Photorefractory Harris' sparrows ( Zonotrichia querula) exposed to a Winter-like daylength gradually regain photosensitivity after a lag

Abstract

The time course of regaining photosensitivity was monitored in intact and in castrated Harris' sparrows ( Zonotrichia querula) held on short days. Measured endpoints, assumed to be equivalent indicators of the photosensitive state, were photoinduced testicular growth in intact males and a photoinduced elevation in plasma LH (luteinizing hormone) concentration in castrated males. Intact males were initially photorefractory when they were moved from long days to short days, and they remained so for at least 3 weeks thereafter. Photosensitivity was partially restored between Weeks 3 and 5 on short days and then gradually increased to a maximum by Week 13. No further change was detected through Week 25 on short days, when the experiment ended. Castrated males, also photorefractory when moved from long days to short days, regained photosensitivity as early as Week 7 on short days, but not uniformly until Week 16. As reflected by linear regression analysis, recovery of photosensitivity was a gradual process through Week 22 on short days, when the experiment ended. Discrepancies between intact and castrated males raise the possibility that the measured endpoints are not equivalent indicators of the photosensitive state. In accordance with an earlier report ( Wilson, 1990), recovery of photosensitivity in castrated Harris' sparrows held on short days was not signaled by a spontaneous increase in plasma LH concentration. Indeed, plasma LH concentrations of castrated males remained suppressed even after photosensitivity had been fully restored in intact males.