ANTH151 Lecture Notes - Lecture 6: Alloparenting, Breastfeeding, Menarche
1 Jun 2018
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Human Sexuality in Evolutionary Perspective – Week 6 ANTH151
Biology, sex and culture:
• Why bother with sex?
• Myths of sexual relations compared to evolutionary theory
• Anatomical and comparative evidence
Sexual reproduction → why not reproduce asexually?
• Some vertebrates do (100% female desert lizard species)
• Costs of sex: decreases potential species growth (only half species can
reproduce) and getting together can introduce risks
Why have sex?
• Recombination – a new mix = new DNA (constant diversity instead of
clones)
• Inheritance that also introduces variation
• The Red Queen
• Deference against infection, parasites and other biological enemies
Sex as exaptation –
• Human sexual activity, affiliative behaviour and even child rearing exceed
what is necessary for survival
• Birth spacing in foraging groups is usually four years minimum
• Our species like others uses the mechanisms of reproduction to solve other
social and psychological problems
• Classic case of an exaptation (but with moral complications)
What does sex do to evolution?
• Sexual competition:
o Whenever the evolutionary interests of the sexes diverge, conflict
arises between the sexes
o A single ‘conflict trait’ – more useful for one of sex than the other
(plumage, pelvis, enlarged canines etc.)
o Anisogamy → energy that goes into male and female contribution
different (size of egg and sperm, female gestation)
Sexual Selection:
• Trivers’ 1972 Hypothesis → “whichever sex invests more effort and more
resources in offspring is, in effect, the limiting resource for the reproduction of
the less investing sex and thus the object of competition”
o That is, the sex investing the most, becomes a limiting resource for the
sex investing the least – so which sex invests the most in reproduction?
• Paternity vs. maternity records → Mrs Feodor Vassilyev (Russia, 1700s)
o 67 children – 16 twins, 7 triplets, 4 quadrupleta
o Interstingly, Wikiepdia only records 6 men who have more children
than Mrs Vassilyev
o Another case was Ismail ibn Sharif (Morocco, died 1727) – 867
children by 1703, 525 sons and 342 daughters in 1741, 700 sons est.
4000 wives and concubines
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find more resources at oneclass.com
• Sexual selection occurs when the potential rate of reproduction on parental
investment differs between sexes
• In humans, at any time, many women are not in the ‘mating pool’ (pregnant,
between ovulation, nursing etc.)
In humans, which sex would we expect to competing??
• MEN - we see this across many species
• Darwin proposed that traits might arise for intraspecific competition (among
members of species of the same sex)
• Direct competition favours weapons (secondary sex traits). In lekking, males
gather to compete
• Mate choice favours ornaments or display
• Signal cost can lead less desirable mates to evolve false signals
Human exceptionalism can come into this → we may want to believe that human
sexuality is categorically not like other animals, and it is true that humans are
distinctive but they are linked.
Reductionism:
• The idea e.g. that we can ignore human distinctiveness and understand
sexuality
• THIS IS A CAUTION → stripping away what makes us human does not get
to our ‘nature’
Sexual dimorphism –
• Do not have the same form, features, capabilities e.g. brain differences
• In primates, the degree of dimorphism suggests whether polygynous or
monogamous
• Differences between male and female often trace of sexual selection
• In some species, competition through display, song, provisioning or nest
construction
• In primates, direct physical male competition and polygyny
What about the evolutionary trend in humans?
• Decreasing dimorphism, increasing female size
Sexual dimorphism??
• Extreme dimorphism suggests male competition and polygynous mating
• Relative equality suggests monogamous mating pattern in primates
• Human females are 80% size of males. Trend is toward convergence.
Mating pattern in humans? = mild polygyny – evolutionary pattern
Sexual homology:
• Hormones and arousal
o Basic anatomy is female
o Hormones shape masculinisation and defeminisation of fetal tissue
o Through embryonic development, bodies diverge
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find more resources at oneclass.com
o Same hormones present in both (androgen, oestrogen) in different
proportions
• Bodily arousal and homology
o Male and female reaction very similar
o Homologous organs
o Blood flows into genitals causing swelling
o Similar changes throughout the body
Why does this matter?
• Inter-sexed individuals not ‘freaks’ – they demonstrate that sex differentiation
is not a simple M/F switch (e.g. Lady Colin Campbell)
Mating strategies?? Is there an evolutionary program for what we desire?
• Risk of naturalizing
o The danger of assuming that what is ‘natural’ is simple
o Compare to native language – problem with evolutionary psychology
o Many traits we have discussed (neoteny, brain development) all
contribute to malleability
o Evidence suggests that mating, reproduction and child rearing variable
and respond to environment
o However, this is a CAUTION
• Because men and women’s selective pressures are different, do mating
strategies also differ? Does evolution influence us to seek certain traits?
o What do women want? An evolutionary psychology perspective
▪ Need enough food and support to carry baby to term, to feed
the baby once its born and to take care of it as long as it needs
▪ In many primates, females form core of social group
o Female mating strategy: evol psych (CAUTION)
▪ Older male
▪ Proven provider
▪ Unlikely to leave
o ‘Ideal’ women’s mating strategy?
o Male concerns: what are men looking for in a woman? Evol psych
(CAUTION)
▪ Seek women with which to mate
▪ Defend against other males
▪ Do not raise other males’ children
o Thus, males tend to group around females, even if male is ‘dominant’
o Male mating strategy: evol psych (CAUTION)
▪ Younger women
▪ Fertile
▪ Not likely to cheat
o ‘Ideal’ mating strategy? Males skew younger than female preferences
But that is asking Western research subjects (sedentary, young, wealthy, sexist) –
what does the evidence say?
• Dangerous assuming that behaviour we observe is universal ‘human’ or
resembles our ancestors
• Better to look at evidence in anatomy (at bodies and what they are telling us)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Human sexuality in evolutionary perspective week 6 anth151. Biology, sex and culture: why bother with sex, myths of sexual relations compared to evolutionary theory, anatomical and comparative evidence. Sexual reproduction why not reproduce asexually: some vertebrates do (100% female desert lizard species, costs of sex: decreases potential species growth (only half species can reproduce) and getting together can introduce risks. Why have sex: recombination a new mix = new dna (constant diversity instead of clones) Inheritance that also introduces variation: the red queen, deference against infection, parasites and other biological enemies. 4000 wives and concubines: sexual selection occurs when the potential rate of reproduction on parental investment differs between sexes. In humans, at any time, many women are not in the mating pool" (pregnant, between ovulation, nursing etc. ) In lekking, males gather to compete: mate choice favours ornaments or display, signal cost can lead less desirable mates to evolve false signals.
