Sexual deprivation and its influence on testosterone levels and sexual behavior of old and middle-aged rhesus males

After approximately 6 mo without access to a receptive female, eight old (19 to 25 yr) and eight middle-aged (9 to 15 yr) males were given nine daily 10-min tests of sexual behavior with receptive females. Three ovariectomized, estrogen-primed females served as partners. Blood samples were taken from the males at 0900 and 2100 h on 2 consecutive days before and after the series of nine behavioral tests. Serum levels of testosterone (T) were measured by means of radioimmunoassay. There were no significant differences between serum T levels after 6 mo of sexual deprivation and after the nine tests of sexual behavior, nor did serum T levels differ between middle-aged and old males. Serum T levels were significantly higher in the 2100-h samples than in the 0900-h samples for both groups. The nocturnal increases in serum T, both absolute amounts and percentages of increase, did not differ between middle-aged and old males. Repeated daily testing had little effect on the major components of male sexual behavior. Middle-aged males outperformed old males; middle-aged males had higher rates of contracting the female, mounting and intromitting. They ejaculated earlier and had a greater percentage of tests with ejaculations than did old males.     

Diurnal patterns of testosterone, dihydrotestosterone, estradiol, and cortisol in serum of rhesus males: Relationship to sexual behavior in aging males

This study was carried out to determine differences between old and young rhesus males in levels and diurnal patterns of testosterone, dihydrotestosterone, cortisol, and estradiol, and to determine correlations between these hormones and sexual behavior of the old males. Blood was drawn from old (n = 9) and young (n = 9) rhesus males over 5 consecutive days at 0900, 1300, and 2100 hr. The two groups of males did not differ in mean serum levels of testosterone, dihydrotestosterone, or estradiol at any time. Although the old and young did not differ in cortisol levels at 0900 and 1300 hr, the cortisol levels at 2100 hr were lower in the old males. Diurnal variations in testosterone, dihydrotestosterone, and cortisol were comparable in old and young males. Mean serum levels of estradiol were significantly higher at 0900 hr than at 1300 hr in the old males, whereas in the young males estradiol levels did not differ with time of day. There was a significant positive correlation between testosterone and yawning rate, and cortisol levels were correlated positively with rate of contacting, rate of mounting, and percentage of tests with erections. The decline in sexual performance of old rhesus males cannot be attributed to changes in the levels or diurnal patterns of testosterone, dihydrotestosterone, or estradiol, but lower cortisol levels in old males may contribute to the decline in sexual behavior.     

Sexual experience changes sex hormones but not hypothalamic steroid hormone receptor expression in young and middle-aged male rats

Testosterone is well known to regulate sexual behavior in males, but this is dependent upon prior sexual experience. Aging is associated with decreased libido and changes in testosterone, but the role of experience in these age-related processes has not been systematically studied. We examined effects of age and sexual experience on serum hormones (total testosterone, free testosterone, estradiol, LH) and on numbers of androgen receptor (AR) and estrogen receptor α (ERα) immunoreactive cells in the hypothalamus. Extensive sexual experience was given to male rats at 4 months of age. Rats were euthanized at either 4 months (young) or 12 months (middle-aged (MA)). Comparable sexually naïve male rats were handled and placed into the testing arena but did not receive any sexual experience. Thus, we had four groups: young-naïve, young-experienced, MA-naïve and MA-experienced. Serum hormone levels were assayed, and numbers of AR and ERα cells were quantified stereologically in the medial preoptic nucleus (MPN) and the anteroventral periventricular nucleus (AVPV). Sexually experienced males had significantly elevated serum testosterone and free testosterone in both age groups. Both total and free testosterone were higher, and estradiol lower, in middle-aged than young rats. Experience did not alter either AR or ERα expression in the preoptic brain regions studied. Aging was associated with increased expression of AR, but no change in ERα. These results show that sexual experience can induce short-term and long-term alterations in serum hormones but these effects are not manifested upon their receptors in the hypothalamus.     

Effects of exogenous testosterone on testicular luteinizing hormone and follicle-stimulating hormone receptors during aging

During aging, the male Japanese quail exhibits a loss of fertility, increased morphological abnormalities in the testes, and a higher incidence of Sertoli cell tumors. Although there is a coincident loss of reproductive behavior, plasma androgen levels remain high until testicular regression occurs in association with senescence. The purpose of this study was to compare mean specific binding of chicken luteinizing hormone (LH) and follicle-stimulating hormone (FSH) as a measure of testicular receptors during identified stages during aging. Males were categorized according to age (young = 9 months, middle aged = 24 months, or old = 36+ months) and sexual behavior (active or inactive). Testicular samples were collected immediately after perfusion with 4% paraformaldehyde from the following groups: young active (n = 8), young photoregressed (n = 5), young photoregressed plus testosterone implant (n = 4), middle-aged active (n = 8), middle-aged inactive (n = 4), old inactive (n = 5), and old inactive plus testosterone implant (n = 6). A crude plasma membrane fraction was prepared from the testes of each bird and an aliquot deriving from 10 mg of testicular tissue was used for binding assay. Specific binding of labeled LH or FSH was expressed as percentage of total radioactive hormone. Results showed significant (P < 0.05) age-related decreases in both FSH and LH receptor numbers. The highest FSH binding was found in young and middle-aged active males, with low binding in old inactive males. Testicular LH binding decreased during aging, with a sharp decrease in middle-aged males, which was similar to old males. Testosterone implants weakly stimulated FSH and LH binding in old males. Both LH and FSH binding decreased in photoregressed young males. However, testosterone implants stimulated increased LH binding, but did not affect FSH binding in young photoregressed males. These results provide evidence for separate regulation of testicular LH and FSH receptors, with testosterone stimulation of LH receptor, but not FSH receptor number in young males. However, during aging there appears to be a loss of this response, potentially because of the reduced efficacy of testosterone stimulation, thereby implying a diminished capacity for response with aging.     

Changes in androgen receptor, estrogen receptor alpha, and sexual behavior with aging and testosterone in male rats

Reproductive aging in males is characterized by a diminution in sexual behavior beginning in middle age. We investigated the relationships among testosterone, androgen receptor (AR) and estrogen receptor alpha (ERα) cell numbers in the hypothalamus, and their relationship to sexual performance in male rats. Young (3 months) and middle-aged (12 months) rats were given sexual behavior tests, then castrated and implanted with vehicle or testosterone capsules. Rats were tested again for sexual behavior. Numbers of AR and ERα immunoreactive cells were counted in the anteroventral periventricular nucleus and the medial preoptic nucleus, and serum hormones were measured. Middle-aged intact rats had significant impairments of all sexual behavior measures compared to young males. After castration and testosterone implantation, sexual behaviors in middle-aged males were largely comparable to those in the young males. In the hypothalamus, AR cell density was significantly (5-fold) higher, and ERα cell density significantly (6-fold) lower, in testosterone- than vehicle-treated males, with no age differences. Thus, restoration of serum testosterone to comparable levels in young and middle-aged rats resulted in similar preoptic AR and ERα cell density concomitant with a reinstatement of most behaviors. These data suggest that age-related differences in sexual behavior cannot be due to absolute levels of testosterone, and further, the middle-aged brain retains the capacity to respond to exogenous testosterone with changes in hypothalamic AR and ERα expression. Our finding that testosterone replacement in aging males has profound effects on hypothalamic receptors and behavior has potential medical implications for the treatment of age-related hypogonadism in men.     

Threshold for behavioral response to testosterone in old castrated male rhesus macaques

The sexual behaviors of old, intact (N = 5) and old, castrated (N = 6) rhesus macaque males were compared in six series of pair tests with receptive females. The castrated monkeys were tested when untreated and when given five doses of testosterone propionate (TP; 0.004, 0.016, 0.064, 0.256, and 1.024 mg/kg of body weight) in consecutive months. The serum testosterone (T) level was determined for each male before and after each series of tests. When untreated, none of the castrated males ejaculated, and yawning was significantly less in these monkeys than in intact males-no other behavioral measures differed significantly. Within 2 weeks of daily injections of 0.004 mg of TP/kg, two males ejaculated, and all differences in measures of ejaculation were eliminated. A third male ejaculated after 1 week of treatment with 0.016 mg of TP/kg. Yawning values did not differ during and after treatment with 0.064 mg of TP/kg. Although final mean serum T levels were six times higher in castrated (24.3 ng/ml) than in intact males (4.2 ng/ml), sexual performance levels did not exceed those of intact males.     

Sexual behavior of laboratory- and wild-born male Rhesus monkeys.

Rearing method has an important influence on the sexual performance of adult male rhesus monkeys. Testosterone levels and sexual behavior of laboratory-born and -reared adult males were compared with those of males born in the wild and brought to the laboratory as adults. The mean sexual performance level of colony-reared animals was significantly below that of males born in the wild but testosterone levels in the two groups did not differ significantly. Not all laboratory-reared males were sexually inadequate but less than 30% ejaculated in tests of sexual behavior. The sexual behavior and testosterone levels of adult males treated prenatally with testosterone propionate were found not to differ from those of untreated males reared in the same way. Conclusions about sexual adequacy based on 10-mm tests of sexual behavior differed very little from those based on 1-hr tests.     

Testosterone-mediated trade-offs in the old age: a new approach to the immunocompetence handicap and carotenoid-based sexual signalling

The immunocompetence handicap hypothesis proposes that testosterone mediates a trade-off between sexual signalling and immunocompetence in males. Such a trade-off could favour the reliability of sexual signals on the basis that testosterone required for signal expression also promotes immunosuppression. However, the immunosuppressive activity of testosterone has not been convincingly demonstrated. We propose that the optimal solution to the testosterone-mediated trade-off should change with age, explaining ambiguous results in the past. Testosterone and ageing would promote two simultaneous immunosuppressive challenges unaffordable for low-quality males. Oxidative stress, as intimately related to ageing and immunosenescence, could contribute to enhance signal reliability. In this context, traits coloured by carotenoids (yellow–red traits) could play a crucial role due to the immunostimulatory and antioxidant properties of these pigments. Here, old and middle-aged male red-legged partridges were treated with testosterone or manipulated as controls. In the presence of high-testosterone levels, middle-aged males increased both circulating carotenoid levels and colour expression, whereas their cell-mediated immunity was not significantly altered. However, in old males, neither circulating carotenoids nor sexual signalling increased when treated with testosterone, but immunosuppression was detected. The link between testosterone and carotenoids could favour the reliability of sexual signals throughout the life.     

Changes in masculine sexual behavior, corticosterone and testosterone in response to acute and chronic stress in male rats

Chronic exposure to stressors increases HPA axis activity and concomitantly reduces HPG axis activity. This antagonistic relationship between both these axes has been proposed to underlie the inhibition of reproductive function due to stress. Sexual behavior in males may be the most vulnerable aspect of male reproduction to acute and chronic stress and it has been suggested that alterations in sexual behavior during stress are due to the antagonistic relationship between testosterone and corticosteroids. However, only in a few studies has a correlation between the levels of testosterone and corticosterone, and sexual behavior been made. In this study, we evaluated the effects of different stressors, applied both acute and chronically, on masculine sexual behavior and whether or not these effects on sexual behavior are accompanied by changes in plasma levels of corticosterone and testosterone. Additionally, we evaluated the effect of testosterone treatment on the effects of stress on sexual behavior. Sexually experienced male rats were exposed to one of the following stressors: immobilization (IMB), electric foot shocks (EFS) or immersion in cold water (ICW). Sexual behavior and plasma levels of testosterone and corticosterone were assessed on days 1, 5, 10, 15, and 20 of stress. In a second experiment, males were castrated, treated with 3 different doses of testosterone propionate (TP) and exposed to ICW for 20 consecutive days. Sexual behavior was assessed on days 1, 5, 10, 15, and 20 and steroids were evaluated on day 20. Parameters of masculine sexual behavior were modified depending on the characteristics of each stressor. Mount, intromission and ejaculation latencies increased significantly, the number of mounts increased, and ejaculations decreased significantly in males exposed to EFS and to ICW but not in males exposed to IMB. Associated with these effects, testosterone decreased in the EFS and ICW groups on days 1, 15, and 20. However, corticosterone increased only in males exposed to ICW. In castrated males, TP treatment failed to block the effects of stress by ICW on sexual behavior and corticosterone. These results indicate that the effects of stress on sexual behavior depend on the characteristics of each stressor, and these effects, as well as the decrease in testosterone are not necessarily associated with the increase in corticosterone. The fact that testosterone treatment did not prevent the effects of stress on sexual behavior suggests that other mediators could be involved in the alterations of sexual behavior caused by stress.     

Persistence of sexual behavior in ovariectomized stumptail macaques following dexamethasone treatment or adrenalectomy

Daily administration over a period of 6 weeks of increasing doses of dexamethasone sodium phosphate (DEX) to seven long-term ovariectomized female stumptail monkeys significantly lowered circulating levels of testosterone without reducing any aspect of the females' sexual behavior or that of their male partners. Since treatment with DEX failed to suppress serum testosterone levels completely an additional experiment was performed in which the sexual behavior of five ovariectomized stumptails was compared before and after bilateral adrenalectomy, combined with chronic administration of both gluco- and mineralocorticoids. Serum levels of both testosterone and estradiol were reduced to very low levels in females after ovariectomy and adrenalectomy, yet no significant depression of females' sexual performance or that of their male partners occurred. Subsequent sc administration of estradiol or estradiol + testosterone in Silastic capsules to ovariectomized, adrenalectomized stumptails had little effect on sexual interaction. In a third experiment five ovariectomized stumptails which initially were relatively unreceptive and unattractive to males were given first testosterone and then testosterone + estradiol sc in Silastic capsules. One of the three indexes of females' receptivity increased significantly after testosterone; however, no other essential aspect of sexual interaction was affected. These findings suggest that sex steroids are normally not required in the female stumptail macaque for activation of preceptive and receptive sexual behaviors or for maintenance of sexual attractivity.     

Activation of sexual behavior in male rats by combined subcutaneous and intracranial treatments of 5 alpha-dihydrotestosterone

When given peripherally, 5 alpha-dihydrotestosterone, the major androgenic metabolite of testosterone, is relatively less effective than testosterone in activating sexual behavior of castrated male rats. In order to test the possible central nervous system effects of dihydrotestosterone more directly, we castrated Long-Evans rats, gave them a behaviorally subthreshold dose of dihydrotestosterone placed subcutaneously in Silastic capsules (ScDHT), and then additionally treated the rats with intracranial implants of crystalline dihydrotestosterone (IcDHT, N = 12), testosterone (IcT, N = 12), or cholesterol (IcCHOL, N = 10) placed in the medial preoptic area. The peripheral ScDHT treatment maintained sexual organ weights of castrated males at levels comparable to those of intact males, but did not in itself significantly activate mating behavior. The addition of IcT or IcDHT to this treatment regimen significantly increased the number of males displaying mounting behavior, intromissions, and ejaculatory behavior (P less than 0.05) compared to males with IcCHOL implants. There were no significant differences between the group given IcT and the group given IcDHT. Results of this study support the hypothesis that the nonaromatizable androgen 5 alpha-dihydrotestosterone can act in the rat brain to influence male sexual behavior. In addition, these data lead us to suggest that the relative ineffectiveness of dihydrotestosterone versus testosterone when given systemically may reflect differences in bioavailability of these hormones to the brain following such treatment.     

Effects of the antiandrogens cyproterone acetate and flutamide on male reproductive behavior in a lizard (Anolis sagrei)

This study examined the effects of the antiandrogens cyproterone acetate (CA) and flutamide (F) on male reproductive behavior in the lizard Anolis sagrei. Reproductively active males were implanted with subcutaneous pellets of either CA, F, or placebo (P). Pellets delivered CA and F at a constant rate of 0.1 mg/day. Three weeks after implantation, males were tested with stimulus males and two days later with stimulus females. Cyproterone acetate inhibited aspects of male aggressive and sexual behavior, and reduced testis weight and size of the renal sex segment. Plasma testosterone (T) levels in CA-treated males were not significantly different than those of P-treated males. Flutamide did not inhibit aggressive or sexual behavior, but did decrease testis weight as well as the size of the renal sex segment. Plasma T levels were significantly higher in F-treated males than in P-treated males. These data suggest that CA, an antiandrogen with antigonadotropic activity, may be used to inhibit reproductive behavior in male lizards.     

Aging and primate male sexual behavior

Old male rhesus macaques display less sexual behavior than young and middle-aged males. The decrease in sexual activity occurs without a statistically significant decline in gonadal hormones or change in diurnal patterns of serum T, DHT, or LH. Levels of sexual activity are not increased by administering T to old intact males. However, the hormone is effective in increasing sexual behavior in old long-term-castrated males. Performance can be increased to levels observed in equally old untreated intact males. Readily detectable physical disabilities of old age have been observed to impair sexual performance, but the observed general decline in sexual activity cannot be accounted for by known physical disabilities. Novelty, as represented by a change in female partner or by a change in environment, has not increased sexual performance in old rhesus males. Only when old males were paired with empirically selected preferred females is their sexual behavior increased to levels displayed by young males. Drugs reported to increase levels of sexual behavior in rats have thus far been less effective in old rhesus males than powdered rhinoceros horn has been in man. The probable absence of a placebo effect in rhesus males should increase their usefulness as an animal model for the study of sexual behavior in aging men.     

Fetal effects on sexual behavior and aggression in young and old female mice treated with estrogen and testosterone

During fetal life female mice (Mus musculus) that develop between two male fetuses (2M females) have higher blood concentrations of testosterone than do females that do not develop next to a male fetus (0M females). In the first experiment reported here, sexual receptivity and sexual attractiveness to males were examined in young (5 month old) and old (17 month old) ovariectomized, estrogen- and progesterone-treated 0M and 2M female mice that were placed in like-age pairs with a male. Most males inseminated the 0M female prior to inseminating the 2M female regardless of age. In addition, 0M females were more likely to exhibit lordosis when mounted than were 2M females. When the same young females were 9 months of age and the old females were 21 months of age, they were treated with testosterone and again placed together in pairs along with a sexually receptive female. Young 2M females exhibited more aggression toward the testosterone-treated female partner, and also exhibited more mounting of the receptive female, than did young 0M females. But, both old 0M and old 2M females were highly aggressive and exhibited mounting. An increase in sensitivity to the effects of testosterone on behavior thus occurs during aging in 0M females, which are relatively insensitive to testosterone in young adulthood. In contrast, when treated with estrogen and progesterone, 0M females were more attractive to males and were more sexually receptive than 2M females regardless of age.     

Some contrasting effects of surgical and “chemical” castration on the behavior of male cynomolgus monkeys (Macaca fascicularis)

In nonhuman primates, surgical castration reduces plasma testosterone levels and male sexual behavior, and testosterone replacement restores them. Chemical castration with compounds that lower plasma testosterone levels is used clinically in the treatment of certain forms of cancer and to reduce aberrant sexual behavior in male sex offenders. In the United States, medroxyprogesterone acetate (MPA) is the drug most used to help reduce serious sexual behavioral problems in men. We were therefore interested in comparing the behavioral effects of MPA treatment (40 mg once a week) in 4 intact male cynomolgus monkeys (4 pairs, 120 tests) with data from an earlier study in our laboratory on 4 males observed before and after surgical castration (16 pairs, 192 tests). Both MPA treatment and surgical castration reduced plasma testosterone to very low levels and decreased ejaculatory activity, but MPA treatment additionally affected measures of male sexual motivation (decreased numbers of male mounting attempts and increased mounting attempt latencies) which were not primarily affected by surgical astration. However, surgical castration decreased intromission ability (percentage of intromitted thrusts per test) and male yawning behavior more rapidly than did MPA treatment. This suggested a hypothesis that different mechanisms could be involved in the behavioral effects—namely, that surgical castration may act primarily via testosterone-dependent peripheral mechanisms, while chemical castration with MPA does so primarily via central mechanisms regulating sexual motivation.     

Restoration of sexual performance in old rhesus macaques paired with a preferred female partner

Experiments were undertaken to determine whether the decline in the ejaculation frequency of old male rhesus macaques is due to a decrease in physical capacity. In the first experiment, the capacity of old males to ejaculate in a series of biweekly tests was investigated. Six old (18–23 years) and six young (8– 12 years) male rhesus macaques were given 10-min tests of sexual behavior with nine different females chosen at random. The old males were also given 10-min tests with a preferred female, 1339. When the old males were tested with nine different females, their sexual performance (e.g., frequency of ejaculation) was significantly less than that of the young males, but their performance was comparable to that of young males in nine tests with female 1339. In a second experiment, the capacity of old males to show repeated ejaculations over a 3-hr period was tested. The same old and young males were given a 3-hr test with female 1339. The sexual performances (number of ejaculatory series completed and behavior displayed within each ejaculatory series) of the old males did not differ significantly from those of the young. Also, no significant differences in behavior were observed between young and old males during the first 10 min of the 3-hr tests. Our data show that the decline in the sexual performance of old rhesus males is not due to a decreased capacity to perform sexually or to physical debilitation.     

Developmental patterns of serum luteinizing hormone, gonadal and adrenal steroids in the sooty mangabey (Cercocebus atys)

Serum levels of luteinizing hormone (LH), testosterone, dehydroepiandrosterone sulfate (DHAS), androstenedione and cortisol were determined in multiple samples from 86 sooty mangabeys of varying ages (0-17 years). Testosterone, androstenedione, DHAS and cortisol were measured by radioimmunoassay; LH was determined by in vitro bioassay. Serum LH concentrations were elevated in neonates (less than 6 months) and in animals older than 72 months of age. The higher LH levels were associated with increased circulating concentrations of testosterone in males but not females. The pubertal rise in serum testosterone at approximately 55-60 months of age in males was coincident with rapid body growth. No pubertal growth spurt was observed in females. Serum levels of androstenedione and DHAS were highest during early postnatal life (less than 6 months) with androstenedione exceeding 600 ng/dl in males and 250 micrograms/dl in females, but declined rapidly in both sexes to a baseline of 150 ng/dl by 19 months of age. Serum androstenedione did not fluctuate significantly in adult animals. The pattern of age-related changes in serum DHAS paralleled those of serum androstenedione, whereas serum cortisol values did not change significantly with age. Developmental changes in serum LH, testosterone and body weight suggest that the sooty mangabey matures substantially later than the rhesus monkey. The pattern of serum gonadal and adrenal steroids during sexual maturation is similar to that seen in the baboon with no evidence of an adrenarche.     

Changes in the secondary sexual adornments of male mandrills (Mandrillus sphinx) are associated with gain and loss of alpha status

Two semifree-ranging mandrill groups, inhabiting large, naturally rainforested enclosures in Gabon, were studied to measure morphological, endocrine, and behavioral changes that occurred when adult males rose, or fell, in dominance rank. Gaining alpha rank (N = 4 males) resulted in increased testicular size and circulating testosterone, reddening of the sexual skin on the face and genitalia, and heightened secretion from the sternal cutaneous gland. Blue sexual skin coloration was unaffected. New alpha males increased in rump "fattedness," but not in body mass, and spent more time associated with other group members, rather than ranging alone. Loss of alpha position (N = 4 males) resulted in less pronounced effects than those that occurred after males had risen to alpha positions. Deposed alpha males showed decreased testicular volume, decreased body mass, a reduction in the extent of red (but not blue) sexual skin coloration, and decreased sternal gland activity. Deposed males did not decrease in the brightness of sex skin coloration. These results demonstrate that male-male competition and rank reversals have remarkable effects upon testicular function, secondary sexual traits, and behavior in the adult male mandrill. Secondary sexual traits respond to changes in male social status and therefore may be important as intrasexual signals of dominance rank.     

Role of the serotoninergic system in the acceleration of sexual maturation in wild Norway rats selected for reduced aggressiveness toward humans

The role of the serotoninergic system in acceleration of the sexual development of domesticated rats (Rattus norvegicus) was assessed. The onset of age-related changes in hypothalamic serotonin during prepubertal period occurred earlier in domesticated than in aggressive male rats. Blockade of the serotoninergic system after p-chlorophenylalanine (PCPA) administration on days 40 and 44 delayed the development of the reproductive system in both aggressive and domesticated males. In 60-day-old rats treated with PCPA, levels of testosterone in plasma and the number of mature spermatozoa in epididymis were decreased compared to controls. At the same time, the administration of PCPA on days 30 and 34 did not modify basal testosterone secretion and other parameters in 60-day-old aggressive rats and produced a decrease similar to PCPA injections on days 40 and 44, although less pronounced, in the weights of testes in domesticated animals. Administration of 5-hydroxytryptophan (5-HTP), a precursor of serotonin synthesis, on days 30, 32, 34, 36 and 38 increased plasma testosterone levels and weights of the sex organs in 60-day-old domesticated males, but did not significantly affect the development of reproductive system in aggressive animals. These data indicate that serotonin stimulates sexual development of males during prepubertal period and this activating effect of serotonin occurs earlier in domesticated than in aggressive males. They also suggest that the acceleration in sexual maturation of domesticated rats could result from changes in the ontogenetic dynamic of hypothalamic serotonin induced by a selection for low aggressiveness towards man.     

Seasonal and social correlates of fecal testosterone and cortisol levels in wild male muriquis (Brachyteles arachnoides)

Fecal testosterone and cortisol levels were analyzed from six wild male muriquis (Brachyteles arachnoides) over a 19-month period at the Esta??o Biológica de Caratinga in Minas Gerais, Brazil, to investigate the hormonal correlates of seasonal sexual behavior and environmental conditions. Group mean testosterone levels based on weekly samples from the six males did not differ between copulatory and noncopulatory periods or between rainy and dry seasons. Cortisol levels did change with copulatory periods, and were significantly higher during the second dry season, when mating continued following an exceptionally heavy rainy season, than during the first dry season, when mating ceased. Males exhibited individual variation in the timing of their hormone shifts relative to their sexual activity, but neither hormone levels nor sexual activity were related to male age. Despite individual differences in the timing of testosterone fluctuations around the onset and offset of the copulatory season, all males exhibited elevated cortisol concentrations following a slight increase in testosterone at the beginning of the copulatory season. Both the lack of significant changes in testosterone levels with the onset of the rainy and copulatory season and the lack of prebreeding increases in cortisol may be related to the low levels of overt aggression displayed by male muriquis over access to mates.