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

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”

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 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 phalarope seahorse

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)