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 formula

5/4, 1/1, 3/3, 4/4 3/3, 1/1, 4/4, 3/3