Biology Reference
In-Depth Information
5.5. The special case of marsupials
Metatherians (marsupials) have a very different development strategy from
that of eutherian mammals. In this group, the newborn, called a joey, born
blind, deaf, and furless. It reaches the ventral pouch of the mother crawling
on the fur where it finds a mammary gland and feeds on it for several months
(
Murphy & Smith, 1970; Wilkes & Janssens, 1986
). The joey terminates its
development in the pouch for several months until it is fully able to feed and
move independently and ensure its thermic autonomy (
Shield, 1966
). In
metatherians, the gestation time in itself lasts several weeks and the postpar-
tum development several months (with differences according to the size of
the animal). If we consider that leaving the pouch is concomitant with the
weaning of the juvenile, metatherians spend more time in postembryonic
development than in gestation itself.
In the tammar wallaby (
Macropus eugenii
), a peak of T4 is observed ca.
180 days after birth when the joey's thyroid gland becomes functional. Max-
imum deiodinase outer-ring deiodination activity is observed near 205 days
after birth and subsequently a peak of T3 occurs between 210 and 235 days
after birth (
Richardson et al., 2002
). The T3 peak occurs between the
young's first exit of the pouch near 200 days and the definitive leaving near
260 days (
Setchell, 1974
). The peaks of TH correspond to the beginning of
homeothermic autonomy, pelage development, and digestive tract remo-
deling of the young (
Janssens, Grigg, Doce, & Hulbert, 1990
). THs are im-
portant for development, as thyroidectomized wallabies are unable to
thermoregulate and exhibit growth and fur development defects (
Janssens
et al., 1990; Setchell, 1974
), indicating that even if the plasma TH peak starts
just after the onset of remodeling in wallaby, the hormone is indeed neces-
sary to achieve these changes.
Very similar pictures emerge from the data coming from two other spe-
cies. In the brushtail possum (
Trichosurus vulpecula
), a peak of T4 is observed
at day 120 after birth (
Buaboocha & Gemmell, 1995
). The T4 peak occurs a
little after the young possum is able to exit the pouch at day 118 (
Buaboocha
& Gemmell, 1995
) and prior the definitive leaving around 180 days postpar-
tum. In the tasmanian bettong (
Bettongia gaimardi
), a peak of T4 is observed at
12-14 weeks after birth which is concomitant with homeothermy and the
fur development (
Rose & Kuswanti, 2004
). The peak of T4 corresponds to
the time when the young exits the pouch for the first time, about 14 weeks,
before the final leaving at 15 weeks.
Indirect clues highlighting the importance of TH level during wallaby
development come from the level of TH carrier proteins. During
