Discussion
Seasonally breeding animals and birds measure the day length, i.e. photoperiod and use these measurements as predictive information to prepare themselves for breeding. Besides melatonin, which of the following hormones is involved in this biological process?
Answer: C
Besides melatonin, the hormone involved is Thyroxine.
In photoperiodic breeders, long or short days are signalled by the duration of nightly melatonin secretion. In mammals melatonin acts on the pars tuberalis, which releases thyroid-stimulating hormone; in birds the light signal acts on the same pathway directly. The resulting change in local deiodinase enzymes alters how much thyroxine (T4) is converted to active T3 in the mediobasal hypothalamus, and that local thyroid hormone signal switches the reproductive axis on or off.
GnRH is the final output of the reproductive axis that the thyroid hormone signal controls, not the hormone that reads day length. Growth hormone and ACTH have no established role in measuring photoperiod.
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