Chapter 10: Reproductive Behaviors

• SRY gene


  Sex-determining region on the Y chromosome that causes undifferentiated gonads to develop into testes in male mammals
• Sexual differentiation in mammals

  1. Undifferentiated gonads

  2. Testes develop in males due to SRY gene

  3. Testes produce androgens

  4. Androgens cause Wolffian ducts to develop into male internal structures

  5. Müllerian ducts degenerate in males

  6. Ovaries develop in females without SRY gene

  7. Müllerian ducts develop into female internal structures

  8. Wolffian ducts degenerate in females
• Steroid hormones


  Hormones containing four carbon rings, including androgens and estrogens
• Effects of steroid hormones

  • Bind to membrane receptors, activate proteins in cytoplasm, activate or inactivate genes
• Organizing effects of hormones

  Long-lasting structural effects during sensitive periods of development, like genital differentiation in first trimester of pregnancy
• Activating effects of hormones

  Temporary effects on behavior and physiology while hormone is present
• Organizing effects

  Set the stage for activating effects
• Genes on X and Y chromosomes also influence sex differences beyond just hormones
• Activating effects

  More temporary, continuing only while a hormone is present or shortly beyond
• Organizing effects

  Set the stage for activating effects, e.g. organizing effects set up the female hypothalamus such that later hormones can activate the menstrual cycle
• Development of human genitals

  1. Undifferentiated before sixth week

  2. Seventh to eighth week: Genital tubercle, Urethral fold, Urethral groove, Genital fold, Anal pit

  3. Fully developed by twelfth week: Penis, Clitoris, Labia minora, Scrotum, Labia majora, Anus
• Testosterone


  Secreted by testes, converted to dihydrotestosterone which promotes growth of penis
• Dihydrotestosterone


  Far more effective than testosterone at promoting growth of penis
• High levels of dihydrotestosterone

  Genital tubercle develops into a penis
• Low levels of dihydrotestosterone

  Genital tubercle develops into a clitoris
• High levels of testosterone and dihydrotestosterone

  Embryo develops scrotum, characteristic of males
• Low levels of testosterone and dihydrotestosterone

  Embryo develops labia, characteristic of females
• Injecting a genetic male with estrogens produces little effect on his external anatomy
• If a genetic male lacks androgens or androgen receptors, he develops the female-typical pattern of anatomy and behavior
• Drugs that interfere with male sexual development

  • Alcohol
  • Marijuana
  • Haloperidol
  • Phthalates
  • Cocaine
  • Even aspirin
• Male development is vulnerable to many sources of interference
• Genetic female that lacks estradiol during early life develops approximately normal female external anatomy but does not develop normal sexual behavior or normal internal anatomy
• Even if a genetic female is given estradiol injections as an adult, she shows little sexual response toward either male or female partners
• Estradiol is essential for female development, including certain aspects of brain differentiation, even if it is not important for external anatomy
• Sex differences in the brain

  • Many brain areas form a larger percentage of the male than female brain, whereas other areas constitute a larger portion of the female brain
  • Differences are not simply a result of the fact that men are larger
• Mechanisms of sex differences in the hypothalamus

  1. In rodents, testosterone is converted to estradiol which exerts masculinizing effects

  2. In different hypothalamic areas, estradiol acts via different mechanisms like increasing prostaglandin E2, activating PI3 kinase, or increasing GABA production
• It is possible for one brain area to become more masculinized or more feminized than another
• Sex differences in the brain involve not only testosterone and estradiol, but also close to a hundred genes that are more active in one sex or the other
• Because genes vary, and so do factors that cause epigenetic changes, the "average" brain differences between males and females do not apply to every individual
• Infants 3 to 8 months old show a predisposition for girls to look at dolls more than toy trucks, and boys to look at both about equally
• This suggests a predisposition for boys and girls to prefer different types of toys, although an alternative explanation is that girls are drawn to faces and social cues in the dolls
• Preference for girl-typical activities at age 13

  Golombok, Rust, Zervoulis, Golding, & Hines, 2012
• Much of this pattern results from socialization, as most parents give their sons and daughters different sets of toys
• Socialization need not be the whole story. Previous generations may have found that boys and girls often differ in their interests from the start
• Infant toy preference study

  1. Infants 3 to 8 months old sat in front of pairs of toys

  2. Researchers monitored eye movements

  3. Girls looked at dolls more than toy trucks

  4. Boys looked at both about equally
• Girls mature faster than boys


  It was harder for boys at this age to show a preference, whatever that preference may have been
• Monkeys

  • Male monkeys played with balls and toy cars more than female monkeys
  • Female monkeys played more with dolls
• Prenatal injections of testosterone to female monkey fetuses

  Led to increased masculine-type play after they were born
• Prenatal testosterone study

  1. Researchers took blood samples from pregnant women, measuring testosterone

  2. When daughters reached age 3.5, researchers observed their toy play

  3. Girls exposed to higher testosterone levels showed slightly elevated preferences for boys' toys
• Infant testosterone study

  1. Researchers measured testosterone levels in infants over the first 6 months

  2. Girls with higher testosterone levels spent more time playing with toy trains

  3. Boys with higher testosterone levels spent less time playing with dolls