A constant 15-hr daily photoperiod led to a prolonged reproductive phase in the female tropical weaver finch Ploceus philippinus. Thyroxine treatment (1.4 and 2.8 μg on alternative days for 5 mo) completely suppressed follicular development, and ovarian and oviducal weight, but had no effect on the liver and body weight in birds maintained on 15-hr daily photoperiods. A "thyroid-sensitive" hypothalamo/hypophyseal/gonadal axis, as exhibited by this bird and many other birds of mid and low latitudes where the degree of insolation is large, may serve as a safety mechanism against unseasonal gonadal growth and hence might be of adaptive significance in tropical photoperiodic finches. © 1974.
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