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manipulation with goitrogens, indicating that (i) no further important mat-
uration controlled by TH seems to occur in these birds after hatching and (ii)
TH metabolism is already mature in these birds as the hatchling respond to
goitrogen in a manner indistinguishable to the adults. As far as other
sauropsids are concerned, indirect evidence indicates that a peak of TH
could exist at hatching in some species such as crocodiles, but few data
are available. As we previously proposed for mammals, the TH peak at birth
could be an indicator of the degree of independence of the hatchling in
sauropsids with a peak of TH just before or at hatching correlating with
an independent autonomous offspring. This acquisition of autonomy
allowed by the TH trigger could be considered as being equivalent to the
amphibian metamorphosis, although this is only a speculation given the data
available.
7. CONCLUSION
In this review, we focused on the data available about TH level in late
embryonic development and early postembryonic development of several
amniote species (
Fig. 14.4
). TH peaks are observed across amniotes around
birth/hatching or at other crucial developmental time points, as weaning,
and these peaks are important for organ maturation, particularly in brain
and digestive system. Moreover, the timing of the peak is variable across spe-
cies and seems related to the degree of autonomy of the young at birth/
hatching with a delayed peak in species with nonautonomous young. On
the contrary, in species with autonomous young, this peak is close to
birth/hatching, allowing a shorter period of organ maturation, particularly
for the intestine (that renders self-feeding possible) and the brain (that allows
an independent behavior and metabolic control).
Among direct developing amphibians, such as
E. coqui
, the young froglet
that hatches is a fully autonomous miniature adult, as amphibians do not
need a pond to develop. In this species, the tadpole-to-frog transformation
occurs very early, within the egg. Thus, we can draw a parallel with amniote
species in which the more the young is autonomous, the earlier the peak of
TH seems to happen. In sauropsids, this would be exemplified by the
Japanese quail with an endothermic autonomous young, and in mammals
this would be exemplified by the guinea pigs with a young than can eat solid
food as soon as 2 days after birth. In metatherian, this period occurs after
birth, when the joey starts to leave its mother and may correspond to the
onset of weaning.
