Sexual selection and mate choice NoH 2006

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Sexual selection and mate choiceNoH 2006
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Sexual Selection Batemans principle (1948)

• Female mammals invest substantially in offspring
  (large gametes, gestation, lactation, infant
  care).
• Reproductive success dependent on resources for
  the offspring and cant be enhanced by mating
  with multiple males
• Male mammals can increase their reproductive
  success by having sex with multiple females
• This had led to behavioural, psychological and
  physical sex differences

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Costs of Sexual Reproduction

• Time and energy is spent identifying and
  attracting a mate of the same species
• Same-sex competition hinders finding a suitable
  mate
• Time and effort spent in defending, and
  copulating with, mate
• Risk of predation may be high during display
  and copulation
• Risk of disease transmission, or physical
  damage during mating
• Risk of homozygosity of dangerous recessive
  genes.

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Benefits of Sexual Reproduction

• Gene recombination helps to create variety to
  exploit different environmental conditions (the
  lottery principle)
• Harmful mutations in DNA may be repaired during
  genetic recombination (the DNA repair hypothesis)
• Sexual reproduction may produce combinations of
  genes that are better able to cope with
  environmental stressors like parasites, or
  predators (the Red Queen hypothesis)

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Intra-sexual selection

• Members of one sex compete amongst each other
  for the access to mates
• Male-male competition more common
• Has led to big size and weaponry in males

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Inter-sexual selection

• Members of one sex prefer a particular trait in
  the opposite sex
• Known as the female choice.
• Has led to fancy colouration, ornaments and
  courtship displays in males

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Runaway SelectionThe Handicap Principle

• Fisher (1930) initial female preference for a
  trait (e.g. long tail) leads to the trait
  becoming more prevalent in the population (The
  sexy son, or runaway selection theory)
• A trait might be an indicator of good genetic
  quality. If a male has a burdening trait, and
  survives, must have good genes (Zahavis Handicap
  Principle)

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• Male-male competition and female choice common in
  conditions where females provide most of the
  parental care (97 of mammals)
• In species with paternal care, female-female
  competition and male choice occurs as well (e.g.
  lion tamarins). In these species, sex differences
  are less accentuated.
• Intra-and inter-sexual selection are not mutually
  exclusive forces

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Characteristics of sexually selected traits

• Normally appear at the time of puberty and are
  controlled by sex hormones
• Result in sex differences
• Sex differences can be found at cognitive,
  physical and behavioural levels
• Aid in obtaining mates (either intra-or inter
  sexual selection)- no survival function!
• What traits in humans might have evolved though
  sexual selection?

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The answer is

• ..parental care! Babies take long time to mature.
  In the past, at least two people were needed in
  order to bring a baby up successfully. Male
  choice as well as female choice has been an
  operational force in human evolution

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Finding a partner.

• What do males and females look for in their
  partners?
• How does this differ amongst different species
  and different cultures?

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Have Human Sexual Emotions In Fact Been Shaped by
Darwinian Sexual Selection?

• First Remember distinction between proximate
  (psychological) and ultimate (evolutionary)
  causation
• Important because modern facts of life are
  different from those we evolved with
• How are we different now from when we were
  cavemen and women?

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• contraception (sex without babies)
• artificial insemination in vitro
  fertilization (babies without sex)
• substitutes for lactation
• equal opportunities for women (no longer
  necessarily tied down by children as dependent
  on male parental investment)

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Studies of mate preferences
Buss (1989) investigated mate preferences across
37 different cultures. Respondents asked to
indicate which characteristics were most, and
least, desirable in a mate, e.g. physical
attractiveness, ambition, chastity, good earning
potential, kindness, intelligence. Across most
cultures, males rated youth and physical
attractiveness as more important, while females
rated ambition and earning capacity as more
important. The biggest cultural differences were
in ratings of the importance of chastity.
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Other theories of intersexual selection suggest
that females using physical characteristics (e.g.
symmetry) as indicators of males genetic quality
(good sense theory).
Females may also choose males on the basis of the
resources that they possess (e.g. nesting sites
in birds). The assumption here is that not only
is the resource useful for the female, but it is
also a possible indication of the quality of the
male (since only strong healthy males can compete
successfully for resources against other males),
and his willingness to invest in her and her
offspring.
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Mating systems

• Monogomy
• Polygyny
• Polyandry
• Polygynandry
• Promiscuity

Which system do humans follow???????
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Polygynandry Dunnock or hedge sparrow, Davies
and Lundberg (1984), Davies (1992)

• Davies (1992) investigated mating patterns in
  hedgesparrows. The reproductive success of males
  and females under each strategy depends upon
  environmental factors such as food distribution,
  and indicates a conflict of interest between the
  genders.

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Sperm Competition
Indirect competition between males can occur
inside the female, as females of some species
(particularly insects) may be able to retain
sperm from different males (e.g.
dragonflies). Sperm competition can also be found
when eggs are fertilised outside of the females
body (e.g. salamanders) Baker Bellis (1995)
made controversial claims that human males engage
in sperm competition (evidence that male
increases number of sperm after a period of
separation from his partner).
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Testis size and sperm competition in primates
Gorilla
Man
Difference in testis size between the primates
can be linked to different mating systems and
strategies. This suggests the existence of sperm
competition.
Orang-utan
Chimp
What do you think about humans now?
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Cues for fertility

• Faces
• want the right sex, and normal hormones.
• Effects of testosterone on jaw, brow ridge.
• Youth ability to have children, oestragen
  eyes, lips, skin.
• Supernormal stimulus exaggerate magnitude
  of psychologically effective physical cues.

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• Infants response to faces Langlois, Roggman
  and Reiser-Danner, 1990. Even young infants gaze
  longer at more attractive faces. Suggesting
  that preference for symmetry etc. does not rely
  on learning as much as was previously thought.
• However, learning does seem to play a part in the
  desired plumpness of females. In cultures
  where food is scarce, plumpness is preferred
  Rosenblatt (1974). In societies where food is
  plentiful, thinness is preferred.
• But, one thing does stay constant, the
  attractiveness of the ratio of waist to hip.

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Human mate choice and lonely heart adverts

• An attractive, slim female seeks a fun, creative
  man to share the future with
• a professional male, into sports and arts, seeks
  an attractive young female for fun nights in and
  out
• Pawlowski and Dunbar (1999) males seek cues
  of attractiveness and youth, females seek cues of
  wealth, commitment, social skills

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Lonely hearts wording (Pawlowski and Dunbar,
2001)

• Attractiveness cute, fit, good-looking
• Wealth/status professional, house-owner,
  college-educated
• Commitment kind, stable, mature, dependable
• Social skills GSOH, witty, lively
• Fidelity faithful, monogamous

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Are there universal rules?

• Buss (1989) a cross-cultural questionnaire study-
  females put importance on wealth and status,
  whereas males find attractiveness more important
• Ideas of beauty seem universal and innate
• In the EEA (environment of evolutionary
  adaptations) status of a man would have been
  important for the survival of the offspring.
• Fertility and health of a female would have been
  important for males
• The mating rules are subconscious

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• If fitness of males is limited by access to
  females
• males should be competitive
• If fitness of females is not limited by access to
  mates, but instead by resources and time invested
  in constructing and rearing offspring
• females should be choosy

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Concealed ovulation

• The existence of concealed ovulation in human has
  generated a number of hypotheses to explain the
  possible adaptive advantage of this
  characteristic
• The daddy-at-home hypothesis ovulation is
  concealed so that males will find females
  constantly attractive and remain with a partner
  rather than seeking another mate
• The sex for food hypothesis women could
  exchange sex for resources without males
  detecting when they were fertile.
• The anticontraceptive hypothesis if women
  could detect when they were ovulating they could
  avoid pregnancy concealed ovulation acts to make
  this difficult, and so this trait is selected.

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However, do women know at some level that they
are ovulating?

• Grammer (1996) women in bars.
• Stanislaw and Rice (1988) X marks the spot
  desire levels and fertility
• Do men know? Symons (1995) during ovulation
  womens skin becomes vascularized gt a glow.
• Estrogen (which rises during ovulation) produces
  a decrease in womens waist-to-hip ratio Symons
  (1995)

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Male Short-term Mating

• Psychological evidence
• 1.Desire for a variety of partners Buss and
  Schmitt (1993)
• 2. Lowering of standards Buss and Schmitt
  (1993), except for key things like hairiness
  low sex drive wanting commitment.
• 3. The closing time phenomenon Gladue and
  Delaney (1990)
• 4. Sex differences in sexual fantasies Ellis
  and Symons (1990)
• 5. Sexual jealousy Buss et al. (1992)

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• Physiological evidence
• testicle size, variations in sperm insemination
  different sperm morphs.
• Behavioural evidence
• Extramarital affairs cross culturally e.g.
  Gregor (1985) and Amazonian Mehinaku men.
• Prostitution Burley and Symanski (1981). USA
  estimates over 100,000 women working as
  prostitutes.

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Symons and Ellis (1980)
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Womens Short-term mating.

• Adaptive benefits
• Resource acquisition Hrdy (1981) get 2 males
  to provide, Smuts (1985) increased protection
• Genetic (sexy sons) Fisher (1958)
• Mate Switching Betzig (1989), men often divorce
  wives who have affairs.

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Supporting evidence

• Buss and Schmitt (1993) women do prefer men who
  will give them lots of presents early on in the
  relationship, supporting the resources
  hypothesis.
• Buss and Schmitt (1993) women do seem to look
  for attractive men to have affairs with
  supporting the sexy son hypothesis.
• Greiling and Buss support for mate switching,
  but the most often reported benefit sexual
  gratification not mentioned in earlier theories!

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Costs to women of short-term mating

• 1. risk impairing desirability as a long-term
  mate.
• 2. lack of physical protection Muehlenhard and
  Linton (1987)
• 3. risk bring up a child without additional
  resources.Daly and Wilson (1988) single women
  delivered only 12 of babies born between 1977
  and 1983, but committed just over 50 of the 64
  maternal infanticides reported to the police.
• 4. loss of resources from current partner.

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Sexual Jealousy

• Possible triggers
• Concealed fertilization, uncertainty of
  paternity, and the cost of cuckoldry
• Similarities differences between men and women
  in jealousy (Buss)
• Men sexual infidelity worst.
• Women emotional infidelity worst.
• Violence against women and male sexual jealousy
  and control
• 34 of American college women report being
  followed or harassed by a rejected man
• Male-male rivalry and jealousy

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Consequences for sexual jealousy Motives for
homicide, Philadelphia, 1948-62

• Altercation of relatively trivial origin insult,
  
• curse, jostling, etc..........................
  .............35
• Domestic quarrel................................
  ...........................14
• Jealousy........................................
  ............................12
• Altercation over money..........................
  ...................... 11
• Robbery.........................................
  ................................. 7
• Revenge.........................................
  ................................ 5
• Accidental......................................
  ................................. 4
• Self defense....................................
  .............................1
• Halting of felon................................
  ................................ 1
• Escaping arrest.................................
  .......................... 1
• Concealing birth................................
  .............................. 1
• Other...........................................
  .................................... 3
• Unknown.........................................
  ................................ 5

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Patriarchy and the"Ownership" of Women

• Marriage as transfer of woman from father to
  husband.
• Brideprices and brideservice.
• Ownership badges rings, names, forms of
  address.
• Control of female sexuality chaperones, veils,
  wigs, chadors burkas, segregation by sex,
  confinement, genital mutilation.

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Cont.
Double standard for adultery. Adultery as a
property violation. Elopement as theft from the
father. Rape as an offense against the husband
(or father). Legality of marital rape.
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Conclusion

• Male evolutionary fitness can be enhanced by
  accessing multiple mates. Female fitness can be
  enhanced by rearing good quality offspring.
• This fundamental difference is reflected on human
  mate choice
• Male provision of parental care in humans has led
  to less prominent sex differences compared to
  many other mammals
• Some morphological and psychological sex
  differences in humans might be a result of sexual
  selection

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• Mate choice is a trade-off between what an
  individual can offer and what they require
• Female mate choice might reflect a trade-off
  between good genes and parenting effort from a
  male
• There seems to be both universal and cultural
  rules of mate choice-both nature and nurture
  important!

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Problems with using evolutionary theory to
explain behaviour

• 1. Much research focuses on basic differences in
  sexual behaviour in males and females, mediated
  by inherited psychological mechanisms. However,
  the complexity of human behaviour suggests the
  important role of learning and socialisation in
  determining sexual preferences and behaviour (see
  Buss, 1992).
• 2. Rather than assuming genetic determination of
  basic preferences, it may be more useful to
  conclude that behavioural plasticity is under
  genetic control (Fausto-Sterling, 2000). In
  other words, we need to focus on variation in
  human behaviour, rather than behavioural
  universals.

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• 3. Detailed studies on male and female strategies
  in nonhumans suggest important sex differences.
  To what extent can we assume that modern human
  behaviour indicates basic ancestral behaviours?
• 4. Problem of lack of evidence of how mating
  behaviours evolved during the Pleistocene period,
  and what environmental conditions shaped them.
• 5. Among humans and nonhuman primates, there
  exists a range of mating patterns to what extent
  can we assume that certain patterns are more
  basic than others?