ADJUSTMENT OF HOUSE SPARROW CIRCADIAN-RHYTHMS TO A SIMULTANEOUSLY APPLIED LIGHT AND FOOD ZEITGEBER

Periodic food availability has been shown to be an effective circadian zeitgeber in many vertebrates. It is still unclear, however, i) wheth er light active species like most birds can synchronize with food cycl es in the presence of a strong light-dark (LD) cycle and ii) whether i t is common among non mammalian vertebrates to use a separate circadia n oscillator to synchronize with food cycles as most mammals do. We in vestigated these questions experimentally by exposing house sparrows s imultaneously to two zeitgebers: light and food. The LD cycle was set at 1410 h; food was always available for 12 hour per day, but at diffe rent phases of the LD cycle. The effects of the two zeitgebers were an alyzed by observing two behavioral outputs of the birds' circadian sys tem, the rhythms of locomotion and feeding. The data revealed that lig ht acted as the dominant zeitgeber in most conditions. Food cycles aff ected the phase of the behavioral rhythms of the birds only when the f ood was presented no later than 3 h after the onset of light. Apart fr om their synchronizing actions both light and food cycles also exerted direct (masking) effects on the behavioral rhythms of the birds. The results suggest that the circadian system of house sparrows can indeed adjust to two simultaneous environmental periodicities, i.e. light an d food. We propose that light is the stronger zeitgeber and plays a 'p ermissive' role in determining the phases at which synchronization wit h food cycles comes into effect. We did not find evidence that the hou se sparrows' behavioral rhythms are controlled by a food-entrainable c ircadian oscillator that is distinct from the light-entrainable system as is the case in most mammals. The differences in the patterns of fo od synchronization and organization of the circadian system that appea r to exist between different species can be interpreted in two ways: i ) species of different phylogenetic origin (e.g., mammals versus birds ) evolved different circadian system or ii) regardless of phylogeny, s pecies with different ecological requirements show a specialization in their circadian organization which is adjusted to the importance of z eitgebers in nature. (C) 1997 Elsevier Science Inc.