Reproductive Behavior

Reproductive Behavior
Chapter 9 Reproductive Behavior Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Sexual Behavior
Hormones are responsible for sexual dimorphism in the structure of the body and its organs. Hormones have organizational and activational effects on the internal sex organs, genitals, and secondary sex characteristics. Naturally, all of these effects influence a person’s behavior. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Female Reproductive Cycles menstrual cycle (men strew al) The female reproductive cycle of most primates, including humans; characterized by growth of the lining of the uterus, ovulation, development of a corpus luteum, and (if pregnancy does not occur) menstruation. estrous cycle The female reproductive cycle of mammals other than primates. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Female Reproductive Cycles The LH surge causes ovulation: The ovarian follicle ruptures, releasing the ovum. Under the continued influence of LH, the ruptured ovarian follicle becomes a corpus luteum (“yellow body”), which produces estradiol and progesterone. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Female Reproductive Cycles corpus luteum (lew tee um) A cluster of cells that develops from the ovarian follicle after ovulation; secretes estradiol and progesterone. progesterone (pro jess ter own) A steroid hormone produced by the ovary that maintains the endometrial lining of the uterus during the later part of the menstrual cycle and during pregnancy; along with estradiol it promotes receptivity in female mammals with estrous cycles. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Human sexual behavior, like that of other mammals, is influenced by activational effects of gonadal hormones and, almost certainly, by organizational effects as well. If hormones have organizational effects on human sexual behavior, they must exert these effects by altering the development of the brain. Although there is good evidence that prenatal exposure to androgens affects development of the human brain, the consequences of this exposure are not yet fully understood. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Activational Effects of Sex Hormones in Women As we saw, the sexual behavior of most female mammals other than higher primates is controlled by the ovarian hormones estradiol and progesterone. As Wallen (1990) pointed out, the ovarian hormones control not only the willingness (or even eagerness) of an estrous female to mate but also her ability to mate. That is, a male rat cannot copulate with a female rat that is not in estrus. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Activational Effects of Sex Hormones in Women In higher primates (including our own species) the ability to mate is not controlled by ovarian hormones. There are no physical barriers to sexual intercourse during any part of the menstrual cycle. If a woman or other female primate consents to sexual activity at any time (or is forced to submit by a male), intercourse can certainly take place. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Activational Effects of Sex Hormones in Women A study by Van Goozen et al. (1997) supports this suggestion. The investigators found that the sexual activity initiated by men and women showed very different relations to the woman’s menstrual cycle (and hence to her level of ovarian hormones). Men initiated sexual activity at about the same rate throughout the woman’s cycle, whereas sexual activity initiated by women showed a distinct peak around the time of ovulation, when estradiol levels are highest. (See Figure 9.9.) Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Activational Effects of Sex Hormones in Women Gangestad and Thornhill (2008) suggest that women’s sexuality changes across the menstrual cycle in a particular way: They do not become indiscriminately more interested in sexual contact during their fertile period, which occurs around the time of ovulation. Instead, because they are more likely to become pregnant if they engage in unprotected sex at that time, they become more choosy. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Activational Effects of Sex Hormones in Women In particular, they become more attracted to male characteristics that might indicate high genetic quality (or did so in the evolution of our species). For example, Gangestad and Thornhill note that studies have shown that at mid-cycle, women’s preference increases for the sight of facial and bodily masculinity, for masculine behavioral displays, for masculine vocal qualities, for androgen-related scents, and for body symmetry, which correlates with genetic fitness. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Activational Effects of Sex Hormones in Men Although women and mammals with estrous cycles differ in their behavioral responsiveness to sex hormones, men resemble other mammals in their behavioral responsiveness to testosterone. With normal levels they can be potent and fertile; without testosterone sperm production ceases, and sooner or later, so does sexual potency. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Activational Effects of Sex Hormones in Men In a double-blind study, Bagatell et al. (1994) gave a placebo or a gonadotropin-releasing hormone (GnRH) antagonist to young male volunteers to temporarily suppress secretion of testicular androgens. Within two weeks, the subjects who received the GnRH antagonist reported a decrease in sexual interest, sexual fantasy, and intercourse. Men who received replacement doses of testosterone along with the antagonist did not show these changes. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation A large-scale study of several hundred male and female homosexuals reported by Bell, Weinberg, and Hammersmith (1981) attempted to assess the effects of these factors. The researchers found no evidence that homosexuals had been raised by domineering mothers or submissive fathers, as some clinicians had suggested. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation The best predictor of adult homosexuality was a self-report of homosexual feelings, which usually preceded homosexual activity by three years. The investigators concluded that their data did not support social explanations for homosexuality but were consistent with the possibility that homosexuality is at least partly biologically determined. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation If homosexuality does have a physiological cause, it certainly is not variations in the levels of sex hormones during adulthood. Many studies have examined the levels of sex steroids in male homosexuals (Meyer-Bahlburg, 1984), and the vast majority of them found these levels to be similar to those of heterosexuals. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation A few studies suggest that about 30 percent of female homosexuals have elevated levels of testosterone (but still lower than those found in men). Whether these differences are related to a biological cause of female homosexuality or whether differences in lifestyles may increase the secretion of testosterone is not yet known. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation A more likely biological cause of homosexuality is a subtle difference in brain structure caused by differences in the amount of prenatal exposure to androgens. Perhaps, then, the brains of male homosexuals are neither masculinized nor defeminized, those of female homosexuals are masculinized and defeminized, and those of bisexuals are masculinized but not defeminized. Of course, these are hypotheses, not conclusions. They should be regarded as suggestions to guide future research. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Failure of Androgenization of Genetic Males As we saw, genetic males with androgen insensitivity syndrome develop as females, with female external genitalia—but also with testes and without uterus or Fallopian tubes. If an individual with this syndrome is raised as a girl, all is well. Normally, the testes are removed because they often become cancerous; but if they are not, the body will mature into that of a woman at the time of puberty through the effects of the small amounts of estradiol produced by the testes. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation and the Brain The human brain is a sexually dimorphic organ. This fact was long suspected, even before confirmation was received from anatomical and functional imaging studies. For example, neurologists discovered that the two hemispheres of a woman’s brain appear to share functions more than those of a man’s brain do. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation and the Brain If a man sustains a stroke that damages the left side of the brain, he is more likely to show impairments in language than will a woman with similar damage. Presumably, the woman’s right hemisphere shares language functions with the left, so damage to one hemisphere is less devastating than it is in men. Also, men’s brains are, on the average, somewhat larger— apparently because men’s bodies are generally larger than those of women. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation and the Brain In addition, the sizes of some specific regions of the telencephalon and diencephalon are different in males and females, and the shape of the corpus callosum may also be sexually dimorphic. Most investigators believe that the sexual dimorphism of the human brain is a result of differential exposure to androgens prenatally and during early postnatal life. Of course, additional changes could occur at the time of puberty, when another surge in androgens occurs. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation and the Brain As we saw earlier, functional imaging studies have found that the brains of heterosexual men and women reacted differently to the odors of AND and EST, two chemicals that may serve as human pheromones. Savic, Berglund, and Lindström (2005) found that the response of brain regions of homosexual men to AND and EST was similar to that of the heterosexual women. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Sexual Orientation and the Brain Berglund, Lindström, and Savic (2006) found that the response of brain regions of homosexual women to these substances was similar to those of heterosexual men. These studies suggest that a person’s sexual orientation affects (or is affected by) his or her response pattern to these potential sexual pheromones. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Heredity and Sexual Orientation Another factor that may play a role in sexual orientation is heredity. Twin studies take advantage of the fact that identical twins have identical genes, whereas the genetic similarity between fraternal twins is, on the average, 50 percent. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Heredity and Sexual Orientation Bailey and Pillard (1991) studied pairs of male twins in which at least one member identified himself as homosexual. If both twins are homosexual, they are said to be concordant for this trait. If only one is homosexual, the twins are said to be discordant. Thus, if homosexuality has a genetic basis, the percentage of monozygotic twins who are concordant for homosexuality should be higher than that for dizygotic twins. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Heredity and Sexual Orientation This is exactly what Bailey and Pillard found: The concordance rate was 52 percent for identical twins and only 22 percent for fraternal twins—a difference of 30 percent. Other studies have shown differences of up to 60 percent (Gooren, 2006). Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Heredity and Sexual Orientation To summarize, evidence suggests that two biological factors— prenatal hormonal exposure and heredity—may affect a person’s sexual orientation. These research findings certainly contradict the suggestion that a person’s sexual orientation is a moral issue. It appears that homosexuals are no more responsible for their sexual orientation than heterosexuals are. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Neural Control of Sexual Behavior
The control of sexual behavior—at least in laboratory animals— involves different brain mechanisms in males and females. This section describes these mechanisms. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Males Erection and ejaculation are controlled by circuits of neurons that reside in the spinal cord. Coolen and her colleagues (Coolen et al., 2004; Coolen, 2005) have identified a group of neurons in the lumbar region of the rat spinal cord that appear to constitute a critical part of the spinal ejaculation generator. However, brain mechanisms have both excitatory and inhibitory control of these circuits. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Males medial preoptic area (MPA) An area of cell bodies just rostral to the hypothalamus; plays an essential role in male sexual behavior. The medial preoptic area (MPA), located just rostral to the hypothalamus, is the forebrain region most critical for male sexual behavior. Electrical stimulation of this region elicits male copulatory behavior. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Males The organizational effects of androgens are responsible for sexual dimorphisms in brain structure. Gorski et al. (1978) discovered a nucleus within the MPA of the rat that is three to seven times larger in males than in females. This area is called (appropriately enough) the sexually dimorphic nucleus (SDN) of the preoptic area. sexually dimorphic nucleus A nucleus in the preoptic area that is much larger in males than in females; first observed in rats; plays a role in male sexual behavior. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Males The motor neurons that innervate the pelvic organs involved in copulation are located in the lumbar region of the spinal cord (Coolen et al., 2004). Anatomical tracing studies suggest that the most important connections between the MPA and the motor neurons of the spinal cord are accomplished through the periaqueductal gray matter (PAG) of the midbrain and the nucleus paragigantocellularis (nPGI) of the medulla. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Males periaqueductal gray matter (PAG) The region of the midbrain that surrounds the cerebral aqueduct; plays an essential role in various species- typical behaviors, including female sexual behavior. nucleus paragigantocellularis (nPGi) A nucleus of the medulla that receives input from the medial preoptic area and contains neurons whose axons form synapses with motor neurons in the spinal cord that participate in sexual reflexes in males. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Males The inhibitory connections between neurons of the nPGI and those of the ejaculation generator are serotonergic. As Marson and McKenna (1996) showed, application of serotonin (5-HT) to the spinal cord suppresses ejaculation. This connection may explain a well-known side effect of selective serotonin reuptake inhibitors (SSRIs). Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Males Men who take SSRIs as a treatment for depression often report that they have no trouble attaining an erection but have difficulty achieving an ejaculation. Presumably, the action of the drug as an agonist at serotonergic synapses in the spinal cord increases the inhibitory influence of nPGi neurons on spinal neurons responsible for ejaculation. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Females Just as the MPA plays an essential role in male sex behavior, another region in the ventral forebrain plays a similar role in female sexual behavior: the ventromedial nucleus of the hypothalamus (VMH). A female rat with bilateral lesions of the ventromedial nuclei will not display lordosis, even if she is treated with estradiol and progesterone. Conversely, electrical stimulation of the ventromedial nucleus facilitates female sexual behavior. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Females ventromedial nucleus of the hypothalamus (VMH) A large nucleus of the hypothalamus located near the walls of the third ventricle; plays an essential role in female sexual behavior. As we saw earlier, sexual behavior of female rats is activated by a priming dose of estradiol, followed by progesterone. The estrogen sets the stage, so to speak, and the progesterone stimulates the sexual behavior. Injections of these hormones directly into the VMH will stimulate sexual behavior even in females whose ovaries have been removed. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Females Daniels, Miselis, and Flanagan-Cato (1999) injected a transneuronal retrograde tracer, pseudorabies virus, in the muscles responsible for the lordosis response in female rats. They found that the pathway innervating these muscles was as previous studies predicted: VMHPAGnPGimotor neurons in the ventral horn of the lumbar region of the spinal cord. Figure 9.13 summarizes the evidence I have presented so far in this section. (See Figure 9.13.) Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Formation of Pair Bonds In approximately 5 percent of mammalian species, heterosexual couples form monogamous, long-lasting bonds. In humans, such bonds can be formed between members of homosexual couples as well. As naturalists and anthropologists have pointed out, monogamy is not always exclusive: In many species of animals, humans included, individuals sometimes cheat on their partners. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Formation of Pair Bonds Several studies have revealed a relation between monogamy and the levels of two peptides in the brain: vasopressin and oxytocin. These compounds are both released as hormones by the posterior pituitary gland and as neurotransmitters by neurons in the brain. In males, vasopressin appears to play the more important role. Monogamous voles have a higher level of vasopressin receptors in the ventral forebrain than do polygamous voles. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Formation of Pair Bonds Many investigators believe that oxytocin and vasopressin may play a role in the formation of pair bonding in humans. For example, after intercourse, at a time when blood levels of oxytocin are increased, people report feelings of calmness and well being, which are certainly compatible with the formation of bonds with one’s partner. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

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