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Marsupial-placental dichotomy

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					Marsupial-placental
        dichotomy
  Biol 455 Mammalogy
  Feb 1, 2005
Marsupial history
   Today, most marsupials are found in:
       Central and South America (~70 sp.)
       Australasia (~200 sp.)
   Radiation occur during Cenozoic (with few
    placental competitors)
   Past marsupial faunas very diverse:
       E.g. In Australia, marsupial herbivore (rhinoceros-
        sized), kangaroos 10 ft tall, carnivorous lion-like
        forms with shearing teeth and retractile claws
       E.g. In S America, no marsupial herbivore, but have
        a carnivorous marsupials (incl. sabertooth marsupial
        “cat”)
Competition with placentals
 Both continents, invasion of placentals
  caused the disappearance of marsupials
 Decline a coincidence? Or competitively
  inferior to placentals?
How are marsupials and
placentals different?
 Coined “marsupial-placental dichotomy”
 Most conspicuous difference between
  marsupials and placentals are:
     Reproductive anatomy and pattern
     Degree of development of young
Reproductive anatomy
   Marsupials:
       Female: paired reproductive tracts
       Right and left vaginae and uteri do not fuse to form a
        single body
       Choriovitelline placenta
       Some have chorioallantoic placenta (no villi)
       Birth takes place through a median canal
        (pseudovaginal canal)
       Have marsupium (not all species) for nursing young
       Male: penis is forked, scrotum in front of penis
Reproductive anatomy con’t
   Placentals:
     Female: right and left vaginae fused, some
      species right and left uteri fused
     Chorioallantoic placenta (extensive villi)

     Male: penis is not forked, scrotum lies
      posterior to penis
     No marsupium
Degree of development of
young
   Marsupials:
       Very altricial, weigh less than 1% of mother’s BM (all
        weigh <1g)
       Short gestation (8-43 days, depending on species),
        equal to length of estrus cycle
       Young born very tiny with few functional organs
        (heart, kidneys, and lungs are barely functional),
        brain is at ontogenetic stage
       Young have well-developed forelimb
       Development takes place in pouch
       Lactation period prolonged
Degree of development of
young con’t
   Placentals:
     Gestation period long
     Young born with functional organs

     Short lactation period
Immunology of marsupials and
placentals
   Immune system recognizes self/non-self
   Fetus is non-self because it receives half
    genome from father
   How do animals get around this problem?
       Egg-laying: separation of fetus/mother; fetus nourish
        by yolk sac
       Marsupials: shell membrane
       Eutherians: trophoblast
         • Chorionic gonadotropin: maintains trophoblast, suppresses
           maternal immune response, longer pregnancy
         • --> precocial young, increased limb structural diversity
Are marsupials inferior?
   Less diversity of habitat types
   Less diversity of locomotion
   Less diversity of foraging
   No really big marsupials
   Social organization is less complex
   Not as speciose
   Most diverse and numerous in Australia, where
    there is negligible competition with eutherians
Is competitive disadvantage
due to mode of reproduction?
   Mode of reproduction limits the environments in which
    they can live
   Maybe not
       Cerebral cortex is smaller and develops more slowly
       Learning and behavioral flexibility is less developed
       Behaviour is less diverse
       Have small number of chromosomes, makes them less
        evolutionary flexible
       Reproductive rate is lower
   Possible advantage of marsupial reproductive strategy:
       Low energy requirements, spread out over a long period of time
       Can quickly replace lost young
       Marsupial-Placental Dichotomy
       Summary
Character                Marsupials                              Placentals
Diversity                6% of living mamm. sp                   94%
Body size                Not very large                          Wide range
Structural adaptations   Less diverse                            Flight (wings), marine
                                                                 (flippers), hooves
Reproduction             Brief gestation, very altricial young   Long gestation. Young more
                                                                 developed
Placenta                 Choriovitelline                         Chorioallantoic
Lactation period         Long                                    Short
Energy investment        Lower                                   Higher, reproduce more rapidly
Cerebral cortex          Smaller, slow development               Larger, fast development
Epipubic bones           Present                                 Absent
Baculum                  Absent                                  Present
Auditory bullae          Derived from alispenoid bone            Varies
Primitive dental         5/4, 1/1, 3/3, 4/4                      3/3, 1/1, 4/4, 3/3
formula

				
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