Those of us who have grown-up watching Giraffe don’t realise it, but they really are weird looking animals. Most of us have been told since childhood that the incredible height of a giraffe (up to 5.5m or 18 feet!) is an adaptation to allow it to reach food material beyond the reach of other browsers. Personally I’m not convinced that this was the driving selective force for that great height. I think that excellent vision combined with great height provide giraffe with their best predator-avoidance mechanism and it is this advantage that is the selective basis for their exceptional height.
Clearly the benefits must be considerable, because there are enormous anatomical and physiological challenges that arise as a result of this giant stature. In order to get blood to their brain, giraffe have massive hearts weighing up to 11 kg and generating twice the blood pressure of a human heart. In order to avoid pooling of blood in their feet resulting from the great blood pressure combined with gravitational force, they have very thick tight skin around their lower legs which acts exactly like compression stockings used medically in humans.
It is all very well to benefit from great height but in the bush water is always at the level of one’s feet and this requires giraffe to get their head all the way down there. When they are drinking giraffe no longer have the advantage of a great vantage and in order to physically get their heads that far down they must spread their legs awkwardly wide apart. In an emergency it takes them a long time to get back-up, turn and gallop away.
Furthermore with their head now down at the level of their feet and the enormous blood pressure now pushing blood downwards to their brain, they would certainly black-out were it not for the fact that the artery delivering blood to the brain divides-up before reaching the brain into a network of smaller vessels thereby decreasing the incoming blood pressure to the brain.
In addition when a giraffe is drinking, the blood in the jugular vein that was supposed to go to the heart would flow back from the heart to the brain if it were not for a set of seven one-way valves.
Clearly being an elegant magical creature is not as simple as it looks!