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Why do the sexes differ?

Sexual selection, which ‘depends on the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction (suggested by Darwin)


Traits favoured by sexual selection

• Intrasexual selection - traits useful in combat between members of the same sex
• Intersexual selection - traits that increase the attractiveness of individuals to members of the opposite sex
• Traits, other than the reproductive organs, that evolve by sexual selection are referred to as secondary sexual characteristics.


Elaborate traits

Sexually selected traits can often be very elaborate and apparently detrimental to survival (especially in males).


Sex differences in reproduction : Female

Females generally make larger parental investments
• Larger gametes (Anisogamy)
• Increased energy producing, caring for offspring
• Lifetime reproductive potential is lower
-Quality of the mates chosen may be more important for females i.e. female mate choice is important


Sex differences in reproduction: Male

Males generally make smaller parental investments
•Smaller gametes
•Less investment
•Lifetime RS is potentially high, but is limited by no. of females
-Number of matings might be the strongest determinant of reproductive success in males


Sexual selection generally operates by male competition and female choice

• Male and female reproductive success are the same, but variance in female reproductive success tends to be less than male variance
• Thus the forces of sexual selection tend to be stronger in males


Male-male competition for partners

• Physical combat
• Body size and weaponry
• Sperm competition
• Infanticide
• Sneaker males….


What are females choosing?

Female mate choice could be for direct benefits (e.g. parental care from male, resources for rearing offspring like wealth)
• But this does not explain mate preferences that operate in species without paternal care (lek species) or in extra-pair matings
• Or why elaborate or disadvantageous traits would be considered attractive


How do elaborate characteristics evolve?

Elaborate characteristics could evolve by female mate choice preferences
• There are two main types of possible mechanism (which are not mutually exclusive) which are:
- Runaway selection
- Good genes


Runaway selection

• Fisher (1930) showed that runaway selection could occur if a female preference for a trait is linked, genetically, to the genes for that trait.
• Once the preference is there, the trait will evolve to become more and more exaggerated. This effect is believed to be operating but it is hard to demonstrate


Good genes

• Zahavi’s (1975) Handicap principle argues that only males with good genes can support expensive displays, thus they represent honest advertising of good genes and females should prefer them. The more costly the trait, the better must be the intrinsic quality of the male to survive. There is substantial empirical evidence that sexual selected characteristics are costly.


Hamilton argued that good genes are all about avoiding parasites

• All organisms are in an arms race - running to stand still (the Red Queen analogy)
• The best genes are those best adapted to survive in the current environment of parasites (who are continually updating their own genetic armoury)
• Sexual selection is about displaying and detecting immunocompetence


How many children can a woman have?

World’s most prolific mother: first wife of Feodor Vassilyev
(1707-1782) 67 surviving offspring from 27 pregnancies 16 twins, 7 triplets, 4 quads
Women limited by:
•Few reproductive opportunities
•Only 400 cycles in a lifetime
•Fertility declines from age 30
•High investment costs
•Pregnancy 9 mths & lactation


How many children can a man have?

World’s most prolific father: Moulay Ismail the Bloodthirsty (1675-1727)
• Many reproductive opportunities, (max 867 kids)
• Sperm quality does decline with age but can still have children in 80’s
• Low investment costs
• Thousands of sperm and potentially thousands of children


Valued Traits

(1) Females value attributes signalling acquisition and possession of resources
(2) Males value attributes signalling fertility and reproductive potential
– Youth / health
(3) Males value chastity when seeking a long term relationship
– Solve paternity uncertainty problem


Attractiveness: Symmetry

Traits that signal reproductive quality should be perceived as attractive
– Symmetry = resistant to pathogens, mutations, injury


Attractiveness: Waist-Hip Ratio

Singh (1993) Fertility cue - for males?
Fertility correlated with fat distribution on hips
– Youth, high oestrogen, strong immune system
– WHR: ratio of waist to hip measurement
– Range for healthy women 0.67 - 0.9


Variations across the menstrual cycle

• Humans have concealed ovulation
• Preference for masculine faces mid-cycle
• More extra-pair copulations (EPCs) when fertile
• Changes in female attractiveness, female satisfaction with own partner
• Controversial topic; small changes, we are a longterm species, slow life history…


But what if women compete for men?

New evidence that a skew in the adult sex ratio might cause preferences to shift
• Study among 8 villages among the Makushi, Guyana (Schacht and Borgerhoff Mulder 2015)
• Only when men are in minroity, more likely to prefer short-term matings than women.
• Women start childbearing early if few men around (Uggla and Mace 2016)
• “Mating market models”, emphasise context
• Supply and demand can relax or increase demands on potential mates