Charles H. Phoenix

Sexual behavior and hormone levels during the menstrual cycles of rhesus monkeys.

Abstract The sexual interactions of 10 pairs of rhesus monkeys were observed during a control and an experimental menstrual cycle of each female. During the experimental cycle the females were treated with an antiandrogen, flutamide. Daily peripheral serum levels of estradiol, testosterone, and progesterone in each female were determined by radioimmunoassay. Sexual behavior did not correlate reliably with female serum concentrations of any hormone measured nor with the menstrual cycle stage. Administration of the antiandrogen to the females did not affect the sexual behavior of the pairs, although female serum levels of estradiol and testosterone were reduced. It was concluded that although female ovarian hormones may influence rhesus sexual interactions under some circumstances, the normal hormonal fluctuations during the menstrual cycle need not regulate this behavior; a knowledge of an intact rhesus females hormonal condition does not allow accurate predictions about behavior displayed during laboratory pair tests with a male.

Sexual behavior in old male rhesus monkeys: Influence of familiarity and age of female partners

Old male rhesus monkeys (Macaca mulatta)whose sexual behavior had declined over a 10-year period were studied. The same ovariectomized females, comparable in age to the males (about 20 years old), served as sexual partners during the 10 years. In the first experiment the old males were paired with unfamiliar females to determine whether changing sexual partners would reverse the decline in performance that had been observed. The unfamiliar females, also about the same age as the males, were ovariectomized and treated with estradiol before pairing, as were the familiar females. Although the males contacted the unfamiliar females more often than they contacted the familiar females, there were no other differences in sexual activity. The two groups of females did not differ in their behavior toward the males. The low level of sexual performance by the males with both familiar and unfamiliar females could have been the result of decreased sexual initiative and responsiveness associated with the advanced age of the females, so a second experiment was undertaken to test this possibility. The old males were paired with two different groups of unfamiliar, intact, cycling females. One group was young (about 4 years old), the other old (about 20 years). The sexual responses of the males to both young and old females were the same. The two groups of females did not differ in their behavior toward the males.

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

Abstract 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.

RELATIONSHIP OF FREE AND BOUND TESTOSTERONE TO SEXUAL BEHAVIOR IN OLD RHESUS MALES

Abstract Total serum testosterone concentrations, percentage of total serum testosterone bound to testosterone-binding globulin (TeBG), and estimates of free testosterone concentrations were determined in old and young male rhesus macaques. Also the sexual performance of the old (20 years and older) males was studied. The two groups did not differ in either the mean level of total testosterone or the index of free testosterone but the old males had a significantly higher percentage of testosterone bound to TeBG than did the young (10 years old) ones. We found significant negative correlations between the percentage of testosterone binding and sexual behavior in the old males. The percentage of bound testosterone was negatively correlated with the rates of contacting females, mounting, and intromission, and with the percentages of tests during which intromission and ejaculation occurred. Neither the total serum testosterone level nor the index of free testosterone correlated with the level of sexual performance.

Ejaculation by male rhesus in the absence of female partners.

Abstract The number of ejaculatory plugs found beneath the cages of male rhesus over a 14-month period indicated a high incidence of ejaculation in the home cage in the absence of a female partner. Frequency of ejaculation in the home cage was not related to concurrent tests of sexual behavior with receptive females. Moreover, ejaculation in the home cage within 22 hr or less of sex tests did not affect the frequency of ejaculation in tests with stimulus females. Long- and short-term vasectomized males ejaculated as frequently in their home cages as nonvasectomized males. No ejaculatory plugs, of course, were found beneath the cages of males castrated 2 years earlier.

Ejaculation by male rhesus as a function of the female partner

Abstract Nine adult male rhesus monkeys were given eight 10-min tests of sexual behavior with eight ovariectomized female rhesus pretreated with estradiol benzoate. Males differed significantly in the number of tests during which they ejaculated, and females differed in the frequency with which specific males ejaculated when tested with them. With phylogenetic increase in neocortex, pair compatibility may be as important as hormonal stimulation in regulating male sexual performance. If this is correct, the problem of understanding the determinants of primate sexual behavior becomes more formidable.

Effects of bilateral amygdaloid lesions on gonadal and pituitary hormones in serum and on sexual behavior in female rhesus monkeys

Bilateral lesions that destroyed the amygdaloid nuclear complex failed to alter either serum concentrations of luteinizing hormone (LH), estradiol (E2), progesterone, and testosterone or sexual behavior in female rhesus monkeys. In 3 of 5 females ovulation occurred in the postoperative cycle, and in 4 of 5 females exogenous E2 treatment released a surge of LH whose pattern was indistinguishable from the normal preovulatory surge. A postcastration rise in serum LH also occurred in the 5 lesioned monkeys, and E2 again induced an LH release. The data suggest that the amygdala does not play a major role in regulation of LH or ovarian steroid secretion during the menstrual cycle. Moreover amygdaloid lesions in female monkeys do not alter their sexual behavior.

Sexual behavior in long-term vasectomized male rhesus monkeys

Abstract It has been suggested that in man vasectomy may lead to problems of psychosexual adjustment such as impotency because of castration anxiety or other superstitions often associated with sexual behavior. An increase in sexual promiscuity has also been attributed to vasectomy and has been explained as a loss of fear of impregnating women companions. In monkeys, any alterations in sexual behavior after vasectomy probably are the result, not of cultural factors but of anatomical or immunologic changes resulting from the operation. The sexual behavior of male rhesus monkeys vasectomized 5 years earlier did not differ from that of sham-vasectomized controls. There was, however, a significant difference in the mean latencies to first mount and first intromission; vasectomized males had shorter latencies. In another set of studies, there were no siginificant differences between males vasectomized 12 years earlier (mean) and nonvasectomized controls. We found no association between the presence or absence of antisperm antibodies in the vasectomized males and their levels of sexual performance.

Endogenous opiate systems and primate reproduction: Inability of naloxone to induce sexual activity in rhesus males

The opiate antagonist naloxone was tested for its effectiveness in inducing sexual behavior in sexually inactive male rhesus macaques (Macaca mulatta).Naloxone was administered to males directly into the carotid artery via a T-tube implant. Naloxone-treated males and sexually receptive females were pair tested for sexual behavior. Despite the positive behavioral cues of the females, the males failed to initiate heterosexual interactions at any level of naloxone therapy. The implications of this finding for the role of the endogenous opiate system in primate reproduction are discussed in relation to the homeostatic nature of the system.

Sexual behavior in aging male rhesus macaques: Effects of test duration on sexual performance

The sexual behavior displayed by nine old (20-year and older) rhesus (Macaca mulatta) males in 10-min tests was compared to that displayed in 1-hr tests. The tests were part of a long-term study on the decline in male sexual activity that accompanies old age. The males were paired with 10 ovariectomized, estrogentreated females in two blocks of 10 tests; each male was tested once with each female in each test block. The percentages of males that achieved intromissions and ejaculated in the two test blocks were the same (P < 0.05). Although the percentage of tests in which males displayed these behaviors was higher in the longer-test block (P > 0.05), there was a significant positive correlation of performance in 10-min tests with performance in 1-hr tests. Assuming a random distribution of contacting, mounting, intromission, and ejaculation throughout the hour, we would have predicted a significantly lower number of these behaviors in the first 10 min of the 1-hr tests than we actually observed. The mean percentages of 1-hr tests with contacting, mounting, intromission, and ejaculation was significantly lower than that of 10-min tests conducted with the same males 11 years earlier. Thus, the decline in sexual performance was not an artifact of the limited (10-min) test duration.