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Sheep use their brains to save water

Cool sheep Sheep can reduce water loss by using a heat exchange system in their brain to cool down their blood, new research suggests.

The animals can save up to 80 per cent of their daily water intake by using this system, says comparative physiologist, Dr Shane Maloney from the University of Western Australia.

Previous research has shown that sheep cool blood flowing into their brains using a structure called a carotid rete at the base of the brain.

In this structure, the artery with blood flowing towards the brain divides into many fine blood vessels about 50 microns in diameter.

Heat in warm arterial blood can transfer out of these fine arteries into cooler venous blood that also passes by the rete. The venous blood, travelling down the body, has been cooled by evaporation in the sheep's nose.

The common wisdom has been that this cooling system is used to protect the sheep's brain from overheating.

But a discovery in a study of South African black wildebeest in the 1990s didn't fit this theory.

While data loggers on the wildebeest showed that brain cooling was often switched on through the day, it was switched off when the animals were at their hottest when being chased by researchers in helicopters.

The fact they were hot but not using brain cooling led to the idea that brain cooling serves another purpose than protecting the brain.

An alternative theory suggests the system is actually a water-saving mechanism that works by cooling the hypothalamus, the body's thermostat, which sits at the base of the brain.

When the hypothalamus is hot it triggers panting and sweating, which helps the animal to lose heat through evaporation.

"By switching on this brain cooling mechanism you lower the hypothalamic temperature and therefore you don't stimulate water use for thermoregulation (panting and sweating)," says Maloney.

Hot sheep

To test this hypothesis, Maloney and colleagues from the University of the Witwatersrand in South Africa studied nine sheep, which naturally varied in the percentage of time they spent brain cooling during the day.

The animals spent six hours a day for eight days in a room heated to 40°C, and were given respite each night when the temperature was reduced to 22°C.

"It was a bit like a Perth summer," says Maloney.

The sheep were deprived of water for the last five days (Maloney says sheep are known to go for a week without water).

Throughout the experiment the researchers measured water loss, and brain and body temperatures every five minutes.

Their findings, reported in a recent issue of PLOS ONE, showed the longer an animal went without water, the more time it spent with its brain cooling system switched on.

This led to a greater difference between the animal's body and brain temperatures, and decreased the amount of water lost.

This is the first study of its kind to lend support to the hypothesis that brain cooling saves water, says Maloney.

He and colleagues calculate that a 50-kilogram sheep can save 2.6 litres of water a day, which is about 60 per cent of its daily water intake, by using this system for 50 per cent of the day.

For an animal using the system 80 per cent of the day, this amount would increase to 3.54 litres a day, or 80 per cent of normal water use.

Need for calm

Maloney says there is some evidence that the more stressed an animal becomes, the less amount of brain cooling it uses.

Brain cooling is switched off by the sympathetic nervous system which in turn is triggered by stress.

This explains why the wildebeest in the 1990s South African study had their system switched off even though they were hot -- they were stressed due to being chased by helicopters. In this situation it was more important for them to lose heat than to save water.

"Whereas if you've got a nice calm docile animal ... they don't switch on the sympathetic nervous system, selective brain cooling is activated so they lower brain temperature and don't pant and sweat."

The researchers want to test the link between temperament and brain cooling in flocks of specially-bred sheep.

"We want to measure water turnover in calm and nervous animals," says Maloney.

He says this line of research could help in the breeding of animals that are able to do better in the heat.

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