Seasonal changes in plasma testosterone and ejaculatory capacity in squirrel monkeys (Saimiri sciureus)

Male fattening is related to increased seminal quality of squirrel monkeys (Saimiri collinsi): Implications for sperm competition

Saimiri are neotropical primates with seasonal reproduction, males develop a seasonal fattening condition that has been suggested as a pre-copulatory sexual selection strategy. Furthermore, females mate with multiple males in the same season. This could also favor the evolution of a postcopulatory sexual strategy by sperm competition. In this study, we tested the hypothesis that there is a relationship between the fatted condition and advantageous seminal characteristics in Saimiri collinsi and evaluated its implications for sperm competition. Adult males (N = 10), aged 5-15 years, housed in mixed or only-male groups, were analyzed from June, 2015 to July, 2016. Measurements of weight, axilla, and arm skinfold, and testicular volume were taken monthly, and semen was collected by electroejaculation. A fattening index was developed to quantify and identify fatted males, biometrics, and seminal parameters were compared between the non-fatted and fatted conditions. The fatted males present a larger testicular volume. This is related to the increase in spermatogenic activity necessary to sustain a high ejaculation frequency during the mating season. An increase in seminal volume and in frequency of semen coagulation were detected in fatted males, advantages related to sperm protection in the female reproductive tract. Age and social context were not significant sources of variation for both morphological and seminal traits. A decrease in response from the fatted males in obtaining semen and an increase in the frequency of azoospermic ejaculates were observed. These unexpected results may be due to intense reproductive activities in a short period. The fattening phenomenon has many implications in the sexual selection of squirrel monkeys, and they are still not entirely unveiled. Our results corroborate the idea that, in S. collinsi, the fatted male condition is related to sexual selection, and we found evidence suggesting it may be also expressed by a post-copulatory component, sperm competition.

The use of anogenital distance as a non-invasive predictor of seminal quality in captive squirrel monkey (Saimiri collinsi Osgood 1961)

BACKGROUND

Anogenital distance is considered a non-invasive measure to assess the development and functionality of sexual organs in different animal species. Hence, this measurement could potentially be used during the selection of non-human primates for reproductive biotechnology programs. The aim of this study was to assess the correlation between anogenital distance and reproductive parameters in captive Saimiri collinsi.

METHODS

Eight mature S. collinsi males were evaluated. Body weight, reproductive hormone levels, testicular volume, and seminal parameters were determined, and their relationship with anogenital distance measurements was assessed.

RESULTS

Anogenital distance was correlated with seminal volume, sperm motility, vigor, and plasma membrane integrity, but not with body weight, reproductive hormones, and testicular volume.

CONCLUSION

The determination of anogenital distance is a non-invasive method to predict seminal quality. This procedure has the advantage of providing andrologic information without a negative impact on animal welfare.

Morphological and hormonal parameters in two species of macaques: impact of seasonal breeding

To compare physiological and developmental differences between two cogeneric species that differ by seasonal vs. aseasonal breeding, values for morphological measurements, testicular volume, serum testosterone, estradiol, and dehydroepiandrosterone-sulfate levels were obtained from 53 rhesus during the early breeding season, as well as 41 pig-tailed macaque males maintained at the Tulane Primate Center. The two species exhibited similar body size, testosterone, and estradiol levels, but differed substantially in testicular volume (3.00 +/- 1.7 vs. 1.72 +/- 1.3 cc), abdominal skinfold measures (15.7 +/- 9.2 vs. 9.0 +/- 7.7 mm), and DHEA-S levels (18.0 +/- 11.7 vs. 7.6 +/- 5.4 microg/dl). Significant interaction effects for species by age group were found for weight, tricep circumference, length, and estradiol level. In addition, length was more closely related to testicular volume among rhesus compared to pig-tailed macaques, suggesting different developmental patterns between the species. Predictors of hormonal levels differed between the two species. In the rhesus, estradiol levels were related to testicular volume and testosterone levels while there were no anthropometric predictors of testosterone or DHEA-S. For the pig-tailed macaques, testicular volume was related to tricep circumference, testosterone to triceps skinfold and testicular volume, and estradiol to weight. It is argued that rhesus have larger testes for body size and more abdominal fat deposits during the early breeding season relative to pig-tailed macaques reflecting the increased demands of sperm competition in a seasonally breeding species. Hormonal differences associated with the difference in breeding system appear to be primarily related to adrenal rather than testicular activity.

Breeding readiness in squirrel monkeys: female-primed females are triggered by males

The South American squirrel monkey (Saimiri) shows pronounced annual variations in reproductive activity. We investigated the contribution of social stimulation to breeding readiness, as reflected in behavior and gonadal hormones. A previous study indicated that formation of new heterosexual pairs can induce breeding readiness in males, but not in females. In the present study we examined the response of individually housed females to the formation of triads of unacquainted females and, subsequently, the response of these triads to the introduction of a single male. The formation of the all-female group was accompanied by a marked reduction in basal cortisol levels. Introduction of a male to such a group promptly induced breeding readiness in the females, as indicated by increased gonadal hormone production and cyclic variation in estrogen and progesterone levels. The contrasting responses of group-living or single females to a new male suggest that at least two processes are involved in the induction of breeding readiness by group formation in females. One is a "priming" effect resulting from interfemale stimulation: the other is a "triggering" effect resulting from the presence of a male.

Behavioral and endocrine consequences of heterosexual pair formation in squirrel monkeys

The South American squirrel monkey (Saimiri sciureus) typically lives in large social groups containing several individuals of all age/sex categories. When living in established heterosexual pairs, reproduction in this seasonally breeding primate is poor. We attempted to induce breeding activity in pair-housed monkeys by forming new heterosexual pairs just prior to the breeding season. Breeding readiness, as reflected in behavior and gonadal hormones, was induced in males, but not in females. Males also showed persistent increases in cortisol levels following formation of new heterosexual pairs; females did not. The results indicate that social stimulation provided by a single novel female is sufficient to enhance breeding readiness in male squirrel monkeys. Females, on the other hand, are apparently unresponsive to a single male whether novel or familiar; this may account for the poor reproductive success in squirrel monkeys housed in heterosexual pairs.

The new world primates as animal models of glucocorticoid resistance

Many New World primate species have greatly increased plasma cortisol concentrations, decreased plasma cortisol binding globulin capacity and affinity, marked resistance of the hypothalamic-pituitary-adrenal axis to suppression by dexamethasone, and no biological evidence of glucocorticoid excess. These primates also have high levels of circulating progesterone, estrogen, mineralocorticoid, androgen and vitamin D. The glucocorticoid target tissues that have been examined (circulating mononuclear lymphocytes and cultured skin fibroblasts) have normal concentrations of glucocorticoid receptors with decreased affinity for dexamethasone. Transformation of B-lymphocytes with the Epstein-Barr virus leads to glucocorticoid receptor induction that is less than that observed with cells from Old World primates. The receptor in these cells has a low affinity for dexamethasone. The low affinity leads to an increased loss of specific bound ligand during thermal activation. Meroreceptor generation is normal. The molecular weight of the receptor, determined by SDS-PAGE, is similar to that of Old World primates (approximately 92,000) and the activation pattern per se, examined in vitro by heating cytosol and performing phosphocellulose chromatography, appears similar to that of human controls. The ratios of nuclear to cytosolic hormone-receptor-complexes and of cytosolic activated to unactivated receptor complexes in intact cells are similar to Old World primates. Results from mixing studies do not support the hypothesis that a binding inhibitor(s) or a deficient cytosolic positive modifier(s) of binding underlies the findings in these primates. The New World primates, unlike men with the syndrome of primary cortisol resistance, have compensated for their condition with intra-adrenal and mineralocorticoid receptor adaptations. Thus, unlike Old World primates, cortisol in New World primates has only weak sodium-retaining potency because the aldosterone receptor has a low affinity for cortisol. The common element that would explain the apparent resistance to six steroid hormones in New World primates remains unknown.