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

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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.