1. Puzzle of sexual dimorphism (differences in males and
females in appearance or behavior)
Darwin “Sight of a peacock makes me sick.”

2. Rat Ovum and Sperm

3. What is “female” and “male”?
• female has larger gametes (eggs larger, and less numerous, than sperm
• female usually invests more resources in reproduction as compared to male
• thus, female often is more ‘particular’ in selecting a mate

4. Eggs are more “expensive” than sperm.
Sperm are very small (“inexpensive”)
Eggs are large and nutrient rich (“expensive”).
So, females INVEST more in gametes, and have more investment in each offspring compared to males;
Asymmetry in sexual reproduction established…
What effects does this have on mating behavior?

5. In general, success at mating:
For a male, is limited by the number of females
he can convince to mate with him.
For a female, is limited by the number of eggs/pregnancies
she can produce.
Leads to:
Mate Competition (male-male, for access to females)
Mate Choice (female choice, for best quality males)

6. Sexual selection is a subset of natural selection that has to do with mating success.
If there is variation in a trait that affects ability to mate successfully, and that trait is heritable, then individuals with that trait will become more common over time.

7. Mate Selection and Competition
Females Males
Greater investment in offspring • Less investment in offspring
Generate fewer offspring • Can generate more offspring
Choosier in mate selection • Less discriminating
Wait for best male • Maximize sexual opportunities
Males = success objects • Females = sex objects
Attract males with: • Attract females with:
Fertility, youth, fitness, beauty Wealth, power, status

8. Lordosis

9. NO testosterone FEMALE MALE XX XY ovary testis testosterone
vas deferens, sex accessory glands 1
oviducts, uterus 1 2
clitoris, labia 2
penis, scrotum
feminine brain differentiation
masculine brain differentiation 3 3

13. “Sex-Typical” Activities in Boys and Girls and in Children With Congenital Adrenal Hyperplasia (CAH)