Darwin’s Puzzle:Why are Males and Females

Different?

Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex. 1st ed., Murray, London.

Parental Investment and Sexual Selection

Trivers 1972

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?

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)

Sperm vs. Egg

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

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

Competitive Males• Males are fighting with each other to mate with as

many females as possible• More females = more offspring (sharp contrast to

females)

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

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

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

• 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”

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…

Sexual Selection and Parental Investment Theory

• Intrasexual selection:weaponry/size differentiation

Sexual Selection and Parental Investment Theory

• Intrasexual selection: lack of differentiation

Sexual Selection and Parental Investment Theory

• Intrasexual selection: behavior

Sexual Selection and Parental Investment Theory

• Intrasexual selection• Causes members of the competitive sex

to die younger than the competed-for sex

• …willingly!!!

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….

Sexual Selection and Parental Investment Theory

• Intersexual selection: ornaments in one sex and not the other

Sexual Selection and Parental Investment Theory

• Intersexual selection: ornaments are not always pretty by human standards

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

Sexual Selection and Parental Investment Theory

Sexual Selection and Parental Investment Theory

Intersexual selection+

Intrasexual selection=

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

seahorse

phalarope

Next Topic

• Altruism

The Problem of Altruism

Kin Selection

Reciprocal Altruism

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…

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)

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

Reciprocal Altruism

• Altruism among Unrelated individuals

• I.e., “exchange”

• Trivers (again)

Reciprocal Altruism

• Not very common in nonhuman primates

• Very common in humans

• Chimps exchange coalitional support

• See also capuchins in Perry book (later)