Sexual Selection

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Sexual Selection Mating systems

• Sexual selection overview
• Hypothesis
• Mechanisms
• Natural Selection v. Sexual Selection
• Males more affected
• Mating systems sexual selection
• Scramble competition
• Mate Guarding Behavior
• Leks Choruses
• Satellites Interferers
• Sperm Competition

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I. Sexual Selection overview

• Defined as Directional selection that acts on
  genetically variable phenotypic traits that
  affect the reproductive success of the
  individuals of a particular sex
• Sexual selection can explain the persistence of
  conspicuous differences between females and males

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A. Hypotheses Why do females prefer certain
phenotypic traits?

• Direct Benefit Certain male characters provide a
  direct benefit to females in terms of increased
  fecundity.

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2) Good Genes

• Male characters are "indicators" of "good genes",
  

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3) Sensory bias or sensory drive

• some aspect of the sensory world biases
• females to "prefer" or notice ascertain
• characters
• eg, if the species feeds on red prey, their
  visual system may be tuned to red wavelengths,
  therefore, females prefer males with red ornaments

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B. POSSIBLE MECHANISMS

• Pre-mating sexual selection
• selection for species recognition (females mating
  w/own species) could possibly lead to elaborate
  secondary sexual characteristics
• females could detect differences in the number of
  deleterious alleles in an male
• Post-mating sexual selection
• sperm competition
• polyandrous species have larger sperm, faster
  swimming sperm, more aggressive sperm

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C. Natural Selection v. Sexual Selection
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D. Males more affected by sexual selection than
females

• Females
• Males (sperm is cheap) spend energy on mate
  selection ability, therefore under greater sexual
  selection pressure

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II. Mating systems sexual selection

• Polygamy - single individual mates with more than
  one individual of the opposite sex (resource use)
• Polyandry
• (male defense, resource defense)
• Polygyny
• (female defense, resource defense, Lek, Scramble
  competition)
• Monogamy 1 male mates with 1 female (mate
  guarding/assistance)

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Larval habitat affects mating strategies
Dendrobates vanzolinii
Dendrobates ventrimaculatus
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A. Scramble competition

• Explosive breeders Female available for short
  periods, spatially aggregated
• Scramble competition males compete to locate
  females as fast as possible, mate as fast as
  possible with as many females as possible

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Rana sylvatica Bufo bufo,
Scaphiopus
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B. Mate Guarding Behavior

• If searching for mate is costly, then it may be
  worthwhile to invest energy in guarding her from
  other males
• Monopolize the female strategy

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C. Leks Choruses

• Lek
• Chorus anuran males calling from particular
  perches females approach the males

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D. Resource Defense

• Monopolizing resources which are attractive to
  females gives males an advantage

Plethodon cinereus
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E. Satellite frogs Sexually interfering
salamanders, lizards snakes

• Satellite males dont vocalize, but rather wait
  near a calling male to intercept females that are
  attracted to the calling male
• Interfering males prevent female from mating
  with another male

Rana clamitans
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Thamnophis sirtalis parietalis males swamp the
few females that emerge
High numbers of male garter snakes die soon after
emerging from hibernation because they are
attacked by crows. She-males at the center of a
mating ball, however, are less exposed to
predators also remain warmer
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F. Sperm Competition
Chiromantis xeramplina African gray treefrog

• Multiple paternity
• Genetically superior sperm more likely to survive
  (better to mate with several males)