A new study by researchers at the University of Pretoria (UP) and the University of Adelaide in Australia has revealed the real function of the giraffe’s long legs ­– to reduce blood pressure.

The study, which was published in the Journal of Experimental Biology, was conducted by Professor Edward (Ned) Snelling, an experimental physiologist from UP’s Faculty of Veterinary Science, and Dr Roger Seymour, a comparative physiologist from the University of Adelaide. It shows that by having long legs, the giraffe is able to reduce the pressure it needs to get blood to its brain, thereby saving the animal energy.

“There are few sites as striking as a giraffe browsing the acacia canopy head and shoulders above all other animals,” Professor Snelling says. “The giraffe’s long neck allows it to access foliage that is out-of-reach for shorter animals, it is also used for fighting, and it provides a high vantage point from which to scan for predators. However, there are also significant energy costs associated with having such a long neck. That energy cost is in the form of blood pressure. And it is a cost that must be paid by the heart.”

In humans, the average blood pressure is around 100mmHg and the heart consumes about 6.7% of daily energy intake just to keep the blood moving around the body, the researchers explain. But in a giraffe, a normal, healthy blood pressure is about 200 to 250mmHg and its heart uses about 16% of its total energy. In fact, the heart of a full-size giraffe uses more energy than the entire body of a resting adult human.

“That’s because the giraffe’s heart has do battle with gravity to develop the pressure needed to get blood to its brain,” Professor Snelling explains. “And because its brain can be more than 2m above its heart, it must develop very high pressures to avoid fainting – and it’s a long way down for a giraffe.  So anything a giraffe can do to lower its blood pressure and save energy, while still getting enough blood to its brain, is going to be a big advantage for the animal. This is where long legs come into the story.”

To understand the trade-offs of having a long neck versus having long legs, the researchers created a computer simulated “elaffe”, a half-giraffe, half-eland creation that enabled them to model the blood pressure and energy costs of animals of different sizes and shapes.

“We have not become Frankensteins by genetically engineering an elaffe in the lab,” Professor Snelling quips. “The elaffe exists only as a series of mathematical computations on our laptops.”

Professor Seymour adds that the elaffe is “essentially the neck of a giraffe attached to the body of an eland, but the neck has been stretched so that the elaffe and giraffe are the same height”. Using the elaffe, the pair modelled how much energy the giraffe saves by achieving its height not only by having a long neck but also by having long legs.

The results of these analyses reveals that by having long legs, the giraffe is able to reduce the pressure it needs to get blood to its brain, saving it energy. Long legs allow the giraffe to achieve height without further increasing an already very high blood pressure and energy burden for the animal. A giraffe’s heart uses about 16% of its body’s energy; in the elaffe, it increases to 21% – so if the giraffe had achieved its height by lengthening its neck alone, it would need to find an additional 3000kJ of energy from food every day. 

“The savings add up to about 1.5 tonnes of food every year, which can be the difference between life and death for a giraffe just managing to scratch a living,” Professor Snelling says. “The long legs effectively raise the heart and bring it closer to the brain, saving the giraffe from even higher energy costs it already has to manage because of its long neck.”

Interestingly, the ancestors of giraffes had long legs before they evolved long necks. “This makes energetic sense, because long legs can only save energy, while a long neck will cost energy, especially if that neck is held erect like a giraffe’s is,” Professor Seymour says.

Long legs save energy not only by reducing the pressure needed to get blood to the brain in an animal of a given height, but they also make locomotion more efficient in terms of the energy needed to move a unit mass over a given distance. However, long legs come with a price: a giraffe cannot gather the acceleration needed to outrun a lion. It is also forced to splay its forelegs while drinking, which makes it awkward to rise up and escape a predator. Statistics show that, of all prey mammals, giraffes are most likely to leave a waterhole without drinking from it.

“Ultimately, the energy saved by the giraffe, thanks to its long legs, can be diverted to other important tasks, like fighting and reproducing,” Professor Snelling says.

Click on the infographic in the sidebar to learn more about giraffes' blood pressure and click on the image in the sidebar to view the elaffe.