Field endocrinology of nonhuman primates: past, present, and future

Peptides and primate personality: Central and peripheral oxytocin and vasopressin levels and social behavior in two baboon species (Papio hamadryas and Papio anubis)

The neurohormones oxytocin (OT) and arginine vasopressin (AVP) are involved in social behaviors and psychiatric conditions. However, more research on nonhuman primates with complex social behaviors is needed. We studied two closely-related primate species with divergent social and mating systems; hamadryas baboons (Papio hamadryas, n=38 individuals) and anubis baboons (Papio anubis, n=46). We measured OT in cerebrospinal fluid (CSF, n=75), plasma (n=81) and urine (n=77), and AVP in CSF (n=45), and we collected over 250 hours of focal behavioral observations. Using Bayesian multivariate models, we found no clear species difference in hormone levels; the strongest support was for hamadryas having higher CSF OT levels than anubis (posterior probability [PP] for females = 0.75, males = 0.84). Looking at nine specific behaviors, OT was associated with affiliative behaviors (approach, proximity, grooming, PP ∼ 0.85 - 1.00), albeit inconsistently across sources of measurement (CSF, plasma, and urine, which were uncorrelated with each other). Most behaviors had low repeatability (R ∼ 0 - 0.2), i.e. they did not exhibit stable between-individual differences (or "personality"), and different behaviors did not neatly coalesce into higher-order factors (or "behavioral syndromes"), which cautions against the use of aggregate behavioral measures and highlights the need to establish stable behavioral profiles when testing associations with baseline hormone levels. In sum, we found some associations between peptides and social behavior, but also many null results, OT levels from different sources were uncorrelated, and our behavioral measures did not indicate clear individual differences in sociability.

Natural Animal Populations as Model Systems for Understanding Early Life Adversity Effects on Aging

Adverse experiences in early life are associated with aging-related disease risk and mortality across many species. In humans, confounding factors, as well as the difficulty of directly measuring experiences and outcomes from birth till death, make it challenging to identify how early life adversity impacts aging and health. These challenges can be mitigated, in part, through the study of non-human animals, which are exposed to parallel forms of adversity and can age similarly to humans. Furthermore, studying the links between early life adversity and aging in natural populations of non-human animals provides an excellent opportunity to better understand the social and ecological pressures that shaped the evolution of early life sensitivities. Here, we highlight ongoing and future research directions that we believe will most effectively contribute to our understanding of the evolution of early life sensitivities and their repercussions.

Developmental and environmental modulation of fecal thyroid hormone levels in wild Assamese macaques (Macaca assamensis)

Thyroid hormones are key modulators of development, as well as mediators of environmental conditions, by regulating developmental processes and metabolism in primates. Hormone measurement in noninvasively collected samples, that is, feces and urine, is a valuable tool for studying the endocrine function of wildlife, and recent studies have demonstrated the feasibility of measuring thyroid hormones in fecal samples of zoo-housed and wild nonhuman primates. Our study aimed to (i) validate the measurement of immunoreactive fecal total triiodothyronine (IF-T3) in wild Assamese macaques (Macaca assamensis) and (ii) to investigate its developmental changes and its response to environmental changes, including stress responses, in immature individuals. Fecal samples and environmental parameters were collected from individuals of three social groups of wild Assamese macaques living at Phu Khieo Wildlife Sanctuary, Northeastern Thailand. Our study confirmed the methodological feasibility and biological validity of measuring IF-T3 in this population. Specifically, the biological validation demonstrated higher IF-T3 levels in immatures compared to adults, and higher levels in females during late gestation compared to the preconception stage. Our analysis of IF-T3 levels in developing immature macaques revealed a significant increase with age. Furthermore, we found a positive association between IF-T3 and immunoreactive fecal glucocorticoid levels, an indicator of the physiological stress response. Neither minimum temperature nor fruit abundance predicted variation in IF-T3 levels in the immatures. Our findings indicate the possibility for differing effects of climatic factors and food availability on thyroid hormone level changes in immature versus adult animals and in wild compared to experimental conditions. Overall, our study provides the basis for further investigations into the role of thyroid hormones in shaping species-specific traits, growth, and overall primate development.

The promise of great apes as model organisms for understanding the downstream consequences of early life experiences

Early life experiences have a significant influence on adult health and aging processes in humans. Despite widespread interest in the evolutionary roots of this phenomenon, very little research on this topic has been conducted in humans' closest living relatives, the great apes. The longitudinal data sets that are now available on wild and captive great ape populations hold great promise to clarify the nature, evolutionary function, and mechanisms underlying these connections in species which share key human life history characteristics. Here, we explain features of great ape life history and socioecologies that make them of particular interest for this topic, as well as those that may limit their utility as comparative models; outline the ways in which available data are complementary to and extend the kinds of data that are available for humans; and review what is currently known about the connections among early life experiences, social behavior, and adult physiology and biological fitness in our closest living relatives. We conclude by highlighting key next steps for this emerging area of research.

Measurement of Salivary Cortisol in Two New World Primate Species

Glucocorticoids (GCs) are mammalian steroid hormones involved in a variety of physiological processes, including metabolism, the immune response, and cardiovascular functions. Due to their link to the physiological stress response, GC measurement is a valuable tool for conservation and welfare assessment in animal populations. GC levels can be measured from different matrices, such as urine and feces. Moreover, especially in captive settings, measuring GCs from saliva samples proved particularly useful as those samples can be collected non-invasively and easily from trained animals. Salivary GC levels can be measured using a variety of analytical methods, such as enzyme immunoassays. However, it is crucial to validate the analytical method for each specific application and species when using a new matrix. Using high-pressure liquid chromatography and a cortisol enzyme immunoassay, we show that the main glucocorticoids secreted in the saliva of squirrel monkeys and brown capuchin monkeys are cortisol and cortisone. Our biological validation found the expected salivary cortisol level to decline throughout the day. Our findings support the reliability of salivary cortisol measurements and their potential to be used as a valid tool in research and welfare assessment for these non-human primates.

Hormonal responses to mating competition in male Tonkean macaques

Glucocorticoid and androgen hormones play a prominent role in male reproductive effort. Their production usually increases in non-human primates during mating competition, which may include rivalry for access to receptive females, struggles for high dominance rank, or social pressure on low-ranking individuals. It is generally assumed that glucocorticoids and androgens are associated with mating challenges rather than dominance status, but the involvement of multiple factors makes it difficult to disentangle the two. In this regard, Tonkean macaques provide a suitable model because they are characterized by relaxed dominance and year-round breeding, meaning that there is typically no more than one receptive female in a group, and thus first-ranking males can easily monopolize her. We studied two captive groups of Tonkean macaques over an 80-month period, recording the reproductive status of females, collecting urine from males and sampling behaviors in both sexes. Male urinary hormone concentrations could be affected by increased competition caused by the mating period, the number of males and the degree of female attractiveness. The highest increases in androgens were recorded in males performing female mate-guarding. Despite the importance of dominance status in determining which males can mate, we found no significant effect of male rank on glucocorticoids and only a marginal effect on androgens during mate-guarding. Both types of hormones were more directly involved in the mating effort of males than in their dominance status. Our results show that their function can be understood in light of the particular competitive needs generated by the species-specific social system.

Measuring short-term changes in stress-associated salivary analytes in free-ranging Japanese macaques (Macaca fuscata)

Monitoring short-term changes of endocrine responses in non-human primates living in wild populations is a challenge. Saliva contains enzymes, steroids, and various analytes that can be broadly useful for helping us understand physiological responses to social and environmental sources of stress as well as other compromises to an individual's health homeostasis. Salivary alpha-amylase and salivary cortisol are known to respond rapidly to stress, which can allow us to use these analytes to monitor stress on a time scale of minutes. We developed a non-invasive methodology for the collection of saliva and verified its applicability by conducting short-term interval sampling of focal individuals under varying social conditions in a group of semi-provisioned free-ranging Japanese macaques (Macaca fuscata) living endemically on the island of Koshima, Miyazaki prefecture, Japan. Monkeys were habituated to sampling by using ingestive attractants applied to cotton ropes. Their receptivity to chewing on the saliva sampling material provided and chewing time was recorded ad libitum during the initial habituation period of the troop to the sampling procedure. Focal animal behavior observations were conducted in conjunction with saliva sampling. Salivary analytes associated with stress, alpha-amylase and cortisol, were measured via enzyme immunoassay. Short-term changes in salivary alpha-amylase and salivary cortisol were examined in relation to short-term changes in social behavior, in particular aggression or grooming. We tested flow-rate effects in both analytes and found strong correlations between original sample results and their respective flow-rate transformed equivalents. Additionally, temperature effects on samples were tested and both analytes showed nearly the same values when stored at - 20, 4, and 30 °C conditions for 6 h; important information expected to facilitate future sampling in field conditions where freezer storage is uncertain. Our study shows that saliva can be repeatedly sampled non-invasively to investigate short-term changes in stress-associated markers in free-ranging Japanese macaques.

Minimally invasive biomarkers in human and non-human primate evolutionary biology: Tools for understanding variation and adaptation

BACKGROUND

The use of minimally invasive biomarkers (MIBs - physiological biomarkers obtained from minimally invasive sample types) has expanded rapidly in science and medicine over the past several decades. The MIB approach is a methodological strength in the field of human and non-human primate evolutionary biology (HEB). Among humans and our closest relatives, MIBs provide unique opportunities to document phenotypic variation and to operationalize evolutionary hypotheses.

AIMS

This paper overviews the use of MIBs in HEB. Our objectives are to (1) highlight key research topics which successfully implement MIBs, (2) identify promising yet under-investigated areas of MIB application, and (3) discuss current challenges in MIB research, with suggestions for advancing the field.

DISCUSSION AND CONCLUSIONS

A range of MIBs are used to investigate focal topics in HEB, including energetics and life history variation/evolution, developmental plasticity, and social status and dominance relationships. Nonetheless, we identify gaps in existing MIB research on traits such as physical growth and gut function that are central to the field. Several challenges remain for HEB research using MIBs, including the need for additional biomarkers and methods of assessment, robust validations, and approaches that are standardized across labs and research groups. Importantly, researchers must provide better support for adaptation and fitness effects in hypothesis testing (e.g., by obtaining complementary measures of energy expenditure, demonstrating redundancy of function, and performing lifetime/longitudinal analyses). We point to continued progress in the use of MIBs in HEB to better understand the past, present, and future of humans and our closest primate relatives.

Plasma Testosterone and Androstenedione Levels Follow the Same Sex-Specific Patterns in the Two Pan Species

Simple Summary

Generally male mammals are more aggressive than their female peers. In these males, aggressive behavior is linked to levels of androgens; higher levels of testosterone are predictive of higher aggression rates or more severe aggression. There are some species where the pattern of sex-specific aggression is reversed, and it was hypothesized that high levels of androgens may be responsible for social dominance and aggressiveness in these females. Studies so far found that females of species with sex-reversed aggression patterns (e.g., spotted hyenas and ring-tailed lemurs) had lower plasma testosterone levels than their male peers, but a precursor of testosterone, androstenedione, was comparable or even higher in females than in males. This supported the idea that selection for female aggressiveness may be facilitated through augmented androgen secretion. Here we show that in two sister species, bonobos and chimpanzees, that differ in terms of sex-specific aggression patterns, females have lower plasma testosterone levels and higher plasma androstenedione levels than their male peers. Thus, our data do not support a theory of a role of female androgen levels on the expression of sex-specific patterns of aggression.

Abstract

In most animals, males are considered more aggressive, in terms of frequency and intensity of aggressive behaviors, than their female peers. However, in several species this widespread male-biased aggression pattern is either extenuated, absent, or even sex-reversed. Studies investigating potential neuro-physiological mechanisms driving the selection for female aggression in these species have revealed an important, but not exclusive role of androgens in the expression of the observed sex-specific behavioral patterns. Two very closely related mammalian species that markedly differ in the expression and degree of sex-specific aggression are the two Pan species, where the chimpanzee societies are male-dominated while in bonobos sex-biased aggression patterns are alleviated. Using liquid chromatography–mass spectrometry (LC-MS) methods, we measured levels of plasma testosterone and androstenedione levels in male and female zoo-housed bonobos (N = 21; 12 females, 9 males) and chimpanzees (N = 41; 27 females, 14 males). Our results show comparable absolute and relative intersexual patterns of blood androgen levels in both species of Pan. Plasma testosterone levels were higher in males (bonobos: females: average 0.53 ± 0.30 ng/mL; males 6.70 ± 2.93 ng/mL; chimpanzees: females: average 0.40 ± 0.23 ng/mL; males 5.84 ± 3.63 ng/mL) and plasma androstenedione levels were higher in females of either species (bonobos: females: average 1.83 ± 0.87 ng/mL; males 1.13 ± 0.44 ng/mL; chimpanzees: females: average 1.84 ± 0.92 ng/mL; males 1.22 ± 0.55 ng/mL). The latter result speaks against a role of androstenedione in the mediation of heightened female aggression, as had been suggested based on studies in other mammal species where females are dominant and show high levels of female aggressiveness.

Testing the role of testosterone versus estrogens in mediating reproductive transitions in female rhesus macaques

In male vertebrates, testosterone is generally known to coordinate reproductive trade-offs, in part by promoting the transition to the next reproduction at the expense of current parental care. The role of testosterone in reproductive transitions has been little tested in female vertebrates, especially in mammals. The present study sought to fill this gap, by first undertaking an experimental study, in which we identified DHT, androstenediol, and in particular etiocholanolone, as fecal androgen metabolites which reflect serum testosterone concentration in female rhesus macaques (Macaca mulatta). Using concentrations of fecal etiocholanolone as proxy for circulating testosterone, we then conducted a field study on 46 free-ranging rhesus macaques of Cayo Santiago, Puerto Rico, to test if testosterone mediates the trade-off between reproductive transition (a higher chance of reproducing in the next year) and current reproduction (providing more care to current offspring). While the evidence for testosterone was weak, the testing of fecal immunoreactive estrogen metabolites suggested a potential role of estrogen in reproductive trade-offs. We found large individual differences in fecal etiocholanolone concentrations during the early postpartum period that were unexplained even after accounting for sociodemographic factors such as age and dominance rank. Further investigation is needed to understand this variation. Our study suggests that the actions of testosterone in females may not have evolved to fulfil the same role in primate reproductive transitions as it does in males, and we encourage more studies to consider the function of testosterone in reproductive behaviors and life history transitions in females of mammalian taxa.

Noninvasive Technologies for Primate Conservation in the 21st Century

Observing and quantifying primate behavior in the wild is challenging. Human presence affects primate behavior and habituation of new, especially terrestrial, individuals is a time-intensive process that carries with it ethical and health concerns, especially during the recent pandemic when primates are at even greater risk than usual. As a result, wildlife researchers, including primatologists, have increasingly turned to new technologies to answer questions and provide important data related to primate conservation. Tools and methods should be chosen carefully to maximize and improve the data that will be used to answer the research questions. We review here the role of four indirect methods—camera traps, acoustic monitoring, drones, and portable field labs—and improvements in machine learning that offer rapid, reliable means of combing through large datasets that these methods generate. We describe key applications and limitations of each tool in primate conservation, and where we anticipate primate conservation technology moving forward in the coming years.

Associations between fecal cortisol and biparental care in a pair-living primate

OBJECTIVES

We quantified variation in fecal cortisol across reproductive periods in Azara's owl monkeys (Aotus azarae) to examine physiological mechanisms that may facilitate biparental care. Specifically, we evaluated evidence for the explanation that owl monkeys have hormonal mechanisms to mobilize energy during periods when each sex is investing heavily in reproduction, that is, the gestation period for females and the infant care period for males.

MATERIALS AND METHODS

Between 2011 and 2015, we monitored 10 groups of Azara's owl monkeys from a wild population in Formosa, Argentina and collected fecal samples from 26 adults (13 males, 13 females). Using enzyme-linked immunosorbent assays, we quantified fecal cortisol as a proxy for evaluating stress responses, including energetic demands, on both sexes during periods of reproduction and parental care.

RESULTS

Male cortisol was lowest during periods when they were caring for young infants (<3 months) compared with periods with older infants or no infant. Female cortisol was elevated during gestation compared with other periods. Mean fecal cortisol in both males and females was lower when an infant was present compared with when females were gestating.

DISCUSSION

Our results do not support the hypothesis that owl monkey males have elevated fecal cortisol during periods when they need to mobilize energy to provide intensive infant care. Our findings are also inconsistent with the Maternal Relief hypothesis. However, results from studies measuring fecal cortisol must be interpreted with care and alternative explanations, such as seasonal fluctuations in diet and thermoenergic demands, should be considered when drawing conclusions.

The Gombe Ecosystem Health Project: 16 years of program evolution and lessons learned

Infectious disease outbreaks pose a significant threat to the conservation of chimpanzees (Pan troglodytes) and all threatened nonhuman primates. Characterizing and mitigating these threats to support the sustainability and welfare of wild populations is of the highest priority. In an attempt to understand and mitigate the risk of disease for the chimpanzees of Gombe National Park, Tanzania, we initiated a long-term health-monitoring program in 2004. While the initial focus was to expand the ongoing behavioral research on chimpanzees to include standardized data on clinical signs of health, it soon became evident that the scope of the project would ideally include diagnostic surveillance of pathogens for all primates (including people) and domestic animals, both within and surrounding the National Park. Integration of these data, along with in-depth post-mortem examinations, have allowed us to establish baseline health indicators to inform outbreak response. Here, we describe the development and expansion of the Gombe Ecosystem Health project, review major findings from the research and summarize the challenges and lessons learned over the past 16 years. We also highlight future directions and present the opportunities and challenges that remain when implementing studies of ecosystem health in a complex, multispecies environment.

Exercise Endocrinology: "What Comes Next?"

Endocrinology is a branch of physiology medical science that many exercise scientists are embracing in their research pursuits, so much so that “exercise endocrinology” is now viewed by many as a viable subdiscipline in the field [...]

Getting Humans Off Monkeys' Backs: Using Primate Acclimation as a Guide for Habitat Management Efforts

Wild primates face grave conservation challenges, with habitat loss and climate change projected to cause mass extinctions in the coming decades. As large-bodied Neotropical primates, mantled howling monkeys (Alouatta palliata) are predicted to fare poorly under climate change, yet are also known for their resilience in a variety of environments, including highly disturbed habitats. We utilized ecophysiology research on this species to determine the morphological, physiological, and behavioral mechanisms howlers employ to overcome ecological challenges. Our data show that howlers at La Pacifica, Costa Rica are capable of modifying body size. Howlers displayed reduced mass in warmer, drier habitats, seasonal weight changes, frequent within-lifetime weight fluctuations, and gradual increases in body mass over the past four decades. These within-lifetime changes indicate a capacity to modify morphology in a way that can impact animals' energetics and thermodynamics. Howlers are also able to consume foods with a wide variety of food material properties by altering oral processing during feeding. While this capability suggests some capacity to cope with the phenological shifts expected from climate change and increased habitat fragmentation, data on rates of dental microwear warn that these acclimations may also cost dental longevity. Lastly, we found that howlers are able to acclimate to changing thermal pressures. On shorter-term daily scales, howlers use behavioral mechanisms to thermoregulate, including timing activities to avoid heat stress and utilizing cool microhabitats. At the seasonal scale, animals employ hormonal pathways to influence heat production. These lines of evidence cumulatively indicate that howlers possess morphological, physiological, and behavioral mechanisms to acclimate to environmental challenges. As such, howlers' plasticity may facilitate their resilience to climate change and habitat loss. While habitat loss in the tropics is unlikely to abate, our results point to a potential benefit of active management and selective cultivation to yield large, interconnected forest fragments with targeted phenology that provides both a complex physical structure and a diversity of food sources. These steps could assist howlers in using their natural acclimation potential to survive future conservation threats.

Adrenal androgens, adrenarche, and zona reticularis: A human affair?

In humans, reticularis cells of the adrenal cortex fuel the production of androgen steroids, constituting the driver of numerous morphological changes during childhood. These steps are considered a precocious stage of sexual maturation and are grouped under the term "adrenarche". This review describes the molecular and enzymatic characteristics of the zona reticularis, along with the possible signals and mechanisms that control its emergence and the associated clinical features. We investigate the differences between species and discuss new studies such as genetic lineage tracing and transcriptomic analysis, highlighting the rodent inner cortex's cellular and molecular heterogeneity. The recent development and characterization of mouse models deficient for Prkar1a presenting with adrenocortical reticularis-like features prompt us to review our vision of the mouse adrenal gland maturation. We expect these new insights will help increase our understanding of the adrenarche process and the pathologies associated with its deregulation.

Exploring the Utility of Hair Endocannabinoids for Monitoring Homeostasis in Bonobos

AbstractQuantifying physiological challenges has gained increasing importance in evolutionary biology, behavioral physiology, and conservation. One matrix that is particularly useful for obtaining long-term records of physiological changes in mammals is hair. Potential markers are components of the endocannabinoid (EC) system, which regulates homeostasis of the brain as well as the endocrine and immune systems. Here, we present results from the first study to measure ECs (anandamide [AEA], 2-archidonyl glycerol [2-AG]) and EC-like compounds (N-palmitoylethanolamine [PEA], N-oleoylethanolamine [OEA], N-stearoylethanolamine [SEA]) in the hair of a nonhuman primate. We found that AEA, SEA, PEA, and OEA can be reliably measured in hair samples. When comparing the measurements of hair from different body parts, we found that variations of some analytes suggest that hair location is likely to affect results. For changes in health status, measurements of ECs and EC-like compounds reflected differences at both intra- and interindividual levels. We concluded that the EC system potentially provides novel tools to assess well-being, health status, and metabolic stress-not only in the hair of humans but also in that of domestic and wild animals. Measuring changes in ECs and EC-like compounds may improve the long-term monitoring of health status in captive and wild primates and may serve as a useful measure in animal welfare programs.

Patterns of urinary cortisol levels during ontogeny appear population specific rather than species specific in wild chimpanzees and bonobos

Compared with most mammals, postnatal development in great apes is protracted, presenting both an extended period of phenotypic plasticity to environmental conditions and the potential for sustained mother-offspring and/or sibling conflict over resources. Comparisons of cortisol levels during ontogeny can reveal physiological plasticity to species or population specific socioecological factors and in turn how these factors might ameliorate or exaggerate mother-offspring and sibling conflict. Here, we examine developmental patterns of cortisol levels in two wild chimpanzee populations (Budongo and Taï), with two and three communities each, and one wild bonobo population (LuiKotale), with two communities. Both species have similar juvenile life histories. Nonetheless, we predicted that key differences in socioecological factors, such as feeding competition, would lead to interspecific variation in mother-offspring and sibling conflict and thus variation in ontogenetic cortisol patterns. We measured urinary cortisol levels in 1394 samples collected from 37 bonobos and 100 chimpanzees aged up to 12 years. The significant differences in age-related variation in cortisol levels appeared population specific rather than species specific. Both bonobos and Taï chimpanzees had comparatively stable and gradually increasing cortisol levels throughout development; Budongo chimpanzees experienced declining cortisol levels before increases in later ontogeny. These age-related population differences in cortisol patterns were not explained by mother-offspring or sibling conflict specifically; instead, the comparatively stable cortisol patterns of bonobos and Taï chimpanzees likely reflect a consistency in experience of competition and the social environment compared with Budongo chimpanzees, where mothers may adopt more variable strategies related to infanticide risk and resource availability. The clear population-level differences within chimpanzees highlight potential intraspecific flexibility in developmental processes in apes, suggesting the flexibility and diversity in rearing strategies seen in humans may have a deep evolutionary history.

Assessing the influence of biotic, abiotic, and social factors on the physiological stress of a large Neotropical primate in Atlantic forest fragments

Wildlife physiological responses to environmental and human-related stressors provide useful clues on animal welfare. Non-invasive biomarkers, such as fecal glucocorticoid metabolites (fGCM), allow researchers to assess whether variations in habitat quality, behavior, and climate influence the animals' physiological stress. We examined the role of fragment size, ambient temperature, ripe fruit availability and consumption, percentage of records moving, sex, female reproductive state, and group composition as predictors of the level of fGCM in adult brown howler monkeys (Alouatta guariba clamitans) inhabiting three small (<10 ha) and three large (>90 ha) Atlantic Forest fragments in southern Brazil. We collected bimonthly behavioral data and fecal samples from adult individuals over three years, and used a multimodel inference framework to identify the main predictors of fGCM. We found that the mean (±SD) fGCM in the study groups ranged from 57 ± 49 ng/g to 93 ± 58 ng/g, which were within the known range for howler monkeys. We found 10 best models including five of the 17 tested variables. Sex and reproductive state were the only variables included in all these models. We found that fGCM was higher in nursing females (mean ± SD = 104 ± 73 ng/g) than in non-nursing females (64 ± 55 ng/g) and males (53 ± 40 ng/g, P < 0.05) and that it decreased with increasing ripe fruit consumption and minimum temperature. However, fragment size did not predict fGCM concentration (groups in small fragments = 71 ± 58 ng/g vs. groups in large fragments = 63 ± 54 ng/g, P > 0.05). We conclude that factors related to the energetic balance of individuals play major roles in modulating the physiological stress of brown howler monkeys. Future studies should investigate the consequences of higher levels of stress hormones on howler monkey health and demography.

Measuring urinary cortisol and testosterone levels in male Barbary macaques: A comparison of EIA and LC-MS

The development of methods to quantify hormones from non-invasively collected samples such as urine or feces has facilitated endocrinology research on wild-living animals. To ensure that hormone measurements are biologically meaningful, method validations are strongly recommended for each new species or sample matrix. Our aim was to validate three commonly used enzyme immunoassays (EIA), one for analysis of cortisol and two for analysis of testosterone, to assess adrenocortical and gonadal endocrine activity, respectively, from the urine of male Barbary macaques. We compared EIA and liquid chromatography-mass spectrometry (LC-MS) results to determine if the EIA measurements truly reflect levels of the target hormone and to determine if antibody cross-reactivities with other steroids were potentially confounding results. Furthermore, we conducted a biological validation of testosterone to ensure that both EIA and LC-MS were able to capture physiologically meaningful differences in hormone levels. We found that cortisol measured by EIA correlated strongly with cortisol measured by LC-MS in both adult and immature males, without the need for deconjugation of steroids in the urine. Both testosterone EIAs correlated strongly with LC-MS in adult males, but only if steroids in the urine were deconjugated by enzymatic hydrolysis prior to analysis. However, in immature males, EIA and LC-MS results did not correlate significantly. Further correlation analyses suggest this is likely due to cross-reactivity of the testosterone antibodies with other adrenal steroids such as cortisol, DHEA, and likely others, which are present at much higher concentrations relative to testosterone in immature males. Testosterone levels were significantly higher in adult compared to immature males as measured by LC-MS but not as measured by EIA. Taken together, our results suggest that the testosterone EIAs are suitable to assess gonadal activity in adult but not immature males, and only if a hydrolysis of the urine is conducted prior to analysis.

Repeatable glucocorticoid expression is associated with behavioural syndromes in males but not females in a wild primate

Behavioural syndromes are a well-established phenomenon in human and non-human animal behavioural ecology. However, the mechanisms that lead to correlations among behaviours and individual consistency in their expression at the apparent expense of behavioural plasticity remain unclear. The 'state-dependent' hypothesis posits that inter-individual variation in behaviour arises from inter-individual variation in state and that the relative stability of these states within an individual leads to consistency of behaviour. The endocrine stress response, in part mediated by glucocorticoids (GCs), is a proposed behavioural syndrome-associated state as GC levels are linked to an individual's behavioural responses to stressors. In this study, in wild Barbary macaques (Macaca sylvanus), consistent inter-individual differences were observed in both sexes for GC activity (faecal glucocorticoid, fGC concentrations), but not GC variation (coefficient of variation in fGC concentrations). The expression of the behavioural syndrome 'Excitability' (characterized by the frequencies of brief affiliation or aggressive interactions) was related to GC activity in males but not in females; more 'excitable' males had lower GC activity. There was no relationship in females between any of the behavioural syndromes and GC activity, nor in either sex with GC variation. The negative relationship between GC activity and Excitability in males provides some support for GC expression as a behavioural syndrome-generating state under the state-dependent framework. The absence of this relationship in females highlights that state-behavioural syndrome associations may not be generalizable within a species and that broader sex differences in state need to be considered for understanding the emergence and maintenance of behavioural syndromes.

Data quality and the comparative method: the case of pregnancy failure in rodents

In mammalian species where infanticide by males is likely, females exhibit counterstrategies to prevent or mitigate the costs of infanticide. One putative mitigation strategy is the “Bruce effect,” in which pregnant or inseminated females exposed to an unfamiliar male experience pregnancy block or failure. Females then mate with the new male, thus shifting investment from a “doomed” pregnancy to a more fruitful one. However, the Bruce effect may be an adaptive response to other factors besides infanticide. For example, if paternal care is necessary for offspring survival, and an unfamiliar male replacing the original mate is unlikely to provide such care to offspring of a litter it did not sire, then a female may terminate a pregnancy to initiate a new one. The infanticide and paternal care hypotheses have not been rigorously tested because comparative data on the Bruce effect across mammals are scarce. We compiled data on the Bruce effect, infanticide, and paternal care from one particularly rich source of information, rodents, but found the data set to be less rich than expected. The Bruce effect, infanticide, and paternal care were common among rodent species, but we found no clear relationship among the traits. However, this was likely due to 1) a bias toward positive results, 2) missing data, and 3) a reliance on studies of captive animals. These are common problems in comparative research, and we outline standards that should be implemented to successfully answer questions of importance in the field.

Sex and seasonal differences in mRNA expression of estrogen receptor α (ESR1) in red-sided garter snakes (Thamnophis sirtalis parietalis)

Estrogens are important regulators of reproductive physiology including sexual signal expression and vitellogenesis. For the regulation to occur, the hormone must bind and activate receptors in target tissues, and expression of the receptors can vary by sex and/or season. By simultaneously comparing circulating hormone levels with receptor expression, a more complete understanding of hormone action can be gained. Our study species, the red-sided garter snake (Thamnophis sirtalis parietalis), provides an excellent opportunity to study the interaction between sex steroid hormones and receptor expression in addition to sexual dimorphism and seasonality. During the spring mating season, male garter snakes rely exclusively on the female's skin-based, estrogen-dependent sex pheromone to direct courtship. Males can be stimulated to produce this sexual attractiveness pheromone by treatment with estradiol (E₂), which also induces male vitellogenesis. Estrogen receptors (ESRs) are required to transduce the effects of estrogens, thus we used quantitative RT-PCR to analyze expression of ESR alpha (ERα; gene ESR1) mRNA in the skin and liver of wild caught male and female garter snakes across simulated spring and fall conditions in the laboratory. While ESR1 was present in the skin of both sexes, there were no sex or seasonal differences in expression levels. Liver expression of ESR1, however, was sexually dimorphic, with females showing greatest expression in fall when circulating E₂ concentrations were lowest. There were no statistically significant correlations between E₂ and ESR1 expression. Our data suggest that the skin of both sexes is sensitive to estrogen signaling and thus the production of sex pheromone is dependent on bioavailable levels of E₂. Female expression of ESR1 in the liver may increase in the fall to prime energy storage mechanisms required for vitellogenesis the following year.

Urinary neopterin levels increase and predict survival during a respiratory outbreak in wild chimpanzees (Taï National Park, Côte d'Ivoire)

Monitoring immune system activation of wild animals has garnered increasing interest within the field of ecological immunology, leading to an urgent need for non-invasive biomarkers measuring these changes. Urinary neopterin, a marker of the cell-mediated immune response, is validated as an immune-related biomarker in captive and laboratory animals. However, wild animals naturally host higher and chronic pathogen loads. Therefore, detection and quantification of additional infections via neopterin might not be possible against the background of a chronically challenged immune system. To assess the suitability of urinary neopterin in wild animals, we measured neopterin corrected for specific gravity with an enzyme immunoassay in 185 samples collected before, during and after a respiratory disease outbreak in 28 individuals from a group of wild chimpanzees (Taï National Park, Côte d'Ivoire). Urinary neopterin levels were significantly higher during periods when individuals showed respiratory symptoms versus before and after the outbreak. Furthermore, urinary neopterin levels were significantly higher in individuals that died, with higher levels already apparent before the outbreak, suggesting individuals may have an already activated immune system. Measuring urinary neopterin levels, with other biomarkers of energetic condition, stress challenges, and reproduction will contribute towards a deeper understanding of life-history trade-offs in wild animals.

Applications for non-invasive thyroid hormone measurements in mammalian ecology, growth, and maintenance

Thyroid hormones (THs) play a pivotal role in the regulation of metabolic activity throughout all life stages. Cross-talk with other hormone systems permits THs to coordinate metabolic changes as well as modifications in growth and maintenance in response to changing environmental conditions. The scope of this review is to explain the relevant basics of TH endocrinology, highlight pertinent topics that have been investigated so far, and offer guidance on measuring THs in non-invasively collected matrices. The first part of the review provides an overview of TH biochemistry, which is necessary to understand and interpret the findings of existing studies and to apply non-invasive TH monitoring. The second part focuses on the role of THs in mammalian ecology, and the third part highlights the role of THs in growth and maintenance. The fourth part deals with the advantages and difficulties of measuring THs in non-invasively collected samples. This review concludes with a summary that considers future directions in the study of THs.

Group-level competition influences urinary steroid hormones among wild red-tailed monkeys, indicating energetic costs

Various theories emphasize that intergroup competition should affect intragroup cooperation and social relationships, especially if the cost of intergroup competition outweighs that of intragroup competition. This cost of intergroup competition may be quantified by changes in physiological status, such as in the steroid hormones cortisol (C) and testosterone (T), which rise or are depressed during periods of energetic stress, respectively. Here we tested for changes in urinary C and T after intergroup encounters (IGEs) among wild red-tailed monkeys (Cercopithecus ascanius), a species that experiences frequent intergroup feeding competition, at the Ngogo station in Kibale National Park, Uganda. We assayed 108 urine samples, of which 36 were collected after IGEs, from 23 individuals in four social groups. Bayesian multilevel models controlling for various confounds revealed that IGEs increased C and decreased T relative to baseline, consistent with an energetic cost to IGEs. The C change was more apparent in samples collected early after IGEs, suggesting an anticipatory increase, whereas the T change was stronger in later samples, suggesting sustained energetic trade-offs. Hormone responses were not affected by the IGE outcome. This cost to intergroup competition, together with little evidence for intragroup competition in redtails and other guenons, establishes an interesting test case for theories emphasizing the effect of intergroup competition on intragroup cooperation.

Faecal glucocorticoid metabolite monitoring as a measure of physiological stress in captive and wild vervet monkeys

The development of non-invasive techniques to analyse physiological stress in mammalian species has revolutionised field-based endocrinology. However, careful validation of the methods used to determine faecal glucocorticoid metabolite (fGCM) and other hormone concentrations are required on a species- and sex-specific basis. In this study, we performed an adrenocorticotropic hormone (ACTH) stimulation test on four (two male and two female) captive vervet monkeys (Chlorocebus pygerythrus) to determine the most appropriate enzyme immunoassay (EIA) from a suite of available EIAs. Furthermore, we took advantage of a potentially stressful event in our wild vervet population from Samara Private Game Reserve, South Africa, to examine if an alpha-beta female rank reversal increases the physiological stress of those individuals directly involved, as well as other group members. Both our physiological and biological validation studies revealed that a cortisol assay was the most appropriate EIA for monitoring fGCM alterations in vervet monkeys. In addition, we found that the observed rank-reversal had no significant effect on the physiological stress levels of uninvolved group members. Our study highlights that physiological validation is imperative and, where possible, should be conducted in parallel with a carefully considered biologically-relevant test under natural conditions. Overall, our results provide a necessary step for future studies to examine physiological stress of vervet monkeys via fGCM monitoring by validating a suitable EIA for this species. This paves the way for future research into the health and welfare of both captive and wild vervet monkeys, and will allow researchers to assess the behavioural, social and ecological correlates of physiological stress levels of this species.

The next step for stress research in primates: To identify relationships between glucocorticoid secretion and fitness

Glucocorticoids are hormones that mediate the energetic demands that accompany environmental challenges. It is therefore not surprising that these metabolic hormones have come to dominate endocrine research on the health and fitness of wild populations. Yet, several problems have been identified in the vertebrate research that also apply to the non-human primate research. First, glucocorticoids should not be used as a proxy for fitness (unless a link has previously been established between glucocorticoids and fitness for a particular population). Second, stress research in behavioral ecology has been overly focused on "chronic stress" despite little evidence that chronic stress hampers fitness in wild animals. Third, research effort has been disproportionately focused on the causes of glucocorticoid variation rather than the fitness consequences. With these problems in mind, we have three objectives for this review. We describe the conceptual framework behind the "stress concept", emphasizing that high glucocorticoids do not necessarily indicate a stress response, and that a stress response does not necessarily indicate an animal is in poor health. Then, we conduct a comprehensive review of all studies on "stress" in wild primates, including any study that examined environmental factors, the stress response, and/or fitness (or proxies for fitness). Remarkably, not a single primate study establishes a connection between all three. Finally, we provide several recommendations for future research in the field of primate behavioral endocrinology, primarily the need to move beyond identifying the factors that cause glucocorticoid secretion to additionally focus on the relationship between glucocorticoids and fitness. We believe that this is an important next step for research on stress physiology in primates.

Testosterone and reproductive effort in male primates

Considerable evidence suggests that the steroid hormone testosterone mediates major life-history trade-offs in vertebrates, promoting mating effort at the expense of parenting effort or survival. Observations from a range of wild primates support the "Challenge Hypothesis," which posits that variation in male testosterone is more closely associated with aggressive mating competition than with reproductive physiology. In both seasonally and non-seasonally breeding species, males increase testosterone production primarily when competing for fecund females. In species where males compete to maintain long-term access to females, testosterone increases when males are threatened with losing access to females, rather than during mating periods. And when male status is linked to mating success, and dependent on aggression, high-ranking males normally maintain higher testosterone levels than subordinates, particularly when dominance hierarchies are unstable. Trade-offs between parenting effort and mating effort appear to be weak in most primates, because direct investment in the form of infant transport and provisioning is rare. Instead, infant protection is the primary form of paternal investment in the order. Testosterone does not inhibit this form of investment, which relies on male aggression. Testosterone has a wide range of effects in primates that plausibly function to support male competitive behavior. These include psychological effects related to dominance striving, analgesic effects, and effects on the development and maintenance of the armaments and adornments that males employ in mating competition.

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.