Transcript Marsupial-placental dichotomy

Marsupial-placental
dichotomy
Biol 455 Mammalogy
Feb 1, 2005
Marsupial history
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Today, most marsupials are found in:
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Central and South America (~70 sp.)
Australasia (~200 sp.)
Radiation occur during Cenozoic (with few
placental competitors)
Past marsupial faunas very diverse:
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E.g. In Australia, marsupial herbivore (rhinocerossized), 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?
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How are marsupials and
placentals different?
Coined “marsupial-placental dichotomy”
 Most conspicuous difference between
marsupials and placentals are:
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Reproductive anatomy and pattern
 Degree of development of young
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Reproductive anatomy
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Marsupials:
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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
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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
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Degree of development of
young
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Marsupials:
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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
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Placentals:
Gestation period long
 Young born with functional organs
 Short lactation period
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Immunology of marsupials and
placentals
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Immune system recognizes self/non-self
Fetus is non-self because it receives half
genome from father
How do animals get around this problem?
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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?
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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?
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Mode of reproduction limits the environments in which
they can live
Maybe not
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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:
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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
formula
5/4, 1/1, 3/3, 4/4
3/3, 1/1, 4/4, 3/3