A One-Time Injection of Thyroxine Programmed Seasonal Reproduction and Postnuptial Molt in Chronically Thyroidectomized Male American Tree Sparrows Spizella arborea Exposed to Long Days

Abstract

Chronically thyroidectomized male American Tree Sparrows were injected with 100 ?tg L-thyroxine (T4) Or with alkaline vehicle either on the first day of photostimulation or 1 week before beginning photostimulation. When exposed to long days, control birds of both experiments showed slight, but statistically reliable, photoinduced testicular growth. That was also true for T4-injected birds moved to long days I week postinjection. Thus, chronically (like acutely) thyroidectomized Tree Sparrows retain the capacity to measure daylength. Tree Sparrows injected with T4 on the day they were moved to long days exhibited robust testicular growth similar to that observed in photostimulated euthyroid Tree Sparrows. Testicular growth later gave way to regression, and beginning at week 7, both T4-injected and control birds were exposed to constant light and given replacement T4 in drinking water for 5 weeks in order to assess their reproductive states. Six of eight T4-injected birds tested photorefractory and, during the assay, began a postnuptial-like molt of the primary remiges. The nine control birds and remaining two T4-injected birds tested photosensitive and did not initiate a postnuptial-like molt. Similarly, all T4-injected and control birds retained 1 week on short days before beginning photostimulation were still photosensitive after 7 weeks on long days. With an estimated half-life of about 4 h, the possibility that the programming effects of injected T4 were activational (i.e. transient and temporally linked to T4 availability) is remote. Accordingly, we conclude that injected T4 programmed chronically thyroidectomized, photostimulated male Tree Sparrows for seasonal reproduction and postnuptial molt by exerting organizational-like actions on the relevant photoperiodic control circuits.