Margaret L. Walker

Dietary influences on growth and sexual maturation in premenarchial rhesus monkeys

The effect of a high-fat diet on growth, sexual maturation, and developmental changes in serum levels of estradiol (E2), growth hormone (GH), somatomedin-C (Sm-C), and insulin were examined in outdoor-housed premenarchial rhesus monkeys. From 16 to 32 months of age, females were fed either a high-fat diet (HFD, N = 5) with 31% calories from fat or a control diet (commercial laboratory chow, N = 10) with 12% of the calories from fat. Maintenance on a HFD did not accelerate physical growth, as all animals exhibited similar increments in body weight, crown-rump length, and weight/height ratios. In contrast, the HFD group exhibited an earlier onset of perineal swelling and menarche despite lower body weights during that time. Moreover, 80% of the HFD animals exhibited an early first ovulation (31-32 months of age) compared with 40% of control animals. These reproductive changes were associated with significant differences in the endocrine profiles of HFD animals. Fasting serum levels of insulin were significantly elevated within 2 months of diet treatment and remained elevated throughout the study period. Levels of Sm-C were elevated relative to those of controls after 3 months of diet treatment, but not thereafter. Serum GH increased after 6 months on the HFD and, overall, concentrations were higher in HFD animals. A significant rise in E2 was observed after only 45 days on the HFD treatment. These concentrations did not differ from 18 to 21 months of age but again were elevated at 27 months in the HFD females. Since these endocrine and physical changes associated with reproductive physiology occurred in the absence of enhanced growth, these data suggest that a high-fat diet may influence the rate of sexual maturation through changes in certain metabolic factors which may act on the developing neuroendocrine system.

Female rhesus monkey aggression during the menstrual cycle

Female aggressive behaviour was studied in a heterosexual group of compound-housed rhesus monkeys (Macaca mulatta) to determine (1) whether the frequency of agonistic encounters fluctuated across the menstrual cycle, and (2) if changes in aggression correlated with fluctuations in serum oestradiol (E2) and testosterone (T) levels or with increased social activity associated with mating behaviour. Aggression involving female increased during the mating phase of the menstrual cycle; this was largely due to a significant increase in the initiation of non-contact aggression. Contact aggression was infrequent and remained stable throughout all cycle phases. Males received less aggression than females independent of female cycle phase, despite the low social rank of some males in the group. Female aggression was directed equally to other mating and non-mating females. Moreover, daily fluctuations in aggressive behaviour did not correlate with changes in serum levels of either E2 or T, but were related to changes in social behaviours, particularly those related to seeking a sexual partner. An analysis of covariance indicated that by holding the measures of directed female activity (Following Males and Initiating Proximity) constant, the mid-cycle increases in aggression were nullified. These data suggest that the significant fluctuations in aggressive behaviour observed throughout the menstrual cycle are not directly related to changes in serum levels of ovarian hormones, but are rather a consequence of a females attempts to copulate with a male.

Ovarian aging in squirrel monkeys (Saimiri sciureus)

In female squirrel monkeys (Saimiri sciureus), the reproductive period normally extends from approximately 2.5 years to the mid-teens. In the present study, we examined the age-associated cytological changes in the ovaries of 24 squirrel monkeys ranging in age from newborn to approximately 20 years. We found a significant, age-related decline in the number of primordial follicles, with the most pronounced loss occurring between birth and 5 years. After approximately 8 years of age, relatively few primordial follicles were evident in the ovarian sections examined. An unusual feature of the aging squirrel monkey ovary is the emergence of highly differentiated, encapsulated clusters of granulosa cells that increase in size and number, particularly after the age of 8 years. Many of these cells express anti-Müllerian hormone, and, histologically, the clusters resemble granulosa cell tumors in humans. However, granulosa cell clusters (GCCs) are present in both ovaries of all older squirrel monkeys, and they display no obvious signs of malignancy, suggesting that they are a normal feature of ovarian aging in this species. Our findings indicate that reproductive senescence in female squirrel monkeys, as in other primates, involves the inexorable depletion of ovarian follicles. In addition, the consistent appearance of abundant, well-differentiated clusters of granulosa cells in older squirrel monkeys, prior to the cessation of reproduction, suggests that these structures may influence the later stages of reproductive potential in this species. Analysis of GCCs in older squirrel monkeys also could yield insights into the pathophysiology of granulosa cell tumors in humans.

Social stimulation and the resumption of copulation in Rhesus (Macaca mulatto) and stumptail (Macaca arctoides) Macaques

Copulatory data derived from observations of social groups of rhesus and stumptail macaques were analyzed to test the hypothesis that pairs of animals would resume copulation significantly sooner if a second male copulated with the female shortly after the first male’s ejaculation. Data from both groups supported the hypothesis. These results, extending previous studies in Macaca nemestrina,suggest that the shortening of copulatory intervals by social stimuli occurs in several species, both in social groups and in experimentally created triads. These findings also are consistent with the hypothesis that socially mediated resumption of mating is related to intrasexual competition among males.