Reproduction and Sexual Behavior Summary

Why Sex

  • Sexual reproduction increases variation in the genetic pool.

  • Benefits of variation:

    • Enables quick evolutionary adaptations.

    • Corrects genetic errors.

Sex and Hormones

  • Hormones: Chemicals secreted by cells, traveling through the bloodstream to target tissues.

  • Prenatal sexual differentiation begins with chromosomes.

  • Early development: Both sexes have Müllerian and Wolffian ducts, plus undifferentiated gonads.

Wolffian and Müllerian Ducts

  • Wolffian ducts: Precursors to male reproductive organs.

    • Develop into vas deferens and seminal vesicles.

  • Müllerian ducts: Precursors to female oviducts, uterus, and upper vagina.

The SRY Gene

  • Male Y chromosome includes the SRY gene.

    • Causes primitive gonads to develop into testes.

  • Developing testes produce:

    • Androgens (increase testes growth).

    • Müllerian inhibiting hormone (MIH).

  • Females lack the SRY gene; gonads develop into ovaries.

Hormones in Males and Females

  • Testes produce more androgens; ovaries, more estrogens.

  • Androgens and estrogens are steroid hormones.

Steroid Hormones

  • Contain four carbon rings; exert effects in multiple ways:

    • Binding to membrane receptors (like neurotransmitters).

    • Binding to chromosomes (activating or inactivating genes).

Androgens and Estrogens

  • Categories of chemicals, not specific chemicals.

  • Androgens: Primarily male hormones; testosterone is most known.

  • Estrogens: Estradiol is most common.

Progesterone

  • Prepares the uterus for implantation of a fertilized ovum.

  • Promotes the maintenance of pregnancy.

Genes and Sex Differences

  • Hormones do not account for all biological sex differences.

  • Genes on X and Y chromosomes also produce sex differences.

  • Specific genes on Y are active in brain areas; one on X is active only in the female brain.

Effects of Sex Hormones

  • Organizing effects:

    • Occur at sensitive development periods.

    • Determine male or female characteristics of brain/body.

  • Activating effects:

    • Occur at any time; temporarily activate a particular response.

Sexual Differentiation

  • Depends mostly on testosterone levels during a sensitive period.

  • Human sensitive period for genital formation: first trimester of pregnancy.

Organizing Effects of Sex Hormones – Androgens

  • Female mammals exposed to testosterone during sensitive periods become partly masculinized.

    • Enlarged clitoris.

    • Masculinized sexual behavior.

  • Male rats lacking androgen receptors or castrated develop female-like anatomy and behavior.

Organizing Effects of Sex Hormones – Estrogens

  • Estrogens don't modify male anatomy but alter internal structures (e.g., prostate gland).

  • Absence of sex hormones generally leads to female-looking external genitalia.

  • Females lacking estradiol during early life do not develop normal sexual behavior.

Sex Differences in the Hypothalamus

  • Sex hormones early in life influence hypothalamus, amygdala, and other areas, producing anatomical and physiological differences.

  • Sexually dimorphic nucleus:

    • Area in the anterior hypothalamus.

    • Larger in males; contributes to control of male sexual behavior.

Sex Differences in the Hypothalamus – Hormone Release

  • Female hypothalamus generates a cyclic pattern of hormone release.

  • During early rodent development, testosterone is converted to estradiol within neurons.

Activating Effects of Sex Hormones

  • Sex hormones exert activating effects to temporarily modify behavior.

  • Behavior can influence hormone secretion.

  • Oxytocin: Important for reproductive behavior; stimulates uterine contractions and mammary glands.

Males – Sexual Arousal

  • Testosterone increases touch sensitivity in the penis.

  • Sex hormones bind to receptors in the hypothalamus.

  • Testosterone triggers dopamine release by the MPOA and other areas.

Males – Effects of Neurotransmitters

  • Dopamine stimulation of D1 and D5 receptors is associated with sexual arousal.

  • Higher dopamine concentrations stimulate D2 receptors and lead to orgasm.

  • Serotonin decreases sexual activity by blocking dopamine release.

Males – Testosterone Levels

  • Testosterone levels correlate positively with sexual arousal and drive to seek partners.

  • Married men have lower testosterone levels.

Males – Sexual Offenders

  • Testosterone reduction has been tried to control sex offenders.

    • Difficulties include medication discontinuation and side effects.

Females – The Menstrual Cycle

  • Hypothalamus and pituitary interact with ovaries to produce the menstrual cycle.

  • Menstrual cycle: Periodic variation in hormones and fertility over ~28 days.

Females – FSH

  • After menstruation, the anterior pituitary releases follicle-stimulating hormone (FSH).

  • FSH promotes follicle growth in the ovary.

  • The follicle nurtures the ovum and produces estrogen.

  • Follicle builds up receptors to FSH,
    -> produces increasing amounts of estradiol.

Females – FSH and LH

  • Increased estradiol causes anterior pituitary to increase FSH and luteinizing hormone (LH) release.

  • FSH and LH cause the follicle to release an ovum.

Females – Pregnancy

  • If pregnant, estradiol and progesterone levels increase gradually.

  • If not pregnant, levels decline, and the uterine lining is cast off (menstruation).

  • High levels of estradiol and progesterone produce heightened activity in serotonin receptors, causing nausea.

Females – Birth Control Pills

  • Birth control pills prevent pregnancy by interfering with the feedback cycle between ovaries and pituitary.

    • Combination pills contain estrogen and progesterone.

    • Thickened cervical mucus makes it harder for sperm to reach the egg.

Females – The Periovulatory Period

  • Point of highest fertility (middle of menstrual cycle).

  • Women become more sexually responsive.

  • Show increased attention to sex-related stimuli and flirting.

Effects of Sex Hormones on Nonsexual Characteristics

  • Prenatal androgens and estrogens influence brain development.

  • Stimulate brain areas important for memory.

Effects of Sex Hormones on Nonsexual Characteristics Part 2

  • Ability to recognize facial expressions.

  • Women receiving testosterone injections became less accurate at recognizing facial expressions of anger.

Parental Behavior – Hormonal Changes

  • Hormonal changes prepare female mammals to provide milk and care for young.

  • Prolactin is necessary for milk production.

  • Females change pattern of hormone receptors during pregnancy.

Parental Behavior – Hormonal Changes (cont’d.)

  • Vasopressin, made by the hypothalamus and secreted by the posterior pituitary, is associated with social behavior.

  • Mice can adopt maternal behavior without hormonal changes.

Parental Behavior – Hormones and the Quality of Care

  • Hormones play an initial role in maternal care quality.

  • Experience replaces hormonal influences as hormone levels decline.

  • Correlation exists between father’s hormones and level of interaction with their child.

Evolutionary Interpretations of Mating Behavior

  • Evolutionary Theory: Genes that help survival lead to more offspring.

  • Sexual selection: Genes that make an individual more appealing increase reproduction probability.

Evolutionary Differences: Men

  • Men seek multiple sex partners (especially short-term).

    • Evolutionary explanation: Spread genes through multiple partners or be loyal to one woman.

Evolutionary Differences: Women

  • Women can gain from multiple sexual partners (increased resources).

  • No direct evidence suggests specific genes influence preference for single or multiple mates.

What Men and Women Seek in a Mate

  • Men prefer a young partner (longer fertility).

  • Women select for long-term traits.

    • Status, ambition, generosity, kindness, emotional openness and willingness to devote resources to offspring.

  • Both prefer healthy, intelligent, honest, and physically attractive partners.

Differences in Jealousy

  • Men are more jealous at indications of a wife’s infidelity.

    • Evolutionary explanation: Ensure children are their own.

  • Cultural attitudes toward infidelity vary.

Sexuality is Cultural

  • Scientific data is not conclusive on the degree sexual behavior is evolutionary-guided or learned.

  • Cultural differences are influential.

Gender Identity and Gender-Differentiated Behaviors

  • Gender identity is how we identify sexually; sex differences are biological differences.

  • Gender differences result from thoughts about oneself as male or female.

Gender Identity and Gender-Differentiated Behaviors (cont’d.)

  • Most have a gender identity that matches their external appearance.

  • Some are dissatisfied with assigned gender.

  • Current evidence strongly suggests that biological factors are important, especially prenatal hormones.

Intersexes

  • People with anatomies intermediate between male and female.

  • Causes: Woman released two ova, each fertilized by different sperm or atypical hormone pattern occurs before birth.

Testicular Feminization

  • Androgen insensitivity: Individuals with XY chromosome pattern have smaller than average penis or female genital appearance.

  • Normal androgen production, but lack the receptor.

Congenital Adrenal Hyperplasia (CAH)

  • Overdevelopment of adrenal glands from birth.

  • Most common cause of intersex condition because cortisol production leads to extra testosterone.

  • Females become partly masculinized.

Genital Ambiguity

  • An estimated 1 out of 100 children is born with some degree of genital ambiguity.

  • 1 in 2,000 has enough genital ambiguity to make the sex uncertain.

  • It's difficult for intersex people to connect due to hospital confidentiality.

Interests and Preferences of CAH Girls

  • Brains of genetic females with CAH were exposed to higher levels of testosterone during prenatal/early postnatal life.

  • Toy preference: intermediate between preferences of non-CAH boys and girls.

  • More physical aggression in adulthood.

Issues of Gender Assignment and Rearing

  • Past recommendations: rear intersex children as girls; perform surgery to make them look more feminine.

  • Current guidelines: Be honest with the person/family and identify gender based on predominant external genital appearance.

Discrepancies of Sexual Appearance

  • 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.

  • Brain exposed to testosterone during early development.

Sexual Orientation

  • Homosexual behavior occurs in many animal species.

  • Most men discover sexual orientation early; process is slower for women.

Genetics

  • One contributing factor in homosexuality.

    • If one twin is homosexual, probability that other twin is homosexual is fairly high (for monozygotic twins).

  • No single particular gene has been identified.

An Evolutionary Question

  • Average homosexual man has one-fifth the number of children of average heterosexual man.

  • Hypotheses as to why genes have not been selected out:

    • Genes may be maintained by kin selection or related to epigenetics.

Prenatal Influences

  • Sexual orientation is not related to adult hormone levels.

  • Homosexual men and women tend to have normal hormonal levels for their biological sex.

Prenatal Stress

  • Mother’s immune system may exert prenatal effects.

  • Stress releases endorphins > Antagonizes the effects of testosterone on the hypothalamus.

Brain Anatomy

  • Differences in brain anatomy between homosexuals and heterosexuals exist.

  • On average, the homosexual brain is shifted towards the opposite sex in some ways.

  • Difficult to decipher why these differences exist.

Do Fetal Hormones Masculinize Human Behaviors in Adulthood?

  • Fetal androgen exposure is not as well understood in homosexual men.

  • Fraternal birth order effect: the more older brothers a boy has, the more likely he is to be homosexual.

Techniques of Modern Behavioral Endocrinology

  • Autoradiography shows where radioactively labeled chemicals accumulate in tissue.

  • Immunocytochemistry (ICC) uses antibodies to locate receptors and chemicals to make them visible.

  • In situ hybridization locates neurons that make mRNA for a receptor.