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

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:
Fertility, youth, fitness, beauty
• Attract females with:
Wealth, power, status
Lordosis
FEMALE
XX
MALE
XY
ovary
testis
NO testosterone
testosterone
1 oviducts, uterus
1 vas deferens, sex
accessory glands
2 clitoris, labia
2 penis, scrotum
feminine brain
3
differentiation
masculine brain
3
differentiation
“Sex-Typical” Activities in Boys and Girls and in Children With
Congenital Adrenal Hyperplasia (CAH)