THE THYROID AND PHOTOPERIODIC CONTROL OF SEASONAL REPRODUCTION IN AMERICAN TREE SPARROWS (SPIZELLA-ARBOREA)

To explore the role of the thyroid gland in the control of seasonal re production in obligately photoperiodic American tree sparrows (Spizell a arborea), the effects of (1) thyroxine administered in drinking wate r to thyroid-intact photosensitive or photorefractory birds, and (2) r adiothyroidectomy before and after photostimulation and during photore fractoriness were examined. Chronic administration of pharmacological doses of thyroxine induced testicular growth and usually regression in initially photosensitive birds held on short or intermediate daylengt hs. Some thyroxine-treated birds with regressed testes were absolutely photorefractory, but most remained photosensitive. Exogenous thyroxin e never induced testicular growth in photorefractory birds moved to sh ort days, though it often impeded, and sometimes even blocked, the rec overy of photosensitivity. Although circumstantial, these effects of e xogenous thyroxine are consistent with an hypothesis that assigns to t hyroid hermones two roles - one stimulatory and the other inhibitory - in the control of seasonal reproduction. Radiothyroidectomy before ph otostimulation inhibited (but did not prevent) photoinduced testicular growth, blocked spontaneous testicular regression, suppressed molt, a nd prevented photorefractoriness. Moreover, as demonstrated by testicu lar growth after thyroxine replacement therapy, radiothyroidectomy dur ing photorefractoriness later restored photosensitivity despite contin ued photostimulation. Thus, euthyroidism is an essential condition for maximizing (but not for initiating) photoinduced testicular growth an d for triggering and maintaining photorefractoriness in photostimulate d tree sparrows. However, when performed early during photostimulation , radiothyroidectomy neither immediately induced nor later blocked spo ntaneous testicular regression. Thus, endogenous thyroid hormones and long days may interact during a critical period to program a sequence of physiological events that plays out as photorefractoriness in chron ically photostimulated birds. Such an organizational event cannot be p ermanent, for seasonal reproduction is episodic and its control mechan ism necessarily cyclic. Because thyroidectomy simulated the well-known restorative effect of short days (and exogenous thyroxine impeded it) , short days may dissipate photorefractoriness by creating a milieu wh erein thyroid hormones are deficient or inactive.