Reproductive behavior Chapter 10 James W. Kalat Biological Psychology 12 edition PDF

Preview

Summary

Chapter 10Females have two X chromosomes, whereas males have an X and a Y chromosome. Biologists used to believe that the chromosomes determine sexual differentiation entirely through hormones.Both have a set of Müllerian ducts (precursors to female internal structures) and a set of Wolffian ducts (...

Description

Chapter 10 Females have two X chromosomes, whereas males have an X and a Y chromosome. Biologists used to believe that the chromosomes determine sexual differentiation entirely through hormones. Both have a set of Müllerian ducts (precursors to female internal structures) and a set of Wolffian ducts (precursors to male internal structures), as well as undifferentiated gonads that are on their way to becoming either testes or ovaries. If you look at an embryo at an early stage of development, you cannot tell whether it is male or female. a gene on the male’s Y chromosome, the SRY (sex-determining region on the Y chromosome) gene, causes those primitive gonads to develop into testes, the spermproducing organs. The testes produce androgens (hormones that are more abundant in males) that increase the growth of the testes, causing them to produce more androgens. Androgens also cause the primitive Wolffian ducts, precursors for other male reproduc- tive structures, to develop into seminal vesicles (saclike structures that store semen) and the vas deferens (a duct from the testis into the penis). The testes also produce Müllerian-inhibiting hormone (MIH), which causes the Müllerian ducts to degenerate. Because females do not have the SRY gene, their gonads develop into ovaries instead of testes, and their Wolffian ducts degenerate. Because their ovaries do not produce MIH, females’ Müllerian ducts develop and mature into oviducts, uterus, and the upper vagina. From then on, the males’ testes produce more androgens than estrogens (hormones that are more abundant in females), whereas the females’ ovaries produce more estrogens than androgens. Steroids exert their effects in three ways: First, they bind to membrane receptors, like neurotransmitters, exerting rapid effects. Second, they enter cells and activate certain kinds of proteins in the cytoplasm. Third, they bind to receptors that bind to chromosomes, where they activate or inactivate certain genes The most widely known androgen is testosterone. The most prominent type of estrogen is estradiol. Progesterone, another predominantly female hormone, prepares the uterus for the implantation of a fertilized ovum and promotes the maintenance of pregnancy. Later research demonstrated that some differences depend directly on control by the X and Y chromosomes independently of hormones. At least three genes on the Y chromosome (found only in men) are active in specific brain areas, and at least one gene on the X chromosome is active only in the female brain. What does the SRY gene do? The SRY gene (sex-determining region on the Y chromosome) causes the undifferentiated gonad of a mammal to develop into a testis, which then produces testosterone and MIH to direct development toward the male pattern. How do sex hormones affect neurons? Sex hormones, which are steroids, bind to receptors on the membrane, activate certain proteins in the cell’s cytoplasm, and activate or inactivate particular genes.

Biologists distinguish between the organizing and activating effects of sex hormones. Organizing effects produce long-lasting structural effects. During a sensitive period in early development, during the first trimester of pregnancy for humans, sex hormones determine whether the body develops female or male genitals. They lead to more receptors, and therefore greater sensitivity, around the female nipples than the male nipples Later researchers recognized that sex hormones produce additional organizing effects at puberty. Activating effects are more temporary, continuing only while a hormone is present or shortly beyond. For example, current hormone levels influence the degree of sex drive. The burst of hormones during pregnancy produce complex, temporary effects on emotional arousal, aggressive behavior, learning, and cognition In mammals, differentiation of the exter- nal genitals and several aspects of brain development depend mainly on the level of testosterone, not estradiol. A high level of testosterone, converted within cells to dihydrotestosterone, causes the external genitals to develop the male pattern, and a low level leads to the female pattern. Estradiol produces important effects on the internal organs, but it has little effect on the external genitals. If levels of dihydrotesterone are high enough, the tiny genital tubercle grows and develops into a penis. If the levels are low, the tubercle develops into a clitoris. Similarly, depending on levels of testosterone and dihydrotes- tosterone, the embryo develops either a scrotum, characteris- tic of males, or labia, characteristic of females.

A mammal exposed to high levels of both male and female hormones will appear male. One exposed to low levels of both will appear female. Genital development depends mostly on the presence or absence of androgens, and is nearly independent of estradiol levels. Pregnant women should avoid alcohol, marijuana, haloperidol, phthalates, and cocaine because these drugs interfere with male sexual development. Even aspirin and the chemicals lining bottles and cans produce mild abnormalities. Obviously, the results depend on both quantities and timing of exposure to these chemicals. one area in the anterior hypothala- mus, known as the sexually dimorphic nucleus, is larger in males than in females and contributes to control of male sexual behavior, in ways that vary from one species to another. Parts of the female hypothalamus generate a cyclic pattern of hormone release, as in the human menstrual cycle. The male hypothalamus cannot produce such cycles, and neither can the hypothalamus of a female who was exposed to extra testosterone early in development. During the early sensitive period, immature mammals have a protein called alphafetoprotein. Alpha-fetoprotein in rodents binds with estradiol and prevents it from entering cells, where it could produce masculinizing effects. Because tes- tosterone does not bind to alphafetoprotein, it enters neurons where enzymes convert it into estradiol. That is, testosterone is a way of getting estradiol to its receptors when estradiol circulat- ing in the blood is inactivated.

Testosterone enters neurons where it exerts its masculinizing effects directly, and it does not need to be aromatized to estradiol What evidence most directly links children’s toy play to prenatal hormones? Girls whose mothers had higher testosterone levels during pregnancy tend to play with boys’ toys more than the average for other girls. Boys whose mothers had higher phthalate exposure tend to play with boys’ toys less than the average for other boys. In addition to the sex hormones, the pituitary hormone oxytocin is also important for reproductive behavior. Oxytocin stimulates contractions of the uterus during delivery of a baby, and it stimulates the mammary gland to release milk. Sexual pleasure also releases oxytocin, especially at orgasm Testosterone, essential for male sexual arousal, acts partly by increasing touch sensitivity in the penis. Sex hormones also bind to receptors that increase responses in parts of the hypothalamus, including the ventromedial nucleus, the medial preoptic area (MPOA), and the anterior hypothalamus. estosterone primes the MPOA and several other brain areas to release dopamine. MPOA neurons release dopamine strongly during sexual activity, and the more dopamine they release, the more likely the male is to copulate Levels of testosterone correlate positively with men’s sexual arousal and their drive to seek sexual partners. Researchers found that, on average, married men and men living with a woman in a committed relationship have lower testosterone levels than single, unpaired men of the same age men with lower testosterone levels are more likely than others to marry and remain married, and research clearly supports that idea as well low testosterone is not the usual basis for impotence, the inability to have an erection. The most com- mon cause is impaired blood circulation, especially in older men. The drug sildenafil (Viagra) increases male sexual ability by prolonging the effects of nitric oxide, which increases blood flow to the peni Testosterone primes hypothalamic cells to be ready to release dopamine. It also increases sensitivity in the genital area. Men with lower testosterone levels are more likely to get married than are men with higher testosterone levels. There may also be a tendency for testosterone levels to drop after marriage.

A woman’s hypothalamus and pituitary interact with the ova- ries to produce the menstrual cycle, a periodic variation in hormones and fertility over the course of about 28 days After the end of a menstrual period, the ante- rior pituitary releases follicle-stimulating hormone (FSH), which promotes the growth of a follicle in the ovary. The follicle nurtures

the ovum (egg cell) and produces several types of estrogen, including estradiol. Toward the middle of the menstrual cycle, the follicle builds up more and more receptors The increased release of estradiol causes an increased release of FSH as well as a sudden surge in the release of luteinizing hormone (LH) from the anterior pituitary One consequence of high estradiol and progesterone levels during pregnancy is fluctuating activity at the sero- tonin 3 (5HT3) receptor, which is responsible for nausea Birth-control pills prevent pregnancy by interfering with the usual feedback cycle between the ovaries and the pituitary. The most widely used birth-control pill, the combination pill, containing estrogen and progesterone, prevents the surge of FSH and LH that would otherwise release an ovum. The estrogen–progesterone combination also thickens the mucus of the cervix, making it harder for a sperm to reach the egg, and prevents an ovum, if released, from implanting in the uterus. Thus, the pill prevents pregnancy in many ways. When researchers compared young married women, they found that frequency of sexual inter- course correlated highly with the amount of testosterone that women produced at the periovulatory period, the days around the middle of the menstrual cycle, when fertility is highest. sexual desire correlated strongly with changes in levels of es- tradiol, not testosterone

women not taking birth-control pills initiate sexual activity more often (either with a partner or by masturbation) during the periovulatory period than at other times of the month At what time in a woman’s menstrual cycle do her estradiol levels increase? When are they lowest? Estrogen levels increase during the days leading up to the middle of the menstrual cycle. They are lowest during and just after menstruation. When someone demonstrates an anatomical difference between male and female brains, on average, what if anything can we conclude about its role in behavior? Until we have further evidence, we should not conclude anything. It is easy to speculate about how brain differences produce behavioral differences between men and women, but it is harder to test those speculations with solid data. Parental behavior • (i) Hormonal changes during pregnancy prepare a female mammal: • To provide milk • To care for the young • She eats and drinks more than usual • Becomes less fearful and more aggressive (in defense of her young) • (ii) After birth she increases her secretion of

• estradiol • prolactin → milk production • decreases production of progesterone

• Estradiol: sensitivity of receptors in the medial preoptic area and anterior hypothalamus increases and maternal brain is more sensitive to estradiol • → Areas responsible for maternal behavior and attention to the young (Rosenblatt et al., 1998), • Vasopressin: synthesized by the the posterior pituitary gland • important for social behavior in many species • facilitating olfactory recognition (Tobin et al., 2010) • Males: testosterone level decreases and his prolactin level increases after a baby is born → higher prolactin, more care for children What factors are responsible for maternal behavior shortly after rats give birth? What factors become more important in later days? The early stage of rats’ maternal behavior depends on a surge in the release of the hormones prolactin and estradiol. A few days later, her experience with the young decreases the vomeronasal responses that would tend to make her reject them. Experience with the young maintains maternal behavior after the hormone levels begin to drop.

summary 1. Male and female behaviors differ because of sex hor- mones that activate particular genes. Also, certain genes on the X and Y chromosomes exert direct effects on brain development. 326 2. Organizing effects of a hormone, exerted during a sensi- tive period, produce relatively permanent alterations in anatomy and physiology. 328 3. 3. In the absence of sex hormones, an infant mammal develops female-looking external genitals. The addition of testosterone shifts development toward the male pattern. 4. During early development in rodents, testosterone is converted within certain brain cells to estradiol, which actually masculinizes their development. Estradiol in the blood does not masculinize development because it is bound to proteins in the blood. In primates, testosterone masculinizes brain development without being converted to estradiol. 329

5. In adulthood, sex hormones activate sex behaviors, partly by facilitating activity in the medial preoptic area and anterior hypothalamus. The hormones prime cells to release dopamine in response to sexual arousal. 331 6. 6. A woman’s menstrual cycle depends on a feedback cycle that controls the release of several hormones. Although women can respond sexually at any time in their cycle, on average, their sexual desire is greatest during the fer- tile period of the menstrual cycle, when estradiol levels are high. 332 7. Sex hormones also influence behaviors not directly related to sexual reproduction, including brain development and the ability to recognize emotional expressions. 334 8. Extra estradiol, within normal limits, does not determine whether the individual looks male or female. 328 At an early stage of embryological development, before the influence of sex hormones, what is the appearance of a fetus’s external genitals? Both sexes have small “unisex” structures that could develop into either a penis or a clitoris. Sex hormones affect neurons in several ways. Which of the following is not one of those ways? Brain enzymes convert them into dopamine and other neurotransmitters. What is the main difference between organizing effects and activating effects of hormones? Organizing effects are long-lasting, whereas activating effects are temporary. If a developing mammal is exposed to high levels of both androgens and estrogens, its external anatomy will appear masculine. If it is exposed to low levels of both, its external anatomy will appear feminine. How does sexual differentiation of the brain differ between rodents and primates? In rodents, testosterone must be aromatized to estradiol before it affects developing neurons. How much a child plays with boys’ toys is apparently increased by prenatal exposure to testosteron and decreased by prenatal exposure to phtalates. In terms of neurotransmitter activity, how does the mechanism of erection differ from that of orgasm? Erection depends on one set of dopamine receptors, and orgasm depends on different dopamine receptors. Compared to other men, what are the testosterone levels of married men? Lower than average At what time, if any, during the menstrual cycle do women’s sexual desires tend to be highest, in women not taking birth-control pills? Sexual desire is highest during the periovulatory period. What effect, if any, do sex hormones have on people’s ability to recognize facial expressions of emotion? Testosterone impairs the ability to recognize emotional expressions. What hormonal levels, if any, correlate with the amount of time and care that human fathers give their young children? High levels of prolactin and low levels of testosterone

Part of Charles Darwin’s theory of evolution by natural selection was that individuals whose genes help them survive will produce more offspring, and therefore the next genera- tion will resemble those with these favorable genes. A sec- ond part of his theory, not so widely accepted at first, was sexual selection: Genes that make an individual more ap- pealing to the other sex will increase the probability of reproduction, and therefore the next generation will resemble those who had these genes. What evolutionary advantage is suggested for why women are more interested in men’s wealth and success than men are interested in women’s wealth? During pregnancy and early child care, a female is limited in her ability to get food and therefore prefers a male partner who can provide for her. A healthy male is not similarly dependent on a female. Gender identity is what we consider ourselves to be. The biological differences between males and females are sex differences, whereas the differences that result from people’s thinking about themselves as male or female are gender differences. Hermaphrodites (from Hermes and Aphrodite in Greek mythology) have anatomies intermediate between male and female, or show a mixture of male and female anatomies. A true hermaphrodite has some testicular tissue and some ovarian tissue. The most common cause of this condition is congenital adrenal hyperplasia (CAH), meaning overdevelopment of theadrenalglandsfrombirth. People whose sexual development is intermediate are called intersexes. An alternative is to use the term disorders of sexual development. What is a common cause for a genetic female (XX) to develop a partly masculinized anatomy? If a genetic female is genetically deficient in her ability to produce cortisol, the pituitary gland does not receive negative feedback signals and therefore continues stimulating the adrenal gland. The adrenal gland then produces large amounts of other hormones, including testosterone, which masculinizes development. A girl who is exposed to extra testosterone during prenatal development is more likely than most other girls to prefer boy-typical toys. A genetic male with a gene that prevents testosterone from binding to its receptors will develop an appearance that partly or completely resembles a female. For which kind of twin pair is the concordance for sexual orientation greatest? Monozygotic twins have higher concordance than dizygotic twins. Be sure to state this point correctly: Do not say that homosexuality is more common in mono- zygotic than dizygotic twins. It is the concordance that is greater—that is, the probability that both twins have the same sexual orientation. According to this hy- pothesis, some unidentified event in the environment can attach an acetyl group or a methyl group to some gene, increasing or decreasing its activity. That gene modification could be passed to the next generation, producing evidence for a hereditary effect, even though there is no “gene for homosexuality.” If events like this happen often enough, the result could be a moderately high prevalence of homosexuality, even if men with the inactivated gene seldom reproduce.

By what route might having an older brother increase the probability of male homosexuality? Having an older brother might increase the probability of male homosexuality by altering the mother’s immune system in the prenatal environment. The effect of the older brother does not depend on growing up in the same home. How might stress to a pregnant rat alter the sexual orientation of her male offspring? Evidently, the stress increases the release of endorphins in the hypothalamus, and very high endorphin levels can block the effects of testosterone. In many species, males and females evolve different appearances and behaviors because of sexual selection. That is, they evolve in ways that make them more ap- pealing to the other sex. 341 Many of the mating habits of people can be interpreted in terms of increasing the probability of passing on our genes. However, it is hard to know to what extent the differences between men and women are evolutionary adaptations and to what extent they are learned. People can develop ambiguous genitals or genitals that don’t match their ch...