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

Rat Ovum and Sperm

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

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?

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)

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.

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

Lordosis

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

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