By  S. Milius

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 fete. 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 2 [degrees] C 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.

Courtesy Science News