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Soci111 – Human Societies
Module 6 – Strategies of Reproduction – Evolutionary
Foundations of Sex Roles
François Nielsen
University of North Carolina
Chapel Hill
September 18, 2016
Outline
Main Themes
Paradoxes of Sexual Reproduction
Parental Investment Theory
Female Strategies
Male Strategies
Human Consequences
Biological Basis of “Gender Roles”?
Main Themes
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why sexual (versus asexual) reproduction is paradoxical
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why male-female distinction evolved
Bateman’s experiments with drosophilae & Trivers’ theory of
parental investment:
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whichever sex invests most in offspring is object of competition
whichever sex invests least competes more
not necessarily the way we think!
generic female strategies
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domestic bliss strategy
he-man strategy
Madame Bovary strategy
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generic male strategies: dad or cad
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human implications of parental investment theory
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evolutionary bases of men-women differences?
Paradoxes of Sexual Reproduction
Why Sex?
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parthenogenesis (asexual
reproduction) has a 2-fold
advantage over sexual
reproduction
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how can sexual reproduction
evolve?
⇐ sexual reproduction:
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increases genetic
variability of offspring
⇒ adaptation to
unforeseen environments
⇒ resistance to
co-evolving parasites
Paradoxes of Sexual Reproduction
Why Just Two Sexes?
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why not 3, 4,. . . , or none?
theory of Parker, Baker, &
Smith:
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male-female distinction
= small gamete vs. large
gamete
evolved by disruptive
selection
= intermediate sizes
selected against
but:
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Joan Roughgarden in
Evolution’s Rainbow
shows intermediate
strategies widespread in
nature
Paradoxes of Sexual Reproduction
Review Questions
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Q – Why is sexual reproduction paradoxical?
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Q – What is the main advantage of sexual reproduction?
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Q – What does parthenogenesis have to do with the Parthenon
in Athens (if anything)?
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Q – What is disruptive selection?
Parental Investment Theory
Bateman’s Experiments
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A. J. Bateman’s (1948) experiments with fruit flies
compare reproductive success of males vs. females
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females gain nothing by mating with more than one male
males gain by mating with many females
males vary more in reproductive success (number of mates &
number of offspring) than females
Bateman (1948) concludes
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“Greater dependence of males for their fertility on frequency of
insemination [is] an almost universal attribute of sexual
reproduction” leading to the evolution of “an undiscriminatory
eagerness in the male and a discriminatory passivity in the
female”
Parental Investment Theory
Male vs. Female Reproductive Success (RS) in Humans
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Xavante Indians in
Matto Grosso jungle
of Brazil
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show pattern of
greater variance in
male RS
effective polygyny
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= ratio of male
to female
variance in RS
measure of
reproductive
competition
among males
12.1/3.9 = 3.1 in
Xavante
Parental Investment Theory
Trivers’s Parental Investment Theory
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Robert Trivers (1972) (again!) generalizes Bateman’s results
as parental investment theory
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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” (p. 139)
sex investing the most constitutes limiting resource for other
sex
“whichever is the sex with greater parental investment will be
the sex that is courted, that competes less, and that survives
better” (SEB p. 159)
in most species females invest most
⇒ males
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compete more
inclined to riskier behavior
have lower life expectancy
Parental Investment Theory
Male Riskier Lifestyle & Greater Vulnerability
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a
Female Soay sheep
survive longer than
malesa
SEB Figure 7-7 p. 94
Parental Investment Theory
Male Riskier Lifestyle & Greater Vulnerability
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a
Females have longer
life expectancy in
peoplea
SEB Figure 5-9 p. 96
Parental Investment Theory
Male RS Related to Rank in Dominance Hierarchy
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a
Correlation of male
RS (copulatory
frequency) with
rank in dominance
hierarchy in yellow
baboonsa
SEB Figure 5-4 p. 86
Parental Investment Theory
Sex-roles Reversal in Seahorses
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But theory says
whichever sex invests
most!
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when male invest
most, “traditional”
sex roles reversed
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e.g. seahorses,
phalarope birds,
giant waterbugsa
a
SEB Figure 7-5 p. 157
Female Strategies
Domestic Bliss Strategy
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female usually invest most in each offspring, so
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three female strategies
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will stick around
is willing and able to invest in offspring
to do this look for male who
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(1) domestic bliss, (2) he-man, (3) Madame Bovary strategies1
in species where male cares for offspring use domestic bliss
strategy = select male who
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can maximize RS by choosing a male judiciously
but by which criteria?
is patient in long courtship
controls resources, e.g. good quality territory
Q – BTW is a long courtship useful only to females?
Terms from Richard Dawkins in Selfish Gene
Female Strategies
Domestic Bliss Strategy & Orians-Verner-Willson Model
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domestic-bliss
strategy basis of
O-V-W model of
resource defense
polygyny in
migratory birdsa
É
Monique
Borgerhoff-Mulder
finds similar
patterns of polygyny
in Kipsigi people of
Kenya
a
SEB Figure 6-2 p. 119
Female Strategies
He-Man Strategy
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in species in which males do not participate in raising the
offspring
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all a female can do is try and select male with “good genes”
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this is the he-man strategy
to do this
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good bet is select older male (age proof of survival ability)
choose male with most conspicuous display, e.g. peacock with
most wonderful tail
originally may be sign of vigor & health
can lead to runaway sexual selection = evolution of
dysfunctional ornaments in males
explained in Richard Dawkins, 1989, Selfish Gene, pp. 158–9
Female Strategies
He-Man Strategy & Runaway Sexual Selection
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peacock tail
epitomizes how
he-man strategy
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can lead to
runaway sexual
selection
producing
dysfunctional
trait in males
Female Strategies
Madame Bovary Strategy
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Emma Bovary is
bored & starts
adulterous affair
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story ends badly
Madame Bovary
strategy = keep
both
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Isabelle Huppert in Claude Chabrol’s
Madame Bovary (1991) after 1856 novel
by Gustave Flaubert
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a husband to
invest parentally
in offspring
a lover to provide
“good genes”
Female Strategies
Madame Bovary Strategy
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clues suggesting that female infidelity played important role
in evolution of human sexuality
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high rates of “illegitimacy” among (supposedly) monogamous
birds, indicating high incidence of extra-pair copulation
in colonial birds (birds living in large colonies) mating in
private is associated with higher prevalence of extra-pair
copulation; by analogy
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absence of estrus (= distinct period of sexual receptivity, or
“heat”) in human female
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humans mate privately
means that extra-pair copulation prevalent in human
evolutionary history?
reduces mate’s ability to monitor her sexual activity
evolved because gives female more control over paternity of her
offspring?
some studies find high rates of illegitimate paternity in
contemporary human populations
Female Strategies
Madame Bovary Strategy
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a big clue about
evolution of human
sexuality
compared to males
in other primates
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SEB Table 12-1 p. 318
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humans have
large testicles
large ejaculates
suggests high
degree of sperm
competition in
human evolution
importance of
Madame Bovary
strategy
Female Strategies
Review Questions
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Q – What might be a male counterstrategy to the Madame
Bovary strategy?
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Q – A female tendency to mate with older males found in
some species where males do not invest parentally is an
instance of what reproductive strategy?
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Q – The assumption that a female may be better off as second
wife to a wealthy male than as first wife to a poor male is part
of what theory?
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Q – What is the predominant theory why human females have
lost the estrus (period of “heat” accompanied by physiological
changes marking ovulation and sexual receptivity) found in
other primates?
Male Strategies: Dad or Cad
Comparative Reproductive Strategies
Table: Types of mating systems in birds and mammals
Mating System
Monogamy
Polygyny
Promiscuity
Polyandry
Birds
92%
2%
6%
<0.5%
Mammals
Sometimes (carnivores)
Predominant
Predominant
Almost never
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Q – Why do birds exhibit a greater propensity to monogamy
(with high levels of paternal investment) compared to
mammals?
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A – Internal fertilization in mammals gives male prior
opportunity to desert. In birds, the egg is detached from the
female earlier, so incentive for male to provide care is greater
Male Strategies: Dad or Cad
Comparative Reproductive Strategies
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across species male strategies range from
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dad strategy = heavy parental investment in offspring, to
cad strategy = no parental investment
Q – A sociobiologist has said “In mammals, the male bring
home the bacon, or nothing at all”. What does he mean?
when males invest parentally
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males are under selective pressure to prevent insemination of
mate by other males
⇒ greater concern about fidelity of mate (“jealousy”)
⇒ guarding behaviors to keep rival males away from mate
Male Strategies: Dad or Cad
Comparative Reproductive Strategies
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across species greater polygyny
(measured as harem size) associated
with greater sexual dimorphism ( =
difference in size and appearance
between male and female)a
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Q – Is there a “natural” human
harem size?
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A – In humans length sexual
dimorphism is 1.08; interpolating
relationship harem size × sexual
dimorphism suggests “slight” or
“mild” polygyny!
a
SEB Figure 7-3 p. 153
Human Consequences
Summary of Generic Male-Female Differences
In most animal species, the female’s greater investment
in each offspring means that her maximal reproductive
potential is lower than the male’s. Males therefore compete
among themselves for fertilization opportunities. Investing
little in each offspring, males are selected to sow their seed
wherever opportunity arises. Investing considerably in each
offspring, females are selected to exhibit greater selectivity
in their choice of mates. One feature on which females may
exercise selectivity is the male’s willingness or ability to
make an effective parental contribution. But wherever
males do in fact invest parentally, they are under selective
pressure to protect themselves against cuckoldry, and
therefore males have a greater concern than females over
the fidelity of their mates.
Martin Daly & Margo Wilson, Sex, Evolution & Behavior, p. 280
Human Consequences
Features of Human Mating Systems & male-Female Differences
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83% of human
societies allow
polygynya
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however majority of
human marriages
are monogamous
(can you see why?)
polyandry rare
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a
Nigeria
(sequential
cohabitation)
Himalayas
(fraternal
polyandry)
SEB Figure 11-1 p. 282
Human Consequences
Features of Human Mating Systems & Male-Female Differences
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in 67% of human
societies groom or
his kin “pays” for
bride througha
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bride-price
bride-service
direct exchange
of women
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only 3% of societies
have dowry (bride or
her kin pays)
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Q – What kind of
society would have
dowry?
a
SEB Figure 11-3 p. 289
Human Consequences
Features of Human Mating Systems & Male-Female Differences
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double standard in adultery laws & customs, reflecting
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sex difference in content of sexual jealousy2
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male parental investment
greater concern of investing male over fidelity of mate
males: more upset by sexual infidelity of partner (intercourse
with someone else)
females: more upset by emotional infidelity (deep attachment
to someone else)
study of difficulty with male partner in aftermath of rape
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physical injury of victim during rape → less difficulty
no physical injury → more difficulty (why?)
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greater attention paid to resemblance of newborn baby with
father rather than mother
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coercive constraints of women’s sexuality (confinement,
genital mutilations, chador, chastity belts, . . .
2
David Buss. 1994. The Evolution of Desire, p. 128
Human Consequences
Features of Human Mating Systems & Male-Female Differences
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greater male
vulnerabilitya
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a
males have
greater mortality
than females at
all ages
difference
especially large
for young
persons
SEB Figure 11-4 p. 298
Human Consequences
Features of Human Mating Systems & Male-Female Differences
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greater male
risk-taking
propensitya
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a
e.g., greater
driver death risk
SEB Figure 11-5 p. 300
Human Consequences
Review Questions
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Q – What is the main ultimate cause of differences between
male and female reproductive strategies in mammals?
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Q – In what species would one predict that males are most
concerned with the fidelity of their mates (i.e., “jealous”)?
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Q - Which marriage practice (monogamy, polygyny,
polyandry) is most common among human societies? Among
human marriages?
Biological Basis of “Gender Roles”?
Prenatal Sexual Differentiation
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genetic basis of sex
behavior differences
paradoxical
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Organizing role of hormones in the womb
discovered by Alfred Jost
sex differentiation is
hormonala
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a
males & females
share most genes
advantageous
traits should be
shared (e.g.,
nipples)
in the womb
(organizing role)
at puberty
(activating role)
SEB Figure 10-3 p. 252
Biological Basis of “Gender Roles”?
Hormonal Changes At Puberty
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a
activating role of
hormones at
pubertya
SEB Figure 10-5 p. 255
Biological Basis of “Gender Roles”?
Sexual Division of Labor in Human Societies
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“social
interpretation” of
sex differences
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a
division of labor
between sexes in
human societiesa
SEB Figure 10-1 p. 262
Biological Basis of “Gender Roles”?
Sex Differences in the Brain
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Are there behavioral/psychological differences between males
and females underlying gender roles?
in classic literature review The Psychology of Sex Differences
(1974) Eleanor Maccoby and Carol Jacklin had identified only
four “real” sex differences
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greater verbal ability of females
greater visual-spatial ability of males
greater mathematical ability of males
greater aggressiveness of males
Maccoby & Jacklin’s method criticized because
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use overly broad concepts (e.g. “sociability” or “dominance”)
which tends to blur sex differences
making their list of “real” differences too conservative
Biological Basis of “Gender Roles”?
Sex Differences in the Brain
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← Doreen Kimuraa
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more recent research finds many
more psychological differences
between males and females
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e.g. Doreen Kimura. 1992. “Sex
Differences in the Brain.” Scientific
American September: 119–125
a
1933–2013. Emerita from Western Ontario U.; Visiting Professo
Fraser U., British Columbia
Biological Basis of “Gender Roles”?
Sex Differences in the Brain
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← Tasks favoring women
Also big difference in techniques to
navigate a route
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women rely on landmarks (e.g.
building or distinctive tree)
men rely on spatial cues (e.g.
distance and direction)
Biological Basis of “Gender Roles”?
Sex Differences in the Brain
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← Tasks favoring men
Are M-F differences relic of sexual
division of labor during human
evolution in hunting-gathering
lifestyle?
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M: hunting large game, using
weapons
F: gathering plant food, tending
home, caring for children
Biological Basis of “Gender Roles”?
Male-Female Differences Discussed by Kimura (1992)
Task
Difference
Target-directed motor skills
Spatial tasks
Mathematical reasoning
Navigating through a route
Arithmetic calculations
Recalling landmarks from route
Precision manual tasks
Vocabulary
Non-verbal reasoning
Verbal reasoning
Verbal fluency
Perceptual speed
Ideational fluency
M>F
M>F
M>F
M>F
F>M
F>M
F>M
F>M (?)
F>M (?)
F>M (?)
F>M
F>M
F>M
3
Size3
0.75
0.70
N/A
N/A
N/A
N/A
N/A
0.20
0.30
0.17
0.22
0.25
0.38
“effect size” is difference in mean scores divided by standard deviation of
scores
Biological Basis of “Gender Roles”?
Origins of M-F Differences in Primordial Division of Labor
É
Doreen Kimura’s
conclusion to “Sex
Differences in the
Brain”