Thyroid hormone manipulation influences development of endothermy and hatching in white leghorn chickens (Gallus gallus).

Chickens experience rapid change in their physiology and metabolism during hatching. We propose that thyroid hormones play a major role in regulating the developmental changes associated with attaining endothermy. To better understand the role thyroid hormones play in hatch timing and development of thermogenic capacity and metabolic rate we manipulated plasma thyroid hormone levels in chicken embryos beginning at 80% development (day 17 of a 21-day incubation) with either a single dose of triiodo-L-thyronine (T3 ) or the thyroperoxidase inhibitor methimazole (MMI). Manipulation of thyroid hormones altered the timing of hatching, accelerating hatching under hyperthyroid conditions, and prolonging hatching with hypothyroid conditions. Effect sizes comparisons of morphological variables between treatment groups revealed larger heart and body masses in hyperthyroid 1-day post hatch animals. Thyroid hormone manipulation influenced the thermal neutral zone for O2 consumption and body temperature during gradual cooling from 35 to 15 °C of externally pipped embryos and 1-day post hatch chicks. Hyperthyroid EP animals had a wider thermal neutral zone during cooling when compared to control animals. At the temperatures tested, the hypothyroid animals did not exhibit a thermal neutral zone. Similar differences between treatments in the breadth of the thermal neutral zone carried through to 1-day post hatch chickens. These findings suggest that thyroid manipulations influence the timing and development of the animal's thermogenic response to cooling.