Biology Reference
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comes from the Japanese quail (
Coturnix japonica
) in which TH levels in the
laid egg yolk has been shown to be similar to the hen. However, impor-
tantly, the THs levels within the embryo have been found as not influenced
by the hens and yolk THs level (
Wilson & McNabb, 1997
).
As in mammals, one can distinguish two sorts of birds according to their
development processes. Precocial birds hatch with their eyes open and with
mature feathers and, consequently, are able to become independent from pa-
rental care early. On the contrary, altricial birds hatch with their eyes closed,
without feathers and stay a long period in the nest totally depending on pa-
rental feeding and warmth (
Ehrlich, Dobkin, &Wheye, 1988
). Of course, this
is a schematic description and all the intermediaries between these two ex-
tremes are found and this is linked to the life history of each species. In the
Japanese quail, a precocial bird, thyroid gland develops early in the embryonic
life, and hormone production begins ca. 5 days before hatching and a peak of
TH is observed in these precocial birds near or at hatching (for review, see
McNabb, 2006
). On the contrary, altricial birds do not exhibit such a TH
peak at hatching and the peak arose much later, ca. 10-15 days after hatching,
as, for example, in the dove (
McNabb, 2006
). In these species, the thyroid
gland seems to become functional later in the development, a few days after
hatching, and the synthesis of TH starts progressively during postembryonic
development (
McNabb, 2006
). However, some data, in turkey (
Meleagris
gallopavo
) shade some trouble on such a link between TH level and the pre-
cocial or altricial mode of development. In turkey, a precocial bird, a peak of
TH, is observed near 12/16 weeks after hatching (unpublished data cited in
Siopes, Millam, & Steinman, 2010
). Unfortunately, these data on TH level
during turkey development have not been published to our knowledge. Thus,
we do not know if there is a peak of TH at hatching in turkey as in other
precocial birds or if that peak is delayed until 12/16 weeks of development.
However, despite these discrepancies, studies performed in bird seem consis-
tent with the existence of a peak of TH after birth in precocial birds and a
smoother increase of TH until adult level in altricial bird. Thus, as in mam-
mals, THmay be implicated in the control of postembryonic life history strat-
egies, although this is far from being proved, yet.
6.2. TH levels in heterotherm sauropsids
Among sauropsids, TH levels vary significantly between birds on the one
hand and tortoise, lizard, and alligator (forming the ancient zoological group
called “reptilia” that is no longer valid on a zoological point of view; see
