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It is interesting to correlate the thyroid metabolism of these species
with their precocial or altricial character. Interestingly, such a correlation
between altricial/precocial strategy and TH metabolism has already been
developed by McNabb, based on studies on birds ( McNabb, Lyons, &
Hughes, 1984; Schew, McNabb, & Scanes, 1996 ). Mice, the less autono-
mous of our examples, complete the development of their brain and intes-
tine during the postembryonic weaning period when a TH peak induces the
maturation of these organs. In humans, the brain exhibits a burst of devel-
opment at birth when a TH peak is observed ( Erenberg et al., 1974 ).
Weaning guinea pigs have their intestine ( Abalo et al., 2009; Matthews,
1998 ) physiologically close to the adult intestine and the main step of brain
maturation occurs before birth, at a time during which TH level is high. If, as
suggested above, the mouse pups are considered as “larvae” and the guinea
pig newborn as “juveniles,” then we may propose that guinea pigs undergo
their TH-controlled maturation at the end of the long gestation period
within the mother. Thus, guinea pig may represent a case of direct devel-
opment in mammals in which the “metamorphosis” occurred during devel-
opment ( Fig. 14.4 )
TH
TH
Horse, cow,
pig, Sheep
Birth
Weaning
Hatching
Metamorphosis
Xenopus
TH
TH
Wallaby,
possum
Birth
Pouch exit
Metamorphosis
Hatching
Eleutrhodactylus
TH
TH
Gray seal,
Harp seal
Mouse,
rat
Birth
Moult
Weaning
Birth
Weaning
TH
TH
Quail
Birth
Weaning
Guinea pig
Hatching
Molting
TH
TH
Human
Crocodile
Hatching
Figure 14.4 Schematic representation of the thyroid level during the development of
the species discussed in this review. The Y-axis represents an arbitrary TH level, and the
X-axis represents the time of development. The X-axis is at an arbitrary scale for the dif-
ferent species. Continuous line means that data are reliable enough to draw the TH
levels during development. Interrupted line means that only partial data are available
and the TH levels during development are still speculative to our knowledge.
Birth
Weaning
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