol metabolite (FCM) levels, but plasma total cortisol levels did not. Differences in FCM concentrations among hares robustly predicted their response to a hormonal challenge. Hares with higher FCM concentrations showed a greater resistance to the suppression of their free plasma cortisol following a dexamethasone injection and a more marked increase of free plasma cortisol following an ACTH injection. Furthermore, we found that changes in FCM concentrations in autumn and winter over two years reliably tracked changes in plasma free cortisol levels obtained from the hormonal challenge test. These results indicate that both fecal and plasma measures of an animal's stress physiology are concordant: they tell the same story.

Social stratification, health, and violence in the very young

Inequalities in human social groups contribute in important ways to the initiation and perpetuation of violence. Because both social inequalities and interpersonal aggression emerge in early life, it is important to study their developmental origins in the collective and individual behavior of young children. As young as 2 years of age, children assemble stable, linearly transitive dominance hierarchies when brought together in novel social groups. Just as the consequences of socioeconomic status may be due in part to experiences of social ordering per se, subjective childhood experiences of occupying a lesser or greater position on a scale of social influence may similarly affect health, safety, and behavior in early development. Children's experiences with social ordering may represent a first, formative encounter with the hierarchical social relations that affect health and susceptibility to violence over the human lifespan. We have studied the health correlates of group stressors, dominance positions, and biobehavioral reactivity within cohorts of preschool children and semi free-ranging rhesus macaques. Social position is ascertained using naturalistic observations of critical dyadic events, and stress-reactivity is measured using assessments of behavioral and biological responses to standardized challenges. Outcome measures, including indicators of mental and physical health and interpersonal injuries, have been assessed--in the case of children--with parent- and teacher-report questionnaires, child self-report protocols, and frequent, objective physical examinations of the child and--in the case of rhesus monkeys--with daily observations of violence-related injuries. Results to date have suggested the following provisional conclusions: (1) Children and monkeys form stable, linearly transitive social hierarchies in which identifiable subgroups of individuals occupy dominant and subordinate social positions. (2) Individuals occupying subordinate positions within the social group show exaggerated adrenocortical and/or autonomic reactivity to challenge and disproportionate rates of chronic medical conditions or violent injuries. (3) Stress-reactivity, naturally occurring stressors, and social position are interactively predictive of rates of morbidity. Monkeys who displayed high biobehavioral reactivity sustained disproportionate numbers of violent injuries during a confinement stressor. Children who were high in the dominance hierarchy and low in reactivity showed significantly higher rates of externalizing mental health symptoms, while those who were in subordinate social positions and high in reactivity showed higher prevalences of internalizing symptoms. In conclusion, the prevention of youth violence--and other forms of early disorder--will require a deeper, developmental understanding of the "headwaters" of aggression and victimization in early childhood and a stronger accounting of how early social inequalities set trajectories toward healthy or disordered behavior.

Female dominance over males in primates: self-organisation and sexual dimorphism

The processes that underlie the formation of the dominance hierarchy in a group are since long under debate. Models of self-organisation suggest that dominance hierarchies develop by the self-reinforcing effects of winning and losing fights (the so-called winner-loser effect), but according to 'the prior attribute hypothesis', dominance hierarchies develop from pre-existing individual differences, such as in body mass. In the present paper, we investigate the relevance of each of these two theories for the degree of female dominance over males. We investigate this in a correlative study in which we compare female dominance between groups of 22 species throughout the primate order. In our study female dominance may range from 0 (no female dominance) to 1 (complete female dominance). As regards 'the prior attribute hypothesis', we expected a negative correlation between female dominance over males and species-specific sexual dimorphism in body mass. However, to our surprise we found none (we use the method of independent contrasts). Instead, we confirm the self-organisation hypothesis: our model based on the winner-loser effect predicts that female dominance over males increases with the percentage of males in the group. We confirm this pattern at several levels in empirical data (among groups of a single species and between species of the same genus and of different ones). Since the winner-loser effect has been shown to work in many taxa including humans, these results may have broad implications.

The implications of stress on male mating behavior and success in a sexually dimorphic polygynous mammal, the grey seal

Studies on primates and other taxa have shown that the physiological response of an individual to stress reflects their social status. We combined behavioral observations with measures of stress to test the hypothesis that stress is an important physiological determinant of mating behavior and success in the male grey seal. Known-age males (N=19) were studied during the breeding seasons of 2004 and 2005 at Sable Island, Canada. The stressor was a capture and restraint period of 35 min and serial samples of cortisol and testosterone were taken as measures of stress. The mean baseline concentrations of cortisol and testosterone were 9.7+/-0.5 ug/dl and 6.2+/-0.6 ng/mL, respectively. The baseline cortisol concentration was negatively correlated with the duration of time a male spent at a site (r=-0.507, P=0.027), which was a strong correlate of mating success (r=0.659, P=0.002). All males experienced an increase in the concentration of cortisol during the restraint period (79.1+/-8.4%; CV=46.1%). The percentage rise in cortisol during restraint was correlated with the mean duration of time spent at a site (r=0.544, P=0.016) and thus success. The concentration of testosterone also increased during the restraint period (32.8+/-9.7%). This might be an adaptive response to maintaining the ability to reproduce while under stress. Our study indicates that stress is an important determinant of success in male grey seals. More successful males might exhibit an adaptive response to stress by maintaining low concentrations of cortisol during breeding.

Physiological stress in ecology: lessons from biomedical research

Increasingly, levels of the 'stress hormones' cortisol and corticosterone are being used by ecologists as indicators of physiological stress in wild vertebrates. The amplitude of hormonal response is assumed to correlate with the overall health of an animal and, by extension, the health of the population. However, much of what is known about the physiology of stress has been elucidated by the biomedical research community. I summarize five physiological mechanisms that regulate hormone release during stress that should be useful to ecologists and conservationists. Incorporating these physiological mechanisms into the design and interpretation of ecological studies will make these increasingly popular studies of stress in ecological settings more rigorous.

Dimensions of health and social structure in the early intermediate period cemetery at Villa El Salvador, Peru

This paper examines relationships between the social structure of a community and the health of its members, based on analysis of human skeletal remains (N = 64) from Villa El Salvador XII (100 BC-AD 100), a prehistoric cemetery located in the lower Lurín Valley, Peru. The ambiguity of social status as conventionally inferred from archaeological context is among the principal complicating factors in such an inquiry. We use multidimensional scaling of skeletal markers to identify the presence of patterned health-based heterogeneity in our sample, without making a priori assumptions about underlying social structure at Villa El Salvador. This procedure situates every skeleton relative to all others in the sample on the basis of multiple health markers, eliciting health groups. Once recognized, the relevance of those groups to social structure can be evaluated by comparison with a broad range of presumptive archaeological status indicators. We test the hypothesis that the distribution of stress indicators in human skeletons covaries with archaeological indicators of social differentiation. Based on multivariate analysis of skeletal indicators, we conclude that the cemetery at Villa El Salvador was utilized by two social groups with different geographic affinities: one of local coastal origin, and the other probably from the upper Lurín Valley or adjacent higher altitudes. These groups differ in skeletal characteristics related to childhood health, probably reflecting systematic contrasts in the growth environments of the studied individuals. This same division is independently supported by the distribution of cranial deformation, a possible marker of ethnicity. We also document some inequality in the distribution of labor among male individuals, as reflected by the relative advancement of degenerative joint disease, and congruent with differences in the number and quality of associated funerary offerings.

Biological sensitivity to context: I. An evolutionary–developmental theory of the origins and functions of stress reactivity

Biological reactivity to psychological stressors comprises a complex, integrated, and highly conserved repertoire of central neural and peripheral neuroendocrine responses designed to prepare the organism for challenge or threat. Developmental experience plays a role, along with heritable, polygenic variation, in calibrating the response dynamics of these systems, with early adversity biasing their combined effects toward a profile of heightened or prolonged reactivity. Conventional views of such high reactivity suggest that it is an atavistic and pathogenic legacy of an evolutionary past in which threats to survival were more prevalent and severe. Recent evidence, however, indicates that (a) stress reactivity is not a unitary process, but rather incorporates counterregulatory circuits serving to modify or temper physiological arousal, and (b) the effects of high reactivity phenotypes on psychiatric and biomedical outcomes are bivalent, rather than univalent, in character, exerting both risk-augmenting and risk-protective effects in a context-dependent manner. These observations suggest that heightened stress reactivity may reflect, not simply exaggerated arousal under challenge, but rather an increased biological sensitivity to context, with potential for negative health effects under conditions of adversity and positive effects under conditions of support and protection. From an evolutionary perspective, the developmental plasticity of the stress response systems, along with their structured, context-dependent effects, suggests that these systems may constitute conditional adaptations: evolved psychobiological mechanisms that monitor specific features of childhood environments as a basis for calibrating the development of stress response systems to adaptively match those environments. Taken together, these theoretical perspectives generate a novel hypothesis: that there is a curvilinear, U-shaped relation between early exposures to adversity and the development of stress-reactive profiles, with high reactivity phenotypes disproportionately emerging within both highly stressful and highly protected early social environments.

The Influence of Social Hierarchy on Primate Health

Dominance hierarchies occur in numerous social species, and rank within them can greatly influence the quality of life of an animal. In this review, I consider how rank can also influence physiology and health. I first consider whether it is high- or low-ranking animals that are most stressed in a dominance hierarchy; this turns out to vary as a function of the social organization in different species and populations. I then review how the stressful characteristics of social rank have adverse adrenocortical, cardiovascular, reproductive, immunological, and neurobiological consequences. Finally, I consider how these findings apply to the human realm of health, disease, and socioeconomic status.

Dominance status predicts response to nonsocial forced movement stress in the green anole lizard (Anolis carolinensis)

We used changes in body color and eyespot formation, two somatic indices of stress controlled mainly by catecholamine activity, to compare the reactions of dominant and subordinate male green anole lizards (Anolis carolinensis) to a nonsocial stressor, forced movement. Individual males were pretested by subjecting them to 10 min of forced movement induced by chasing them around their home cage with a slender wooden stick. Stress responses were assayed via changes in body color (progressive darkening from green to brown indicating increasing stress) and expression of a black postorbital eyespot (which appears with increasing catecholaminergic stress responses). Lizards were paired and allowed to form stable dominant/subordinate relationships for 2 weeks. After that period of stable social status, dominants and subordinates were separated and subjected to the same forced-movement stress. There was no difference between experimental groups in the pretest. After assuming positions in the dominance hierarchy, however, dominant males showed reduced somatic indicators of stress and were quicker to recover from the stress. The data suggest that animals that assumed the dominant position decreased their stress response relative to the pretest, while animals that assumed the subordinate position increased their stress response relative to the pretest. The results indicate that dominant social status may have advantages beyond the realm of social interactions by enhancing an individual's ability to tolerate other, nonsocial stressful events.

A pacific culture among wild baboons: its emergence and transmission

Reports exist of transmission of culture in nonhuman primates. We examine this in a troop of savanna baboons studied since 1978. During the mid-1980s, half of the males died from tuberculosis; because of circumstances of the outbreak, it was more aggressive males who died, leaving a cohort of atypically unaggressive survivors. A decade later, these behavioral patterns persisted. Males leave their natal troops at adolescence; by the mid-1990s, no males remained who had resided in the troop a decade before. Thus, critically, the troop's unique culture was being adopted by new males joining the troop. We describe (a) features of this culture in the behavior of males, including high rates of grooming and affiliation with females and a "relaxed" dominance hierarchy; (b) physiological measures suggesting less stress among low-ranking males; (c) models explaining transmission of this culture; and (d) data testing these models, centered around treatment of transfer males by resident females.

Reproduction and resistance to stress: when and how

Environmental and social stresses have deleterious effects on reproductive function in vertebrates. Global climate change, human disturbance and endocrine disruption from pollutants are increasingly likely to pose additional stresses that could have a major impact on human society. Nonetheless, some populations of vertebrates (from fish to mammals) are able to temporarily resist environmental and social stresses, and breed successfully. A classical trade-off of reproductive success for potential survival is involved. We define five examples. (i) Aged individuals with minimal future reproductive success that should attempt to breed despite potential acute stressors. (ii) Seasonal breeders when time for actual breeding is so short that acute stress should be resisted in favour of reproductive success. (iii) If both members of a breeding pair provide parental care, then loss of a mate should be compensated for by the remaining individual. (iv) Semelparous species in which there is only one breeding period followed by programmed death. (v) Species where, because of the transience of dominance status in a social group, individuals may only have a short window of opportunity for mating. We suggest four mechanisms underlying resistance of the gonadal axis to stress. (i) Blockade at the central nervous system level, i.e. an individual no longer perceives the perturbation as stressful. (ii) Blockade at the level of the hypothalamic-pituitary-adrenal axis (i.e. failure to increase secretion of glucocorticosteroids). (iii) Blockade at the level of the hypothalamic-pituitary-gonad axis (i.e. resistance of the reproductive system to the actions of glucocorticosteroids). (iv) Compensatory stimulation of the gonadal axis to counteract inhibitory glucocorticosteroid actions. Although these mechanisms are likely genetically determined, their expression may depend upon a complex interaction with environmental factors. Future research will provide valuable information on the biology of stress and how organisms cope. Such mechanisms would be particularly insightful as the spectre of global change continues to unfold.

Individual differences versus social dynamics in the formation of animal dominance hierarchies

Linear hierarchies, the classical pecking-order structures, are formed readily in both nature and the laboratory in a great range of species including humans. However, the probability of getting linear structures by chance alone is quite low. In this paper we investigate the two hypotheses that are proposed most often to explain linear hierarchies: they are predetermined by differences in the attributes of animals, or they are produced by the dynamics of social interaction, i.e., they are self-organizing. We evaluate these hypotheses using cichlid fish as model animals, and although differences in attributes play a significant part, we find that social interaction is necessary for high proportions of groups with linear hierarchies. Our results suggest that dominance hierarchy formation is a much richer and more complex phenomenon than previously thought, and we explore the implications of these results for evolutionary biology, the social sciences, and the use of animal models in understanding human social organization.

Response of fecal cortisol to stress in captive chimpanzees (Pan troglodytes)

This study examined whether fecal cortisol could be used as an index of stress responses. The stress responsiveness of fecal cortisol was tested with a stressor known to stimulate adrenal activity, the stress of anesthesia. Daily fecal and urine samples were collected from four captive chimpanzees (Pan troglodytes) before and after anesthetizations with Telazol/Ketasat. Tests of assay validity indicated that cortisol was measurable in chimpanzee fecal extracts. Fecal cortisol concentrations were significantly elevated 2 days after anesthetization, with elevations in seven of the eight treatments. The posttreatment peak was significantly greater than baseline values in three of the four subjects. Both fecal concentrations and proportionate increases in responses to stress were significantly correlated with the corresponding values in urinary cortisol, confirming the stressfulness of these procedures and the stress responsiveness of fecal cortisol. These findings provide evidence for the application of fecal cortisol as a noninvasive index of physiologic stress in nonhuman primates.

Darting terrestrial primates in the wild: a primer

While a goal of many field primatologists is to observe subjects in as undisturbed a setting as possible, it is often necessary to anesthetize animals for any of a variety of reasons. In this paper, we review techniques for anesthetizing wild primates, based on our experience with more than a thousand such procedures carried out on baboons in East Africa. We consider the following: 1) rationales for anesthetizing a wild primate; 2) systems for the delivery of anesthetic and choice of anesthetic; 3) the darting process itself and issues relevant to the period between darting and the safe removal of the animal; 4) handling of an anesthetized primate; 5) medical complications associated with darting; 6) when to reanesthetize an animal; 7) the process of recovery from anesthesia and release of an awake animal; 8) safety issues for humans. The range of information that can be obtained through field anesthetizations, when carried out successfully, as well as the potential benefit for the animals involved, can be enormous. However, this process is not trivial from the standpoint of the dangers involved to both the subjects and to humans and because of the potential disruption of social behavior to the group. As such, anesthetizations in the field should not be carried out without a strong rationale and without a tremendous priority given to the safety and care of all involved.

Pituitary-adrenocortical responses to the first dyadic encounters in male rhesus monkeys: effect of dominance relationship

Male rhesus monkeys unfamiliar with each other were paired in a cage, and blood samples were collected before and a few hours after pair formation. Adrenocorticotrophic hormone (ACTH) and cortisol levels in each blood sample were measured. Dominant-subordinate status was ascertained through two rank tests, the food competition test and the agonistic behavior test, which were performed immediately after pair formation. As a result, the dominance relationship was determined in seven pairs formed from five animals, and the differences in ACTH and cortisol values between the dominant and subordinate animal in these pairs were compared statistically. The day after the first encounter, a second encounter was conducted in randomly selected pairs of monkeys. In the first encounters, higher levels of both ACTH and cortisol were detected in dominant animals in comparison to subordinate animals. Changing the animal's partner altered the stress responses whenever the animal's dominant-subordinate status changed. The elevated levels of ACTH and cortisol in dominant animals disappeared on the day after the first encounter. In dominant animals, the pituitary-adrenocortical stress response reacts sharply to situational demands, whereas subordinate animals have a weaker response. This acute stress response is different from a chronic stress response. When the subordinate animal cannot escape, its hypothalamic-pituitary-adrenocortical axis appears to be suppressed.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.