The Evolution of Reproductive Behavior

Sexual Selection- Traits that lower survival, but enhance the ability to mate are sexually selected traits.

  • Elaborate costly traits exhibited by male birds

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- The number of sexually operational females in much less than the number of sexually operational males. It becomes male-biased. This allows the females to choose the best one and causes the males to be competitive with each other.

  • The actual sex ration tends towards 50:50

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

Conventional Sex Roles

  • Male-Male Competition
  • Female Mate Choice

Sex Role Reversal- In those 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

Food Supply Affects Sex Roles in the Skinny Australian Katydid

  • When food abundance is low, spermatophores are harder to produce and a 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

Alternative Mating Tactics

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