Biopsychology of Sex Behavior

1. Biopsychology of Sex Behavior Lecture 1: The Sexual Brain LeVay, Ch. 1-4

2. Chapter 1 • Two ideas about sexuality & its development: • We’re all born with similar brains, molded by external stimuli (nurture) • We’re totally preprogrammed (nature) • LeVay: “strong influence of nature, & only modest influence of nurture • Most people place emphasis on the influence of external factors (nurture)

3. Chapter 1 • Widespread tendency to exaggerate the importance of external factors • perception that parents teach children to walk. In reality, children may be able to make transition from crawling to walking w/out help from parents • Language acquisition: Children have an inborn ability to pick up verbal patterns of any human language. This can’t be taught.

4. Chapter 1 • Differences in external genitalia remain the commonly accepted criteria for defining sex • However, consider: chromosomal sex (XX=female, XY=male, & other combinations); internal sex (ovaries= female, testes=male), sexual orientation, & gender identity • Usually, external genitalia, chromosomal sex, , secondary sexual characteristics, and sexual identity are similar, but there are exceptions.

5. Chapter 2 • Sexual reproduction is not universal; some organisms reproduce asexually • Given that sex is cumbersome and inefficient, why do organisms use sex as a means of propagation? • Advantages conferred by sexual reproduction

6. Chapter 2 • 2 classes of hypotheses for the nature of this benefit: • (1) Interaction of organism & environment: mixing of genes (sexual reproduction) -> more variability in population = certain members of the population better able to adapt to changing environment • (2) Sex as mechanism to get rid of harmful mutations: Random mutations occur in natural populations. With asexual reproduction, parents pass on all mutations to offspring. In sexual reproduction, some offspring may inherit all mutations, but others may carry fewer mutations.

7. Chapter 2 • Cooperativity: 2 mutations may have more than additive effects • “Weeding out” process eliminates more bad mutations from sexually reproducing populations, even assuming the same number of individuals are eliminated • Kondrashov: Sex is mechanism for generating unfit individuals (runts), which take a disproportionate share of bad mutations w/ them • Although sexual reproduction is more advantageous, some species have made transition from sexual to asexual reproduction (e.g., asexual lizards). In addition, organisms that reproduce asexually may flourish under certain conditions. However, as LeVay stated, Muller’s ratchet may spell their doom • Still not clear which theory is closest to the truth

8. Chapter 2 • Why are the 2 sexes different? • In the remote past, the 2 sexes were probably similar in size (unicellular organisms) • Under natural selection, gametes gradually developed into 2 types: • Larger (female) • Smaller (male) • Both have advantages: • Larger=more nutritive material • Smaller=produced more rapidly; w/ fewer nutrients • Average=no special advantages (selected against)

9. Chapter 2 • Female investment is so great in producing an egg (trapped in nurturing role) • Males is like a parasite, taking advantage of her dedication. However, sexual reproduction produces offspring that are on average more fit • It would seem more advantageous to produce an excess of females. However, selective forces maintain a 50:50 sex ratio

10. Chapter 2 • It appears that evolution has made females the exploited sex. However, a female can choose her mate (sexual selection) • This allows female to make considerable demands on the male (e.g., selecting males that invest more in their offspring) • Kinship & Altruism: close relatives share genes, and evolution has favored behaviors that promote altruistic behaviors toward close relatives.

11. Chapter 2 • Dominant male langur monkeys regularly kill babies that aren't their own; so females, pregnant by the previous male, come into false heat and mate with the new dominant male. Sarah Hardy hypothesized that females engage in this behavior to protect their offspring. In most cases, this behavior ensures that the new male will not attempt to kill the female’s offspring

12. Chapter 3 • In mammals, sex is determined genetically (not true for all organisms). E.g., in alligators, sex is determined by temperature at which eggs are incubated • Sex chromosomes are different from other 22 pairs of chromosomes (autosomes) • In women, sex chromosomes resemble each other (XX). In men, one chromosome is long & the other is short (XY) • In mammals, presence of a gene, sry (aka testes determining factor, TDF), on Y chromosome  organism develops into a male (XY, Y, XXY, & XXYY all develop into males). Absence of a Y chromosome  organism develops into a female (XX, X, & XXX all develop into female). Thus, in mammals, intrinsic (default) program is to produce females. • TDF works in the undifferentiated gonad  becomes male gonad (testis) • In the absence of TDF, undifferentiated gonad becomes female gonad (ovary) • TDF also influences neurons and other tissues

13. Chapter 3 • Gonad cells become either support cells or hormone secreting cells: • Males: if TDF is present in gonads, gonad cells develop into Sertoli (supporting) or Leydig (hormone producing) cells. • Females: gonad cells develop into Granulosa (supporting) or thecal (hormone producing) cells. Must have XX ova in ovary; E.g., in XO females (Turner’s syndrome), ovaries atrophy • The founders of the germ cells (gametes) originate in the yolk sac and they migrate to the gonads

14. Chapter 3 • Females: Developing ovary does not produce significant amounts of hormones. Thus, the rest of the body develops according to an intrinsic program—to produce a female. Later on in life, during puberty, female development is dependent on hormones • Males: Turning on of TDF in supporting cells of the gonads causes these cells to send a message to hormone producing cells, instructing them to produce steroid hormones. Leydig cells synthesize and release testosterone (T) and other androgens. These hormones alter the default (female) developmental plan and the organism develops into a male • In males, Supporting cells also produce Müllerian inhibiting hormone (MIH), which prevents the müllerian duct from developing into the female genital tract. In females, the müllerian duct, embryonic structure, develops into oviducts, uterus, cervix, and part of the female’s external genitalia (vagina)

15. Chapter 3 • Gonadal steroids are a class of compounds that are synthesized from cholesterol. They are fatty molecules that dissolve in other fats but are poorly soluble in water. They can pass through cell membranes • These steroids control the activity of genes in cells—turning these genes on or off , and influencing growth processes & biochemical activities. Target cells contain receptor molecules that bind steroid hormones

16. Chapter 3 Hormone Effects Growth & biochemical processes Hormone receptor (in target cells) Gonadal Steroid Enzymes can convert a hormone molecule into another type of hormone molecule: Converting enzyme (e.g., aromatase) Different hormone molecule (e.g., estradiol) Gonadal Steroid (e.g., testosterone) In this example, testosterone is converted to estradiol by the converting enzyme aromatase. Aromatase is not present in the testes, therefore leydig cells cannot synthesize estradiol. Aromatase is present in other cells (e.g., neurons)

17. Chapter 3 • The male & female gonads develop from a common precursor—undifferentiated gonad • The male & female external genitalia develop from a common precursor—tissue around urogenital membrane • On the other hand, the male & female internal genitalia arise from different precursors: • Wolffian duct develops into male internal genitalia. This requires testosterone (T). In the absence of T, the wolffian duct regresses (this occurs in females) • Müllerian duct develops into female internal genitalia. This occurs by default. MIH prevents the Müllerian duct from developing into female internal genitalia (this occurs in males)

18. Development of male and female reproductive tracts

19. Chapter 3 • To become a normal male, embryo must turn off müllerian pathway (w/ MIH) and turn on wolffian pathway (w/ T) • In the presence of T, both internal and external reproductive structures develop in the male direction. However, the two processes are separated in time • Intersexuality: • Congenital adrenal hyperplasia: adrenal glands secrete larger than normal amounts of androgens; in females fetuses this can lead to masculinization of external genitalia (and the brain in some cases); but internal genitalia were formed before the adrenal glands produced all that androgen—they develop in the female direction • 5-alpha-reductase-deficiency syndrome: 5-alpha reductase is an enzyme that converts T to another androgen, dihydrotestosterone (DHT), and DHT is needed for the external reproductive structures to develop in the male direction. If a fetus is chromosomally male but lacks this enzyme, this leads to ambiguous genitals.

20. Congenital Adrenal Hyperplasia

21. 5alpha-Reductase Deficiency Appearance at puberty Child at Birth

22. Chapter 4 • LeVay: The mind is just the brain doing its job • As a network the brain is vastly underconnected: • most connections (probably well over 90%) are local • Neurons in each small region of the brain only process limited amounts of information from neighboring cells • Small regions of the brain are made up of cells working on a common task; each small region has a specific function • Although there is considerable specialization; other regions of the brain may also be involved in a given function. The idea that there is a “center” for each behavior or task is not accurate.

23. Chapter 4 • White matter of the brain contains axons covered by myelin sheath • Corpus callosum – well known piece of white matter; huge band of axons connecting the left and right hemispheres of the cerebral cortex. • Gray matter of the brain is dominated by cell bodies of neurons. • There are two kinds of gray matter: • Cortical (layered): cortex is gray matter arranged in layers at the outside surface of the brain • Nuclear (nonlayered): nuclear structures located within the substance of the brain (e.g., thalamus)