Biology Reference
In-Depth Information
If we consider weaning in mammals as an ecological change during
which the young is no longer maternally protected but exposed to the out-
side environment, it could be analogous to metamorphosis in biphasic ver-
tebrates such as amphibians since, as mentioned earlier, metamorphosis
always corresponds to an ecological transition. According to this view,
the TH peak could trigger in amphibian and mammals the remodeling of
intestine and brain and other organs necessary to reach the adult stage and
to cope with all the changes induced by the living in a new ecological en-
vironment. In amphibian, this peak indeed correlates with metamorphosis.
In most Anurans, it leads to the spectacular and complete metamorphosis of
tadpole into frog. In mammals, as in some amphibians, we would observe
some variations between the TH peak and birth (or hatching) as an example
of heterochrony that is an evolutionary change in developmental timing.
According to this view, both the guinea pig and
E. coqui
are direct developers
that undergo their TH-controlled remodeling as embryos and hatch
(or born) as juveniles. The TH system, that is connected to the environment
via the HPT axis, has all the necessary flexibility to allow plastic response to
environmental cues. Such a system allows for heterochronic changes on the
respective timing of birth and “metamorphosis” in various species.
6. SAUROPSIDS
In this review, we took the phylogenetic point of view to assess a phys-
iological and developmental question among amniotes. Thus, if the
hypothesis stands and THs control organ remodeling in newborn amniotes,
we should observe the same relation between TH level and organ remo-
deling in sauropsids, as in mammals. Only few studies specifically address
TH levels during hatching (
McNabb et al., 1984
), and as most sauropsids
lay eggs, as do the well-studied amphibians or teleost fishes, this simplifies
the situation because in mammals, it is obvious that maternal change of
TH levels during pregnancy can have a profound influence on the develop-
ing embryo.
6.1. TH levels during development in birds
In birds, there is neither clear larval to adult transition as in amphibian nor
weaning as in mammal, but there is independent feeding (as in mammals)
and changes of ecological niche: egg/non-egg or nest/non-nest. Neverthe-
less, TH has been identified as important in the late development of preco-
cial birds particularly during stages close to hatching. A first piece of evidence
