Why don’t racing horses fry their brains?

Air sacs bulging from a horse’s hearing system may solve the mystery of how such an athletic animal cools its brain during exercise without the standard anatomical gadgetry, argues an international team of researchers.

In gazelles, cheetahs, dogs, and a herd of other zoological athletes, brain cooling depends on a structure called the carotid rete mirabile, explains Keith E. Baptiste of the Danish Veterinary Laboratory in Copenhagen. In the rete, hot blood surging from the heart via the carotid artery flows into smaller arteries surrounded by cool blood returning from the nose and face.

Horses lack a rete. In the Jan. 27 Nature, Baptiste and Canadian colleagues propose new evidence for one of the more novel notions of what horses use instead of a rete: guttural pouches, or lumpy sacs of auditory tubes around the internal carotid arteries.

“People have had lots of weird and wacky ideas of what [guttural pouches] might do,” says Baptiste, such as enhancing vocalization or hearing, or buoying the head during swimming. 

Never mind that horses don’t vocalize, hear, or swim better than animals without pouches. Analyzing the pouches in discarded carcasses gave Baptiste the cooling idea. “Nobody would believe us—we only had dead horses,” he says. Finally he persuaded the University of Saskatchewan in Saskatoon to let a research team implant sensors into arteries in live horses.

“The first horse was the most exciting,” Baptiste recalls. The project struck some prominent veterinarians as a waste of funds, but the school consented as long as the first horse yielded good data. A dozen bystanders gathered around a barn treadmill when Baptiste tested a little gray Arabian named Dusk. As Dusk worked up a sweat, data from three points on the artery showed a temperature drop along the path around the pouch. Baptiste’s project was saved.

After testing four horses, the researchers attribute up to 2ºC of cooling to the pouch. “It’s possible,” comments Finola McConaghy of Nature Vet in Richmond, Australia, who first showed brain cooling in horses. Baptiste’s idea doesn’t immediately grab her, she says, because “I have always understood that the guttural pouches are not well ventilated during exercise.”

She prefers the idea that blood returning from the nose through a sinus cavity cools the brain directly. Perhaps both mechanisms work, suggests brain-cooling pioneer Mary Ann Baker of the University of California, Riverside. She too has favored the sinus-cooling mechanism, because she wonders whether the carotid artery has the surface area for sufficient cooling. However, Baker says she hopes Baptiste will make more detailed measurements since he’s raised “a very interesting possibility.”

The view of brain cooling changed in the past decade, she notes, after Claus Jessen of the University of Giessen in Germany showed that goat brains survive higher temperatures than predicted.

He proposes that brain cooling mainly fools the body’s thermostat and delays cool-down mechanisms that demand a lot of water. Cooling might be less important to prevent frying than to prevent sweating and panting.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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