Lecture 4: Sexual Selection & Differential Investmentpeople.fas.harvard.edu/~anth1380/Slides/Lecture 4.pdf · 2007-02-16 · 1 Lecture 4: Sexual Selection & Female Choice •Differential

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Lecture 4: Sexual Selection &

Female Choice

• Differential investment• Classic views of male-female behavioral differences

• Natural Selection • Sexual Selection• Inter-sexual choice

• Intra-sexual competition

• Inter-sexual coercion

Behavioral Biology of Women — 2007

Differential Investment

Differential Investment in

Mammals

• Large gametes• Internal gestation• Lactation• Lengthy parental investment

• Small gametes• Paternal Investment variable

Females Males

Triver’s Hypothesis

“The sex investing most becomes a limiting

resource for the sex investing least”

ORANGUTANS:

Females most investing sex

YELLOW SEA HORSE

Males most investing sex

Classical view of male-

female Behavioral

Differences

• Compete for females• Aggressive• Sexual

• Choosy• Passive• Non-sexual

Males Females

Especially true when:

• No investment in offspring by males

• No cost of reproduction for males

Natural Selection

• There must be variation in a trait

• The trait must be heritable

• There must be differential reproductive success

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Reproductive Success (RS)

Successfully leaving more

copies of your genes in

subsequent generations than do

other individuals.

The Enigma of the Peacock

“The sight of the peacock’s

tail makes me sick”

Darwin - letter to a friend

SOLUTION: Sexual Selection Sexual Selection

• Inter-Sexual Choice(Female Choice)

• Intra-Sexual Competition (Male-Male Competition)

Sexual Selection

• Rejected by Victorians as ‘ludicrous’

• Didn’t receive much attention until 1970’s with Trivers

Features such as the peacock’s

tail due to ‘superabundant

energy of males’

Co-discoverer of evolution

through natural selection

Abraham Lincoln

February 12, 1809 -

April 15, 1865

Charles Darwin

February 12, 1809 -

April 19, 1882

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•One sex choosing to mate with a member(s) of the opposite sex

Inter-Sexual Choice(classically called Female Choice)

Birds of Paradise

Bower Birds Why be Choosy?

• Possession of ‘good’ genes• Investment in offspring

Two types of explanation:

Why by Choosy?

• Possession of “good”genes

• Sexy Son Hypothesis

Why be choosy?

• Possession of “good” genes• Handicap principle

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Why be choosy?

• Possession of “good” genes• Low pathogen load

Indicated by lower

Fluctuating Asymmetry

Why be choosy?

• Possession of “good” genes• Sperm Quality

> Lower Fluctuating Asymmetry =

better sperm quality

Why be choosy?

• Ability to invest in offspring• Reproductive ability

Why be choosy?

• Ability to invest in offspring• Resources/territory

Ache Kipsigis

Female Choice:

Sperm Donor, Mate, “EPC”

• Character• Kind

• Understanding

• Dependable

• Considerate

• Honest

• Self Confident

• Health• Family health

• History

• Family longetivity

• Record

• Health Background Scheib, 1997

•Physical Abilities•Hair color

•Handsome

•Eye color

•Height

•Abilities•Creative

•Musical talent

•Intelligent

Hypotheses

• Women value those attributes they believe would affect a child (i.e. can be passed on) such as health and physical attributes

• Women value the same attributes they do in a long term mate.

Scheib, 1997

OR

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Female Choice:

Sperm Donor vs Mate

Scheib, 1997

1

1.5

2

2.5

3

3.5

4

4.5

5

Mean Importance

Rating

Character Health Physical

Attributes

Abilities

Sperm donor Mate

“Heritability” 1.93 4.05 4.69 3.36

Female Choice:

Sperm Donor vs Mate

Scheib, 1997

1

1.5

2

2.5

3

3.5

4

4.5

5

Mean Importance Rating

Character Health Physical

Attributes

Abilities Resources

Sperm donor Mate Extra Pair Copulation

“Heritability”2.15 3.59 4.71 3.43 1.83

When not to be choosy?

• Males: where there is no limit on insemination and low

investment in offspring can

afford not to be choosy

When not to be choosy?

• Females: where advantageous to confuse

paternity

Intra-Sexual Competition =

(classically called Female Choice)

• Competition among members of one sex for members (or a member) of the

opposite sex.

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Is Male-Male Competition a

Constraint on Female Choice?

• Can female only mate with the victors?• Is female choice negated by outcome of male-male competition?

Sexual DimorphismFemale choice constrained by

male-male competition only if:

• Dominant males are able to keep all other males away from females, so female has no

choice.

Female choice constrained by

male-male competition only if:

• Dominant males are able to keep all other males away from females, so female has no

choice.

• If winners can’t keep other males away, then gain a reproductive advantage only if:

• Coerce females into mating

• Are freely chosen by females

Why Compete?

• Genetic quality of mate• Resources of mate• Parenting ability• Reproductive Ability• Sperm Competition

• Fecundity

• Mates are limited• Energetic/time Constraints

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How to compete?

• Direct Competition

How to compete?

Reproductive Competition in Baboons

• Females in first part of the

menstrual cycle

and pregnant

females received

the most

aggression from

other females.-10

-5

0

5

10

15

20

25

Observed-expected attacks

E1 E2 E3 P1 P2 P3 L1 L2 L3

(Wasser, 1983)

LactatingPregnantCycling

0

0.1

0.2

0.3

0.4

0.5

0.6

Estrous Post-estrous Pregnant Nursing

Reproductive State (focal subject)

Frequen

cy per female-hour

**

Mangabeys

• Recipient of estrous

female

aggression

(Emery, unpublished)

Why are women often the ‘Showier’ Sex?

• Compete for high status males where:

• Male parental investment important

• Differential male status

• Long term pair bonds

Women of Surma

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How to compete?

• Compete to be chosen

How to compete?

• Compete to be chosen

Sperm Competition Sperm Competition

• Highest rank female gets to mate first

Sperm Competition

Martin Muller

Sexual Selection

• Inter-Sexual Choice

• Intra-Sexual Competition

• Inter-Sexual Coercion

Barbara Smuts

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Inter-Sexual Coercion

“The use of force or threat of force by members of

one sex to coerce mating from a member(s) of the

opposite sex.”

Inter-Sexual Coercion

• Can override the results of inter-sexual choice and intra-sexual competition.

• Imposes a cost on the sex that is coerced.

“Flanged” Males “Unflanged” Males

0 20 40 60 80 100

Gunung Palung

Kutai

Tanjung Puting

Ketambe

Suaq Balimbing

Study site

% of total copulations

% Forced % Cooperative

Forced vs. Cooperative Copulations

Flanged Males

BORNEO

SUMATRA

Forced vs. Cooperative Copulations

Unflanged Males

BORNEO

SUMATRA

0 20 40 60 80 100

Gunung Palung

Kutai

Tanjung Puting

Ketambe

Suaq Balimbing

Study site

% of total copulations

% Forced % Cooperative

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Chimpanzee Sexual Coercion

• Males attack parous females more often during swelling stages than during non-swelling stages

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07Hits/Hour

Nulliparous Parous

non-Swelling Swelling

(Kahlenberg & Wrangham, unpublished data)

Chimpanzee Sexual Coercion & Cortisol

• Cortisol (an indicator of metabolic or psychological stress) levels are associated with rate of aggression received by females from males.

0

100

200

300

400

500

600

700

Pmol Cor tisol /mg Cr eatinine

Nulliparous Parous

Non-Swelling Swelling

(Muller, unpublished data)

Female Strategies to Combat

Sexual Coercion

• Escape from males

• Aggressive towards males

• Coalitions amongst females

• Association with a particular male

Mate Choice Criteria

Kind and understanding

Exciting personality

IntelligentEasygoing

Good earning capacity

College graduate

Healthy

Physically attractive

Creative

Wants children

Sex Differences in Mate Choice

� Kind and

understanding

� Exciting

personality

� Intelligent

� Kind and

understanding

� Exciting

personality

� Intelligent

FEMALE PREFERENCE MALE PREFERENCE

Sex Differences in Mate Choice

� Kind and understanding

� Exciting personality

� Intelligent

� Physically attractive

� Healthy

� Easygoing

� Creative

� Wants children

� College graduate

� Good earning capacity

� Kind and understanding

� Exciting personality

� Intelligent

� Easygoing

� Creative

� Healthy

� Physically attractive

� College graduate

� Good earning capacity

� Wants children

FEMALE PREFERENCE MALE PREFERENCE

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Mate Preference Cross-Culturally

� Females value mates with a high earning

potential more than do males.

� Females value mates with high ambition

and industriousness more than do males.

� Both males and females prefer that the

husband be older than the wife.

� Males value mates who are physically

attractive more than do females.

� Males value mates with no prior sexual

experience more than do females.

Yes NoPREDICTION

36 1

29 8

37 0

34 3

23 14

Significant Difference?

Next Time...

• The Menstrual Cycle

• Changes in Cognition with

Cycle Phase