Coupling between light- and food-entrainable circadian oscillators in pigeons.

Two experiments were conducted with a restricted feeding schedule to determine whether pigeons have a separate food-entrainable oscillator (FEO) and to assess the coupling strength between the FEO and the light-entrainable oscillator (LEO). In the baseline condition, the body temperature (Tb) and O2 consumption of two pigeons increased prior to light onset (LD 12:12 cycle) and food presentation (at the ninth hour of the light phase). In one experiment, when the LD cycle was phase delayed or advanced by 4 h while feeding time remained unchanged, the Tb and O2 rises prior to light-onset showed the expected delaying or advancing transients, but the rises prior to feeding also delayed or advanced for several days before returning to their proper phase position. In the second experiment, food was presented at 23.5-h intervals for 10 days while the LD cycle continued with a 24-h period. Entrainment to the LD cycle was unaffected, and Tb and O2 consumption continued to rise prior to the changing feeding time, but with a reduced lead time. When the feeding time was subsequently delayed by 5 h, Tb and O2 consumption with regard to the LD cycle were unaffected, but delaying transients occurred until both measures reentrained to the new feeding time. The results provide support for the existence of a separate FEO in pigeons and indicate asymmetrical coupling between the LEO and FEO, with the former having a stronger effect on the latter.