Download 1 All discussion of mating strategies and sex differences begins with

All discussion of mating
strategies and sex differences
begins with Darwin’s theory of
Sexual Selection
Intrasexual
Natural Selection
Sexual Selection
Survival
Mating Success
Intrasexual
Intrasexual
Competition
Competition
Parental
Care
Mate
MateChoice
Choice
(Epigamic)
(Epigamic)
Sexual Selection
Intrasexual Selection
Elephant Seal
Intersexual or Epigamic
Epigamic Selection
http://www.youtube.com/watch?v=fmmp3wrf9gg
Wilson’s Bird of Paradise
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Epigamic Selection
http://www.youtube.com/watch?v=6gAxbxxmYZ8
Superb bird of paradise
Pronghorn: an interesting case. Female gets
only sperm (genes) from male. She spends
lots of time choosing among males. Kids of
‘popular’ males are healthier and develop
faster. But in pronghorn, female choice is
based not on ornaments or displays, but on
the behavior of the male, in particular his
ability to keep her in the ‘harem’!
sexual dimorphism
1. Not always obvious whether trait due
to epigamic or intrasexual selection
♂♀
2. Not all dimorphism due to sexual
selection.
selection equally on both
3. Multiple selection pressures on these
traits
♂
♂♀
conventional
♂
♀
♀
sex role reversed
♂ ♀
~ no selection on either
Red-winged
Blackbird
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Sexual selection can occur even in
monogamous species with biparental
care – its just less intense
Northern Cardinal
♀
strong epigamic selection on both sexes?
Anisogamy
♂
Anisogamy = “Unequal gametes”
Eggs: few, large, energetically expensive
Sex role reversal: Wattled Jacana
Sperm: many, small, cheap
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SEXUAL SELECTION
PARENTAL INVESTMENT (PI)
1. initial PI: females produce large egg
cells, males produce small sperm
2. specialized adaptations directly
related to reproduction (e.g.,
gestation, nursing): usually the female
3. behavioral adaptations (and costs):
egg-guarding, protecting and feeding
young: females or males (more
commonly the female)
) In most animals, female PI
is much greater than male PI
Trivers: Key to sexual selection is
parental investment (PI).
Trivers: Sexual selection =
1. Sex investing less will compete for
sex investing more (intrasexual
selection)
2. Sex investing more will be
discriminating in choosing mates
(epigamic selection)
Typically: Female PI > Male PI
so usually its males competing among
themselves for females and females
choosing among males.
Let’s consider some consequences…
General Sex Differences in Behavior
Females as the discriminating sex
♀
Male: ardent, indiscriminant
Female: reluctant, discriminating
♀
™ FEMALES – “Don’t you Dare.”
™ MALES – “Laissez Faire.”
Males as the less discriminating sex
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Mate Choice
• Direct benefits – Female choice
leads to increased fecundity (more
offspring).
Kinds of Mate Choice
1) Good looks ?
2) Good genes
3) Good parent
4) Good resources
“Rational choice”
Mate Choice and Sexual Selection
how signal?
how detect?
indirect benefits
boil down
to PI
direct benefits
• Good genes – Female choice leads
to improved genetic quality of
offspring re survival.
* Runaway selection – Female
preference increases because it is
linked to ‘sexy son’ advantage.
• Handicap selection – Female
prefers signals that are handicaps.
* Sensory exploitation – Male
evolves display trait that exploits
pre-existing sensory bias in female.
* non-rational choice hypotheses
The field is now wide open and many
opinions are possible. Indeed, it remains to be
seen whether female choice is an arbitrary
and amusing evolutionary sideshow or a
powerful force driving male evolution
for female benefit. – Trivers 1985
Direct Benefits
Polygyny threshold model
Female chooses mated male
(polygyny) rather than unmated
male (monogamy) if she gets more
direct benefits from that choice.
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Good Genes
Good Genes
controls
-20 eyespots
Petrie & Halliday 1994
Malaria parasites
Uakari monkey
Cut out eyespots on
some males and did
mate choice tests.
http://www.youtube.com/watch?v=ZfCqelgwf0I
Honest signals are uncheatable, and
Good Genes
Honest signaling: Handicap model
Alcock Fig 10.44
Petrie 1992
Mean area father’s eyespots (mm2)
8 males X 4 females each
96 offspring released
Apparently #eyespots does
signal better quality males
(pass on better genes)
Fitness cost or benefit
Female peacocks definitely prefer
males with more eyespots ... but are
they better quality males?
% of chicks surviving after 2 years
provide accurate information about
health, vigor, and general condition
cost low
quality
benefit
optimum
cost high
quality
Signal intensity
An epigamic signal will be an honest
indicator of the condition (quality) of the
male if it is less costly (a smaller handicap)
for good condition (high quality) males
than for low condition (low quality) males.
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Sensory Exploitation
Sensory biases in zebra finches:
Human Mating Systems
& Mate Choice
• Females prefer males with certain
color bands
• Females prefer males with artificial
feather hats over normal males!
• Monogamy most common
• but sequential, also extra-pair mating
• Polygyny (mild) also fairly common
• Polyandry (milder) also found (rare)
• Bi-parental care, division of labor
Buss (1994):
The Strategies of Human Mating
1. Human mating is inherently
strategic. Strategies solved
specific problems in human
evolutionary history. The
manifestation of these strategies
need not be through conscious
psychological mechanisms; in fact
they generally will not be.
Sex Differences in Mate Choice
Human mean mate preference scores in
9,474 people from 37 different cultures.
Rating scores vary from 0.00 (irrelevant or
unimportant) to 3.00 (indispensable).
2. Mating strategies are contextdependent – in particular, we need
to distinguish short-term vs. longterm strategies.
3. Men and woman have faced
different problems, therefore have
evolved different strategies.
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Hypothesis 1: Short-term mating is more
important for men than women
Hypothesis 2: Men seeking a short-term
mate will solve the problem of identifying
women who are sexual accessible
Hypothesis 3: Men seeking a short-term
mate will minimize commitment and
investment
Hypothesis 4 and 5: Men seeking a shortterm mate will solve the problem of
identifying fertile women, whereas men
seeking a long-term mate will solve the
problem of identifying reproductively
valuable women
Hypothesis 6: Men seeking a long-term mate
will solve the problem of paternity
confidence
Hypothesis 7: Women seeking a short-term
mate will prefer men willing to impart
immediate resources
Hypothesis 8: Women will more selective
then men in choosing a short-term mate
Hypothesis 9: Women seeking a long-term
mate will prefer men who can provide
resources for their offspring
Hypothesis 6:
Fidelity more important to men
Hypothesis 4 and 5: Men seeking a shortterm mate will solve the problem of
identifying fertile women, whereas men
seeking a long-term mate will solve the
problem of identifying reproductively
valuable women
Fertility: probability woman is currently
able to conceive (ex: 14 yo < 24 yo)
Reproductive value: individual’s expected
future reproduction (ex: 14 yo > 24 yo)
Importance of physical attractiveness
3 important, 0 unimportant
Short-term
Long-term
Men
Women
2.71
2.31
2.43
2.10
Hypothesis 8: Women will more
selective then men in choosing a
short-term mate
Study: imagine two scenarios:
1) your partner having sex with
someone else, or
2) your partner falling in love and
forming a deep emotional
attachment to someone else
Men: 1 is worse than 2
Women: 2 is worse than 1
3 definitely yes
-3 definitely no
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