Brain temperature in pigeons: Effects of anterior respiratory bypass

Summary

During heat stress in domestic pigeons (Columba livia, mean mass 0.43 kg) brain temperature (T _B) varied in parallel with colonic temperature (T _c). The difference between these (T _C−T _B=ΔT) averaged 0.7°C and was not significantly altered when the animal breathed through a trachael cannula bypassing the buccopharyngeal cavity. When we sealed the nares and beak in bypass animals, ΔT was significantly reduced but was nevertheless maintained at 0.4°C. When the eyes were sealed as well, however, ΔT was reversed, amounting to −0.4°C. Conversely, with eyes sealed but beak and nares open, ΔT was indistinguishable from that in controls. These results suggest a role for the cornea in evaporative cooling, at least when respiratory evaporation is impaired, and are consistent with the hypothesis that buccopharyngeal and corneal evaporation are coupled to brain cooling. The probable mechanism for this coupling is the flow of venous blood from evaporative surfaces through theretia mirabilia in the temporal areas. Here heat is transferred from the warmer arterial blood flowing through theretia toward the brain to the centrally flowing, cooler venous blood.