Food search demand effort effects on behavior and cortisol in adult female squirrel monkeys

Effect of food limitation and reproductive activity on fecal glucocorticoid metabolite levels in banded mongooses

Background

Glucocorticoids mediate responses to perceived stressors, thereby restoring homeostasis. However, prolonged glucocorticoid elevation may cause homeostatic overload. Using extensive field investigations of banded mongoose (Mungos mungo) groups in northern Botswana, we assessed the influence of reproduction, predation risk, and food limitation on apparent homeostatic overload (n=13 groups, 1542 samples from 268 animals). We experimentally manipulated reproduction and regulated food supply in captive mongooses, and compared their glucocorticoid responses to those obtained from free-living groups.

Results

At the population level, variation in glucocorticoid levels in free-living mongooses was explained by food limitation: fecal organic matter, recent rainfall, and access to concentrated anthropogenic food resources. Soil macrofauna density and reproductive events explained less and predation risk very little variation in glucocorticoid levels. Reproduction and its associated challenges alone (under regulated feeding conditions) increased glucocorticoid levels 19-fold in a captive group. Among free-living groups, glucocorticoid elevation was seasonal (occurring in late dry season or early wet season when natural food resources were less available), but the timing of peak glucocorticoid production was moderated by access to anthropogenic resources (groups with fewer anthropogenic food sources had peaks earlier in dry seasons). Peak months represented 12- and 16-fold increases in glucocorticoids relative to nadir months with some animals exhibiting 100-fold increases. Relative to the captive group nadir, some free-living groups exhibited 60-fold increases in peak glucocorticoid levels with some animals exhibiting up to 800-fold increases. Most of these animals exhibited 1- to 10-fold increases relative to the captive animal peak.

Conclusions

Banded mongooses exhibit seasonal chronic glucocorticoid elevation, associated primarily with food limitation and secondarily with reproduction. Magnitude and duration of this elevation suggests that this may be maladaptive for some animals, with possible fitness consequences. In late dry season, this population may face a convergence of stressors (food limitation, agonistic encounters at concentrated food resources, evictions, estrus, mate competition, parturition, and predation pressure on pups), which may induce homeostatic overload.

Maternal regulation of the infant's hypothalamic-pituitary-adrenal axis stress response: Seymour 'Gig' Levine's legacy to neuroendocrinology

Thirty years ago, Seymour 'Gig' Levine published a serendipitous, yet, seminal finding with respect to the regulatory role of maternal presence on the corticosterone stress response of neonatal rats during the developmental period known as the stress hyporesponsive period. At the same time, his group of students also investigated the stress response of infant monkeys with respect to maternal separation, as a means of understanding the stress to the primary caregiver resulting from disruptions of attachment. Gig and his group of students and collaborators, mainly in the USA and the Netherlands, investigated how initial social relationships buffer the stress response of nonhuman primates and rodent infants. His work in rodents involved determining how prolonged deprivation of maternal care disinhibits the stress response of neonates and how maternal behaviours regulate specific aspects of the hypothalamic-pituitary-adrenal axis. Maternal deprivation for 24 hours was useful for determining the importance of nutrition in suppressing the corticosterone stress response, whereas anogenital licking and grooming inhibited stress-induced adrenocortoctrophic hormone release, with the combination of both behaviours preventing the effects of maternal deprivation on the central hypothalamic stress response. Levine's group also studied the consequences of maternal deprivation on basal and stress-induced activity of the hypothalamic-pituitary-adrenal axis in juveniles and the persistent effects of the replacement of maternal behaviours on these parameters. Gig's legacy allowed many groups around the world to use the 24-hour maternal deprivation paradigm as an animal model of vulnerability and resilience to stress-related psychiatric disorders, as well as in studies of the neurobiological underpinnings of disruption of the mother-infant relationship and loss of parental care, a highly prevalent condition in humans. This review pays homage to a great scientist and mentor, whose discoveries paved the way for the understanding of how early social relationsships build resilience or lead to susceptibility to emotional disorders later in life.

Transgenerational effects of variable foraging demand stress in female bonnet macaques

Stress coping is an important part of mammalian life, influencing somatic and mental health, social integration, and reproductive success. The experience of early psychological stress helps shape lifelong stress coping strategies. Recent studies have shown that the effects of early stress may not be restricted to the affected generation, but may also be transmitted to offspring. Understanding whether early stress influences development in subsequent generations may help us understand somewhat why many stress-related traits and diseases, for which little genetic basis has been discovered, run in families. Experimental early life "variable foraging demand" (VFD) stress has been associated with behavioral hypo-responsiveness to stress in infant and adolescent bonnet macaques. The present study examined the behavioral effects of experimental early VFD stress in adult bonnet macaques, and further investigated whether non-exposed adolescent offspring of VFD macaques were also affected. Thirty female bonnet macaques from four rearing histories were observed for behavioral response during stress: adults which had been VFD reared as infants (n = 11), adults which had been Control reared as infants (n = 9), and foraging demand naïve adolescents whose mothers were VFD (n = 4) or Control reared (n = 6). Subjects were observed for behavioral response during two experimental stressor conditions, including: (1) relocation to a novel environment; and (2) relocation with exposure to a "human intruder" making eye contact. Factor analysis yielded five factors that described categories of behavior across stress conditions. While adult VFD and Control reared females unexpectedly did not differ significantly, non-exposed adolescent offspring of VFD reared mothers displayed significant hypo-responsiveness in all behavioral categories compared with non-exposed adolescent offspring of Control females. We suggest that stress hypo-responsiveness previously observed in adolescent VFD reared animals may abate with age, but is nonetheless observed in the next generation. We conclude that VFD stress affects behavioral development of subsequent generations in non-human primates.

Seasonal energetic stress in a tropical forest primate: proximate causes and evolutionary implications

Animals facing seasonal variation in food availability experience selective pressures that favor behavioral adjustments such as migration, changes in activity, or shifts in diet. Eclectic omnivores such as many primates can process low-quality fallback food when preferred food is unavailable. Such dietary flexibility, however, may be insufficient to eliminate constraints on reproduction even for species that live in relatively permissive environments, such as moist tropical forests. Focusing on a forest-dwelling primate with a flexible diet (Cercopithecus mitis) we investigated whether females experience seasonal energetic stress and how it may relate to reproductive seasonality. We used fecal glucocorticoids (fGCs) as an indicator of energetic stress, controlling for the potentially confounding effects of social interactions and reproductive state. We modeled within-female fGC variation with General Linear Mixed Models, evaluating changes in feeding behavior and food availability as main effects. Regardless of reproductive state, fGCs increased when females shifted their diet towards fallback foods (mature leaves and other non-preferred items) and when they spent more time feeding, while fGCs decreased with feeding time on preferred items (insects, fruits, young leaves) and with the availability of young leaves. Changes in fruit availability had no general effects on fGCs, likely because fruits were sought out regardless of availability. As predicted, females in the energetically demanding stages of late pregnancy and early lactation showed greater increases in fGCs between periods of low versus high availability of fruits and young leaves than females in other reproductive states. Potential social stressors had no measurable effects on fGCs. Preliminary evidence suggests that seasonal energetic stress may affect the timing of infant independence from mothers and contribute to unusually long inter-birth intervals compared to closely related species of similar body size. Our findings highlight how the study of stress responses can provide insights into the proximate control of reproductive strategies.

Effects of single-use and group-use enrichment on stereotypy and intragroup aggressive and affiliative behaviors of a social group of squirrel monkeys (Saimiri sciureus) at the Singapore Zoo

Four food-based enrichment devices were used to test the effects of single-use and group-use enrichment devices on stereotypy, intragroup aggression, and affiliation in a compatible group of 5 squirrel monkeys (Saimiri sciureus). All enrichment devices were found to reduce overall stereotypic behavior from baseline levels (without enrichment). The occurrence of stereotypic behavior differed between individual squirrel monkeys with an adult female showing the highest level of stereotypic behavior. This individual also showed the highest usage of enrichment devices, and stereotypic behavior was significantly reduced when enrichment was applied. The occurrence of stereotypic behavior did not differ significantly between single-use and group-use enrichment treatments. Higher intragroup aggression and lower affiliation were observed during the provision of enrichment compared with baseline levels. However, aggressive behavior was higher and affiliation lower during single-use enrichment compared with group-use enrichment. The results of this study showed that enrichment had positive effects on alleviating stereotypic behavior in a group of zoo-housed squirrel monkeys and such effects were similar when group-use and single-use enrichment devices were used, but with variations between individuals. The application of enrichment, particularly single-use enrichment devices, elicited higher levels of aggression within the group and lower affiliation. Such effects could curtail the benefits of original enrichment goals as higher intragroup aggression could lead to higher stress levels within the group. When food-based enrichment for social nonhuman primates is implemented, the most appropriate methods to alleviate undesirable behavior without additional negative effects such as increased group aggression should be considered.

Social status, glucocorticoids, immune function, and health: can animal studies help us understand human socioeconomic-status-related health disparities?

For humans in developed nations, socioeconomic status (SES)--relative income, education and occupational position in a society--is a strong predictor of morbidity and mortality rates, with increasing SES predicting longer life span (e.g. Marmot et al., 1991). Mechanisms underlying this relationship have been examined, but the relative role of each mechanism still remains unknown. By understanding the relative role of specific mechanisms that underlie dramatic health disparities between high and low social status individuals we can begin to identify effective, targeted methods to alleviate health disparities. In the current paper, we take advantage of a growing number of animal studies that have quantified biological health-related correlates (glucocorticoid production and immune function) of social status and compare these studies to the current literature on human SES and health to determine if and how animal studies can further our understanding of SES-associated human health disparities. Specifically, we compared social-status related glucocorticoid production and immune function in humans and animals. From the review, we show that our present understanding of the relationships between social status and glucocorticoid production/immune function is still growing, but that there are already identifiable parallels (and non-parallels) between humans and animals. We propose timely areas of future study focused on (1) specific aspects of social status that may influence stress-related physiology, (2) mechanisms underlying long-term influences of social status on physiology and health, and (3) intervention studies to alleviate potentially negative physiological correlates of social status.

Social behavior, foraging strategies, and fecal glucocorticoids in female blue monkeys (Cercopithecus mitis): potential fitness benefits of high rank in a forest guenon

Socioecological theory predicts that aggressive feeding competition is associated with linear dominance hierarchies and reproductive advantages for high-ranking females. Female blue monkeys contest fruits and have a linear dominance hierarchy, yet previous research has shown no evidence that high-ranking females benefit from greater feeding success or fertility. Here, we assess whether individuals differ in fecal glucocorticoid (fGC) excretion and examine proximate determinants of such differences to infer potential fitness correlates of rank, using data collected from two study groups in the Kakamega Forest, Kenya. We found that higher ranking females had preferential access to fruits in both groups, although the behavioral mechanisms leading to this effect varied between groups. Despite a consistent rank difference in feeding on fruits, an overall rank effect on fGCs emerged in only one group; females of this group spent comparatively more time feeding on fruits, fruits accounted for a greater proportion of the diet, and females engaged in more frequent food-related agonism. In addition, more females in this group were lactating during a period of low fruit availability, when rank effects on fGCs were particularly strong. Regardless of fruit availability, among lactating females of both groups higher rank was associated with lower fGC levels, indicating lower energetic stress in higher ranking females when energy demands were particularly high. Individual rates of agonism, a potential psychological stressor, were unrelated to fGCs at all times. After we accounted for rates of agonism and feeding on fruits, females of one group who groomed others more had lower fGCs, suggesting that variable social coping behavior can contribute to fGC variation in some groups. This study provides the first empirical evidence that high-ranking female blue monkeys may obtain fitness benefits from their social status, by gaining priority of access to fruits during critical times in the reproductive cycle.

Fecal glucocorticoids as indicators of metabolic stress in female Sykes' monkeys (Cercopithecus mitis albogularis)

Because of their mediating role in the stress response and potential effects on fitness, glucocorticoid (GC) hormones are increasingly used to assess the physiological costs of environmental and behavioral variation among wild vertebrates. Identifying the proximate causes of GC variation, however, is complicated by simultaneous exposure to multiple potentially stressful stimuli. Here, we use data from a partially provisioned social group of Sykes' monkeys to evaluate the effects of potential psychological and metabolic stressors on temporal and individual variation in fecal GC (fGC) excretion among 11 adult females. Despite high rates of agonism over provisioned foods fGCs declined during periods of high provisioning frequency when fruit availability was dominated by neem (Azadirachta indica), an item requiring great feeding effort. Provisioned foods did not prevent fGC increases when availability of the most preferred main fruit item, tamarind (Tamarindus indica), declined drastically. Although rank-related differences in access to provisioned foods and rates of agonism did not lead to an overall effect of rank on fGCs, low-ranking females excreted more fGCs than high-ranking females during a period of high provisioning intensity and low fruit availability. The emergence of this rank effect was associated with elevated feeding effort in all females, a greater access to provisioned items by high-ranking females, and a higher proportion of time spent moving in low-ranking females. Our findings suggest that metabolic stressors were the primary determinants of both temporal and individual variation in fGCs, indicating potential fitness benefits for high-ranking females when food availability is limited.

Enrichment and aggression in primates

There is considerable evidence that primates housed under impoverished conditions develop behavioural abnormalities, including, in the most extreme example, self-harming behaviour. This has implications for all contexts in which primates are maintained in captivity from laboratories to zoos since by compromising the animals' psychological well-being and allowing them to develop behavioural abnormalities their value as appropriate educational and research models is diminished. This review examines the extensive body of literature documenting attempts to improve living conditions with a view to correcting behavioural abnormalities and housing primates in such a way that they are encouraged to exhibit a more natural range and proportion of behaviours, including less self-directed and social aggression. The results of housing, feeding, physical, sensory and social enrichment efforts are examined with specific focus on their effect on aggressive behaviour and variation in their use and efficacy. It is concluded that while inappropriate or poorly distributed enrichment may encourage aggressive competition, enrichment that is species, sex, age and background appropriate can dramatically reduce aggression, can eliminate abnormal behaviour and substantially improve the welfare of primates maintained in captivity.

Early maternal availability and prefrontal correlates of reward-related memory

Early emotional experiences affect developing brain systems that subsequently mediate adult learning and memory in rodents. Here we test for similar effects in squirrel monkeys (Saimiri sciureus) four years after disruptions in early maternal availability. These conditions were previously shown to generate differences in emotional behavior, hypothalamic-pituitary-adrenal stress physiology, and right ventral medial prefrontal volumes determined in adulthood by magnetic resonance imaging. This report identifies in the same monkeys variability in reward-related memory on tests with a spatial reversal. Adult monkeys that more often selected locations repeatedly rewarded before each reversal had larger right ventral medial prefrontal volumes, but not hippocampal nor dorsolateral prefrontal volumes on the left or right brain side. Differences in performance were also discerned after each spatial reversal. These findings indicate that maternal availability alters developing ventral medial prefrontal brain regions involved in reward-related memory.

Enduring maternal influences in a precocial rodent

The guinea pig is highly developed at birth and requires little active maternal care. Yet the mother and other social figures markedly influence biobehavioral processes of the offspring. Here, responses of guinea pigs and nonhuman primates to maternal and other social separation procedures are compared, and influences of social partners on endocrine responses and behavior in periadolescent guinea pigs are described.

Feeding demand conditions and plasma cortisol in socially-housed squirrel monkey mother-infant dyads

Previous studies have demonstrated that experimentally altering the accessibility and availability of food can have profound impact on behavior and adrenocortical activity in nonhuman primate species. In this study, groups of mother-infant squirrel monkey dyads were housed in either high demand (HFD: 120% normal daily food intake provided), low demand (LFD: 600% normal daily food intake provided) or variable demand (VFD: alternating two-week blocks of low demand and high demand) conditions for 12 weeks. During the 12-week experimental foraging phase, animals in the HFD group exhibited prolonged and consistent cortisol elevations. The cortisol levels in the VFD group reflected the ambient demand condition, with higher levels exhibited during the high demand phases of the study, and lower values when the low demand condition was in effect. Overall, mothers were more affected by the experimental manipulation than were infants. The experimental condition did not affect the infants' response to a 24-h separation from their mothers. A suppression of cortisol levels, particularly in the HFD group, was observed upon resumption of ad-libitum feeding.

Social influences on endocrine activity in guinea pigs, with comparisons to findings in nonhuman primates

Guinea pigs exhibit a rich and varied social organization. Studies in recent years have demonstrated that social stimuli have widespread neuroendocrine effects in guinea pigs. Here, effects on the hypothalamic-pituitary-adrenal, adrenal medullary/sympathetic, and hypothalamic-pituitary-gonadal systems of both adult and developing guinea pigs are reviewed. These systems respond to various social variables, or factors that affect social variables, including: separation from attachment objects, housing conditions, changes in housing, the familiarity of the environment in which social interactions occur, foraging conditions, surrogate-rearing, agonistic interactions, and the establishment of dominance rank. Similarities and differences between these findings and those in nonhuman primates are discussed. It is argued that the guinea pig is well suited for the study of socioendocrine effects throughout the life span, and can provide a valuable complement to nonhuman primate research in this area.

Chronic corticosterone impairs memory performance in the Barnes maze

Chronic stress has been reported to impair spatial memory and cause hippocampal impairment in rodents. Glucocorticoids are believed to be the active agent in this impairment. Studies have demonstrated that chronic glucocorticoid administration results in animals being impaired in the Morris water maze (MWM) or eight-arm radial maze. Although both of these methods are well established means of testing spatial memory, neither might be considered optimal for studying the behavioral effects of stress. The Morris maze is itself highly stressful to the animals. The eight-arm maze relies on a food reward to motivate the animals, and glucocorticoids have profound effects on hunger and satiety. We therefore investigated behavioral deficits of corticosterone-treated animals in the two previously used mazes and the Barnes circular platform maze (BCM), a test similar in design to the Morris maze, but one that does not require the animal to perform a highly stressful swim. Consistent with results in other tests, we found that animals that had been treated for 3 months with stress-equivalent concentrations of glucocorticoids showed significantly impaired behavior in the Barnes maze.

Interspecies variations of corticosteroid-binding globulin parameters

In mammalian plasma, cortisol binds to a specific alpha 1-glycoprotein: corticosteroid-binding globulin (CBG). In this study, we measured the protein binding of cortisol by equilibrium dialysis in seven species in which plasma cortisol concentrations varied from 0.02 to 0.05 (ewe, dog, cow) to 0.1 to 0.6 (horse, human, cynomolgus monkey) to reach 1.6 microM (squirrel monkey). No binding of cortisol to CBG was discernible in plasma from squirrel monkey. In all other species examined, we showed that the CBG maximal capacity (Bmax) was 3 (1.7 to 5.2) times more than the plasma cortisol levels, with cow, dog, ewe exhibiting the lowest and cynomolgus monkey exhibiting the highest values. We also noted the existence of a linear relationship between Bmax and the corresponding dissociation constant (Kd), Bmax being systematically 10 (8.5 to 11.8) times more than Kd. The low binding affinity of cortisol assigned to albumin did not differ between species. The free (6 to 14%), CBG-bound (67 to 87%), and albumin-bound (7 to 19%) cortisol fractions calculated from the estimated binding parameters and measured plasma cortisol concentrations were similar within species, except for squirrel monkey, in which half of the cortisol was albumin bound, and the other half remained protein free. Our most appealing finding was that in most species, as much as 68% of plasma CBG remained free of cortisol under physiologic conditions. These results are discussed with respect to the theories concerning the role of CBG in plasma transport and the local delivery of cortisol and free CBG as a proper hormone.