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

Hormonal Control of Sexual Behavior
Hormonal Control of Female Reproductive Cycles Hormonal Control of Sexual Behavior of Laboratory Animals Organizational Effects of Androgens on Behavior: Masculinization and Defeminization Effects of Pheromones Human Sexual Behavior Sexual Orientation Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Neural Control of Sexual Behavior Males Females Formation of Pair Bonds Section Summary Parental Behavior Maternal Behavior of Rodents Hormonal Control of Maternal Behavior Neural Control of Maternal Behavior Neural Control of Paternal Behavior Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Learning Objectives 1. Describe mammalian sexual development and explain the factors that control it. 2. Describe the hormonal control of the female reproductive cycle and of male and female sexual behavior. 3. Describe the role of pheromones in reproductive physiology and sexual behavior. 4. Discuss the activational effects of gonadal hormones on the sexual behavior of women and men. 5. Discuss sexual orientation and the effects of prenatal androgenization of genetic females and the failure of androgenization of genetic males. 6. Discuss the neural control of male sexual behavior. 7. Discuss the neural control of female sexual behavior. 8. Describe the maternal behavior of rodents and discuss the hormonal and neural mechanisms that control maternal behavior and paternal behavior. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Reproductive behaviors constitute the most important category of social behaviors, because without them, most species would not survive. These behaviors—which include courting, mating, parental behavior, and most forms of aggressive behaviors—are the most striking categories of sexually dimorphic behaviors, that is, behaviors that differ in males and females (di + morphous, “two forms”). Copyright © 2014 Pearson Education, Inc. All Rights Reserved

sexually dimorphic behavior A behavior that has different forms or that occurs with different probabilities or under different circumstances in males and females. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization gamete (gamm eet) A mature reproductive cell; a sperm or ovum. sex chromosome The X and Y chromosomes, which determine an organism’s gender. Normally, XX individuals are female, and XY individuals are male. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization There are two types of sex chromosomes: X chromosomes and Y chromosomes. Females have two X chromosomes (XX); thus, all the ova that a woman produces will contain an X chromosome. Males have an X and a Y chromosome (XY). When a man’s sex chromosomes divide, half the sperms contain an X chromosome and the other half contain a Y chromosome. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization A Y-bearing sperm produces an XY-fertilized ovum and therefore a male. An X-bearing sperm produces an XX-fertilized ovum and therefore a female. (See Figure 9.1.) Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Development of the Sex Organs The X chromosome and the twenty-two nonsex chromosomes found in the cells of both males and females contain all the information needed to develop the bodies of either sex. Exposure to sex hormones, both before and after birth, is responsible for our sexual dimorphism. What the Y chromosome does control is the development of the glands that produce the male sex hormones. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Gonads There are three general categories of sex organs: the gonads, the internal sex organs, and the external genitalia. gonad (rhymes with moan ad) An ovary or testis. Sry The gene on the Y chromosome whose product instructs the undifferentiated fetal gonads to develop into testes. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Internal Sex Organs Early in embryonic development, the internal sex organs are bisexual; that is, all embryos contain the precursors for both female and male sex organs. However, during the third month of gestation, only one of these precursors develops; the other withers away. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Internal Sex Organs Müllerian system The embryonic precursors of the female internal sex organs. Wolffian system The embryonic precursors of the male internal sex organs. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Internal Sex Organs anti-Müllerian hormone A peptide secreted by the fetal testes that inhibits the development of the Müllerian system, which would otherwise become the female internal sex organs. defeminizing effect An effect of a hormone present early in development that reduces or prevents the later development of anatomical or behavioral characteristics typical of females. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Internal Sex Organs androgen (an dro jen) A male sex steroid hormone. Testosterone is the principal mammalian androgen. masculinizing effect An effect of a hormone present early in development that promotes the later development of anatomical or behavioral characteristics typical of males. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Internal Sex Organs testosterone (tess tahss ter own) The principal androgen found in males. dihydrotestosterone (dy hy dro tess tahss ter own) An androgen, produced from testosterone through the action of an enzyme. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Internal Sex Organs The fact that the internal sex organs of the human embryo are bisexual and could potentially develop as either male or female is dramatically illustrated by two genetic disorders: androgen insensitivity syndrome and persistent Müllerian duct syndrome. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Internal Sex Organs androgen insensitivity syndrome A condition caused by a congenital lack of functioning androgen receptors; in a person with XY sex chromosomes, causes the development of a female with testes but no internal sex organs. persistent Müllerian duct syndrome A condition caused by a congenital lack of anti- Müllerian hormone or receptors for this hormone; in a male, causes development of both male and female internal sex organs. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization Internal Sex Organs Turner’s syndrome The presence of only one sex chromosome (an X chromosome); characterized by lack of ovaries but otherwise normal female sex organs and genitalia. People with Turner’s syndrome have no gonads at all, they develop into females, with normal female internal sex organs and external genitalia. They cannot bear children, because without ovaries they cannot produce ova. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization External Genitalia The external genitalia are the visible sex organs, including the penis and scrotum in males and the labia, clitoris, and outer part of the vagina in females. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Production of Gametes and Fertilization External Genitalia The external genitalia do not need to be stimulated by female sex hormones to become female; they just naturally develop that way. In the presence of dihydrotestosterone the external genitalia will become male. Thus, the gender of a person’s external genitalia is determined by the presence or absence of an androgen. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Sexual Maturation The primary sex characteristics include the gonads, internal sex organs, and external genitalia. These organs are present at birth. The secondary sex characteristics, such as enlarged breasts and widened hips or a beard and deep voice, do not appear until puberty. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Sexual Maturation gonadotropin-releasing hormone (go nad oh trow pin) A hypothalamic hormone that stimulates the anterior pituitary gland to secrete gonadotropic hormone. gonadotropic hormone A hormone of the anterior pituitary gland that has a stimulating effect on cells of the gonads. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Sexual Maturation follicle-stimulating hormone (FSH) The hormone of the anterior pituitary gland that causes development of an ovarian follicle and the maturation of an ovum. luteinizing hormone (LH) (lew tee a nize ing) A hormone of the anterior pituitary gland that causes ovulation and development of the ovarian follicle into a corpus luteum. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Sexual Maturation The secretion of GnRh, which directs the production of the gonadoptropic hormones, which in turn stimulate puberty and production of the sex hormones secreted by the gonads, is under the control of another peptide: kisspeptin. Kisspeptin, produced by neurons in the arcuate nucleus of the hypothalamus, is essential for the initiation of puberty and the maintenance of male and female reproductive ability (Millar et al., 2010). Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Sexual Development Sexual Maturation estradiol (ess tra dye ahl) The principal estrogen of many mammals, including humans. estrogen (ess trow jen) A class of sex hormones that cause maturation of the female genitalia, growth of breast tissue, and development of other physical features characteristic of females. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

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

Hormonal Control of Sexual Behavior of Laboratory Animals The interactions between sex hormones and the human brain are difficult to study. We must turn to two sources of information: experiments with animals and various developmental disorders in humans, which serve as nature’s own “experiments.” Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Sexual Behavior of Laboratory Animals Males The sexual behavior of rats has been studied more than that of any other laboratory animal (Hull and Dominguez, 2007). When a male rat encounters a receptive female, he will spend some time nuzzling her and sniffing and licking her genitals, mount her, and engage in pelvic thrusting. He will mount her several times, achieving intromission on most of the mountings. After eight to fifteen intromissions approximately 1 minute apart (each lasting only about one-quarter of a second), the male will ejaculate. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Sexual Behavior of Laboratory Animals Males After ejaculating, the male refrains from sexual activity for a period of time (minutes, in the rat). Most mammals will return to copulate several times, finally showing a longer pause, called a refractory period. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Sexual Behavior of Laboratory Animals Males refractory period (ree frak to ree) A period of time after a particular action (for example, an ejaculation by a male) during which that action cannot occur again. Coolidge effect The restorative effect of introducing a new female sex partner to a male that has apparently become “exhausted” by sexual activity. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Sexual Behavior of Laboratory Animals Females It is true that in some species the female’s role during the act of copulation is merely to assume a posture that exposes her genitals to the male. This behavior is called the lordosis response (from the Greek lordos, meaning “bent backward”). lordosis A spinal sexual reflex seen in many four-legged female mammals; arching of the back in response to approach of a male or to touching the flanks, which elevates the hindquarters. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Sexual Behavior of Laboratory Animals Females The sequence of estradiol followed by progesterone has three effects on female rats: It increases their receptivity, their proceptivity, and their attractiveness. Receptivity refers to their ability and willingness to copulate—to accept the advances of a male by holding still and displaying. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Sexual Behavior of Laboratory Animals Females Proceptivity refers to a female’s eagerness to copulate, as shown by the fact that she seeks out a male and engages in behaviors that tend to arouse his sexual interest. Attractiveness refers to physiological and behavioral changes that affect the male. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Organizational Effects of Androgens on Behavior: Masculinization and Defeminization The dictum “Nature’s impulse is to create a female” applies to sexual behavior as well as to sex organs. That is, if a rodent’s brain is not exposed to androgens during a critical period of development, the animal will engage in female sexual behavior as an adult (if it is then given estradiol and progesterone). Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Organizational Effects of Androgens on Behavior: Masculinization and Defeminization Behavioral defeminization refers to the organizational effect of androgens that prevents the animal from displaying female sexual behavior in adulthood. Behavioral masculinization refers to the organizational effect of androgens that enables animals to engage in male sexual behavior in adulthood. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Effects of Pheromones Hormones transmit messages from one part of the body (the secreting gland) to another (the target tissue). Another class of chemicals, called pheromones, carries messages from one animal to another. Some of these chemicals, like hormones, affect reproductive behavior. pheromone (fair oh moan) A chemical released by one animal that affects the behavior or physiology of another animal; usually smelled or tasted. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Effects of Pheromones Lee-Boot effect The slowing and eventual cessation of estrous cycles in groups of female animals that are housed together; caused by a pheromone in the animals’ urine; first observed in mice. Whitten effect The synchronization of the menstrual or estrous cycles of a group of females, which occurs only in the presence of a pheromone in a male’s urine. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Effects of Pheromones Vandenbergh effect The earlier onset of puberty seen in female animals that are housed with males; caused by a pheromone in the male’s urine; first observed in mice. Bruce effect Termination of pregnancy caused by the odor of a pheromone in the urine of a male other than the one that impregnated the female; first identified in mice. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Effects of Pheromones Detection of odors is accomplished by the olfactory bulbs, which constitute the primary olfactory system. However, many of the effects that pheromones have on reproductive cycles are mediated by another sensory organ—the vomeronasal organ (VNO)—which consists of a small group of sensory receptors arranged around a pouch connected by a duct to the nasal passage. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Effects of Pheromones vomeronasal organ (VNO) (voah mer oh nay zul) A sensory organ that detects the presence of certain chemicals, especially when a liquid is actively sniffed; mediates the effects of some pheromones. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Effects of Pheromones The vomeronasal organ, which is present in all orders of mammals except for cetaceans (whales and dolphins), projects to the accessory olfactory bulb, located immediately behind the olfactory bulb (Wysocki, 1979). (See Figure 9.8.) accessory olfactory bulb A neural structure located in the main olfactory bulb that receives information from the vomeronasal organ. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Effects of Pheromones It appears that at least some pheromone-related phenomena occur in humans. McClintock (1971) studied the menstrual cycles of women attending an all-female college. She found that women who spent a large amount of time together tended to have synchronized cycles—their menstrual periods began within a day or two of one another. In addition, women who regularly spent some time in the presence of men tended to have shorter cycles than those who rarely spent time with (smelled?) men. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Effects of Pheromones Whether or not pheromones play a role in sexual attraction in humans, the familiar odor of a sex partner probably has a positive effect on sexual arousal—just like the sight of a sex partner or the sound of his or her voice. It is likely that men and women can learn to be attracted by their partners’ characteristic odors, just as they can learn to be attracted by the sound of their voice. In an instance like this, the odors are serving simply as sensory cues, not as pheromones. 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 Prenatal Androgenization of Genetic Females Evidence suggests that prenatal androgens can affect human social behavior and sexual orientation, as well as anatomy. In a disorder known as congenital adrenal hyperplasia (CAH), the adrenal glands secrete abnormal amounts of androgens. congenital adrenal hyperplasia (CAH) (hy per play zha) A condition characterized by hypersecretion of androgens by the adrenal cortex; in females, causes masculinization of the external genitalia. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Prenatal Androgenization of Genetic Females Children typically show sex differences in behaviors such as toy preferences (Alexander, 2003). Boys generally prefer toys that can be used actively, especially those that move or can be propelled by the child. Girls generally prefer toys that provide the opportunity for nurturance. Of course, it is an undeniable fact that both caregivers and peers encourage “sex-typical” toy choices. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Human Sexual Behavior Prenatal Androgenization of Genetic Females However, evidence suggests that biology may play a role in the nature of these choices. For example, even at one day of age, baby boys prefer to watch a moving mobile and baby girls prefer to look at a female face (Connellan et al., 2000). Alexander and Hines (2002) found that young vervet monkeys showed the same sexually dimorphic preferences in choice of toys that children do: Males chose to play with a car and a ball, whereas females preferred to play with a doll and a pot. (See Figure 9.10.) 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 In female voles, oxytocin appears to play a major role in pair bonding. Mating stimulates the release of oxytocin, and injection of oxytocin into the cerebral ventricles facilitates pair bonding in female prairie voles (Williams et al., 1994). In contrast, a drug that blocks oxytocin receptor disrupts formation of pair bonds. 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

In most mammalian species, reproductive behavior takes place after the offspring are born as well as at the time they are conceived. This section examines the role of hormones in the initiation and maintenance of maternal behavior and the role of the neural circuits that are responsible for their expression. Most of the research has involved rodents; less is known about the neural and endocrine bases of maternal behavior in primates. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Although most research on the physiology of parental behavior has focused on maternal behavior, some researchers are now studying paternal behavior shown by the males of some species of rodents. It goes without saying that the paternal behavior of human fathers is very important for the offspring of our species, but the physiological basis of this behavior has not yet been studied. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Maternal Behavior of Rodents The final test of the fitness of an animal’s genes is the number of offspring that survive to a reproductive age. Just as the process of natural selection favors reproductively competent animals, it favors those that care adequately for their young, if their young in fact require care. Rat and mouse pups certainly do; they cannot survive without a mother to attend to their needs. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Maternal Behavior of Rodents parturition (par tew ri shun) The act of giving birth. As a pup begins to emerge, she assists the uterine contractions by pulling the pup out with her teeth. She then eats the placenta and umbilical cord and cleans off the fetal membranes—a quite delicate operation. After all the pups are born and cleaned up, the mother will probably nurse them. Milk is usually present in the mammary glands very near the time of birth. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Maternal Behavior As we saw earlier in this chapter, most sexually dimorphic behaviors are controlled by the organizational and activational effects of sex hormones. Maternal behavior is somewhat different in this respect. First, there is no evidence that organizational effects of hormones play a role; as we will see, under the proper conditions even males will take care of infants. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Maternal Behavior Second, although maternal behavior is affected by hormones, it is not controlled by them. Most virgin female rats will begin to retrieve and care for young pups after having infants placed with them for several days (Wiesner and Sheard, 1933). And once the rats are sensitized, they will thereafter take care of pups as soon as they encounter them; sensitization lasts for a lifetime. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Hormonal Control of Maternal Behavior Although pregnant female rats will not immediately care for foster pups that are given to them during pregnancy, they will do so as soon as their own pups are born. The hormones that influence a female rodent’s responsiveness to her offspring are the ones that are present shortly before parturition. Figure 9.14 shows the levels of the three hormones that have been implicated in maternal behavior: progesterone, estradiol, and prolactin. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Neural Control of Maternal Behavior The medial preoptic area, the region of the forebrain that plays the most critical role in male sexual behavior, appears to play a similar role in maternal behavior. Numan (1974) found that lesions of the MPA disrupted both nest building and pup care. The mothers simply ignored their offspring. However, female sexual behavior was unaffected by these lesions. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Neural Control of Maternal Behavior An fMRI study—this time with humans—found that when mothers looked at pictures of their infants, brain regions involved in reinforcement and those that contain receptors for oxytocin and vasopressin showed increased activity. Regions involved with negative emotions, such as the amygdala, showed decreased activity (Bartels and Zeki, 2004). We already know that mothers (and fathers too, for that matter) form intense bonds with their infants, so it should not come as a surprise that regions involved with reinforcement should be activated by the sight of their faces. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Neural Control of Paternal Behavior Newborn infants of most species of mammals are cared for by their mother, and it is, of course, their mother that feeds them. However, males of a few species of rodents share the task of infant care with the mothers, and the brains of these nurturing fathers show some interesting differences compared with those of nonpaternal fathers of other species. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Neural Control of Paternal Behavior Little is known about the brain mechanisms involved in human paternal behavior, but evidence suggests that both prolactin and oxytocin facilitate a father’s role in infant care. Fleming et al. (2002) found that fathers with higher blood levels of prolactin reported stronger feelings of sympathy and activation when they heard the cries of infants. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Neural Control of Paternal Behavior Gordon et al. (2010) found that higher levels of prolactin were associated with more exploratory toy-manipulating play behavior with their infants, and that higher levels of oxytocin were associated with synchronous, coordinated emotional behavior between father and infant. Copyright © 2014 Pearson Education, Inc. All Rights Reserved

Copyright © 2014 Pearson Education, Inc. All Rights Reserved

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