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) whether light-active species like most birds can synchronize with food cycles in the presence of a strong light-dark (LD) cycle and ii) whether it is common among non-mammalian vertebrates to use a separate circadian oscillator to synchronize with food cycles as most mammals do. We investigated these questions experimentally by exposing house sparrows simultaneously 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 different phases of the LD cycle. The effects of the two zeitgebers were analyzed by observing two behavioral outputs of the birds' circadian system, the rhythms of locomotion and feeding. The data revealed that light acted as the dominant zeitgeber in most conditions. Food cycles affected the phase of the behavioral rhythms of the birds only when the food was presented no later than 3 h after the onset of light. Apart from 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 and food. We propose that light is the stronger zeitgeber and plays a 'permissive' role in determining the phases at which synchronization with food cycles comes into effect. We did not find evidence that the house sparrows' behavioral rhythms are controlled by a food-entrainable circadian oscillator that is distinct from the light-entrainable system as is the case in most mammals. The differences in the patterns of food synchronization and organization of the circadian system that appear 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, species with different ecological requirements show a specialization in their circadian organization which is adjusted to the importance of zeitgebers in nature.