THYROID-HORMONES ARE REQUIRED FOR THE EXPRESSION OF SEASONAL-CHANGES IN RED DEER (CERVUS-ELAPHUS) STAGS

Experiment 1, from 6 July 1990 (winter) to 8 November 1990 (spring), e xamined the effect of an early elevation of plasma thyroid hormone con centrations on the expression of seasonal changes in 20-month-old stag s. Four stags were injected subcutaneously with 1 mg thyroxine (T-4) d aily, and four with vehicle only. Plasma T-4 concentration was elevate d by injection of T-4; the mean concentration 24 h after injection was 192+/-31 nmol L(-1) compared with 58+/-5 nmol L(-1) in controls on 21 September, However, live weight, testicular diameter, plasma testoste rone concentration and moulting underwent similar changes in both grou ps of animals. Experiment 2 tested the hypothesis that thyroid hormone s are required for the expression of changes that occur during spring and early summer in red deer stags. Thyroidectomized (THX) stags (n = 4) of mixed age (>22 months old) were given a replacement dosage of T- 4 or vehicle only (controls) during spring (from 7 September). T-4-tre ated THX stags underwent testicular regression, antler regeneration an d spring moult during spring and early summer (October-December), wher eas control THX stags did not display these changes. During spring and summer, T-4-treated stags gained more live weight (P < 0.05) than con trol stags. These results show that, in stags, thyroid hormones are re quired for the expression of seasonal changes and that these changes a re not caused by the seasonal elevation of plasma concentrations of th yroid hormones.