BIO 132

1. BIO 132 Lecture 37 Sex and the Brain Neurophysiology

2. Sexual versus Asexual Reproduction • Asexual reproduction consists of a cell dividing into two genetically identical offspring (clones) • Bacteria reproduce this way • Sexual reproduction requires the genetic material from two individuals combine to make a new genetically unique offspring. • Advantage: • Increased diversity in the population offers protection against disasters • Disadvantage: • Slower proliferation and is costlier

3. Sex and Gender • The Y-chromosome (only found in men) will change the gender from the default female to male. • The SRY-gene (sex-determining region of the Y-chromosome) turns on other sex-determining genes. • Genitalia of men are derived from the same tissue as women. The presence of high levels of testosterone trigger the transformation of the tissue. • Clitoris becomes the head of the penis • Labia fuse to form scrotum • Ovaries descend and become testes

4. Sexual Behavior • Sexual behavior in humans is a complex thing that most certainly has powerful genetic and social components. • Not well understood. • Sexual Behavior in non-human animals have less social pressures and is easier to study.

5. Monogamy and Promiscuity • Monogamy (“one spouse”) – a male and female form a tightly bound relationship that includes exclusive mating with each other. • Only 3% of mammals practice monogamy • 12% of primates practice monogamy • 90% of bird species practice monogamy • Promiscuity – mating with many partners • Polygyny (“many women”) and Polyandry (“many men”). • Polygyny is far more common than is polyandry

6. Studying Sexual Behavior • Choosing an animal model • Voles (very similar to mice) were chosen to be studied because there are two species that are very similar (physically and genetically) in most respects except that one species uses promiscuous and the other uses monogamous sexual behavior • Prairie Vole – Monogamous • Male forms a tight pair-bound with female right after mating • Both parents live in same nest and raise offspring • Montane Vole – Promiscuous • Male forms no pair-bound and tries to mate with many females • Male lives in an isolated nest and takes no part in raising offspring • Females care for offspring only briefly

7. Brain Differences in Voles • Studies looking at the size of different brain areas were not significantly different between the two species. • Studies looking at differences in receptors found little difference between the two species except for the receptor locations of oxytocin and anti-diuretic hormone (ADH).

8. Oxytocin/ADH Receptor Mapping Montane Voles Prairie Voles Oxytocin receptors ADH receptors

9. Different Neural Pathways Activated • Different neural pathways are activated by oxytocin and ADH in the two vole species. • Receptor mapping of female Montane voles shows plasticity, temporarily looking more like that of the Prairie voles (monogamous) during the brief period they were raising young. • Pharmacological studies were needed to further test the hypothesis that oxytocin and ADH are key components of sexual behavior in voles.

10. Pharmacological Evidence • It was found that in Prairie voles levels of ADH rise sharply in males and levels of oxytocin rise sharply in females during copulation (sex). • Further evidence: • Injection of male Prairie voles with an ADH antagonist before copulation abolished the pair-bonding and paternal care. • Oxytocin injections had no effect. • Injection of male Prairie vole without copulation but in the presence of a female resulted in the forming of a tight pair-bond. • In Prairie voles, levels of ADH in males and oxytocin in females was proportional to the time they spent parenting.

11. Pharmacological Evidence in Montanes • Injection of Montane vole males and females with either ADH or oxytocin, or injection of the antagonists of the hormones had no significant effect on sexual behavior. • The reason is probably that the Montane voles do not have the receptors in the right brain areas to elicit the behavior seen in the Prairie voles. • Imagine the implications if injecting a male human with a single hormone caused him to form a tight life-long pair-bond with the first female he saw.

12. Gender Differences in Cognition • Many studies have looked for physical differences in the brains of human men and women but nothing concrete or conclusive has been found. • In humans are women better than men at some mental tasks and vice versa? • Evidence seems to suggest that on average… • women are better at certain verbal and language skills: • e.g. naming objects of the same color, listing words starting with the same letter, verbal memory, etc. • Men are better at spatial and mathematical skills: • 3D puzzles, reading maps, learning mazes, etc.

13. Cautionary Notes • Differences in performance between individuals of the same gender are far larger than difference across the genders. • Pick any man and woman and there is almost a 50/50 chance that one will be better at verbal or spatial tasks. • Not all studies reveal the same results. • It is unclear how much social influence and upbringing plays a role.

14. Sex Hormones and Behavior • There are real and significant differences in the brains of men and women. • How do we know this? • All behavior comes from the brain and men and women behave differently. Therefore, there must be difference in the brains. • If genitalia development is very different in men and women and strongly effected by testosterone, then perhaps brains are as well.

15. Masculinization of the Brain • Evidence from other animals supports the idea that testosterone levels during development masculinize the brain, leading to male-like behavior. • If a female rat is injected with high levels of testosterone during her pregnancy, female rat offspring show male-like behavior. • Mount other females • Overly aggressive • Show no maternal instincts • Mother not affected – her brain was already developed • Male offspring not affected – they already had high testosterone levels

16. Masculinization of the Brain (cont) • Pregnant cows can have twin calves that are male and female. • If some of the testosterone from the male twin reaches the female twin during development, she will have non-functioning ovaries and show bull-like behaviors as an adult. • Farmers have long recognized this and call the female a “Freemartin”.

17. Pathway of Masculinization • Women have two X sex chromosomes (XX) • Men have one X and one Y sex chromosome (XY) • The SRY gene (found only on the Y-chromosome) turns on genes and leads to the expression of testis-determining factor which turns ovaries into testes. • Testes produce male sex hormones (androgens) like testosterone. • Androgens affect changes in nearly all tissues (including the brain) during development.

18. Masculinization Errors in Humans • Androgen insensitivity – Males (XY) that have testes and produce androgens but lack androgen receptors because of a genetic defect on the X chromosome. (1 in 13,000 births) • Individuals appear female • Have vagina, clitoris, labia (but no menstruation; infertile) • Develop breasts and a female body shape • Individuals behave like a female • Consider themselves women, dress and act like women • Choose men as sex partners

19. Androgen Insensitivity

20. Masculinization Errors in Humans • Congenital adrenal hyperplasia (CAH) – Females (XX) that have overdeveloped adrenal glands at birth that produce an excess of testosterone. (1 in 13,000 births) • Extent of the syndrome dependent on the levels of testosterone during development. • Individuals have ovaries but have intermediate-sized external genitalia (between size of penis and clitoris). • Surgery and medication are the usual treatments • Individuals more likely to have “male” traits • Tomboyish, aggressiveness, etc. • Most are heterosexual but a greater proportion than average are homosexual.