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In the less heavily forested woodlands, animals travelling between trees are exposed to more heat from the sun.

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Peter Wheeler, an ecological physiologist from Liverpool’s John Moores University, has studied the heat stress the ancestral apes would have experienced as they moved through the wooded savannahs of Africa.

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His calculations show that an animal which walks upright receives up to a third less radiant heat from the sun, especially during the middle of the day when the sun is at its hottest.

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This is simply because less of the body surface is exposed to the direct rays of the sun when standing upright than when walking on all fours.

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It is a point intuitively obvious to sunbathers: they always lie down to expose as much of the body surface as possible.

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You’ll never get brown quickly standing up.

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Moreover, on two legs you benefit from the slight increase in wind speed that occurs above the surface of the earth.

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Friction from the vegetation and even the ground itself slows the wind down close to the earth’s surface in much the same way that a brake acts on a wheel.

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The increase in wind speed has a significant cooling effect from about three feet above the ground.

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Large animals of course benefit from this, but smaller animals can benefit too if they stand on their hind legs.

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Animals about the size of chimpanzees are in the narrow range of body size where standing upright is worthwhile.

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Smaller species like baboons are not tall enough for standing on two legs to make any difference.

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So by standing tall these apes kept cool, which enabled them to travel further into more open habitats in search for food.

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In the process, another device came into play.

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With less body surface exposed to the direct rays of the sun, there is less need of the fur that normally serves to keep animals’ skin cool.

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Fur is inert and a good insulator, so the outer tips of the hair can heat up dramatically without the heat being transferred to the body underneath.