Why giraffes' necks aren't longer

Festival of Science

Scientists have discovered that the main thing that stops giraffes from growing longer necks - and so eating from even higher trees - is their hearts.

Giraffes, whose necks typically make up half their 5m (16ft) height, already have enormous hearts, which make up 2.3 per cent of their total body mass, compared to just 0.5 per cent in humans.

According to Professor Tim Pedley, of the University of Cambridge, the extra heart size is necessary to pump blood up the neck to the brain, and ensure that it still has some residual pressure when it leaves the skull, so it will flow down the jugular vein back to the heart.

Evolutionary theory would suggest that a giraffe which was taller than its peers and had no side-effects from it would have an advantage, since it should be able to reach food higher up. The fact that that has not happened suggests that long necks present a significant problem for the circulatory system. Professor Pedley and his co-workers have developed mathematical models to look at what that means.

"An important example is what happens when a giraffe wants to raise its head quickly after having its head down to drink. The prediction of our model is that it takes nearly 10 seconds for the usual upright steady state flow to be established." This implies that giraffes feel faint when startled. "We have only begun to think about the implications for the giraffe when it is startled and tries to escape," he said.