Photoperiodism in House Sparrows: Testing for Induction with Nonphotic Zeitgebers

Abstract

In birds, as in other animals, the mechanism for photoperiodic time measurement is based on circadian rhythmicity. In models of this mechanism, the entrainment of circadian rhythms is a key feature. One can therefore ask whether any synchronizing agent (zeitgeber) that can entrain circadian rhythms could also induce photoperiodic responses. We addressed this question using known nonphotic circadian zeitgebers (playbacks of conspecific vocalizations, and nonspecific acoustic disturbance) in house sparrows. We subjected photosensitive male sparrows to stimulatory photoperiods (2L: 10D:2L: 10D and 12.5L: 11.5D) and to similar "phonoperiods" in which part or all of the light of the photoperiods was replaced by a combination of conspecific vocalization playbacks and nonspecific acoustic disturbance. Direct observation with an infrared visionscope revealed that almost all birds remained alert during the sound presentation. Their circadian rhythms were also entrained to the sound. However, whereas sparrows exposed to stimulatory photoperiods experienced testicular growth (from 6 mg to 311 mg), sparrows exposed to the corresponding phonoperiods did not. These results suggest that nonphotic zeitgebers cannot mimic the photoperiodic effect of light. Conceivably, however, photoperiodic-like effects might still be obtained with stronger nonphotic zeitgebers, as yet undiscovered in birds, or with nonphotic zeitgebers whose action would reveal itself only when the zeitgebers occurred at circadian phases other than those at which light is effective.