As pigeons swoop into accelerated flight, the air rushing past their eyes selectively cools their brain, its air blast effectively employing corneal convection to maintain their brain temperature below their body temperature. This situation is especially present in flight when internal heat production can exceed resting heat by 10 times or more, causing body temperatures to rise by several degrees C.
I have come across the interesting article titled: Regulation of brain temperature in pigeons; effect of corneal convection by B Pinshow, MH Bernstein, GE Lopez and S> Kleinhaus American Physiological Society 1981 R577-R581 as part of my on going series of essays on how nature achieves brain cooling as an object strategy through a variety of birds, mammals and even dolphins.
Thermogram of the hot eye of a dolphin.
Thermoregulation in dolphins goes in another direction due to the variation of a water temperature environment especially during deep dives.
But back to pigeons. Because the pigeon during flight can store more core body heat to levels above their brain’s tolerance limit, active brain cooling adjusts to this thermal gradient demand between body and brain.
The investigators used very thin insulated copper-constantan thermocuples positioned within their colons and in the pre-optic hypothalamus, confirmed following post mortem inspection. Each thermocouple was calibrated to an accuracy of +-0.05C. The implanted eye thermocouples were teflon coated copper-constantan wires, 0.1 mm in diameter. Once healed after the thermister surgery, the pigeons eye movements and blinking occurred at normal frequencies.
“The model offered to explain avian-brain temperature differences holds that heat is conducted away from relatively hot arterial blood flowing toward the brain to cooler venous blood returning from the evaporative surfaces of the head. The heat transfer occurs in the ophthalmic retia located bilaterally in the temporal regions of the skull in all birds so far investigated. In many mammals the analogous carotid rete at the base of the brain serves a similar function. The anatomy of the avian and mammalian structures indicates that heat transfer is facilitated by the large surface contact between arterioles and venules interchanging within countercurrent flow. An additional portion of the venous flow in the rete appears to arise from the eye. The ocular vessels, including those in the iris, drain to the orbital veins, which lead in tern directly to the ophthalmic rete. It is therefore possible that part of the cooled occular blood perfuses the rete to contribute to cooling of arterial blood.”
The experiments elegantly revealed that wind flow at 18 m/sec affected brain temperature by cooling with open eyes, but not when the eyes were covered. If a vasoconstrictor, phenylephrine, available usually as an over the counter decongestant, knocks out temporarily by constricting the corneal blood vessels response. Because phenylephrine is a direct selective α-adrenergic receptor agonist, it does not cause the release of endogenous noradrenaline, as pseudoephedrine does. Phenylephrine’s effectiveness as a decongestant stems from its vasoconstriction of nasal blood vessels, thereby decreasing blood flow to the sinusoidal vessels, leading to decreased mucosal edema. Phenylephrine given to a pigeon’s cornea caused a marked decrease measured as intraocular temperature but not brain temperature. “This could be explained by vasoconstriction, observed as an immediate marked reduction in iridial engorgement that undoubtedly reduced ocular blood flow and heat delivery.” The authors speculate that compensation cooling occurred probably in the buccophayngeal cavity.
“From the available evidence, it is possible to speculate that venous blood returning from the buccophayrngel cavity (the bird’s mouth) he eyes, and other cranial cooling surfaces may flow to the opthalmic retia in parallel. The resistance to heat loss of each of these cooling surfaces may then be independently controlled via vasomotor adjustments so that the venous admixture would arrive at the rete at a temperature appropriate for the degree of arterial cooling.”
So the next time you watch a pigeon in flight think of a sophisticated brain cooling apparatus in operation as air moves over the pigeon’s cornea.
The eye of the dolphin (Tursiops truncatus) is also a major heat loss in its cool water environment, evidence seen in the eye thermogram shown as high thermal emission color when compared to adjacent skin areas. The anatomical blood vessel density behind the dolphin eye stresses the numerous multivessel plexuses covering the bigger portion of the back of the eye of the opthalmic rete acts effectively like a radiator to prevent heat loss from the retina and optic nerve. The tangle of blood vessels within the rete is a huge arterial mass. This anatomical arterial spaghetti distinguishes the dolphin from terrestrial mammals.
Veterinary Opthamology (2007) 10, 4 231-238 Functional anatomy of the ocular circulatory system: vascular corrision casts of the cetacean eye by H Ninomiya and E Yoshida
“Such rich retinal vasculature may compensate for a decrease in arterial blood supply, thereby enabling relative stability within a physiological range of arterial blood pressure and oxygen during steep dives. The rete is bathed in venous blood from the orbit, relating to thermoregulation by the countercurrent heat exchange mechanism by heating the eye during a deep dive.”
The authors did not have the luxury of measuring the eye temperature plus the brain/hypothalamus temperature compared to the dolphin colon temperature which was the case for the pigeon brain cooling observation. So we still do not know what happens to eye temperature compared to brain temperature for a diving dolphin. But what is clear is the existence of an exquisite thermoregulation between eye and brain of the dolphin. So if you ask the query: does Nature cool the brain? The mammalian evolutionary answer appears to be accomplished in a large variety of ways.