Giraffes’ long legs reduce heart’s energy burden

A study by researchers from the University of Pretoria and the University of Adelaide has found giraffes’ long legs play a crucial role in managing the high blood pressure required to pump blood to their brains.

Giraffes are known for their towering necks which help them reach high leaves, fight for dominance and scan for predators. According to the study, that height comes with a significant physiological cost as the hearts must generate exceptionally high pressure to push blood all the way to the brain.

The study shows giraffe’s long legs help offset the cost. By lifting the animal’s body higher off the ground, the legs reduce the pressure the heart needs to generate, ultimately saving energy.

The study, published in the Journal of Experimental Biology, was conducted by Dr Edward 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, a giraffe is able to reduce the pressure it needs to get blood to its brain, thereby saving the animal energy.

Snelling said there are few sights as striking as a giraffe browsing the acacia canopy head and shoulders above all other animals.

“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 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,” Snelling said.

According to the study, in humans the average blood pressure is around 100mmHg and the heart consumes about 6.7% of daily energy intake to keep blood moving around the body. In a giraffe a normal, healthy blood pressure is about 200mmHg to 250mmHg and its heart uses about 16% of its total energy.

The heart of a full-size giraffe uses more energy than the entire body of a resting adult human.

The heart of a full-size giraffe uses more energy than the entire body of a resting adult human.

Snelling explained that it is because the giraffe’s heart has to battle with gravity to develop the pressure needed to get blood to its brain.

“Because its brain can be more than 2m above its heart it must develop very high pressures to avoid fainting. Anything a giraffe can do to lower its blood pressure and save energy while getting enough blood to its brain is a big advantage. This is where long legs come into the story,” said Snelling.

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 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. The elaffe exists only as a series of mathematical computations on our laptops.” Snelling said.

Seymour said the elaffe is “essentially the neck of a giraffe attached to the body of an eland, but the neck has been stretched so the elaffe and giraffe are the same height”.

Using the elaffe, the two modelled how much energy the giraffe saves by achieving its height, not only by having a long neck but also by having long legs.

Long legs allow the giraffe to achieve height without further increasing a very high blood pressure and energy burdenl. A giraffe’s heart uses about 16% of its body’s energy. In the elaffe it increases to 21%, so if the giraffe achieved its height by lengthening only its neck, 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 managing to scratch a living,” Snelling said.

Seymour said the long legs effectively raise the heart and bring it closer to the brain, saving the giraffe from higher energy costs it 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 the neck is held erect like a giraffe,” said Seymour.

Snelling said the energy saved by the long legs can be diverted to other important tasks such as fighting and reproducing.

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