Infra-class Metatheria
The taxonomy, life history, & ecology of:
Marsupials
“animals with pouches”
Mammalia
But first...
Prototheria
Theria
Eutheria
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Metatheria
Prototheria (4 species) = egg-laying monotremes
Theria = live birth
Metatheria (~280 species) = viviparous
Eutheria (~4500 species) = placental birth
Luo, Z. 2007. Transformation
and diversification in early
mammal evolution. Nature, 450,
1011-1019.
7 Orders (18 families):
Didelphimorphia (American opossums)
Paucituberculata ("shrew" opossums)
Microbiotheria (monito del monte & extinct relatives)
Dasyuromorphia (Australasian carnivorous marsupials)
Peramelemorphia (bandicoots and bilbies)
Notoryctemorphia (marsupial "moles")
Diprotodontia (kangaroos, wallabies, possums, koalas, gliders, wombats, etc)
*Two primary divisions within Marsupialia denote American marsupials
and Australian marsupials.
Adaptive Radiation
largest
smallest
Fossil Evidence Suggests Marsupials
Much More Common
 Marsupial origination is cited as
Mongolia
 Advantages of placental development
must have out-competed marsupial
 Australia has maintained diversity and
dominance of marsupials because of
isolation
Distribution of Metatheria
Virginia Opossum
 Only Marsupial in North
America
 Very generalized
 Immune to rabies
 Can remain in involuntary
comatose state for
1min. – 6 hrs.
 13day gestation, when
born size of honeybee
 More teeth than any other
NA land mammal = 50
Convergent
Evolution
 Filling the same
ecological niche in diff.
parts of the world.
 Seperated from common
ancestor 100-150mya
 Marsupials still maintain
diversity and dominance
in Australia
Flying
Termite
eating
Foregut Fermentation
Sugar glider
arboreal marsupials.
Flying squirrel,
Arboreal placental
Numbat
(above)
Anteater
(right)
Similar Habits
Marupial (left)
Antechinus stuartii
Placental (right)
Peromyscus maniculatus
Burrowing
Marsupial mole
of Australia
Placental:
Golden mole of
S. Africa
Metatheria are different because…
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Development
Ecretory/reproductive systems
Cranial differences
Epipubic bone
Teeth
Teeth
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Different number of incisors on top and bottom
P3/3, M4/4
Delayed development of teeth
The milk teeth are represented by a single premolar in each
jaw
 Only replace one tooth postnatally
Reproduction & Development
 Short gestation (8-43 days)
 Young born extremely small relative to adult
size
 Organs not completely developed
 Well-developed front limbs for grasping fur
 Move to pouch or (if pouchless)
grab ahold of teats
Newborn Kangaroo
0.003% mother’s weight
Major investment during Lactation
 Teat swells in its mouth to make a
semi-permanent attachment
 Stays in pouch for 1 week - 1 year
 Leaves pouch at weight roughly
equivalent to birthweight of a similarlysized placental
 Milk changes composition during
lactation
 Early: high protein, low fat (for structural development)
 Later: low protein, high fat (for rapid increase in mass)
Marsupial vs. Placental
Character
Diversity
size
structural adaptations
Reproduction
[Figs. 10.7]
Placenta [Fig. 9.11]
Lactation period
[Fig. 10.7]
Investment of energy in
motherhood
Cerebral Cortex/
Braincase
Behavioural plasticity
Territoriality
Marsupial state
6% of living mammal species
not as large
less diverse
brief gestation; semiembryonic young;
body mass 1% of mother [Fig. 10.11].
Need for precocious grasping forelimbs
may constrain adaptability
choriovitelline (usually)
Placental state
94% of living mammal species
larger range of body sizes
flying (wings), marine (fins)
long
short
lower, altricial young
higher, reproduce more rapidly (usually),
precocial young
long gestation period; young more
developed at birth; body mass up to 50%
of mother
chorioallantoic
Epipubic bones
Baculum [Fig. 9.3]
Auditory bullae
smooth and complex, slow development,
fast development, greater volume (maybe)
smaller volume (maybe)
uncommon
great range of behaviours
uncommon
common and important
highly developed, particularly in herding
not well developed
animals; capable of sustained high speeds
present
absent
absent
present in most
derived from alisphenoid bone
varies, but not alisphenoid
primitive dental formula
5/4-1/1-3/3-4/4
Antipredator behaviour
3/3-1/1-4/4-3/3
TOP
Avg. weight gain much
greater in younger
possums suggesting
older mothers put put
more effort into
reproduction than
maintenance/survival
Middle
Common
bushtail
possum
Survival to breed the
following year. Notice
year 5 - ??
Bottom
Mean Reproductive
Effort measured as
relative mass lost
during lactation
*Terminal Reproductive
Hypothesis exemplified
in Marsupials
RE as loss of weight
shows transfer of
energy protrayed in
yearling weight.
Isaac, J.L. & Johnson, C.N. 2010.
Terminal reproductive effort in a
marsupial. Biology Letters. 1: 271275.
Theoretical Constraints of
Lactation
 Young have limited control over energy
input - Marsupial mammary gland is
relatively unresponsive to changes in
suckling
 In placental animals, fetal hormones
control more energy transfer
 Mother-offspring conflict constrains
adaptive variation
Sources
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http://www.biology.iastate.edu/InternationalTrips/1Australia/04papers/keuderDigSys.htm
http://users.tamuk.edu/kfjab02/Biology/Mammalogy/systematics/A1metatheria.htm
http://www.ucmp.berkeley.edu/mammal/marsupial/marsupial.html
http://users.tamuk.edu/kfjab02/Biology/Mammalogy/systematics/A1metatheria.htm
http://animaldiversity.ummz.umich.edu/site/accounts/information/Metatheria.html
Google Images
Tyndale-Biscoe, H. 2005. Life of Marsupials.