Males and females often look different

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Males and females often look different
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Sexual Dimorphism

• Males and females differ in appearance (secondary
  sexual characteristics)
• Primary sexual characteristics reproductive
  organs
• Secondary sexual characteristics other external
  features of an organism that differ between males
  and females

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Examples of Sexual Dimorphism
Black Widow
bugguide.net/ node/view/7426
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Examples of Sexual Dimorphism
Great-tailed Grackle
Gila Topminnow
Desert Pupfish
Pheasant
Elephant Seal
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Why do males and females often look different
from each other?
In Elk?
In Great-tailed Grackles?
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BackgroundNatural Selection has 2 components

• Ecological Selection
• Selection for attributes (physical or behavioral)
  that contribute to survival.
• Sexual Selection
• Selection for attributes (physical or behavioral)
  that contribute to reproductive advantage.

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How does sexual dimorphism arise?
1. Give ecological selection hypotheses
2. Give sexual selection hypotheses
3. Give deductions for each
4. Devise tests that will allow you to observe
whether a prediction for the ecological
selection, sexual selection, or both hypotheses
occurs.
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Sexual Selection 2 components

• Intrasexual selection
• within sex
• usually male-male competition
• Intersexual selection
• between sexes
• usually female choice

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Intrasexual Selection Strategies

• Mate Defense
• Whoever defends mates best against rivals gets to
  mate
• Examples deer, elk, beetles

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Intrasexual Selection Strategies

• Resource Defense
• Whoever defends best resources (e.g., food,
  shelter, etc.) against rivals gets to mate
• Examples creosote bush grasshopper, elephant
  seal, hummingbird

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Intrasexual Selection Strategies

• Scramble Competition
• Whoever gets to most mates first gets to mate
• May or may not lead to sexual dimorphism
• Examples cactus bees, ground squirrels,
  horseshoe crabs

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Intersexual Selection Choice

• Good Gene Selection
• Also called Direct Fitness Benefits
• Chooser gains better nest site, territory,
  provider (of food/protection), parental care,
  lack of contagious disease or parasites, etc.

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Intersexual Selection Choice

• Runaway Selection
• Also called Indirect Fitness Benefits
• Choosers offspring gain better sexual
  attractiveness, often at expense of survival

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Intersexual Selection Choice

• Lek
• Female chooses male holding best spot at ritual
  gathering place (note males often have to
  compete for best spot). The spot has no
  resources.
• Example sage grouse, tarantula hawk

www.ownbyphotography.com
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Intersexual Selection Choice

• Prenuptial Gifts
• Whoever offers best gifts (e.g., food)
• Example shrike

The suitor
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Intersexual Selection Strategies

• Good Looks
• Whoever looks/smells, sounds, etc. the best
• Example 1 widowbird
• Female selects male with longest tail

Is this good gene or runaway selection?
Her male offspring will have long tail and be
selected first, both by females and predators,
but Also may indicate the male is free of
parasites or disease.
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Intersexual Selection Strategies

• Good Looks
• Whoever looks/smells, sounds, etc. the best
• Example 2 northern mockingbird
• Females select male with largest song repertoire.

Is this good gene or runaway selection?
Her male offspring will have large song
repertoire and be selected first butalso may
indicate male longevity
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So, why do male elk and deer have antlers, but
not females?

• Competition Mate Defense
• Males defend groups of females from other males

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So why are male birds often more brightly colored
than females?

• Choice
• Females just choose more brightly colored males
  (so do predators!)

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The amount of competition/choice occurring in a
species depends on the mating system
Monogamy Polygamy Polyandry Polygyny
Promiscuity
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Mating 2 Main Strategies

• Monogamy single mate per mating season
• Pros two-parent care
• Cons out-reproduced
• Example 90 of birds rare in mammals
• Caveat extra-pair copulations

http//www.paulnoll.com/
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Mating 2 Main Strategies

• Polygamy multiple mates per mating season
• Pros choose best partners
• Cons single parent care
• Example most mammals

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Polygamous Mating Systems

• Polyandry one female with multiple male mates
  (sets up for competition/choice)
• Example phalarope, seahorse

male
female
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Polygamous Mating Systems

• Polygyny one male with multiple female mates
  (sets up for competition/choice)
• Example deer, elk

www.peopleandwildlife.org.uk
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Polygamous Mating Systems

• Promiscuity males and females both have multiple
  mates (sets up for competition/choice)
• Example snowshoe hare

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Mating Systems

• Monogamy mate-guarding, mate-assistance
• Polygamy
• Polygyny one male, multiple females
• Mate defense
• Resource defense
• Scramble competition
• Lek
• Polyandry one female, multiple males
• More material benefits/parental care
• Better sperm/fertility insurance
• Promiscuous males and females both have multiple
  mates