1. Darwin’s Puzzle: Why are Males and Females Different?
Darwin, C The Descent of Man and Selection in Relation to Sex. 1st ed., Murray, London.

2. Parental Investment and Sexual Selection
Trivers 1972

3. Assumption
Assumption: Every organism has adaptations that function to facilitate reproduction
Members of a population/species live in the same environment, so why do some animals have different adaptations than others?
Morphs: age, sex, others
SEX: male and female adaptations are different
WHY?

4. Parental Investment
Any investment by the parent in an individual offspring that increases the offspring’s chance of surviving (and hence reproductive success) at the cost of the parent’s ability to invest in other offspring” (Trivers 1972)

5. Sperm vs. Egg
In sexually-reproducing species, the relative size of gametes define who is male and who is female.

6. Nurturant Females
In most animals, and almost all mammals, females provide far more parental investment than just the egg
Internal fertilization protects, but at a cost
Cod vs. gorillas
Humans (mammals):
Prolonged internal gestation (pregnancy)
Placentation
Lactation

7. Competitive Males
Males are fighting with each other to mate with as many females as possible
More females = more offspring (sharp contrast to females)

8. High male Variance: Elephant seals
One breeding season, 115 males were present, but the 5 highest ranking ones in the hierarchy (the big, tough ones) performed 123 of 144 observed copulations

9. Sexual Selection and Parental Investment Theory
For members of the sex that invests more in offspring, reproductive success is limited by the amount of resources an individual can secure for itself and it’s offspring
For members of the sex that invests less in offspring, reproductive success is limited by the number of mates one can acquire

10. Sexual Selection and Parental Investment Theory
What of it?
Selection acted on males differently than it acted on females
Specifically, differences in parenting strategies cause differences in adaptations
Sex that invests more: adaptations to survive and get resources for offspring
Sex that invests less: adaptations to help them get as many mates as possible
It explains why, in many species, males look and behave differently than females

11. Sexual Selection and Parental Investment Theory
Explains primary sex differences (uteruses vs. testes)
Explains secondary sex differences
Differences in weaponry
(intrasexual selection)
Differences in ornaments
(intersexual selection)
When the sexes have different adaptations, they are “sexually dimorphic”

12. Sexual Selection and Parental Investment Theory
Intrasexual selection occurs when members of one sex fight with each other to gain sexual access to members of the other sex
Results in weaponry
Examples…

13. Sexual Selection and Parental Investment Theory
Intrasexual selection:weaponry/size differentiation

14. Sexual Selection and Parental Investment Theory
Intrasexual selection: lack of differentiation

15. Sexual Selection and Parental Investment Theory
Intrasexual selection: behavior

16. Sexual Selection and Parental Investment Theory
Intrasexual selection
Causes members of the competitive sex to die younger than the competed-for sex
…willingly!!!

17. Sexual Selection and Parental Investment Theory
Intrasexual selection occurs when members of one sex fight with each other to gain sexual access to members of the other sex
Intersexual selection….

18. Sexual Selection and Parental Investment Theory
Intersexual selection: ornaments in one sex and not the other

19. Sexual Selection and Parental Investment Theory
Intersexual selection: ornaments are not always pretty by human standards

20. Sexual Selection and Parental Investment Theory
Intersexual selection: why ornaments?
Arbitrary (you just know you want your offspring to inherit those traits)
Good genes
Low parasite load
Handicap principle
Combination

21. Sexual Selection and Parental Investment Theory
Intersexual selection +
Intrasexual selection =

22. Sexual Selection and Parental Investment Theory
Why not just say “females are limited by resources and males are limited by access to females”?
The exceptions that prove the rule
phalarope
seahorse

23. Next Topic
Altruism

24. The Problem of Altruism
Kin Selection
Reciprocal Altruism

25. Altruism
Doing something that benefits another individual’s reproductive success at a cost to one’s own reproductive success
If natural selection is all about competition to reproduce, how could there possibly be adaptations that cause one to be altruistic?
Two solutions…

26. Kin Selection William Hamilton (bees)
Not about helping the individual but about helping the gene
Hamilton’s Rule: C<Br
You share genes with your relatives
A gene that causes its bearer to be altruistic will only spread in a population if the cost to the altruist (C) is less than the benefit to the recipient (B), multiplied (devalued) by the coefficient of relatedness (r)

27. Kin Selection Mom, dad, full siblings: 50% (1/2)
Grandparents, 1st uncles and aunts, and half siblings: 25% (1/4)
First cousins: 12.5% (1/8)
Identical twins: 100% (very un likely to be a significant factor in the evolution of nonhuman primate behavior)
Bees: weird; workers are more closely related to each other than to the queen; Hamilton’s work

28. Reciprocal Altruism Altruism among Unrelated individuals
I.e., “exchange”
Trivers (again)

29. Reciprocal Altruism Not very common in nonhuman primates
Very common in humans
Chimps exchange coalitional support
See also capuchins in Perry book (later)