This feature gives the giraffe the ability to reproduce year-round and to withstand drought better than smaller animals.
But a long neck comes at a price: a giraffe's heart must generate enormous pressure to pump blood to its head, with its blood pressure reaching over 200 mmHg (more than double that of most mammals). As a result, a giraffe's heart consumes more energy at rest than a human heart, and indeed more than the heart of any other mammal of its size.
A new study, published in the Journal of Experimental Biology , has shown that giraffes' long legs act as an unexpected aid to their hearts in the face of gravity. Raising the heart closer to the head reduces the energy required to pump blood, saving about 5% of the energy from food consumed throughout the year (equivalent to more than 1.5 tons of food, a difference that could be life-or-death in the savanna).
In the study, researchers compared the cost of pumping blood in an adult giraffe to that of a fictional creature called "Elavi," which had shorter legs and a longer neck to reach the same tree height. The model showed that Elavi used 21% of its total energy to power its heart, compared to 16% for the giraffe and 6.7% for humans, illustrating the importance of longer legs in reducing the strain on the heart.
In his book "How Giraffes Work," zoologist Graham Mitchell revealed that giraffe ancestors had long legs before developing long necks. Long legs facilitate heart function, while lengthening the neck increases strain and energy expenditure.
However, this development carries risks, as giraffes are forced to spread their front legs while drinking, making them slow and vulnerable to predators, and may be the reason why they are the mammals most likely to leave a waterhole without drinking.
The report was prepared by Roger S. Seymour, Professor Emeritus of Physiology, University of Adelaide; and Edward Snelling, Faculty of Veterinary Medicine, University of Pretoria.