Testicular Responses of House Sparrows and White-Crowned Sparrows to Short Daily Photoperiods with Low Intensities of Light

Abstract

Male Passer domesticus were subjected to daily photoperiods of 2, 4, 7, and 20 hrs. The positive responses to 2, 4, and 7 hrs. confirm the earlier findings of Threadgold and with the much greater responses to 20 hrs. show that the photoperiodic testicular response as a function of the length of the daily photoperiod is parabolic with a minimum response at 7 - 8 hrs. Male Zonotrichia leucophrys gambelii showed no response whatsoever to daily photoperiods of 2, 4, and 7 hrs. under identical conditions but showed characteristically rapid responses to 20-hr. photoperiods. This suggests that the photoperiodic testicular response mechanisms of these 2 species are fundamentally different.