Marsupial-Placental Dichotomy

I. Why do marsupials and placentals differ in their mode of reproduction?

Marsupials Placentals
very short gestation usually longer gestation
neonate incompletely developed neonate anatomically complete

  1. Why do marsupials get expelled from the uterus so early? May relate to immunology

    1. Immune system recognizes self/not-self
    2. Fetus is not-self because it receives 1/2 its genome from the father
    3. How do animals get around this problem?

      1. Egg laying vertebrates: separation of fetus/mother; fetus nourished with yolk sac
      2. Marsupials: shell membrane
      3. Eutherians: trophoblast

    4. Chorionic gonadotropin

      1. Maintains the trophoblast
      2. Suppresses immune response from maternal cells
  1. Ramifications of long pregnancy

    1. Precocial young
    2. Increased structural diversity (eg limb structure)

II. Are marsupials inferior?

  1. Lines of reasoning that they are:

    1. Less diversity of habitat types
    2. Less diversity of locomotion
    3. Less diversity of foraging
    4. No really big marsupials
    5. Social organization is less complex
    6. Not as speciose
    7. Most diverse and numerous in Australia, where there is negligible competition with eutherians
  1. Is the competitive disadvantage the result of their mode of reproduction?

    1. Maybe. Their mode of reproduction limits the environments in which they can live.
    2. Maybe not. Other factors include:

      1. Cerebral cortex of marsupials is smaller and develops more slowly
      2. Learning and behavioral flexibility is less developed in marsupials.
      3. Behavior is less diverse.
      4. They have a small number of chromosomes, which may make them less evolutionarily flexible.
      5. Their reproductive rate is lower.
  1. An alternate view is that marsupials are not necessarily at a competitive disadvantage in all circumstances. Possible advantages of a marsupial reproductive stratgey include:

    1. Low energy requirements, spread out of a long period of time
    2. Can quickly replace lost young