19 - Human sexuality Flashcards
sexual selection
“[…] differences in reproduction that arise from variation among individuals in traits that affect success in competition over mates and fertilisations”
Malte Anderson
Peacocks
“the sight of a feather in a peacocks tail, whenever I gaze at it, it makes me sick!”
Darwin, 1860
Traits can evolve to increase reproductive success, even if costly for survival
Peacoks tail:
intersexual selection better territory defence
intersexual selection more visits from female
Darwin females and males
Descent of Man
“That the males of all mammals eagerly pursue
the females is notorious to every one”
“The female, on the other hand, with the rarest
exceptions, is less eager than the male”
Batemans principle
- Greater variation in mating success (MS) in males
- Greater variation in reproductive success (RS) in males
- Stronger correlation between MS and RS in males
i.e. males have more to gain from competition than female
Parental investment
Trivers (1972): asymmetries in reproductive costs affect mating strategies.
In mammals
- anisogamy (larger female gametes)
- internal fertilisation and gestation
- lactation, long juvenile periods
- min f.input > min m.input
- female parental care universal
- male parental care optional (-10%) (-20% primates)
i.e. females have more to gain from choosiness than males
are mammalian sex differences in parental care universal?
no
examples:
birds (90-95% species have biparental care)
fish and frogs - m care > f care
mammals exception rather than rule in vertebrates
sex differences in mammals
highly variable, female passivity over-stated:
e.g. meerkat, bonobos, elephant
human reproductive biology
we share basic reproductive biology with other mammals:
i.e.
internal fertilisation, gestation, lactation, high parental investment
mating
most animals seasonal, limited to specific periods
other primates: receptivity variable, often seasonal/cyclic (but not always - bonobos constant!)
In humans: diverse, prolonged, elaborate, frequent (not just for reproduction)
ovulation
other mammals: often noticeable changes in behaviour, odour or visual cues
other primates: variable, sometimes highly exaggerated ‘sexual swellings’
in humans - not obviously advertised through behaviour, odour or visual cues
sperm competition
other mammals: can be intense in species with multi-male mating
humans: testes volume suggests low-moderate levels of sperm competition
reproduction
Humans: extremely costly due to larger, more helpless babies
-5% adult mass at birth
sexual dimorphism
expected where costs/benefits of mating unequal between sexes
e.g. greater sexual dimorphism in body size in polygynous primates
sexual dimorphism in modern humans
modern males - 15% larger than females
appears to declined in human evolution
human marriage systems
marriage is universal but marriage systems highly variable
some form of polygyny most common marriage system worldwide
are humans a largely polygynous species like most other mammals?
- males typically have more variation in reproductive success than females
- sex differences in reproductive skew smaller in humans
- polygyny rare among closest primate relatives
- human polygyny may be constrained by costs of childcare
cultural variation in reproductive and sexual behaviour
“human behaviour is juggle influenced by culture”
“there is no clear case for a typical human ecology”
- Mace, R (2000)
marriage doesn’t equal mating systems
rates of concurrent partnerships highly variable across cultures e.g. 49% extra-pair parternity rate in Himba vs 1.8% in Dogon
increasing freedom in sexuality and relationships in may countries . showing in increasing divorce rates worldwide
sex differences not fixed
differences in age preference decline with gender equality
partner preferences influenced by mass media
e.g. preferences for skinnier female bodies increase with TV exposure
