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Wednesday, 26 January, 2000, 19:00 GMT
Horses have air-conditioned brains


Horses have air-cooled brains to stop them overheating when galloping at full speed, say Canadian scientists.

They have an unusual anatomical arrangement at the base of their skulls that acts to cool the blood entering the brain, the researchers report in the journal Nature.

Athletic animals like horses must keep their heads cooler than 40degC during vigorous exercise or risk permanent brain damage. But how horses do this has long been a mystery, as they appear to lack any of the usual temperature control devices found in other animals.

Now, Keith Baptiste and his colleagues at the University of Saskatchewan, Saskatoon, Canada, believe they have found the answer.

The internal carotid arteries taking blood to the brain, the researchers find, are enveloped by a pair of pouches containing about 300 to 500 ml (just under a pint) of air that comes from the respiratory system. As the horse works up a sweat, heat in the blood is transferred across to the air in the pouches.

Air bags

Similar pouches are also found in other horse-like animals such as zebras and donkeys, also in tapirs, some six species of bats, and even a South American forest mouse.

The pouches were first discovered in horses in 1756 and there have been many theories put forward since then to explain their function.

"Someone in the 1800s even thought they were there to help horses to swim by acting as air bags to keep the animals' heads above water," Keith Baptiste told BBC News Online. "But horses never evolved to swim, they're not very good at it, and so it's hard to believe the pouches evolved for something like that."

The pouches must have some useful function, the veterinary scientist said, because the horse takes a big risk by having them.

"They are prone to bacterial and fungal infections that can prove fatal. So the device has to do something."

Temperature probes

The team used probes to measure the changes in temperature at three different points as the blood moved through the arteries, over the pouches, and into the brain. Four animals were implanted with the probes and put through a series of exercises on a treadmill.

This showed clearly that heat was lost as the blood flowed across the pouches.

"It depends on what type of exercise the horse is going through, but we found a temperature difference of up to two degrees from the first probe to the last."

Keith Baptiste, who has now moved to the Danish Veterinary Laboratory in Copenhagen, Denmark, said the discovery would force many to reverse their thinking on horse anatomy.

"Before this research, my theories on this were based on dead horses and I was subjected to tremendous criticism and no-one would believe me. Now, with this paper, a lot of people are very happy with it."

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