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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.
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.
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.
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.
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.
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
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.
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.
Variations in Sexual Behavior • 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.
Variations in Sexual Behavior • 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.