Photoperiodic control of the timing of testicular regression in white-throated sparrows

Photosensitive white-throated sparrows were transferred from a nonstimulatory LD 8:16 light cycle to either a stimulatory LD 13:11 or LD 20:4 light cycle on Day O. While some birds remained on these stimulatory photoperiods throughout the study, other birds were transferred from LD 13:11 to LD 20:4 on Day 50, or from LD 20:4 to LD 13:11 on Day 30 after the testes had reached a width of about 4.0 mm. The testes of birds continuously exposed to LD 13:11 grew more slowly, reached a larger maximum size, and spontaneously regressed later than the testes of birds continuously maintained on LD 20:4. Spontaneous testicular regression occurred at the same time (between Days 90 and 120) in sparrows that were transferred from LD 13:11 to LD 20:4 on Day 50 as in sparrows that were maintained continuously on LD 13:11. Following testicular growth in sparrows that were exposed initially to LD 20:4, the transfer to a presumably photostimulatory LD 13:11 cycle induced rapid testicular regression suggesting that LD 13:11 was no longer being interpreted as a long day. These results suggest that in nature, the timing of spontaneous testicular regression which occurs after a prolonged exposure to photostimulatory long days may be (1) programmed by the LD cycle the bird is exposed to during gonadal growth, and/or (2) due to a decrease in day length after the summer solstice even though the length of the day is still longer than that which previously initiated testicular growth.