Infra-class Metatheria The taxonomy, life history, & ecology of: Marsupials “animals with pouches”

But first...  Prototheria (4 species) = egg-laying monotremes  Theria = live birth  Metatheria (~280 species) = viviparous  Eutheria (~4500 species) = placental birth

Luo, Z Transformation and diversification in early mammal evolution. Nature, 450,

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 mya  Marsupials still maintain diversity and dominance in Australia

Burrowing Flying squirrel, Arboreal placental Marupial (left) Antechinus stuartii Placental (right) Peromyscus maniculatus Similar Habits Placental: Golden mole of S. Africa Marsupial mole of Australia Flying Foregut Fermentation Sugar glider arboreal marsupials. Termite eating Numbat (above) Anteater (right)

Metatheria are different because…  Development  Ecretory/reproductive systems  Cranial differences  Epipubic bone  Teeth

Teeth  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

 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 similarly- sized placental  Milk changes composition during lactation  Early: high protein, low fat (for structural development)  Later: low protein, high fat (for rapid increase in mass) Major investment during Lactation

Marsupial vs. Placental CharacterMarsupial statePlacental state Diversity6% of living mammal species94% of living mammal species sizenot as largelarger range of body sizes structural adaptationsless diverseflying (wings), marine (fins) Reproduction [Figs. 10.7] brief gestation; semiembryonic young; body mass 1% of mother [Fig ]. Need for precocious grasping forelimbs may constrain adaptability long gestation period; young more developed at birth; body mass up to 50% of mother Placenta [Fig. 9.11]choriovitelline (usually) chorioallantoic Lactation period [Fig. 10.7] longshort Investment of energy in motherhood lower, altricial young higher, reproduce more rapidly (usually), precocial young Cerebral Cortex/ Braincase smooth and complex, slow development, smaller volume (maybe) fast development, greater volume (maybe) Behavioural plasticityuncommongreat range of behaviours Territorialityuncommoncommon and important Antipredator behaviournot well developed highly developed, particularly in herding animals; capable of sustained high speeds Epipubic bonespresentabsent Baculum [Fig. 9.3]absentpresent in most Auditory bullaederived from alisphenoid bonevaries, but not alisphenoid primitive dental formula5/4-1/1-3/3-4/43/3-1/1-4/4-3/3

Common bushtail possum TOP Avg. weight gain much greater in younger possums suggesting older mothers put put more effort into reproduction than maintenance/survival Middle 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

Isaac, J.L. & Johnson, C.N Terminal reproductive effort in a marsupial. Biology Letters. 1: RE as loss of weight shows transfer of energy protrayed in yearling weight.

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       Google Images  Tyndale-Biscoe, H Life of Marsupials.