Sexual Selection Flashcards
What is sexual selection?
How mate choice drives evolution - characteristics develop to attract the opposite sex
Mating systems
Polygamy - everyone has multiple mates
Polyandry - females have multiple mates
Polygyny - males have multiple mates
Monogomy - long standing pair bonds between individuals
What is sexual dimorphism?
When males and females withing a species have different physical traits e.g. lion’s mane, peacock’s feathers…
Depends on mating system:
- bonobo - polygamy = little dimorphism
- chimps - polygyny = slight dimorphism
- gorillas - polygyny = large difference
What are intra- and inter-sexual competition?
Intra-sexual competition
- within-sex competition
- typically males compete for access to females (male elephant seals are much bigger bc of this)
Inter-sexual competition
- one sex tries to attract the other with ornaments and displays
- typically females do the choosing
Ornaments
Used to attract the opposite sex
Evolution of male ornaments driven by aesthetic choices of females
Costly!
- require energy to grow & maintain –> display fitness
- can be more easily caught by predators (e.g. peacock)
- can become too costly e.g. Irish elk’s antlers
Parental investment theory
Trivers (1972)
Females are the choosie ones because their costs greater than male…
- limited egg supply
- 9 months gestation
- dangerous birth
- lactation
- years of child-rearing
- feeding
Choosiness
Females pay high price for mistakes (parental investment theory)
Male choosiness increases with investment….
- if they rear (e.g. seahorses)
Depends on mating system
- monogamous species have similar choosiness between sexes
Ornamentation and mate choice for the long-tailed widow bird
Anderson, 1982
Artificially enhanced tail length - greater mating success
Shortened tail length - reduced mating success
Fisherian runaway
Positive feedback between heritable ornaments and heritable preferences
- offspring inherit parents’ ornaments and preference for it –> they spread through the species together
In humans…?
- Stulp et al., 2015
- height increased globally with diet and nutrition - 8” over 150yrs
- People in Netherlands taller than UK
- Taller men have more children (11% more on avg.)
Honest signalling
Hamilton & Zuk (1982)
- Signal of quality - can’t be faked
- improves population fitness
Loyau et al., 2005
- measure no. antibodies in male peacocks
- measure no. displays per hour
- more antibodies = fewer displays
Thompson et al., 1997
- house finches
- high parasite load = reduced expression of sexually selected traits (bright plumage), greater reduction in males with low parasite resistance (genetic!)
Handicap principle
Zahavi, 1975
- only ‘fittest’ can grow ornaments
- spend more = more ornaments
- high cost handicap = reduced species fitness
- waste is necessary for ornaments, can’t be faked
- therefore… greater ornaments = greater input = better health
Conspicuous consumption
- time - nest building, performing songs
- life - bright colours, useless appendages
- resources - luxury goods
Handicap principle in humans
Masculine facial features
- require lots of T
- T is an immunosuppressant
- Only healthiest can “spent” T
Resources
- luxury goods
Sexual selection in humans
Hadza people
Mild dimorphism suggests some kind of competition for mates
Apicella, 2014
- Hadza people
- males w/ greater upper body strength reported more mating success
- mediated by hunting success
- more resources (food) allows access to more females
Sexual selection in humans
Male secondary secual characteristics
Many have greater influence on dominance than attractiveness to women - suggests competition between males for mates
e.g.
- full beard
- masculine voice
- masculine face
- muscular/brawny
Swaddle & Reierson, 2002
- increased facial masculinity increases perceived dominance but has no effect on attractiveness ratings from females
Sexual selection in humans
Sexual dimorphism
Body fat distribution
Female fat deposits signal sexual maturity, used for development of offspring, attractive to males