Studies on the role of thyroid gland function in seasonal physiology of red deer (Cervus elaphus) stags

Abstract

This thesis investigated the role of thyroid function in the regulation of seasonal changes in live weight, reproductive activity, pelage and antler development in red deer stags.

Experiment 1 examined the effects of removal of thyroid hormones in the regulation of seasonal changes by use of 3 THX stags with 5 euthyroid stags as controls. Thyroidectomy blocked the seasonal decline in testicular diameter, neck girth, plasma testosterone concentration and responsiveness of testosterone and LH to a stimulus of exogenous GnRH, which occurred in controls in spring and early summer. THX stags also failed to undergo the spring growth spurt, antler and pelage changes. Furthermore a seasonal pattern of plasma T3 concentrations was recorded with low values in winter and high values in summer.

Experiments 2 and 3 examined the importance of the seasonal patterns of plasma thyroid hormone concentration in regulation of seasonal changes. In Experiment 2 injections of T3 during autumn and winter elevated plasma T3 concentrations in 4 treated stags compared with 4 controls, and in Experiment 3 injections of T4 from winter to spring raised both plasma T4 and T3 concentrations in 4 stags. In spite of the elevation of plasma thyroid hormones there were no changes in seasonal patterns of live weight, reproductive activity or pelage development. These results suggest that the seasonal changes in secretion of thyroid hormones are not instrumental in regulating seasonality of stags.

Experiment 4 utilised 4 THX stags treated with T4 and another 4 THX stags as controls to test the hypothesis that thyroid hormones are required for the expression of seasonal changes. Treatment with T4 commencing in spring stimulated gain of live weight induced reproductive regression, antler development and the spring moult, thus confirming the hypothesis.

Twenty-four hour melatonin profiles between THX (n=3) and euthyroid stags (n=3) were similar (Experiment 5), which indicated that the blockade of seasonal changes following thyroidectomy was not due to inappropriate secretion of melatonin. Also in Experiment 5, treatment of THX stags with MPA was utilised to check whether development of antlers could be altered by thyroidectomy. The treated stags cast hard antlers and grew a set of new antlers. This indicated that blockade of antler development in THX stags was due to the maintained secretion of testosterone and not to hypothyroidism, and that antler bone growth could proceed normally in the absence of thyroid hormones.

In conclusion, results of this thesis demonstrate that in red deer stags thyroid hormones are required for the normal expression of seasonal changes in spring and early summer. Thyroid function affects development of antlers indirectly through its influence on seasonal testicular activity.