Chapter 11 Reproductive Behaviors

Chapter 11 Reproductive Behaviors

Sex and Hormones Sexual reproduction between two individuals increases variation in the gene pool. Variation in the gene pool of a species enables quick evolutionary adaptations to change in the environment.

Sex and Hormones Widespread communication throughout the body is accomplished through the release of hormones. Two kinds of hormones include: Steroid hormones Sex hormones

Sex and Hormones Steroid hormones are derived from cholesterol, contain four carbon rings and exert their effects in three ways. Binding to membrane receptors like neurotransmitters. Entering cells and activate certain kinds of proteins in the cytoplasm. Binding to chromosomes where they activate or inactivate certain genes.

Fig. 11-1, p. 326 Figure 11.1: Steroid hormones.
Note the similarity between the sex hormones testosterone and estradiol. Fig. 11-1, p. 326

Figure 11.2: Three routes of action for steroid hormones.
Steroid hormones such as estrogens and androgens bind to membrane receptors, activate proteins in the cytoplasm, and activate or inactivate specific genes. (Source: Revised from Starr & Taggart, 1989) Fig. 11-2, p. 327

Sex and Hormones The sex hormones are a special kind of steroids, released mostly by the gonads and to a lesser degree by the adrenal glands. affect the brain, genital and other organs Two types of sex hormones include: Androgens Estrogens Both sexes have both hormones.

Sex and Hormones Androgens are a groups of sex hormones that include testosterone and others. Generally referred to as “male hormones” because men have higher levels than women.

Sex and Hormones Estrogens include estradiol and others and are referred to as “female hormones” because women have higher levels. Progesterone is a type of hormone that prepares the uterus for the implantation of a fertilized ovum and promotes the maintenance of pregnancy.

Sex and Hormones Sex limited genes are those activated by androgens or estrogens and control most of the differences between male and female. Example: estrogens activate the gene for breast growth; androgens activate the gene for the growth of facial hair in men. Sex hormones increase or decrease the rate of apoptosis in various regions of the brain. Certain areas are slightly larger in males or females

Sex and Hormones Sex hormones can have the following effects:
Organizing effects- occur mostly at sensitive stages of development. -Determine whether the brain and body will develop male or female characteristics Activating effects- occur at any time of life and temporarily activate a particular response.

Sex and Hormones The distinction between the activating and organizing effects of hormone is not absolute. Example: hormones early in life can exert temporary effects; during puberty hormones can also induce long-lasting structural changes

Sex and Hormones Obvious differences can exist between the reproductive organs and the gonads of males and female. Sexual differentiation begins with the chromosomes and Female mammal has two x chromosomes and a male has an X and a Y. During an early stage of prenatal development, both male and female have a set of Mullerian ducts and a set of Wolffian ducts as well as primitive gonads.

Sex and Hormones Wolffian ducts are the precursors to other male reproductive organs. Develop into the vas deferens and seminal vesicles. Mullerian ducts are precursors to the female’s oviducts, uterus, and upper vagina.

Sex and Hormones The male Y chromosome includes the SRY gene which causes the primitive gonads to develop into testes, the sperm-producing organ. The developed testes produce the hormone testosterone. Testosterone induce sthe development of the penis and scrotum. Females are not exposed to high testosterone levels and their gonads develop into ovaries, the egg-producing organs.

Sex and Hormones Sensitive periods are early periods when hormones have long-lasting effects. Sexual differentiation depends mostly on the level of testosterone during a sensitive period. The human sensitive period for genital formation is about the third and fourth month of pregnancy.

Sex and Hormones Female rats exposed to testosterone shortly before or after birth are partly masculinized in anatomy and behavior. Clitoris grows larger than normal At maturity, pituitary and ovaries produce steady levels of hormones instead of cycles Parts of the hypothalamus appear more male Sexual behavior becomes masculinized

Sex and Hormones Extra estradiol does not determine whether the individual looks female or male. Estradiol and other estrogens do modify various aspects of the development of the brain and the internal sexual organs. The absence of sex hormones generally leads to female-looking external genitalia If a male rat lacks androgen receptors or is castrated, it develops female-like anatomy and behavior.

Fig. 11-3, p. 328 Figure 11.3: Differentiation of human genitals.
The male’s SRY gene causes the gonad to become a testis, and the testis produces testosterone, which masculinizes development. In the absence of testosterone, development follows the female pattern. (Source: Based on Netter, 1983) Fig. 11-3, p. 328

Sex and Hormones Sex hormones early in life bind to receptors in specific areas of the hypothalamus, amygdala, and other brain areas and produce anatomical and physiological differences. The sexually dimorphic nucleus is an area in the anterior hypothalamus that is larger in the male and contributes to control of male sexual behavior. Parts of the female hypothalamus generate a cyclical pattern of hormone release; the hypothalamus of a male cannot.

Sex and Hormones During early development in rodents, testosterone is converted within certain brain cells to estradiol, which masculinizes development. Alpha-fetoprotein is found in the blood during early sensitive periods and binds to estrogen and prevents it from entering developing cells. Testosterone does not bind to alpha-fetoprotein and freely enters the cell.

Sex and Hormones Differences in the cerebral cortex also exist between men and women: Men tend to have more white matter. Women tend to have a greater density of neurons in parts of the temporal lobe dedicated to language. The language-related areas are larger in the left than right hemisphere for both sexes, but it is more pronounced in women.

Sex and Hormones Sex hormones have also been shown to influence intellectual performance in specific domains: Females typically do better in most school subjects than men, except for math and science. Boys perform better at mental rotation tasks and line orientation tasks. Performance on these tasks most likely is attributed to organizational effects.

Fig. 11-4, p. 330 Figure 11.4: A spatial rotation task.
People are presented with a series of pairs such as this one and asked whether the first figure could be rotated to match the second one. Here the answer is no. For the line-angle question, the correct answer is e. On the average, men perform better on this task than women, and women perform better while they are menstruating (and their sex hormone levels are low) than at other times. However, women perform verbal fluency and manual dexterity tasks better at other times than they do while menstruating. Fig. 11-4, p. 330

Sex and Hormones Men excel in tasks involving spatial reasoning, but performance depends on effectiveness of directional strategy. Men are more likely to use directional (north, south, etc.) orientations to navigate. Women are more likely to use landmarks Evolutionary explanations suggest that differences exist because the males of many species travel over greater geographical areas than do females.

Figure 11.5: Relative use of different kinds of directions.
Women used more landmark terms than men; men were more likely than women to use terms relating to north-south-east-west or to distances. (Based on data of Rahman, Andersson, & Govier, 2005) Fig. 11-5, p. 330

Sex and Hormones In adulthood, sex hormones can activate behavior.
Behavior can also influence hormone secretion. Hormones do not cause behavior but rather alter the activity in various brain areas to change the way the brain responds to certain stimuli. Hormones also change sensitivity in the penis, vagina and cervix.

Sex and Hormones Sex hormones activate sexual behavior partly by facilitating activity in areas of the brain. Estrogens increase the sensitivity of the pudendal nerve, which transmits tactile stimulation from the pubic area to the brain.

Sex and Hormones Sex hormones also increase responses of certain areas of the hypothalamus. the ventromedial nucleus, the medial preoptic area (MPOA), and the anterior hypothalamus. Stimulation of an area known as the sexually dimorphic nucleus (SDN) increases sexual behavior in males of many species.

Sex and Hormones Testosterone and estradiol trigger the release of dopamine by the MPOA and other areas. Dopamine release is associated with sexual arousal. Facilitates erection of the penis and sexually receptive postures in females Higher concentrations of dopamine stimulate D2 receptors and leads to orgasm. Serotonin activity decreases sexual activity by blocking dopamine release.

Sex and Hormones Humans are less dependent on current sex hormones than other species but changes can increase or decrease sexual arousal. For males, sexual excitement is generally highest when testosterone levels are highest. The hormone oxytocin contributes to sexual pleasure. The body releases enormous amounts of oxytocin during orgasm.

Sex and Hormones Decreases in testosterone levels generally decrease male sexual activity and interest. Example: castration Impotence is the inability to maintain an erection. usually caused by impaired blood circulation, not low testosterone. Erection partially depends on testosterone increasing the release of nitric oxide. facilitates the hypothalamic neurons and increases blood flow to the penis.

Sex and Hormones Although most sex offenders have normal testosterone levels, testosterone reduction has sometimes been tried as a means of controlling sex offenders. Cyproterone is a drug that blocks the binding of testosterone to receptors. Medroxyprogesterone inhibits gonadotropin, the pituitary hormone that stimulates testosterone production.

Sex and Hormones In women, the hypothalamus and pituitary interact with the ovaries to produce the menstrual cycle. The menstrual cycle is the periodic variation in hormones and fertility over the course of about 28 days.

Sex and Hormones After the end of a menstrual period:
the anterior pituitary releases follicle-stimulating hormone (FSH) FSH promotes the growth of a follicle in the ovary. The follicle nurtures the ovum and produces estrogen. Towards the middle of the menstrual cycle, the follicle builds up receptors to FSH. As a result, the follicle produces increasing amounts of estradiol, a type of estrogen.

Figure 11.6: Blood levels of four hormones during the human menstrual cycle.
Note that estrogen and progesterone are both at high levels during the midluteal phase but drop sharply at menstruation. Fig. 11-6, p. 332

Sex and Hormones Increased estradiol causes the anterior pituitary to increase release of FSH and luetinizing hormone (LH). FSH an LH cause the follicle to release an ovum. The remnants of the follicle release the hormone progesterone. prepares the uterus for implantation of a fertilized ovum inhibits the further release of LH

Figure 11.7: Interactions between the pituitary and the ovary.
FSH from the pituitary stimulates a follicle of the ovary to develop and produce estradiol, triggering a release of a burst of FSH and LH from the pituitary. Those hormones cause the follicle to release its ovum and become a corpus luteum. The corpus luteum releases progesterone while the ovary releases estradiol. Fig. 11-7, p. 333

Sex and Hormones If the ovum is not fertilized, the lining of the uterus is cast off and menstruation occurs. If the ovum is fertilized, the levels of estradiol and progesterone increase gradually throughout pregnancy.

Sex and Hormones Birth control pills prevent pregnancy by interfering with the usual feedback cycle between the ovaries and pituitary. The “combination-pill” contains both estrogen and progesterone and prevents the surge of FSH and LH that would release an ovum. also thickens the mucus of the cervix making it harder for the sperm to reach the egg.

Figure 11.8: Female-initiated sexual activities during the monthly cycle.
The top graph shows autosexual activities (masturbation and sexual fantasies); the bottom graph shows female-initiated activities with a male partner. “Intrusive” birth-control methods are diaphragm, foam, and condom; “nonintrusive” methods are IUD and vasectomy. Note that women other than pill users increase self-initiated sex activities when their estrogen levels peak. (Source: From “Rise in female-initiated sexual activity at ovulation and its suppression by oral contraceptives,” by D. B. Adams, A. R. Gold, and A. D. Burt, 1978, New England Journal of Medicine, 299, pp. 1145–1150. Reprinted by permission of The New England Journal of Medicine.) Fig. 11-8, p. 334

Sex and Hormones The periovulatory period is the time of maximum fertility and increased estrogen levels when ovulation occurs. Studies suggest that women become more sexually responsive during this time when estrogen levels are high. Show increased attention to sex-related stimuli. show increased mate preference towards men who act and look more masculine.

Sex and Hormones Premenstrual syndrome (PMS) occurs days before menstruation and is associated with anxiety, irritability and depression. Studies suggest that fluctuation in hormone levels are not the direct cause. Women with PMS have about the same fluctuation as women without PMS. Research has focused on the metabolism of progesterone, into allopregnanolene and other chemicals.

Sex and Hormones Hormones released around the time of giving birth facilitate maternal behavior in females. Late in pregnancy, the female secretes large amounts of estradiol, prolactin, and oxytocin. Prolactic is responsible for milk production. Oxytocin is associated with maternal behavior and social attachment.

Sex and Hormones Females also change patterns of hormone receptors.
Late in pregnancy, the brain increases its sensitivity to estradiol in areas responsible for maternal behavior, but not for sexual behavior. The hormonal changes increase the attention of the mother to the young after birth. Hormones also increase activity in the medial preoptic area and the anterior hypothalamus.

Sex and Hormones Vasopressin is a hormone synthesized by the hypothalamus and secreted by the posterior pituitary gland. associated with establishing long-term bonds in some species. Mothers are also stimulated by the odors of their babies. Infant rats release chemicals that stimulate the mother’s vomeronasal organ.

Sex and Hormones Mammals have two mechanisms for stimulating maternal behavior: Hormones in the early phase compensate for the lack of familiarity with the young. Later experience maintains the maternal behavior as hormones decline. Although hormonal changes are necessary to nurse a baby, they are not necessary to elicit care for a baby by humans.

Variations in Sexual Behavior
A wide degree of variation exists between people in terms of frequency of sexual behavior, preferred types of sexual activity, and sexual orientation. One perspective of explaining differences in behavior is from an evolutionary perspective.

Gender differences in sexual behavior include the following: Men are more likely to seek multiple sex partners, especially for short-term encounters. Women are more likely to be concerned about a mates earning potential: men are more likely to be concerned about a mate’s youth. Men usually show greater jealousy at indications of sexual infidelity.

Buss (2000) argues that gender differences reflect past evolutionary pressures. Men are interested in brief sexual relationships with multiple partners because such a strategy increases the likelihood of his genes being passed along to the next generation.

Research has also suggested that women can gain from having multiple sexual partners as well. Multiple mates increases resources available to her child and herself. No direct evidence suggests that specific genes influence whether people prefer single or multiple mates.

Both men and women prefer a mate that is healthy, intelligent, honest, and physically attractive. In almost all cultures, women prefer mates who are likely to be good providers. Evolutionary explanations suggest that choosing a father who is likely to be a good provider aids the women while she is pregnant or caring for a small child.

Men tend to prefer a young partner. Evolutionary explanations suggest that this preference exists because younger women are more likely to be fertile than older women. Men remain fertile well into old age so preference for a young mate for women is not as pronounced.

Evolutionary explanations of gender differences in jealousy suggest that men need to be sure that the children he supports are his own. Unfaithful wives threaten this certainty. Women are always sure that the child is their own. Although cultures vary in attitudes towards infidelity, no culture exists where infidelity is more acceptable for women.

Although mating habits of people can be explained in terms of increasing the probability of passing on one’s genes, we can not assume a genetic basis. Behaviors and preferences may be a product of learning. More data, especially about the effects of particular genes, is needed to draw conclusions.

Gender identity refers to how we identify sexually and what we call ourselves. Biological differences are generally referred to as “sex differences”. Differences that result from people’s thoughts about themselves as male or female are referred to as “gender differences”.

Most people have a gender identity that matches their external appearance. Some people have a gender identity that is opposite their biological sex. Psychologists and researchers once believed that gender identity was learned and more a product of rearing and experience. Current evidence strongly suggests that biological factors, especially prenatal hormones, play a large role in gender identity.

Intersex people are people are intermediate between being male or female. Some XY males with a mutation of the SRY gene have poorly developed genitals. Some are born with an XX chromosome pattern but an SRY gene that translocates from the father’s Y chromosome causes ambiguous genitalia. Can also occur because of an atypical hormone pattern or mutation of testosterone receptors before birth.

Congenital adrenal hyperplasia (CAH) is an overdevelopment of the adrenal glands from birth and the most common cause of the intersex condition. Caused by a genetic defect in which cortisol production leads to overstimulation of the adrenal gland. Overstimulation of the adrenal gland leads to extra testosterone production. The female fetus becomes partly masculinized.

Hermaphrodites are individuals whose genitals do not match the usual development for their genetic sex. An estimated 1 out of 100 children is born with some degree of genital ambiguity. 1 in 2000 has enough genital ambiguity to make the sex uncertain.

Figure 11.12: Toy preferences by CAH girls, unaffected girls, and unaffected boys.
CAH girls were intermediate between unaffected girls and boys. These data show results when the children played alone. Results changed slightly when the mother or father was present, but in each case, the CAH girls were intermediate between the other groups. (Source: Based on data of Pasterski et al., 2005) Fig , p. 342

The brains of genetic females with CAH are exposed to high levels of testosterone during prenatal and early postnatal life. Research indicates that CAH girls show a greater preference for boy-typical toys than do other girls. During adolescence and early adulthood, they also show partly masculinized interests. Sexual interest and activity also differs for CAH girls as well.

Androgen insensitivity or testicular feminization is a condition in which individuals with an XY chromosome pattern have the genital appearance of a female. Production of androgens remains normal but they lack the androgen receptor that enables it to activate genes in a cell’s nucleus. Condition occurs in various degrees from a smaller than average penis to genitals that develop a female appearance.

Physicians have traditionally recommended that intersex people be reared as girls. surgery was often conducted to make them look more feminine. Assumed that children consistently raised as female would accept that identity. Many intersex people protest against such surgery and suggest that an informed consent would have been preferred.

Some genetic males fail to produce an enzyme that converts testosterone to dihydrotestosterone. Most look female at birth but a penis develops during adolescence and puberty. Most then accept a male gender identity. Brain is exposed to testosterone during early development.

Many genetic males born without a penis or who had the penis accidentally removed and who were raised as a girl ask to be reassigned as males. Many who remain female feel discontent or conflict with being female. Such cases indicate that although hormones do not determine gender identity, they do play an important role.

Studies of twins suggest sexual orientation is influenced by genetic factors. Probability of homosexuality is highest in monozygotic twins and lower in dizygotic twins, and even lower in siblings and adopted brothers or sisters. Because monozygotic twins can have opposite sexual orientations, genes are not the only factor.

Figure 11.14: Sexual orientations in adult relatives of a homosexual man or woman.
Note that the probability of a homosexual orientation is highest among monozygotic twins of a homosexual individual, lower among dizygotic twins, and still lower among adopted brothers or sisters. These data suggest a genetic contribution toward the development of sexual orientation. (Source: Based on the data of Bailey & Pillard, 1991; Bailey, Pillard, Neale, & Agyei, 1993) Fig , p. 346

Studies also suggest a higher incidence of homosexuality among the maternal relatives of homosexual men. These results suggest a gene on the X chromosome that a man receives from his mother may play a role. Other studies have not replicated this result and thus the findings are inconclusive.

Sexual orientation may be influenced by testosterone levels during sensitive periods of brain development. Studies of male animals deprived of testosterone early in life show sexual interest in other males as adults. Studies of female animals exposed to testosterone during early development show an increased likelihood of mounting behavior.

Subtle physiological differences between heterosexual and homosexual men and women include: Length of the arms, legs, and hands. Ratio of index finger length to ring finger. Homosexual males and females do not differ in terms of adult hormone levels. Homosexual men are partly “feminized” and homosexual women are partly “masculinized”, but not all homosexuals correspond to this trend.

Research also suggests that certain brain structures differ in size between heterosexual and homosexual men and women. On average, the male homosexual brain is shifted towards a female development in some (but not all) ways; the female is shifted in some ways towards male development. Studies emphasize the role of testosterone at certain times of development as certain areas of the brain have altered sensitivities to testosterone.

The probability of homosexual orientation is also higher among men with older brothers. Number of previous sisters has no effect nor do these effects apply to females. Results suggest that a mother’s immune system may react against a protein in a son and attacks subsequent sons to alter development.

Laboratory research has also shown that prenatal stress can alter sexual development. Male subjects subjected to either prenatal stress or alcohol developed male sexual behavior in addition to female sexual behaviors. Male subjects exposed to both stress and alcohol during prenatal development had decreased sexual behavior.

On average, differences in brain anatomy exist between heterosexual and homosexuals. Homosexual men tend to have: Larger anterior commissure and suprachiasmatic nucleus Smaller neurons in the third interstitial nucleus of the anterior hypothalamus (INAH-3) Difficult to decipher why differences exist.

Figure 11.17: Volumes of the interstitial nucleus 3 of the anterior hypothalamus (INAH-3).
Samples are females (F), heterosexual males (M), and homosexual males (HM). Each filled circle represents a person who died of AIDS; each triangle represents a person who died from other causes. The one open circle represents a bisexual man who died of AIDS. (Source: Reprinted with permission from “A difference in hypothalamic structure between heterosexual and homosexual men,” by S. LeVay, Science, 253, pp. 1034–1037. Copyright © 1991 American Association for the Advancement of Science.) Fig , p. 349

Fig. 11-18, p. 349 Figure 11.18: Another comparison of INAH-3.
In this study, the mean volume for homosexual men was larger than that of women but smaller than that of men. (Source: Based on data of Byne et al., 2001) Fig , p. 349

Chapter 11 Reproductive Behaviors

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