SEXUAL SELECTION

SEXUAL SELECTION

1. SEXUAL DIMORPHISM

   • body size - orang utan
   • colouration - orange throat for male (Morethia obscura)
   • size and morphology - flowers (to attract vectors for pollination) - male
   • Primary sexual characters – males and females differ in reproductive organs
   • Secondary sexual characters – males and females differ in traits not specifically associated with reproduction
     • These traits can not be explained by natural selection – Why just in one sex? – If costly, why are they maintained?
     • Under sexual selection they are adaptive if they help in obtaining a mate

2. NATURAL SELECTION - differential reproductive success due to variation among individuals in SURVIVAL & REPRODUCTION

3. SEXUAL SELECTION - differential reproductive success due to variation among individuals in success at GETTING MATES &/ FERTILIZATION

   • CAN TEST THE 4 POSTULATES AS IN NATURAL SELECTION

4. DIFFERENTIAL SELECTION ON MALES & FEMALES

   • which sex is more costly? if limitations for males, then the strong sexual selection falls on males (they compete each other)
   • Parental investment differs between the sexes
     • Differential parental care
     • Eggs are more expensive than sperm
   • Limitation to lifetime reproductive success:
     • Males limited by access to mates
     • Females limited by number of eggs or young cared for

5. ASSYMETRIES IN REPRODUCTIVE SUCCESS

   • e.g. Rough skinned newt Taricha granulosa
     • males rarely mate & have offsprings all females mate and have offspring - competition among males
   • competitive - males
   • choosy - females

6. Pre-copulatory sexual selection- MECHANISMS

   • Intrasexual selection
     • interactions among members of a single sex (e.g. male-male competition)
     • male-male competition - overt combat
       • Direct fight to control females or resources
       • Selects for specialized traits in males (large body size, armaments)
         • - e.g. rock wallabies have bigger arms -e.g. deer have antlers e.g. marine iguanas - larger body size - fight to get the area where the females want (rock - source of food)
     • Male-male competition - covert competition
       • No direct fighting – ‘scramble’ competition
       • Males compete through knowledge of the whereabouts of reproductively active females
       • e.g. Thirteen-lined ground squirrel
   • Intersexual selection
     • interactions among members from opposite sexes (female choice)
     • Female choice
       • Some male traits are not used in direct combat (ornamental only)
       • They are maintained because females show preferential mating
       • Cost to male (reduced survival) offset by increased matings
       • e.g. – Tail length in birds – Song in frogs
       • what’s the advantage?
         • Acquisition of resources
           • – Mate choice based on food, parental care etc – Insects (e.g. bush cricket) – females mate based on size of spermatophore, greater spermatophore = increased fecundity
         • ‘Good genes’
           • e.g. peacocks
             • • Females choose mates based on tail eyespots • Offspring of more attractive males have greater fitness
         • Secondary character is an indicator of intrinsic genetic quality:
           • 1. Secondary character is a handicap to male 2. Males who can survive despite this and afford to invest highly in this trait must be good quality males 3. Offspring of these males (and the females that choose them) inherit these ‘good genes’ and have greater viability
         • ‘Sexy son’
           • 1. Female preference arbitrary 2. Once male trait is favoured by females then it will be maintained 3. If a female happens to show a different preference, her sons won’t attract female mates
       • e.g. stalk-eyed flies

7. Post-copulatory sexual selection/sperm competition

   • Sperm competition
     • def: ‘competition between the sperm of two or more males for the fertilization of a given set of ova’
     • The ‘competition’ arises from female multiple mating (=mating with more than one male in single oestrus)
       • – Sperm of two or males trying to fertilize the same set of ova – Copulations must occur near the time of ovulation – Sperm of different males must coexist
     • e.g. California/Colombian ground squirrel
     • Female tract – selective arena
       • hostile environment to sperm
         • obstacles - cervic
     • What does sperm competition select for in males?
       • Testes mass scales with body size
         • e.g. primates, bats, ungulates, other eutherian mammals, marsupials, birds, fish, frogs, butterflies
         • testes size predicts mating system (e.g. Aust. marsupials)
         • What do larger testes do?
           • more sperm producing tissue
           • Greater volume ejaculate + more sperm per ejaculate
           • males with more sperm win
           • If it’s a ‘race’ then long sperm would be an advantage
             • correlation is unclear  – Mammals - no correlation – Birds, butterflies & cichlid fishes +ve correlation
       • Male initiation of mating
         • – Males in polyandrous taxa more active – Rodents – polyandrous sp. males waited less time to copulate than monandrous sp.
       • Repeated copulation
         • – E.g. socially monogamous birds
       • Post-copulatory mate guarding
         • – Fighting other males, prolonged mounting, physical restraint of female – Reduces risk of insemination by another male – E.g. damselflies
       • Removal of rival sperm from female tract
     • sperm competition vs mate acquisition
       • - Sperm production is costly  - Males with low risk of sperm competition invest in mate acquisition  - Males with high risk of sperm competition invest in testes
   • Multiple paternity
   • Conflicts between males & females
     • Evolutionary interests of males and females differ
     • Females may incur physical costs from male adaptations to sperm competition
       • – Energy costs – Physical injury
     • Females may incur decrease in reproductive success
       • – Females obtain benefits by multiple mating (acquisition nutrients, ‘good genes’, incompatibility avoidance, genetic diversity, ‘sexy sperm’) – Restriction of ability to multiply mate may lead to decrease in offspring number and/or quality

8. E.G. RESEARCH : HONEY POSSUM

   • Large investment in spermatogenesis
     • -longest sperm in mammals (4.2% body mass)
     • prediction: high risk of sperm competition
   • Asymmetry
   • Sexual dimorphism - * Prediction: males don’t compete by fighting
     • - female larger, dominant - Solitary lives, male and female home ranges overlap
   • Implications of findings
     • Multiple mating & multiple paternity are intrinsic to the mating system
       • Females actively seek multiple partners
     • Evidence of sperm competition
     • High risk and high intensity of sperm competition
   • The mating system of the honey possum
     • Multiple mating by females (and probably males)
     • Likely to be sperm-based ‘scramble’ competition
       • – Competition amongst males to locate receptive females – Males have larger home ranges than females
     • Benefit of multiple paternity for female honey possums
       • Genetic benefits – good genes? Incompatibility avoidance? Selection of best quality embryos/young?

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