PowerPoint Presentation for Physiology of Behavior 11 th Edition by Neil R. Carlson Prepared by Grant McLaren, Edinboro University of Pennsylvania This.

PowerPoint Presentation for Physiology of Behavior 11 th Edition by Neil R. Carlson Prepared by Grant McLaren, Edinboro University of Pennsylvania This multimedia product and its contents are protected under copyright law. The following are prohibited by law: any public performance or display, including transmission of any image over a network; preparation of any derivative work, including extraction, in whole or in part, of any images; any rental lease, or lending of the program. COPYRIGHT © ALLYN & BACON 2012

Reproductive Behavior COPYRIGHT © ALLYN & BACON 2012 4 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 Section Summary

Reproductive Behavior COPYRIGHT © ALLYN & BACON 2012 6 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”). sexually dimorphic behavior A behavior that has different forms or that occurs with different probabilities or under different circumstances in males and females.

Reproductive Behavior COPYRIGHT © ALLYN & BACON 2012 7 As you will see, hormones that are present both before and after birth play a very special role in the development and control of sexually dimorphic behaviors. This chapter describes male and female sexual development and then discusses the neural and hormonal control of two sexually dimorphic behaviors that are most important to reproduction: sexual behavior and parental behavior.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 8 A person’s chromosomal sex is determined at the time of fertilization. However, this event is merely the first in a series of steps that culminate in the development of a male or female. This section considers the major features of sexual development.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 9 Production of Gametes and Fertilization All cells of the human body (other than sperms or ova) contain twenty-three pairs of chromosomes. The genetic information that programs the development of a human is contained in the DNA that constitutes these chromosomes. The production of gametes (ova and sperms; gamein means “to marry”) entails a special form of cell division. gamete ( gamm eet ) A mature reproductive cell; a sperm or ovum.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 10 Production of Gametes and Fertilization This process produces cells that contain one member of each of the twenty-three pairs of chromosomes. The development of a human begins at the time of fertilization, when a single sperm and ovum join, sharing their twenty-three single chromosomes to reconstitute the twenty-three pairs.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 11 Production of Gametes and Fertilization A person’s genetic sex is determined at the time of fertilization of the ovum by the father’s sperm. Twenty-two of the twenty-three pairs of chromosomes determine the organism’s physical development independent of its sex. The last pair consists of two sex chromosomes, which contain genes that determine whether the offspring will be a boy or a girl. sex chromosome The X and Y chromosomes, which determine an organism’s gender. Normally, XX individuals are female, and XY individuals are male.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 12 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. 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 10.1. )

Sexual Development COPYRIGHT © ALLYN & BACON 2012 14 Development of Sex Organs Men and women differ in many ways: Their bodies are different, parts of their brains are different, and their reproductive behaviors are different. Are all these differences directly encoded on the tiny Y chromosome, the sole piece of genetic material that distinguishes males from females?

Sexual Development COPYRIGHT © ALLYN & BACON 2012 15 Development of Sex Organs The answer is no. 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 16 Gonads There are three general categories of sex organs: the gonads, the internal sex organs, and the external genitalia. The gonads —testes or ovaries—are the first to develop. Gonads (from the Greek gonos, “procreation”) have a dual function: They produce ova or sperms, and they secrete hormones. Through the sixth week of prenatal development, male and female fetuses are identical. gonad (rhymes with moan ad ) An ovary or testis.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 17 Gonads Sry The gene on the Y chromosome whose product instructs the undifferentiated fetal gonads to develop into testes. Both sexes have a pair of identical undifferentiated gonads, which have the potential of developing into either testes or ovaries. The factor that controls their development appears to be a single gene on the Y chromosome called Sry (sex-determining region Y). This gene produces a protein that binds to the DNA of cells in the undifferentiated gonads and causes them to become testes.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 18 Gonads Once the gonads have developed, a series of events is set into action that determines the individual’s gender. These events are directed by hormones, which affect sexual development in two ways. During prenatal development these hormones have organizational effects, which influence the development of a person’s sex organs and brain. organizational effect (of hormone) The effect of a hormone on tissue differentiation and development.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 19 Gonads These effects are permanent; once a particular path is followed in the course of development, there is no going back. The second role of sex hormones is their activational effect. These effects occur later in life, after the sex organs have developed. Because the bodies of adult males and females have been organized differently, sex hormones will have different activational effects in the two sexes.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 20 Gonads Because the bodies of adult males and females have been organized differently, sex hormones will have different activational effects in the two sexes. activational effect (of hormone) The effect of a hormone that occurs in the fully developed organism; may depend on the organism’s prior exposure to the organizational effects of hormones.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 21 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. The precursor of the internal female sex organs, which develops into the fimbriae and Fallopian tubes, the uterus, and the inner two-thirds of the vagina, is called the Müllerian system. Müllerian system The embryonic precursors of the female internal sex organs.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 22 Internal Sex Organs The precursor of the internal male sex organs, which develops into the epididymis, vas deferens, and seminal vesicles, is called the Wolffian system. (These systems were named after their discoverers, Müller and Wolff. See Figure 10.2. ) Wolffian system The embryonic precursors of the male internal sex organs

Sexual Development COPYRIGHT © ALLYN & BACON 2012 24 Internal Sex Organs The gender of the internal sex organs of a fetus is determined by the presence or absence of hormones secreted by the testes. If these hormones are present, the Wolffian system develops. If they are not, the Müllerian system develops. The Müllerian (female) system needs no hormonal stimulus from the gonads to develop; it just normally does so. (Turner’s syndrome, a disorder of sexual development that I will discuss later, provides evidence for this assertion.)

Sexual Development COPYRIGHT © ALLYN & BACON 2012 25 Internal Sex Organs In contrast, the cells of the Wolffian (male) system do not develop unless they are stimulated to do so by a hormone. Thus, testes secrete two types of hormones. The first, a peptide hormone called anti-Müllerian hormone, does exactly what its name says: It prevents the Müllerian (female) system from developing. 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 26 Internal Sex Organs It therefore has a defeminizing effect. 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. The second, a set of steroid hormones called androgens, stimulates the development of the Wolffian system. This class of hormone is also aptly named: Andros means “man,” and gennan means “to produce.”) Androgens have a masculinizing effect.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 27 Internal Sex Organs The second, a set of steroid hormones called androgens, stimulates the development of the Wolffian system. (This class of hormone is also aptly named: Andros means “man,” and gennan means “to produce.”) androgen ( an dro jen ) A male sex steroid hormone. Testosterone is the principal mammalian androgen. Andrograns have a masculinizing effect. masculinizing effect An effect of a hormone present early in development that promotes the later development of anatomical or behavioral characteristics typical of males.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 28 Internal Sex Organs Two different androgens are responsible for masculinization. The first, testosterone, is secreted by the testes and gets its name from these glands. An enzyme called 5  reductase converts some of the testosterone into another androgen, known as dihydrotestosterone.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 29 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 the enzyme 5  reductase.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 30 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. Some people are insensitive to androgens; they have androgen insensitivity syndrome, one of the more aptly named disorders (Money and Ehrhardt, 1972; MacLean, Warne, and Zajac, 1995). androgen insensitivity syndrome A condition caused by a congenital lack of functioning androgen receptors; in aperson with XY sex chromosomes, causes the development of a female with testes but no internal sex organs.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 31 Internal Sex Organs The cause of androgen insensitivity syndrome is a genetic mutation that prevents the formation of functioning androgen receptors. (The gene for the androgen receptor is located on the X chromosome.) The primitive gonads of a genetic male fetus with androgen insensitivity syndrome become testes and secrete both anti-Müllerian hormone and androgens. The lack of androgen receptors prevents the androgens from having a masculinizing effect; thus, the epididymis, vas deferens, seminal vesicles, and prostate fail to develop.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 32 Internal Sex Organs However, the anti-Müllerian hormone still has its defeminizing effect, preventing the female internal sex organs from developing. The uterus, fimbriae, and Fallopian tubes fail to develop, and the vagina is shallow The external genitalia are female, and at puberty the person develops a woman’s body. Of course, lacking a uterus and ovaries, the person cannot have children.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 33 Internal Sex Organs The second genetic disorder, persistent Müllerian duct syndrome, has two causes: either a failure to produce anti-Müllerian hormone or the absence of receptors for this hormone (Warne and Zajac, 1998). When this syndrome occurs in genetic males, androgens have their masculinizing effect but defeminization does not occur. 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 34 Internal Sex Organs Thus, the person is born with both sets of internal sex organs, male and female. The presence of the additional female sex organs usually interferes with normal functioning of the male sex organs.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 35 Internal Sex Organs What about prenatal sexual development in females? A chromosomal anomaly indicates that the hormones produced by female sex organs are not needed for development of the Müllerian system. This fact has led to the dictum “Nature’s impulse is to create a female.” People with Turner’s syndrome have only one sex chromosome: an X chromosome. 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 36 Internal Sex Organs Thus, instead of having XX cells, they have X0 cells—0 (zero) indicating a missing sex chromosome. In most cases the existing X chromosome comes from the mother, which means that the cause of the disorder lies with a defective sperm (Knebelmann et al., 1991). Because a Y chromosome is not present, testes do not develop.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 37 Internal Sex Organs In addition, because two X chromosomes are needed to produce ovaries, these glands are not produced either. But even though people with Turner’s syndrome have no gonads at all, they develop into females, with normal female internal sex organs and external genitalia—which proves that fetuses do not need ovaries or the hormones they produce to develop as females. Of course, they must be given estrogen pills to induce puberty and sexual maturation. And they cannot bear children, because without ovaries they cannot produce ova.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 38 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. (See Figure 10.3. ) As we just saw, 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 40 External Genitalia Thus, the gender of a person’s external genitalia is determined by the presence or absence of an androgen, which explains why people with Turner’s syndrome have female external genitalia even though they lack ovaries. People with androgen insensitivity syndrome have female external genitalia too, because without androgen receptors their cells cannot respond to the androgens produced by their testes. Figure 10.5 summarizes the factors that control the development of the gonads, internal sex organs, and genitalia. (See Figure 10.4. )

Sexual Development COPYRIGHT © ALLYN & BACON 2012 42 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 43 Sexual Maturation The onset of puberty occurs when cells in the hypothalamus secrete gonadotropin- releasing hormones (GnRH), which stimulate the production and release of two gonadotropic hormones by the anterior pituitary gland. The gonadotropic (“gonad-turning”) hormones stimulate the gonads to produce their hormones, which are ultimately responsible for sexual maturation. (See Figure 10.5. )

Sexual Development COPYRIGHT © ALLYN & BACON 2012 44 Sexual Maturation gonadotropin-releasing hormone (GnRH) ( 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 46 Sexual Maturation The two gonadotropic hormones are follicle-stimulating hormone (FSH) and luteinizing hormone (LH), named for the effects they produce in the female (production of a follicle and its subsequent luteinization, to be described in the next section of this chapter). 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 47 Sexual Maturation However, the same hormones are produced in the male, where they stimulate the testes to produce sperms and to secrete testosterone. If male and female pituitary glands are exchanged in rats, the ovaries and testes respond perfectly to the hormones secreted by the new glands (Harris and Jacobsohn, 1951– 1952).

Sexual Development COPYRIGHT © ALLYN & BACON 2012 48 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 A peptide produced by neurons in the arcuate nucleus of the hypothalamus under the control of leptin receptors; essential for initiation of puberty and maintenance of reproductive ability. The unusual name of this peptide does not refer to a behavior that sometimes serves as the beginning of a sexual encounter. Instead, it refers to Hershey, Pennsylvania, the location of the laboratory where the gene that encodes for the peptide was discovered.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 49 Sexual Maturation In response to the gonadotropic hormones (usually called gonadotropins ) the gonads secrete steroid sex hormones. The ovaries produce estradiol, one of a class of hormones known as estrogens. As we saw, the testes produce testosterone, an androgen. Both types of glands also produce a small amount of the hormones of the other sex.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 50 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.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 52 Section Summary: Sexual Development Gender is determined by the sex chromosomes: XX produces a female, and XY produces a male. Males are produced by the action of the Sry gene on the Y chromosome, which contains the code for the production of a protein that turn causes the primitive gonads to become testes. Testosterone and dihydrotestosterone (androgens) stimulate the development of the Wolffian system (masculinization), and anti-Müllerian hormone suppresses the development of the Müllerian system (defeminization).

Sexual Development COPYRIGHT © ALLYN & BACON 2012 53 Section Summary: Sexual Development Androgen insensitivity syndrome results from a hereditary defect in androgen receptors, and persistent Müllerian duct syndrome results from a hereditary defect in production of anti-Müllerian hormone or its receptors. By default the body is female (“Nature’s impulse is to create a female”); only by the actions of testicular hormones does it become male. Masculinization and defeminization are referred to as organizational effects of hormones; activational effects occur after development is complete.

Sexual Development COPYRIGHT © ALLYN & BACON 2012 54 Section Summary: Sexual Development Sexual maturity occurs when neurons in the arcuate nucleus of the hypothalamus begin secreting kisspeptin, which stimulates the secretion of gonadotropin-releasing hormone (GnRH), which in turn stimulates the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) by the anterior pituitary gland These hormones stimulate the gonads to secrete their hormones, thus causing the genitals to mature and the body to develop the secondary sex characteristics (activational effects).

Sexual Development COPYRIGHT © ALLYN & BACON 2012 55 Section Summary: Sexual Development Leptin, a hormone secreted by well-nourished fat tissue, appears to be one of the signals that stimulates the onset of puberty, at least in females. Leptin also enables neurons in the arcuate nucleus to secrete kisspeptin.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 56 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. Simply having the physique and genitals of a man or a woman exerts a powerful effect. But hormones do more than give us masculine or feminine bodies; they also affect behavior by interacting directly with the nervous system.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 57 Androgens that are present during prenatal development affect the development of the nervous system. In addition, both male and female sex hormones have activational effects on the adult nervous system that influence both physiological processes and behavior. This section considers some of these hormonal effects.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 58 Hormonal Control of Female Reproductive Cycles The reproductive cycle of female primates is called a menstrual cycle (from mensis, meaning “month”). 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 59 Hormonal Control of Female Reproductive Cycles Females of other species of mammals also have reproductive cycles, called estrous cycles. estrous cycle The female reproductive cycle of mammals other than primates. Estrus means “gadfly”; when a female rat is in estrus, her hormonal condition goads her to act differently than she does at other times.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 60 Hormonal Control of Female Reproductive Cycles A cycle begins with the secretion of gonadotropins by the anterior pituitary gland. These hormones (especially FSH) stimulate the growth of ovarian follicles, small spheres of epithelial cells surrounding each ovum. ovarian follicle A cluster of epithelial cells surrounding an oocyte, which develops into an ovum.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 61 Hormonal Control of Female Reproductive Cycles As ovarian follicles mature, they secrete estradiol, which causes growth of the lining of the uterus in preparation for implantation of the ovum, should it be fertilized by a sperm. Feedback from the increasing level of estradiol eventually triggers the release of a surge of LH by the anterior pituitary gland. (See Figure 10.6 )

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 62 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. (See Figure 10.6. ) corpus luteum ( lew tee um ) A cluster of cells that develops from the ovarian follicle after ovulation; secretes estradiol and progesterone.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 63 Hormonal Control of Female Reproductive Cycles 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. The latter hormone promotes pregnancy (gestation ).

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 65 Hormonal Control of Female Reproductive Cycles It maintains the lining of the uterus, and it inhibits the ovaries from producing another follicle. Meanwhile, the ovum enters one of the Fallopian tubes and begins its progress toward the uterus. If it meets sperm cells during its travel down the Fallopian tube and becomes fertilized, it begins to divide, and several days later it attaches itself to the uterine wall.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 66 Hormonal Control of Female Reproductive Cycles If the ovum is not fertilized or if it is fertilized too late to develop sufficiently by the time it gets to the uterus, the corpus luteum will stop producing estradiol and progesterone, and then the lining of the walls of the uterus will slough off. At this point, menstruation will commence.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 67 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.” Let us first consider the evidence gathered from research with laboratory animals.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 68 Hormonal Control of Sexual Behavior of Laboratory Animals Males Male sexual behavior is quite varied, although the essential features of intromission (entry of the penis into the female’s vagina), pelvic thrusting (rhythmic movement of the hindquarters, causing genital friction), and ejaculation (discharge of semen) are characteristic of all male mammals. Humans, of course, have invented all kinds of copulatory and noncopulatory sexual behavior.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 69 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). Male rats reach sexual maturity. The sexual behavior of rats has been studied more than that of any other laboratory animal (Hull and Dominguez, 2007). Male rats reach sexual maturity between 45 and 75 days of age.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 70 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. (The term comes from the Latin refringere, “to break off.”) refractory period (ree frak to ree) particular action (for example, an ejaculation by a male) during which that action cannot occur again.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 71 Hormonal Control of Sexual Behavior of Laboratory Animals Males Other hormones play a role in male sexual behavior. Oxytocin is a hormone produced by the posterior pituitary gland that contracts the milk ducts and thus causes milk ejection in lactating females. oxytocin ( ox ee tow sin ) A hormone secreted by the posterior pituitary gland; causes contraction of the smooth muscle of the milk ducts, the uterus, and the male ejaculatory system; also serves as a neurotransmitter in the brain.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 72 Hormonal Control of Sexual Behavior of Laboratory Animals Males It is also produced in males, where it obviously plays no role in lactation. Oxytocin is released at the time of orgasm in both males and females and appears to contribute to the contractions of the smooth muscle in the male ejaculatory system and of the vagina and uterus (Carmichael et al., 1987; Carter, 1992). The effects of this hormonal release can easily be seen in lactating women, who often eject some milk at the time of orgasm. Oxytocin plays a role in establishment of pair bonding, a phenomenon that will be discussed later in this chapter.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 73 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 74 Hormonal Control of Sexual Behavior of Laboratory Animals Females Sexual behavior of female rodents depends on the gonadal hormones present during estrus: estradiol and progesterone. In rats, estradiol increases about 40 hours before the female becomes receptive; just before receptivity occurs, the corpus luteum begins secreting large quantities of progesterone (Feder, 1981). Ovariectomized rats (rats whose ovaries have been removed) are not sexually receptive.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 75 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 lordosis when he attempts to mount her. 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 76 Hormonal Control of Sexual Behavior of Laboratory Animals Females Attractiveness refers to physiological and behavioral changes that affect the male. The male rat (along with many other male mammals) is most responsive to females who are in estrus (“in heat”). Males will ignore a female whose ovaries have been removed, but injections of estradiol and progesterone will restore her attractiveness (and also change her behavior toward the male). The stimuli that arouse a male rat’s sexual interest include her odor and her behavior.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 77 Hormonal Control of Sexual Behavior of Laboratory Animals Females Even though women do not show obvious physical changes during the fertile period of their menstrual cycle, some subtle changes do occur. Roberts et al. (2004) took photos of women’s faces during fertile and nonfertile periods of their menstrual cycle and found that both men and women judged the photos taken during the fertile period to be more attractive than those taken during a nonfertile period.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 78 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 the animal is then given estradiol and progesterone). Fortunately for experimenters this critical time comes shortly after birth for rats and for several other species of rodents that are born in a rather immature condition.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 79 Organizational Effects of Androgens on Behavior: Masculinization and Defeminization Thus, if a male rat is castrated immediately after birth, permitted to grow to adulthood, and then given injections of estradiol and progesterone, it will respond to the presence of another male by arching its back and presenting its hindquarters. In other words, it will act as if it were a female (Blaustein and Olster, 1989). In contrast, if a rodent brain is exposed to androgens during development, two phenomena occur: behavioral defeminization and behavioral masculinization. Behavioral defeminization refers to the organizational effect of androgens that prevents the animal from displaying female sexual behavior in adulthood.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 80 Organizational Effects of Androgens on Behavior: Masculinization and Defeminization Behavioral masculinization refers to the organizational effect of androgens that enables animals to engage in male sexual behavior in adulthood. This effect is accomplished by stimulating the development of neural circuits controlling male sexual behavior. For example, if the female rodent in my previous example is given testosterone in adulthood rather than estradiol and progesterone, she will mount and attempt to copulate with a receptive female. (See Breedlove, 1992, and Carter, 1992, for references to specific studies.) (See Figure 10.7. )

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 82 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. Karlson and Luscher (1959) coined the term, from the Greek pherein, “to carry,” and horman, “to excite.”

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 83 Effects of Pheromones Pheromones are released by one animal and directly affect the physiology or behavior of another. In mammalian species most pheromones are detected by means of olfaction. pheromone ( fair oh moan ) A chemical released by one animal that affects the behavior or physiology of another animal; usually smelled or tasted.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 84 Effects of Pheromones Pheromones can affect reproductive physiology or behavior. First, let us consider the effects on reproductive physiology. When groups of female mice are housed together, their estrous cycles slow down and eventually stop.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 85 Effects of Pheromones This phenomenon is known as the Lee-Boot effect (van der Lee and Boot, 1955). 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 86 Effects of Pheromones If groups of females are exposed to the odor of a male (or of his urine), they begin cycling again, and their cycles tend to be synchronized. This phenomenon is known as the Whitten effect (Whitten, 1959). 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 87 Effects of Pheromones The Vandenbergh effect (Vandenbergh, Whitsett, and Lombardi, 1975) is the acceleration of the onset of puberty in a female rodent caused by the odor of a male. Both the Whitten effect and the Vandenbergh effect are caused by a group of compounds that are present only in the urine of intact adult males (Ma, Miao, and Novotny, 1999; Novotny et al., 1999); the urine of a juvenile or castrated male has no effect. 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. Thus, the production of the pheromone by a male requires the presence of testosterone.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 88 Effects of Pheromones 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. The Bruce effect (Bruce, 1960a, 1960b) is a particularly interesting phenomenon: When a recently impregnated female mouse encounters a normal male mouse other than the one with which she mated, the pregnancy is very likely to fail. This effect, too, is caused by a substance secreted in the urine of intact males—but not of males that have been castrated.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 89 Effects of Pheromones Thus, a male mouse that encounters a pregnant female is able to prevent the birth of infants carrying another male’s genes and subsequently impregnate the female himself. This phenomenon is advantageous even from the female’s point of view. The fact that the new male has managed to take over the old male’s territory indicates that he is probably healthier and more vigorous, and therefore, his genes will contribute to the formation of offspring that are more likely to survive.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 90 Effects of Pheromones As you learned in Chapter 7, 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 91 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 10.8. ) 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. accessory olfactory bulb A neural structure located in the main olfactory bulb that receives information from the vomeronasal organ.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 93 Effects of Pheromones Removal of the accessory olfactory bulb disrupts the Lee-Boot effect, the Whitten effect, the Vandenbergh effect, and the Bruce effect; thus, the vomeronasal system is essential for these phenomena (Halpern, 1987). The accessory olfactory bulb sends axons to the medial nucleus of the amygdala, which in turn projects to the preoptic area and anterior hypothalamus and to the ventromedial nucleus of the hypothalamus. medial nucleus of the amygdala ( a mig da la ) A nucleus that receives olfactory information from the olfactory bulb and accessory olfactory bulb; involved in the effects of odors and pheromones on reproductive behavior.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 94 Effects of Pheromones Thus, the neural circuit responsible for the effects of these pheromones appears to involve these regions. As we shall see, the preoptic area, the medial amygdala, the ventromedial nucleus of the hypothalamus, and the medial preoptic area all play important roles in reproductive behavior. (See Figure 10.9. )

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 96 Effects of Pheromones The males of some species produce sex-attractant pheromones that affect the behavior of females. For example, a pheromone present in the saliva of boars (male pigs) elicits sexual behavior in sows. This response persists even after the sow’s VNO is destroyed, which indicates that the main olfactory system can detect the pheromone and elicit the behavior (Dorries, Adkins, and Halpern, 1997).

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 97 Effects of Pheromones Other male pheromones that attract females are also detected by the main olfactory system. For example, Mak et al. (2007) found that the odor of soiled bedding taken from the cage of a male mouse activated neurons in the main olfactory system and hippocampus of female mice. The odor even stimulated neurogenesis (production of new neurons); Mak and her colleagues found new neurons in the olfactory bulb and hippocampus.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 98 Effects of Pheromones Moreover, bedding from cages of dominant males stimulated neurogenesis more effectively than did bedding from subordinate males As we will see in Chapter 13, one function of newly formed neurons appear to participate in memory formation. Perhaps neurogenesis stimulated by the odor of male mice provides the means by which females learn and remember the odor of that particular male.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 99 Effects of Pheromones What sensory organ detects the presence of human pheromones? Although humans have a small vomeronasal organ located along the nasal septum (bridge of tissue between the nostrils) approximately 2 cm from the opening of the nostril (Garcia-Velasco and Mondragon, 1991), the human VNO to be a vestigial, nonfunctional, organ. 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 100 Effects of Pheromones The density of neurons in the VNO is very sparse, and investigators have not found any neural connections from this organ to the brain (Doty, 2001). Evidence clearly shows that human reproductive physiology is affected by pheromones, but it appears that these chemical signals are detected by the “standard” olfactory system—the receptor cells in the olfactory epithelium—and not by cells in the VNO.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 101 Effects of Pheromones Whether or not pheromones play an important 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. We are not generally conscious of the fact, but we can identify other people on the basis of olfactory cues.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 102 Effects of Pheromones For example, a study by Russell (1976) found that people were able to distinguish by odor between T-shirts that they had worn and those previously worn by other people. They could also tell whether the unknown owner of a T-shirt was male or female. Thus, 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 103 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 104 Human Sexual Behavior 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. The evidence pertaining to these issues is discussed later, in a section on sexual orientation.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 105 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 106 Human Sexual Behavior Activational Effects of Sex Hormones in Women Although ovarian hormones do not control women’s sexual activity, they may still have an influence on women’s sexual interest. In stable, monogamous relationships in which the partners are together on a daily basis, sexual activity can be instigated by either of them.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 107 Human Sexual Behavior Activational Effects of Sex Hormones in Women Normally, a husband does not force his wife to have intercourse with him, but even if she is not interested in engaging in sexual activity at that moment, she may find that she wants to do so because of her affection for him. Thus, changes in sexual interest and arousability might not always be reflected in changes in sexual behavior. In fact, a study of lesbian couples (whose menstrual cycles are likely to be synchronized) found a significant increase in sexual interest and activity during the middle portions of the women’s cycles (Matteo and Rissman, 1984), which suggests that ovarian hormones do influence women’s sexual interest.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 108 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).

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 109 Human Sexual Behavior Activational Effects of Sex Hormones in Women 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 10.10. ) Bullivant et al. (2004) found that women were more likely to initiate sexual activity and were more likely to engage in sexual fantasies just before and during the surge in luteinizing hormone that stimulates ovulation.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 111 Human Sexual Behavior Activational Effects of Sex Hormones in Women Several studies suggest that women’s sexual interest can be stimulated by androgens. There are two primary sources of androgens in the female body: the ovaries and the adrenal glands.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 112 Human Sexual Behavior Activational Effects of Sex Hormones in Women The primary ovarian sex steroids are, of course, estradiol and progesterone, but these glands also produce testosterone. The adrenal glands produce another androgen, androstenedione, along with other adrenocortical steroids. However, the available evidence indicates that androgens by themselves (in the absence of estradiol) do not directly stimulate women’s sexual interest but appear to amplify the effects of estradiol.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 113 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 114 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 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 115 Human Sexual Behavior Activational Effects of Sex Hormones in Men The decline of sexual activity after castration is quite variable. As reported by Money and Ehrhardt (1972), some men lose potency immediately, whereas others show a slow, gradual decline over several years. Perhaps at least some of the variability is a function of prior experience; practice not only may “make perfect” but also may forestall a decline in function.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 116 Human Sexual Behavior Activational Effects of Sex Hormones in Men By the way, women also show similar changes in testosterone level. (I mention it here rather than in the previous subsection because it makes a good follow-up to the study I just described.) Hamilton and Meston (2010) studied women in long-distance relationships who saw their partners only intermittently. The investigators found that the women’s testosterone levels increased the day before they rejoined their partners—presumably in anticipation of their reunion.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 117 Sexual Orientation What controls a person’s sexual orientation, that is, the gender of the preferred sex partner? Some investigators believe that sexual orientation is determined by child- hood experiences, especially interactions between the child and parents.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 118 Sexual Orientation The researchers found no evidence that homosexuals had been raised by domineering mothers or submissive fathers, as some clinicians had suggested. 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 119 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 found these levels to be similar to those of heterosexuals.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 120 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 121 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 122 Sexual Orientation 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. ( Hyperplasia means “excessive formation.”) The secretion of androgens begins prenatally; thus, the syndrome causes prenatal masculinization. Boys born with CAH develop normally; the extra androgen does not seem to have significant effects.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 123 Sexual Orientation Prenatal Androgenization of Genetic Females The secretion of androgens begins prenatally; thus, the syndrome causes prenatal masculinization. Boys born with CAH develop normally; the extra androgen does not seem to have significant effects. 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 124 Sexual Orientation Prenatal Androgenization of Genetic Females Children typically show sex differences in 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 often encourage “sex- typical” toy choices.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 125 Sexual Orientation 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: Males chose to play with a car and a ball, whereas females preferred to play with a doll and a pot. (See Figure 10.11. )

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 127 Sexual Orientation 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 128 Sexual Orientation Failure of Androgenization of Genetic Males Normally, the testes are removed because they often become cancerous; but if they are not removed, 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. (If the testes are removed, the person will be given estradiol to accomplish the same result.) At adulthood the individual will function sexually as a woman, although surgical lengthening of the vagina may be necessary. Women with this syndrome report average sex drives, including normal frequency of orgasm in intercourse. Most marry and lead normal sex lives.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 129 Sexual Orientation Effects of Rearing on Sexual Identity and Orientation of Prenatally Androgenized Genetic Males A developmental abnormality known as cloacal exstrophy results in the birth of a boy with normal testes but urogenital abnormalities, often including the lack of a penis. In the past, many boys born with this condition were raised as females, primarily because it is relatively easy to surgically construct a vagina that can function in intercourse but very difficult to construct a functioning penis.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 130 Sexual Orientation Effects of Rearing on Sexual Identity and Orientation of Prenatally Androgenized Genetic Males Such people are almost always sexually oriented toward females. If we consider the social and parental pressure against someone who has been raised as a girl subsequently adopting a male sex role, 50 percent is an impressively large number. However, studies have shown that approximately 50 percent of such people later expressed dissatisfaction with their gender assignment and began living as men, often undergoing sex-change procedures (Meyer-Bahlberg, 2005; Reiner, 2005; Gooren, 2006).

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 131 Sexual Orientatio 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 132 Sexual Orientatio Sexual Orientation and the Brain 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. 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 133 Sexual Orientation Sexual Orientation and the Brain Presumably, the woman’s right hemisphere shares language functions with the left, so damage to one hemisphere is less devastating in women than it is in men. Also, men’s brains are, on average, somewhat larger—apparently because men’s bodies are generally larger than those of women. 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. (For specific references, see Breedlove, 1994; Swaab, Gooren, and Hofman, 1995; and Goldstein et al., 2001.)

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 134 Sexual Orientation Sexual Orientation and the Brain 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 135 Sexual Orientation Sexual Orientation and the Brain Sexual dimorphism in the human brain could even be a result of differences in the social environments of males and females. We cannot manipulate the hormone levels of humans before and after birth as we can with laboratory animals, so it might be a long time before enough evidence is gathered to permit us to make definite conclusions.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 136 Sexual Orientation Sexual Orientation and the Brain Approximately 8 percent of domestic rams (male sheep) show a sexual preference for other males. These animals do not show female-typical behavior; they show typical male mounting behavior but direct this behavior toward other males in preference to females (Price et al., 1988).

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 137 Sexual Orientation Sexual Orientation and the Brain A study by Roselli et al. (2004) discovered a sexually dimorphic nucleus in the medial preoptic/anterior hypothalamic area that was significantly larger in males than in females. They also found that this nucleus was twice as large in female-oriented (heterosexual) rams as in male-oriented (homosexual) rams. (See Figure 10.12. )

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 139 Sexual Orientation Sexual Orientation and the Brain Although this section has been considering sexual orientation—the sex of those to whom an individual is sexually and romantically attracted—another sexual characteristic is related to structural differences in the brain. In a study of postmortem brains, Zhou et al. (1995) found that the size of a particular region of the forebrain, the central subdivision of the bed nucleus of the stria terminalis (BNST ), is larger in males than in females.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 140 Sexual Orientation Sexual Orientation and the Brain They also found that in male-to-female transsexuals this nucleus is as small as it is in normal females. The size of this nucleus was as large in male homosexuals as in male heterosexuals.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 141 Sexual Orientation Sexual Orientation and the Brain Thus, its size was related to gender identity, not to sexual orientation. (Most male homosexuals have male sexual identities; although they are romantically and sexually oriented toward other men, they do not regard themselves as women, nor do they wish to be.) Kruijver et al. (2000) replicated these results and found that the size of this region in female-to-male transsexuals was within the range of sizes seen in normal males. (See Figure 10.13. )

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 143 Sexual Orientation Sexual Orientation and the Brain We cannot necessarily conclude that any of the brain regions I mentioned in this section are directly involved in people’s sexual orientation (or sexual identity). The real differences—if indeed sexual orientation is determined by prenatal exposure to androgens—may lie elsewhere in the brain, in some regions as yet unexplored by researchers. However, the observation that differences in brain structure are related to sexual orientation and gender identity does suggest that biological factors such as exposure to prenatal hormones have an effect on the nature of a person’s sexuality.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 144 Sexual Orientation Possible Causes of Differences in Brain Development If sexual orientation is, indeed, affected by differences in exposure of the developing brain to androgens, what factors might cause this exposure to vary? Presumably, something must decrease the prenatal androgen levels to which male homosexuals are exposed and increase the levels to which female homosexuals are exposed.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 145 Sexual Orientation Possible Causes of Differences in Brain Development As we saw, congenital adrenal hyperplasia exposes the developing fetus to increased levels of androgens, but most homosexual women do not have CAH. So far, no other plausible sources of high levels of prenatal androgens have been proposed.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 146 Sexual Orientation 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. Bailey and Pillard (1991) studied pairs of male twins in which at least one member identified himself as homosexual.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 147 Sexual Orientation Heredity and Sexual Orientation 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. 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).

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 148 Sexual Orientation Heredity and Sexual Orientation Genetic factors also appear to affect female homosexuality. Bailey et al. (1993) found that the concordance of female monozygotic twins for homosexuality was 48 percent, while that of dizygotic twins was 16 percent. Another study, by Pattatucci and Hamer (1995), found an increased incidence of homosexuality and bisexuality in sisters, daughters, nieces, and female cousins (through a paternal uncle) of homosexual women.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 149 Sexual Orientation 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.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 150 Section Summary: Hormonal Control of Sexual Behavior Sexual behaviors are controlled by the organizational and activational effects of hormones. The female reproductive cycle (menstrual cycle or estrous cycle) begins with the maturation of one or more ovarian follicles, which occurs in response to the secretion of FSH by the anterior pituitary gland.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 151 Section Summary: Hormonal Control of Sexual Behavior The sexual behavior of males of all mammalian species appears to depend on the presence of androgens. Oxytocin has a facilitatory effect on erection and ejaculation. The proceptivity, receptivity, and attractiveness of female mammals other than primates depend primarily on estradiol and progesterone. In particular, estradiol has a priming effect on the subsequent appearance of progesterone.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 152 Section Summary: Hormonal Control of Sexual Behavior In most mammals, female sexual behavior is the norm, just as the female body and female sex organs are the norm. In other words, unless prenatal androgens masculinize and defeminize the animal’s brain, its sexual behavior will be feminine.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 153 Section Summary: Hormonal Control of Sexual Behavior Behavioral masculinization refers to the androgen-stimulated development of neural circuits that respond to testosterone in adulthood, producing male sexual behavior. Behavioral defeminization refers to the inhibitory effects of androgens on the development of neural circuits that respond to estradiol and progesterone in adulthood, producing female sexual behavior.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 154 Section Summary: Hormonal Control of Sexual Behavior Pheromones can affect sexual physiology and behavior. Odorants present in the urine of female mice affect their estrous cycles, lengthening and eventually stopping them (the Lee-Boot effect). Odorants present in the urine of male mice abolish these effects and cause the females’ cycles to become synchronized (the Whitten effect). (Phenomena similar to the Lee-Boot effect and the Whitten effect also occur in women.)

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 155 Section Summary: Hormonal Control of Sexual Behavior Connections between the olfactory system and the amygdala appear to be important in stimulating male sexual behavior. The main olfactory system detects volatile chemicals that signal the presence of another animal, and the vomeronasal organ determines the other animal’s sex, estrous condition, and identity.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 156 Section Summary: Hormonal Control of Sexual Behavior Because the human vomeronasal organ does not appear to have sensory functions, these effects must be mediated by the main olfactory bulb. The search for sex attractant pheromones in humans has not produced definitive results, but the odors of AND (produced by males) and EST (produced by females) have effects on brain activity and changes in ratings of peoples attractiveness. Research also indicates that we might well recognize our sex partners by their odors.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 157 Section Summary: Hormonal Control of Sexual Behavior Testosterone has an activational effect on the sexual behavior of men, just as it does on the behavior of other male mammals. Women do not require estradiol or progesterone to experience sexual interest or to engage in sexual behavior.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 158 Section Summary: Hormonal Control of Sexual Behavior Studies with women suggest that variations in levels of ovarian hormones across the menstrual cycle do affect sexual interest but that other factors (such as initiation of sexual activity by partners or a desire to avoid or attain pregnancy) can also affect sexual behavior. In addition, the presence of androgens may facilitate the effect of estradiol on women’s sexual interest.

Hormonal Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 159 Section Summary: Hormonal Control of Sexual Behavior Sexual orientation (that is, heterosexuality or homosexuality) may be influenced by prenatal exposure to androgens. Studies of prenatally androgenized girls suggest that organizational effects of androgens influence the development of sexual orientation; androgenization appears to enhance interest in activities and toys usually preferred by boys and to increase the likelihood of a sexual orientation toward women.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 160 The control of sexual behavior—at least in laboratory animals—involves different brain mechanisms in males and females. This section describes these mechanisms.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 161 Males Spinal Mechanisms Some sexual responses are controlled by neural circuits contained within the spinal cord. Men with injuries that totally transect the spinal cord have become fathers when their wives have been artificially inseminated with semen obtained by mechanical stimulation (Hart, 1978).

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 162 Males Spinal Mechanisms Brackett et al. (1998) found that vibratory stimulation of the penis successfully elicited ejaculation in most men with complete spinal cord transection above the tenth thoracic segment, which suggests that the circuits in the spinal cord that control the ejaculatory response are located below this region. Because the spinal damage prevents sensory information from reaching the brain, these men are unable to feel the stimulation and do not experience an orgasm.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 163 Brain Mechanisms Brain mechanisms have both excitatory and inhibitory control of these circuits. Although tactile stimulation of a man’s genitals can stimulate erection and ejaculation, these responses can be inhibited by the context.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 164 Brain Mechanisms For example, the outcomes of tactile stimulation of a man’s penis will have different outcomes when his physician is carrying out physical examination or when his partner touches him while they are lying in bed. In addition, a man’s penis can become erect when he sees his partner or has erotic thoughts—even when his penis is not being touched. Thus, there must be brain mechanisms that can activate or suppress the spinal mechanisms that control genital reflexes.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 165 Brain Mechanisms The medial preoptic area (MPA), located just rostral to the hypothalamus, is the forebrain region most critical for male sexual behavior. medial preoptic area (MPA) An area of cell bodies just rostral to the hypothalamus; plays an essential role in male sexual behavior.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 166 Brain Mechanisms Domingues, Gil, and Hull (2006) found that mating increased the release of glutamate in the MPA and that infusion of glutamate into the MPA increased the frequency of ejaculation. Finally, destruction of the MPA abolishes male sexual behavior (Heimer and Larsson, 1966/1967).

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 167 Brain Mechanisms 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.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 168 Brain Mechanisms This area is called (appropriately enough) the sexually dimorphic nucleus (SDN) of the preoptic area. (As we saw in the previous discussion of brain mechanisms of gender identity, this brain region, which is called the uncinate nucleus in humans, is sexually dimorphic in our species as well.) 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.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 169 Brain Mechanisms The size of this nucleus is controlled by the amount of androgens present during fetal development. According to Rhees, Shryne, and Gorski (1990a, 1990b), the critical period for masculinization of the SDN appears to start on the eighteenth day of gestation and end once the animals are five days old. (Normally, rats are born on the twenty-second day of gestation.) De Jonge et al. (1989) found that lesions of the SDN decrease masculine sexual behavior.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 171 Brain Mechanisms Androgens also exert their activational effects on neurons in the MPA and associated brain regions. If a male rodent is castrated in adulthood, its sexual behavior will cease. However, the behavior can be reinstated by implanting a small amount of testosterone directly into the MPA or in two regions whose axons project to the MPA: the central tegmental field and the medial amygdala (Sipos and Nyby, 1996; Coolen and Wood, 1999). Both of these regions contain a high concentration of androgen receptors in the male rat brain (Cottingham and Pfaff, 1986).

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 172 Brain Mechanisms As we saw in the previous subsection, the lumbar region of the spinal cord contains a group of neurons that play a critical role in ejaculation (Coolen et al., 2004). Anatomical tracing studies suggest that the most important connections between the MPA and the spinal ejaculation generator are accomplished through the periaqueductal gray matter (PAG) of the midbrain and the nucleus paragigantocellularis (nPGi) of the medulla.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 173 Brain Mechanisms 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.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 174 Brain Mechanisms The nPGi normally inhibits spinal cord sexual reflexes, so one of the tasks of the pathway originating in the MPA is to suppress this inhibition. The MPA suppresses the nPGi directly through an inhibitory pathway and does so indirectly by inhibiting the activity of the PAG, which normally excites the nPGi.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 175 Brain Mechanisms The inhibitory connections between neurons of the nPGi and those of the spinal ejaculation generator are serotonergic. As Marson and McKenna (1992) showed, application of serotonin (5-HT) to the spinal cord suppresses ejaculation. This connection may explain a well-known side effect of specific serotonin reuptake inhibitors (SSRIs).

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 176 Brain Mechanisms 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 the serotonergic synapses in the spinal cord increases the inhibitory influence of nPGi on the spinal ejaculation generator. Figure 10.15 summarizes the evidence I have presented so far in this section. (See Figure 10.15. )

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 178 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). 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.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 179 Females 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 (Pfaff and Sakuma, 1979).

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 180 Females The mechanism by which estradiol primes a female’s sensitivity to progesterone appears to be simple: Estradiol increases the production of progesterone receptors, which greatly increases the effectiveness of progesterone. Blaustein and Feder (1979) administered estradiol to ovariectomized guinea pigs and found a 150 percent increase in the number of progesterone receptors in the hypothalamus.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 181 Females Presumably, the estradiol activates genetic mechanisms in the nucleus that are responsible for the production of progesterone receptors. Figure 10.17 shows two slices through the hypothalamus of ovariectomized guinea pigs, stained for progesterone receptors. One of the animals had previously received a priming dose of estradiol; the other had not. As this figure shows, the estradiol dramatically increased the number of cells containing progesterone receptors. See Figure 10.16. )

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 183 Females As we saw in the previous subsection, the brain regions that control male genital reflexes include the MPA, PAG, and nPGi. Anatomical tracing studies (Marson, 1995; Marson and Murphy, 2006) injected pseudorabies virus into the clitorises and vaginas of female rats and found heavy retrograde labeling in these three brain structures (and some others as well).

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 184 Females Thus, it seems likely that erections of the penis and clitoris are controlled by similar brain mechanisms. This finding is not surprising, because these organs derive from the same embryonic tissue. Figure 10.18 summarizes the evidence I have presented so far in this section. (See Figure 10.18. )

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 186 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.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 187 Formation of Pair Bonds In addition, some people display serial monogamy—intense relationships that last for a period of time, only to be replaced with similarly intense relationships with new partners. And, of course, some cultures condone (or even encourage) polygamy. But there is no doubt that pair bonding occurs in some species, including our own.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 188 Formation of Pair Bonds Several studies have revealed a relationship 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 V1a vasopressin receptors in the ventral forebrain than do polygamous voles (Insel, Wang, and Ferris, 1994). This difference appears to be responsible for the presence or absence of monogamy.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 189 Formation of Pair Bonds In female voles, oxytocin appears to play a major role in pair bonding. Mating stimulates the release of oxytocin, and peripheral injection of oxytocin or injection 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 (Cho et al., 1999).

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 190 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.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 191 Formation of Pair Bonds However, it is difficult to envision ways to perform definitive research on this topic. Experimenters can study the effects of these hormones or their antagonists on pair bonding in laboratory animals, but they certainly cannot do so with humans. However, Heinrichs et al. (2003) found that injections of oxytocin caused relaxation and a reduction of anxiety in human subjects.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 192 Section Summary: Neural Control of Sexual Behavior Sexual reflexes such as sexual posturing, erection, and ejaculation are organized in the spinal cord. Vibratory stimulation of the penis can elicit ejaculation in men with complete transection of the spinal cord, as long as the damage is located above the tenth thoracic segment. The sexually dimorphic nucleus, located in the medial preoptic area, develops only if an animal is exposed to androgens early in life. A sexually dimorphic nucleus is found in humans as well. Destruction of the SDN (part of the MPA) in laboratory animals impairs mating behavior.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 193 Section Summary: Neural Control of Sexual Behavior The most important forebrain region for female sexual behavior is the ventromedial nucleus of the hypothalamus (VMH). Its destruction abolishes copulatory behavior, and its stimulation facilitates this behavior. Both estradiol and progesterone exert their facilitating effects on female sexual behavior in this region, and studies have confirmed the existence of progesterone and estrogen receptors there. Orgasm in women is accompanied by increased activity in regions similar to those activated during ejaculation in men and, in addition, in the periaqueductal gray matter.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 194 Section Summary: Neural Control of Sexual Behavior The most important forebrain region for female sexual behavior is the ventromedial nucleus of the hypothalamus (VMH). Its destruction abolishes copulatory behavior, and its stimulation facilitates this behavior. Both estradiol and progesterone exert their facilitating effects on female sexual behavior in this region, and studies have confirmed the existence of progesterone and estrogen receptors there.

Neural Control of Sexual Behavior COPYRIGHT © ALLYN & BACON 2012 195 Section Summary: Neural Control of Sexual Behavior The priming effect of estradiol is caused by an increase in progesterone receptors in the VMH. The steroid-sensitive neurons of the VMH send axons to the periaqueductal gray matter (PAG) of the midbrain; these neurons, through their connections with the medullary reticular formation, control the particular responses that constitute female sexual behavior. Orgasm in women is accompanied by increased activity in regions similar to those activated during ejaculation in men and, in addition, in the periaqueductal gray matter.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 196 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. 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.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 197 Maternal Behavior of Rodents At birth, rats and mice resemble fetuses. The infants are blind (their eyes are still shut), and they can only wriggle helplessly. They are poikilothermous (“cold-blooded”); their brain is not yet developed enough to regulate body temperature. They even lack the ability to release their own urine and feces spontaneously and must be helped to do so by their mother.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 198 Maternal Behavior of Rodents During gestation, female rats and mice build nests. The form this structure takes depends on the material available for its construction. In the laboratory the animals are usually given strips of paper or lengths of rope or twine. A good brood nest, as it is called, is shown in Figure 10.19. This nest is made of hemp rope; a piece of the rope is shown below the nest. The mouse laboriously shredded the rope and then wove an enclosed nest, with a small hole for access to the interior. (See Figure 10.19. )

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 199 Maternal Behavior of Rodents This nest is made of hemp rope; a piece of the rope is shown below the nest. The mouse laboriously shredded the rope and then wove an enclosed nest, with a small hole for access to the interior. (See Figure 10.18. )

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 201 Maternal Behavior of Rodents At the time of parturition (delivery of offspring) the female begins to groom and lick the area around her vagina. As a pup begins to emerge, she assists the uterine contractions by pulling the pup out with her teeth.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 202 Maternal Behavior of Rodents parturition ( par tew ri shun ) The act of giving birth. She then eats the placenta and umbilical cord and cleans off the fetal membranes—a quite delicate operation. (A newborn pup looks as though it is sealed in very thin plastic wrap.) After all the pups have been born and cleaned up, the mother will probably nurse them. Milk is usually present in the mammary glands very near the time of birth.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 203 Maternal Behavior of Rodents Besides cleaning, nursing, and purging her offspring, a female rodent will retrieve pups if they leave or are removed from the nest. The mother will even construct another nest in a new location and move her litter there, should the conditions at the old site become unfavorable (for example, when an inconsiderate experimenter puts a heat lamp over it).

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 204 Maternal Behavior of Rodents The way a female rodent picks up her pup is quite consistent: She gingerly grasps the animal, managing not to injure it with her very sharp teeth. (I can personally attest to the sharpness of a mouse’s teeth and the strength of its jaw muscles.) She then carries the pup with a characteristic prancing walk, her head held high. (See Figure 10.19. )

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 206 Maternal Behavior of Rodents Under normal conditions, one of the stimuli that induce a female rat to begin taking care of pups is the act of parturition. Female rodents normally begin taking care of their pups as soon as they are born. Some of this effect is caused by prenatal hormones, but the passage of the pups through the birth canal also stimulates maternal behavior: Artificially distending the birth canal in nonpregnant females stimulates maternal behavior, whereas cutting the sensory nerves that innervate the birth canal retards the appearance of maternal behavior (Graber and Kristal, 1977; Yeo and Keverne, 1986).

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 207 Hormonal Control of Maternal Behavior 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. (Obviously, they cannot provide the infants with milk.) 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.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 208 Hormonal Control of Maternal Behavior Figure 10.21 shows the levels of the three hormones that have been implicated in maternal behavior: progesterone, estradiol, and prolactin. Note that just before parturition the level of estradiol begins rising, then the level of progesterone falls dramatically, followed by a sharp increase in prolactin, the hormone produced by the anterior pituitary gland that is responsible for milk production. (See Figure 10.20. )

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 209 Hormonal Control of Maternal Behavior If ovariectomized virgin female rats are given progesterone, estradiol, and prolactin in a pattern that duplicates this sequence, the time it takes to sensitize their maternal behavior is drastically reduced (Bridges et al., 1985.) Prolactin A hormone of the anterior pituitary gland, necessary for production of milk; also facilitates maternal behavior.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 211 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.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 212 Neural Control of Maternal Behavior However, female sexual behavior was unaffected by these lesions. Del Cerro et al. (1995) found that the metabolic activity of the MPA, measured by 2-DG autoradiography increased immediately after parturition. They also found that virgin females whose maternal behavior had been sensitized by exposure to pups showed increased activity in the MPA. Thus, stimuli that facilitate pup care activate the MPA.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 213 Neural Control of Maternal Behavior Prolactin appears to exert its stimulating effect on maternal behavior by acting on receptors in the medial preoptic area. Bridges et al. (1997, 2001) found that an infusion of prolactin into the MPA of virgin female rats that has been primed with estradiol and progesterone stimulated maternal behavior, and an infusion of a prolactin antagonist into the MPA delayed the onset of maternal behavior.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 214 Neural Control of Maternal Behavior Numan (2007) has reviewed research from his laboratory that has traced the neural pathways responsible for the two forms of sensitization to pups: inhibition of circuits responsible for the aversion to the odor of pups and activation of circuits responsible for caring for pups. It turns out that the MPA is involved in both forms of sensitization. First, let’s consider the aversive response that virgin females make toward the odor of pups.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 215 Neural Control of Maternal Behavior As we saw earlier in this chapter, the olfactory system provides input to the medial amygdala, which plays an important role in sexual behavior Lesions of the medial amygdala abolish the aversion of virgin females to pups just as treatment with zinc sulfate does, which indicates that the medial amygdala plays a role in mediating the aversive effects of pup odors. Some neurons in the medial amygdala send axons to the anterior hypothalamus (AH), which in turn projects to the periaqueductal gray (PAG).

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 216 Neural Control of Maternal Behavior Because studies have shown that connections between the AH and the PAG play a role in defensive behavior and avoidance responses, Numan (2007) suggests that the role of the MPA in habituation to the odor of pups may be to inhibit the activity of the AH  PAG circuit. (See Figure 10.21. )

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 218 Neural Control of Maternal Behavior The neural circuitry of the system responsible for caring for pups is somewhat more complex. As you learned earlier, in the discussion of the neural basis of male sexual behavior, the MPA sends axons to the midbrain and lower brain stem. Numan and Numan (1997) found that neurons of the MPA that were activated by the performance of maternal behavior sent their axons to the ventral tegmental area (VTA). In turn, dopaminergic neurons in the VTA send axons to the nucleus accumbens (NAC).

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 219 Neural Control of Maternal Behavior Numan and his colleagues have shown that dopamine is released in the NAC during maternal behavior, lesions of the NAC or injection of a dopamine antagonist into the NAC disrupts maternal behavior (Numan, 2007). Finally, axons from the NAC project to the ventral pallidum, a region of the basal ganglia involved in control of motivated behaviors, and injection of muscimol into the ventral pallidum suppressed maternal behavior. (Muscimol, a GABA agonist, inhibits neural activity.) (See Figure 10.22. )

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 221 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.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 222 Section Summary: Parental Behavior Exposure of virgin females to young pups stimulates maternal behavior within a few days. The stimuli that normally induce maternal behavior are those produced by the hormones present during pregnancy and around the time of birth. Injections of progesterone, estradiol, and prolactin that duplicate the sequence that occurs during pregnancy facilitate maternal behavior. The hormones appear to act in the medial preoptic area (MPA).

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 223 Section Summary: Parental Behavior Connections between the MPA and the medial amygdala are responsible for the suppression of the aversive effects of the odor of pups, and a different circuit starting with the MPA is involved in establishing the reinforcing effect of pups and enhancing motivation to care for them: MPA  VTA  NAC  ventral pallidum. Oxytocin, which facilitates the formation of pair bonds in female rodents, is also involved in formation of a bond between mother and her pups. An fMRI study with rats showed activation of brain mechanisms of reinforcement when the mothers were presented with their pups. Women who look at pictures of their infants show increased activity in similar brain regions.

Parental Behavior COPYRIGHT © ALLYN & BACON 2012 224 Section Summary: Parental Behavior Paternal behavior is relatively rare in mammalian species, but research indicates that sexual dimorphism of the MPA is less pronounced in male voles of monogamous, but not promiscuous, species. Lesions of the MPA abolish paternal behavior of male rats. Some aspects of parental care may be related to blood levels of prolactin and oxytocin in human fathers.

PowerPoint Presentation for Physiology of Behavior 11 th Edition by Neil R. Carlson Prepared by Grant McLaren, Edinboro University of Pennsylvania This.

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PowerPoint Presentation for Physiology of Behavior 11 th Edition by Neil R. Carlson Prepared by Grant McLaren, Edinboro University of Pennsylvania This.

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