Mating
•
•
•
•
•
•
Cost of males
Red Queen hypothesis
Reproductive value
Waist-hip ratio (WHR)
Body mass index (BMI)
Parental investment (PI)
Disadvantages of Sex
• Disrupting adaptive combinations.
• Cost of finding a mate and mating.
• Individual gene’s chances of replication
are halved (cost of males).
Sex Differences in Parental Investment
• Minimum (obligatory) Investment
– Males - sperm
– Females - egg and 9 months gestation
• Typical Investment
– Males - resources, some child care
– Females - lactation, child care, some
resources
Why Sex?
• Most genes have a 50% chance of
being replicated in sexually produced
offspring.
• All genes have a 100% chance of
getting into asexually produced
offspring.
• The advantages of sexual reproduction
must outweigh this large and obvious
disadvantage, plus others.
Advantages of Sex
• Genetic diversity of offspring
• Reduce competition between offspring
• Some may be successful if the
environment changes
• Immune system diversity
to keep ahead of
parasites and pathogens
(Red Queen Hypothesis)
What Would We Expect?
• What do men want from women?
– Many partners
– Fertile mates
– Healthy mates
– Parental investment
– Fidelity (paternity certainty)
• What do women want from men?
– Resources
– Parental investment
– Healthy mates
1
Men Desire More Partners and are
Less Choosy
Sex with a Stranger
I’ve noticed you around and I find you
very attractive…
a) …would you go out with me tonight?
b) …would you come over to my apartment
tonight?
c) …would you go to bed with me tonight?
Question
a
b
c
Women
50%
6%
0%
(Buss & Schmidt 1993)
Female Reproductive Value
Men
50%
69%
75%
(Clark & Hatfield 1989)
Female Fertility Declines with Age
Age-specific RV =
S (probability of attaining age) * (age-specific fertility)
Reproductive Value
3
2
1
0
10
20
30
Age in Years
40
50
Male Fertility Declines Less
Canadian Age-Specific Fertility Rate
children per 1000 women
250
1961
1971
1981
1991
200
150
100
50
0
15-19 20-24 25-29 30-34 35-39 40-44 45-49
age
(Dunson, Colombo & Baird 2002)
2
Signs of Fertility in the Face
Androgens cause:
Wider and longer lower jaw
Wider nose
More pronounced brow ridges
Thicker eyebrows
Estrogens cause:
Fuller lips
Fat deposits on hips
Fat deposits on breasts
Cultural Variance in WHR Preference
• The Matsigenka
people of Peru
prefer 0.9 WHR
over 0.7.
• The Hazda of
Tanzania prefer
women with higher
weight and do not
differentiate based
on WHR.
WHR versus BMI
Waist-Hip Ratio (WHR) and Fertility
• Much research has found that a low
(0.7) waist-hip ratio is most attractive.
• WHR is influenced by levels of
androgens.
• This could signal non-pregnancy and
fertility.
• Women with a WHR greater than 0.8
have a more difficult time getting
pregnant through IVF.
Body Mass Index (BMI) and Fertility
• Body mass index = kg/m2
• BMI under 19 and over 25 have been
shown to reduce fertility.
Are Face and Body Signals the Same?
In a study where undergraduate men rated pictures of
women’s bodies, both WHR and BMI significantly pedicted
attractiveness ratings. BMI accounted for 73.5% of the
variance while WHR accounted for only 1.8%. Waist-bust
and bust-hip ratios were not correlated with attractiveness.
http://evolution.humb.univie.ac.at/institutes/urbanethology/beautypro.html
(Toveé et al. 1998)
(Thornhill & Grammer 1999)
3
Skin Colour
• Women have lighter skin than do men.
• People become darker with age, but women’s
skin gets lighter at puberty.
• Women’s skin is darker during pregnancy and
during infertile phases of the menstrual cycle.
• There is a preference for lighter skin colour in
47 of the 51 HRAF societies with relevant
information. 30 mention this preference only
for females, while 4 mention it only for males.
• Light skin may be a sign of youth and fertility.
Resources
• Women rate earning
potential as a more
important factor in a
mate than do men.
• Both women and
men rate earning
capacity as more
important for longterm than short-term
relationships.
(van den Berghe & Frost 1986)
Facial Dominance
Facial Dominance and Rank Attainment
Rated facial dominance of photographs of the
West Point class of 1950 was significantly
correlated with rank attainment.
(Mueller & Mazur 1997)
Parental Investment
Male Body Type and Attractiveness
Women rate men with lower waist-chest ratios
and intermediate BMI to be most attractive.
Women find the man interacting
positively with the baby
considerably more attractive.
Comparable photos of women
produced no effect on men’s
judgments of attractiveness
(La Cerra 1994)
BMI
Waist-Chest Ratio
(Maisey et al. 1999)
4
Sperm Competition
Competition between sperm from different
males for fertilization of eggs produced by a
single female.
Do Women Double Mate?
How long do sperm last in the female reproductive tract?
1) How much opportunity is there for sperm
competition in humans?
2) What evidence is there that sperm
competition affects human sexual response
and behaviour?
3) How might women choose which sperm
fertilize their eggs?
How Long Do Sperm Live?
“Double Matings”
100
Vagina - only after 12 hours (too acidic)
80
Cervix - up to 10 days
Behavioural Evidence
The greater the risk of sperm competition,
the more sperm should be inseminated
during copulation
35 couples collected semen in condoms as
part of an on-going diary study.
They measured the number of sperm
inseminated (concentration times volume),
the time between episodes of intercourse,
and also the amount of time partners spent
together.
40
Min days bet. men
20
1
0
5
1-50
201-500
51-200
1001-3000
501-1000
>3000
Lifetime copulations
50% of extra-pair copulations (EPCs) involved
copulation with different partners within five days.
Males Ejaculate More the Less Time a
Couple Spends Together
1000
Number of sperm inseminated (millions)
Women must mate with two different men
within about 5 days for sperm competition to
occur.
Percentage of females
60
Fertilization can occur 5 days after copulation.
800
600
400
Hrs since last ejacn
<24 h
200
24-72 h
0
> 72 h
0-20
21-40
41-60
61-80
81-100
Proportion of time together
(Baker & Bellis 1989, 1993)
5
Female Choice
Phase I (days 1-5)
6.5
6.0
5.5
80
5.0
4.5
70
I
Phase II (days 6-14) most likely to conceive
Phase III (days 15-28)
7.0
90
Number of copulations/day
In a survey study, 2546 females stated when they
last copulated, whether it was with the main partner
or with a lover, and the day of their menstrual cycle
on which it occurred.
7.5
100
Number copulations/day
Women might have evolved various devices to bias
sperm competition in favour of certain partners,
such as short-term extra-pair males (because they
have different genes than the main partner).
Women Bias EPCs in Phase II
(Baker & Bellis 1990)
II
I
III
PHASE
Intra-pair copulations
II
III
PHASE
Extra-pair copulations
6