Lec 12 Sexual Strategies

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  • 1. Sexual Strategies
  • 2. Sexual Strategies
    • Sexual strategies
      • Intraselection (competition within a gender for mates)
      • Interselection (mate choice)
  • 3. Sexual Strategies
    • Historically intrasexual selection was studied
      • (e.g., male male competition)
    • Now more attention to interselection
      • (e.g., female choice of mates)
  • 4. Sexual Strategies
    • Both may occur
      • For example the peacock feathers
        • Intrasexual selection male/male size of feathers
        • Intersexual selection female/male – females prefer males with most dense occelli
  • 5. What will we learn?
    • 'Lek Paradox'
    • mate choice & genetic diversity
    Animal Behavior Introduction, BIOL 4518
  • 6. Sexual Strategies
    • More recently competition between the sexes
  • 7. Sexual Stragies
      • e.g., strategies that tradeoff fitness of male and female
    • What are Sexual strategies?
      • Finding a mate, securing reproductive success, reproductive success of males vs females (intersexual conflict)
    • This is distinct from “Mating Systems”
      • Monogamy
      • Polyandry
      • polygamy
  • 8. Sexual strategies:
    • You will need to be able to answer the following questions:
      • How have the strategies resulted in positive fitness/evolutionarily beneficial (provide a description)?
  • 9. Sexual strategies:
    • You will need to be able to answer the following questions (cont):
      • What are the costs/benefits that have selected for certain animal characteristics, such as sexual dimorphism, and multiple male morphs?
  • 10. Sexual strategies
    • You will need to be able to answer the following questions (cont.):
      • What are the selective pressures on males and females (e.g., sexual conflict between genders within a species)?
  • 11. Sexual strategies
    • You will need to be able to answer the following questions (cont.):
      • Why have multiple sexual strategies remained in population?
  • 12. Sexual Strategies
    • Use theoretical models to examine strategies
  • 13. Topics
    • Intrasexual strategies
      • Competition for mates
      • Conditional Strategies
    • Intersexual competition
      • Female choice
    • Intersexual conflict
      • Honest signalling
  • 14. Concept behind competition for mates
    • Factors
      • Energy investment in reproduction
      • Availability of mates
    • Concept is that females have greater investment in reproduction therefore tend to be choosier
      • Greater gamete size, and typically greater gestation, parental care than males
    • Males therefore compete for females;
  • 15. Conditional strategies
    • A conditional strategy is a strategy only adopted under certain conditions
    • Example
      • Reverse gender role of aggression with:
        • high male parental care,
        • low male availability
      • Different strategies by Multiple morphs
        • Sneaker strategy
        • Gender switching
  • 16. Intraselection Mate selection systems
    • Male dominance hierarchy system established for mating
    • Males compete with each other until they win the right to be around a female unmolested by males until she is ready to mate. A losing male may mate with a female if more than one female is in estrous at a time
      • E.g., mountain sheep;
    • Exception – sneaker role, where very low ranking male mates with a female while more dominant males are fighting for highest rank.
  • 17. Ritualized aggression in mating systems
    • Aggression in mating systems is often ritualized.
      • e.g., weaponry for killing is often not used
    • Venomous snakes wrestle without biting, fish lock jaws but do not bite; antelopes push and fence with the probable winner.
    • Communication system
      • e.g., roaring in red deer and outcome of competition
  • 18. Intraselection Mate selection systems, Lek:
    • Single area seemingly randomly selected area “matrix” within the territory that all males go to and compete for and defend a spot nearest the center (even when they doen’t stand a chance of mating).
  • 19. Mate selection systems, Lek:
    • Leks may last for weeks. Area is same year to year. Area offers no food or place to rest.
    • Number of copulations is highly skewed to one male with the territory in the center who may do 50-75% of the copulations (e.g., kobs and grouse), and neighbors may account for the remainder while the majority of males do not mate at all.
    • Female “honor” the lek system.
  • 20. Mate selection systems, Lek:
    • E.g., Hawaiian drosophila, 2 species of grouse, 2 species of tropical birds, African antelope (e.g., the Uganda kobs), prairie chicken, kakapo (flightless parrot in New Zealand)
    • Some species leks are brothers or half brothers
    • Female lek
      • (females aggregate and compete for males)
      • yellow spotted millipede
  • 21. Male Competition: Interference
    • Previous examples have been male/male competition before females arrive
    • Also, competition during attempt to copulation
      • Larger bodied males interrupt copulation between smaller bodied males and females
        • Solicitation help by females in elephant seals
      • Earwigs, elephant seals
  • 22. Male Competition: Cuckoldry
    • Bluegill sunfish
    • 3 male morphs
      • Parental male
      • Sneaker male
      • Satellite male
    • Parental – maintain territory, mate with female
    • Sneaker - no territory, sneak to fertilize eggs when parental mates with femal
    • Satellite – looks like a female; fools male (who tries to mate with satellite) and fertilizes egg
  • 23. Male Competition: Sperm competition
    • Sperm competition
      • Parental male
      • Sneaker male
    • Sneaker male –
      • more sperm per ejaculate
      • Short lived sperm
      • Increases chance of fertilization
    • Parental male –
      • less dense longer lived high quality sperm
    • When same density, equal fertilization by parental and sneaker sperm
  • 24. Mate guarding
    • Males guard females from other males
    • Monogamous systems
  • 25. Male mating strategies: Sexual Dimorphism
    • Hypothesize
      • Sexual dimorphism is largest when harem size is large and hence when male/male competition is most intense
      • 38 species of pinnipeds (harbor seals, elephant seals etc.)
      • Independent contrasts phylogenetic analysis
      • Found support for this hypothesis
  • 26. Alternative male strategies
    • Sneaker role
    • Different strategies by Multiple morphs
      • Sneaker strategy
      • Gender switching
  • 27. Alternative male strategies
    • Horseshoe crab
      • Males which grasps onto female have higher chance of fertilizing eggs
  • 28. Female Choice
    • Theories behind female choice of mates:
      • What are females using to select mates? Does the selection criteria increase females fitness or fitness of offspring?
    • Theories:
      • Good genes
      • Gift
      • Runaway selection hypothesis
  • 29. Female choice
    • Good genes
      • Pronghorn harem size as indicator of genes, and correlation with growth rate of offspring
  • 30. Female Choice
    • Good genes
      • MHC complex
      • MHC number of alleles
  • 31. What will we learn?
    • Birth control pills, mice, smelly shirts, MHC and pregnancy...
    • Disruption of mate choice in humans?
    Animal Behavior Introduction, BIOL 4518
  • 32. Female Choice
    • Good genes
      • Number of MHC alleles
      • Female sticklebacks
      • Number of MHC peptides – smell (proximate mechanism)
      • Optimal number - offspring with intermediate MHC diversity
      • Experiment
        • Female and male fish of known allele number (2-8)
        • Supplement water with MHC peptides increasing the apparent number of MHC alleles
        • Examine female choice
  • 33. Female Choice
    • Males have large symbols
      • Bower bird – size of the bower and amount of decoration
      • Size of antlers
      • Density of occelli in peacock’s tail (Size of peacock tail is part of male dominance hierarchy)
  • 34. Female choice
    • Gifts
      • e.g., insects, provide a gift to female
  • 35. Female choice
    • Males present with gifts
    • Example
      • Male spider presents a wrapped up fly
      • Body of male praying mantis (removal of head removes reproduction FAP inhibitors)
  • 36. Female Choice: Mate Choice Copying
    • Grouse in leks
      • Evidence that young females tend to copy mate choice of older females
      • Supported with experiment
  • 37. Female choice
    • Removal of spermatophore
    • Sperm storage
    • Choice of stored sperm to use
  • 38. Female Choice
    • Do females always choose the best mates?
      • Choice of non-dominant male
        • Primates; females may mate with persistent non-dominant males
        • Direct fitness benefits? Indirect?
      • Avoid non-dominant male
        • Elephant seals, female will increase calls if non-dominant male attempts to mate
      • Mate choice copying
  • 39. Female Choice
    • When is female mate choice not beneficial?
        • Example of Drosophila largest males result in a reduced lifespan of the female
        • Direct fitness benefits? Indirect?
    • This leads us to intersexual (male/female) conflict
  • 40. Intersexual conflict
    • Conflict between fitness of males and females
      • Examples:
        • “Dishonest” signaling of health
        • “Dishonest” gifts
      • Arms race of detection of genetic quality and dishonest signaling
  • 41. Signaling
    • Healthy male
      • Female preferences based on displays of male health
      • Males display gene quality
    • Runaway selection theory
        • Endow offspring with genes to prefer traits mothers prefer
  • 42. Intersexual conflict: “Dishonest” signaling
    • Dishonest gift to females
        • Only exoskeleton of body is presented
    • How can this strategy persist in the population?
  • 43. Summary