Avian brain cooling--a review

The capability of birds to keep the brain cooler than the body over a wide range of ambient temperatures is a major thermoregulatory characteristic enabling the defense of the central nervous tissue from overheating, heat storage and saving of water. Brain cooling is related to the presence of a paired extra-cranial vascular heat exchanger, the rete ophthalmicum. The combination of data from retial morphometry and from the experimental responses of brain temperature to changing environmental conditions provides a unique manifestation of the correlation between structure and function in thermal physiology. Recently I have used this key function as an index for the ontogenetic development of the avian thermoregulatory system. The findings suggest that neonates of different developmental patterns differ in the ontogeny of brain and body temperature regulation in association with the degree of precocity at hatching and the growth rate thereafter. However, they share a common, mass-dependent pattern of changes in the morphometry of the ophthalmic retia and a linear increase in brain cooling capacity with age.