Normal hypothalamic-pituitary-adrenal (HPA) axis activity leading to rhythmic and episodic release of adrenal glucocorticoids is essential for body homeostasis and survival during stress. Acting through specific intracellular receptors in the brain and periphery, glucocorticoids regulate behavior, metabolic, cardiovascular, immune, and neuroendocrine activities. In contrast to chronic elevated levels, circadian and acute stress-induced increases in glucocorticoids are necessary for hippocampal neuronal survival and memory acquisition and consolidation, through inhibiting apoptosis, facilitating glutamate transmission and inducing immediate early genes and spine formation. In addition to its metabolic actions leading to increasing energy availability, glucocorticoids have profound effects on feeding behavior, mainly through modulation of orexigenic and anorixegenic neuropeptides. Evidence is also emerging that in addition to the recognized immune suppressive actions of glucocorticoids by counteracting adrenergic proinflammatory actions, circadian elevations have priming effects in the immune system, potentiating acute defensive responses. In addition, negative feedback by glucocorticoids involves multiple mechanisms leading to limiting HPA axis activation and preventing deleterious effects of excessive glucocorticoid production. Adequate glucocorticoid secretion to meet body demands is tightly regulated by a complex neural circuitry controlling hypothalamic corticotrophin releasing hormone (CRH) and vasopressin secretion, the main regulators of pituitary adrenocorticotrophic hormone (ACTH). Rapid feedback mechanisms, likely involving non-genomic actions of glucocorticoids, mediate immediate inhibition of hypothalamic CRH and ACTH secretion, while intermediate and delayed mechanisms mediated by genomic actions involve modulation of limbic circuitry and peripheral metabolic messengers. Consistent with their key adaptive roles, HPA axis components are evolutionarily conserved, being present in the earliest vertebrates. Understanding these basic mechanisms may lead to novel approaches for the development of diagnostic and therapeutic tools for disorders related to stress and alterations of glucocorticoid secretion.

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The mating system of callitrichids has been reported to be monogamous, polygynous and polyandrous. In Callithrix jacchus, groups with 2 breeding females and groups with 1 breeding female have been reported. Our purpose was to evaluate the occurrence of occasional reproduction by subordinate females in free-ranging C. jacchus groups characterized as monogamous. Four groups were monitored at a field site of IBAMA-Brazil for between 20 and 72 months. We recorded the birth of 7 sets of twins to subordinate females that had never reproduced before. Sexual activities were recorded opportunistically: dominant females copulated with only the resident male, while subordinates copulated with extra-group males. We suggest these were essentially monogamous groups that occasionally had 2 reproductive females. Between-group copulations seem to be an alternative strategy used by the subordinates. Despite the costs, there would probably be benefits as a result of scanning for vacancies for reproductive positionsin neighbouring groups and the establishment of ties with extra-group males that might become a reproductive partner in the future.

The use of fecal steroid analysis to assess gonadal and adrenal function in primates has rapidly increased in recent years due to the ability to collect feces from nonhuman primates living in wild conditions. These techniques offer an exciting new potential for enhancing our knowledge of the endocrine status of free‐living animals. Prior to using these techniques under field conditions, it is important to determine the diurnal variation of fecal excreted steroids for assessing possible time limitations on fecal collections. The following study investigates the diurnal frequency of defecation and patterns of steroid levels excreted in feces from four female common marmosets, Callithrix jacchus, living in a family group. These females represented three reproductive conditions: early pregnancy, ovarian cycling, and noncycling (postpubertal). Cortisol, estradiol, and progesterone were extracted and analyzed by enzyme immunoassay. Diurnal variations in steroid levels were found by ANOVA for cortisol and progesterone but not for estradiol. Significantly higher levels of cortisol were found in the afternoon, while the reverse was found for progesterone. All females showed the same pattern of steroid level change, except for cortisol in the pregnant female. Since all females defecated within the first hour after they awoke in the morning, this time was determined to be the most effective time to collect feces. The consistency of our findings reinforces the usefulness of this approach for studying reproductive and adrenocortical function in marmosets and also indicates that fecal collection should be limited to either morning or afternoon collections. Am. J. Primatol. 46:105–117, 1998. © 1998 Wiley‐Liss, Inc.

Social relationships are crucial for the development and maintenance of normal behavior in non-human primates. Animals that are raised in isolation develop abnormal patterns of behavior that persist even when they are later reunited with their parents. In rodents, social isolation is a stressful event and is associated with a decrease in hippocampal neurogenesis but considerably less is known about the effects of social isolation in non-human primates during the transition from adolescence to adulthood. To investigate how social isolation affects young marmosets, these were isolated from other members of the colony for 1 or 3 weeks and evaluated for alterations in their behavior and hippocampal cell proliferation. We found that anxiety-related behaviors like scent-marking and locomotor activity increased after social isolation when compared to baseline levels. In agreement, grooming—an indicative of attenuation of tension—was reduced among isolated marmosets. These results were consistent with increased cortisol levels after 1 and 3 weeks of isolation. After social isolation (1 or 3 weeks), reduced proliferation of neural cells in the subgranular zone of dentate granule cell layer was identified and a smaller proportion of BrdU-positive cells underwent neuronal fate (doublecortin labeling). Our data is consistent with the notion that social deprivation during the transition from adolescence to adulthood leads to stress and produces anxiety-like behaviors that in turn might affect neurogenesis and contribute to the deleterious consequences of prolonged stressful conditions.

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This chapter presents data on the relations between reproduction and aging in both captive and free-ranging marmosets and tamarins. The relationship is examined from two perspectives. First, the relation of age to physiological impairments in reproductive function is explored. Callitrichid females, in common with many other nonhuman primates, begin to display anovulation associated with follicular depletion at a point relatively close to the maximum life span. Unlike Old World primates, however, they continued to display significant steroidogenic activity in the ovary. There are age effects on some reproductive output variables, such as litter size and inter-birth interval, though the effects are often small. Like other mammals, male marmosets display a change in levels of androgens with age, although the magnitude of the decrease is not large and they actually mount an elevated response to GnRH challenge as they age. We also examined whether age affects either the establishment or maintenance of a breeding position, the factors most important in determining lifetime reproductive success. Infant mortality did increase with increasing parturitions, suggesting that there may have been aging or parturition effects on lactation. Generally, marmoset females were well past the age of sexual maturity at the beginning of the tenure and approaching 8-9 years at the end of it. Reproductive decline did not appear to be a gradual process, but a rather abrupt one, often causing the dismantling of the group. There are potentially interesting relations among maternal age, mass and declining reproductive performance, given the known importance of mass as a determinant of reproductive success in female callitrichids.

New insights into the mating systems of common marmosets suggest that they are mainly monogamous, although polygyny and polyandry occasionally occur. Long-term monitoring of wild common marmosets has shown that some reports of polygynous groups (i.e., groups that contain more than one reproducing female) in fact indicate an unbalanced reproductive output associated with extragroup copulation. In this study we describe the behavioral and hormonal profiles of common marmoset (Callithrix jacchus) females living in three wild monogamous groups (Q, PBf, and T), varying from five to 11 individuals, at Nísia Floresta field station, RN, Brazil. The mating system of the groups was previously characterized in terms of affiliative, sexual, and mate-guarding behaviors. Behavioral data were collected once a week, and fecal samples were collected at least twice a week for 10-16 months, depending on the group. A preferential allogrooming relationship was recorded between dominant males and females. Under field conditions the reproductive inhibition of subordinate females appears to be more behavioral than hormonal, since subordinate females of the three groups ovulated and two conceived during the study. In these cases, the subordinate and dominant females reproduced 1 month apart, and infanticide (one case confirmed and one suspected) appeared to be part of the reproductive strategy of dominant females. Following the infanticide, ovarian inhibition (group T) or emigration and return to the natal group (group PBf) were observed. In the third group (Q) the subordinate female showed hormonal profiles compatible with pregnancy, but no infants were seen. These findings reflect the different alternatives that wild subordinate common marmoset females use to reproduce.