Animal Reproduction

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Animal Reproduction

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Vertebrate Evolution

• As animals evolved to live on land, their means
  of reproduction had to change
• Water dwelling animals tend to release swimming
  gametes into the water and their young developed
  in the water
• In order to live on land, animals had to devise
  ways to keep gametes and zygotes from drying out
  in the air, and evolved to have internal
  fertilization and development

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Order of Evolution

• ? ?
• ? ?

Amphibians
Fish
Reptiles
Birds
Mammals
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Fish

• Fertilization External, Fish release eggs and
  sperm into the water
• (sharks are an exception they can have
  internal fertilization)
• DevelopmentExternal Fertilized eggs develop in
  the water

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Amphibians

• Fertilization External, Male grabs female in
  position called amplexus, squeezes out her eggs
  and he deposits sperm on them
• Development External.
• Two stages, larval and adult

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Reptiles

• Fertilization Internal with a male penis
  inserting sperm into female
• Development Internal, either oviparous or
  viviparous

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Oviparous vs. Viviparous

• Oviparous means to lay eggs, with organisms
  hatching after eggs are laid (turtles, lizards,
  alligators, some snakes)

• Viviparous means the young develop in the body
  and live young are released (some snakes)

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Amniotic Eggs

• Reptiles and Birds evolved the amniotic egg,
  which has protective membrane layers and a shell
  to avoid drying out on land.
• Note functions of various membranes

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Birds

• Fertilization Internal, though most birds dont
  have a penis. (Why?) Male oozes sperm from cloaca
  and rubs on females cloaca
• Development External, Oviparous

Many male birds have elaborate courtship displays
to attract females
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Mammals

• Fertilization internal fertilization
• Development three groups of mammals based on
  development
• Monotremes - oviparous (lay eggs) ex. platypus
  and spiny echidna
• Marsupials - viviparous, underdeveloped young
  finish development in pouch ex. kangaroo,
  koala, opossum
• Placentals - viviparous, nourished by placenta
  in womb, born more fully developed ex. all other
  mammals