Sexual Selection: Traits that lower survival, but enhance the ability to mate are sexually selected traits Ex: Elaborate costly traits exhibited by male birds. Bower building might be an indicator of brain size

-Variance in reproductive success is greater for males than for females in the satin bowerbird

-Male and female gametes differ greatly in size

-Operational Sex Ratio Actual 50:50 / Male biased operational sex ratio. Can differ between seasons

Ex: Male sex drive is intense that scientists were able to lure seals with a tape recording

Parental investment takes many forms

  • Males can engage in parental investment
  • Females protecting their young
  • Feeding their young


Differences between the sexes in sexual behavior may arise from fundamental differences in parental investment

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Photo taken from reproduction slides

Conventional Sex Roles

  • Male-Male Competition
  • Female-Male Choice

Sex Role Reversal- In species where males make the larger parental investment we would expect…

  • The operational sex ratio to be biased towards females
  • Competition among females
  • Mate choice by males

Ex: Male Pot-bellied seahorse giving birth, Female laying eggs in males brood pouch of seahorse acting like a female placenta

Nuptial gifts

Food supply affects sex roles in the Skinny Australian Katydid

-When food abundance is low, spermatophores are harder to produce and more valuable nuptial gift to females. Males become choosy, females compete for access

-Sexual selection involves trade-offs

Pecking order= Dominance Hierarchy

Options for subordinate males:

  • Female friendships
  • Male friendships

Conditional Strategy

  • Low ranking males can “make the best of a bad situation
  • Higher fitness payoff by adapting an alternative then try to fight their dominant opponents






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