In the less heavily forested woodlands, animals travelling between trees are exposed to more heat from the sun.
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.
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.
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.
It is a point intuitively obvious to sunbathers: they always lie down to expose as much of the body surface as possible.
You’ll never get brown quickly standing up.
Moreover, on two legs you benefit from the slight increase in wind speed that occurs above the surface of the earth.
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.
The increase in wind speed has a significant cooling effect from about three feet above the ground.
Large animals of course benefit from this, but smaller animals can benefit too if they stand on their hind legs.
Animals about the size of chimpanzees are in the narrow range of body size where standing upright is worthwhile.
Smaller species like baboons are not tall enough for standing on two legs to make any difference.
So by standing tall these apes kept cool, which enabled them to travel further into more open habitats in search for food.
In the process, another device came into play.
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.
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.