Physiological Basis of Repeated Testicular Cycles on Twelve-Hour Days (12L 12D) in White-Crowned Sparrows Zonotrichia leucophrys gambelii

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Abstract
The testes of male white-crowned sparrows, Zonotrichia leucophrys gambelii, subjected to a photoregimen of 12-h days (12L 12D) undergo quasiannual cycles of repeated growth and regression. These cycles decrease successively in period and amplitude and are not accompanied by postnuptial molts. This suggests that they are not physiologically equivalent to those of birds subjected to natural photocycles. Our results demonstrate that transfer of white-crowned sparrows undergoing testicular regression on 12L 12D to long days (20L 4D) induces both testicular growth and increases in plasma levels of luteinizing hormone at normal rates for 20L 4D. These data indicate that testicular regression on 12L 12D must be induced by mechanisms other than the development of photorefractoriness, which is the normal cause of regression on natural photocycles. This suggests very strongly that daylength functions as a driver of the independent components of the annual cycle rather than, as in some other species, a Zeitgeber for an endogenous circannual cycle.