For many years, researchers wondered why this does not harm the giraffe’s brain when it bends down to drink or graze. The biologist Tobias Wang (b.1967) and the doctor Christian Aalkjær (b.1954), both at Aarhus University, found an explanation for this problem when they analysed the giraffe’s unique circulatory system.

As a researcher, Wang has always been fascinated by exotic animals, particularly snakes, as well as basic research and sought to find the explanations behind nature’s phenomena. As for the giraffe, researchers have long debated why it has a long neck.

Is it the evolutionary result of competition for food, because a long neck enables the giraffe to reach leaves at the tops of trees, or is it a symbol of sexual potency that makes it easier for it to attract a mate? Wang believes the answer is probably a bit of both.

The giraffe needs the high blood pressure to supply blood to its brain, which is two metres higher than its heart and must therefore overcome gravity to pump the blood all the way up through its long neck. If the pressure was lower, the brain would not receive sufficient blood when the giraffe is upright, and the animal would faint – or worse.

Even for a giraffe, however, this high blood pressure is not without its challenges. One major issue is what happens when the giraffe bends down, thus suddenly placing its brain lower than its heart. All things being equal, we would expect the pressure in the brain to rise precipitously at the risk of causing brain damage.

In 2020, Wang and Aalkjær, working with a large research team, showed that the giraffe not only has a very high blood pressure; it also has a circulatory system that is fine-tuned to the animals’ height. While the high pressure is necessary for bringing blood all the way up to the brain, special mechanisms in the legs protect the blood vessels and tissue from excess strain.

When the giraffe bends down, its veins and heart quickly adapt: blood accumulates in the jugular veins, and the heart pumps out less blood, alleviating the rise in blood pressure that would otherwise would have occurred when the head is lowered all the way to the ground. When the giraffe then raises its head, it avoids fainting because blood vessels in the brain expand, and pressure-sensitive sensory cells keep the blood pressure up until the balance has been restored.

At a cellular level, Aalkjær’s studies showed that the giraffe’s blood vessels – particularly in its legs – have unusually thick, strong walls. This enables them to withstand the high pressure without being damaged. Furthermore, the vessels can actively contract and relax to distribute the blood properly throughout the body. 

The research shows that the giraffe’s circulatory system is not just dependent on a heart with a strong pump function but also on the close interaction with the body’s ability to regulate the blood flow.

The study of the giraffe’s blood pressure has had both scientific and practical implications. It has given zoologists and zookeepers new knowledge to enable improved care for captive giraffes. Particularly during anaesthesia and surgery, it is essential to position the head correctly and to monitor the blood pressure closely, as sudden changes in pressure can lead to circulatory collapse or brain bleeds. The study of the giraffe’s extreme circulatory system also illustrates how evolution can solve seemingly unsurmountable physiological challenges.

This example reminds us that basic research into the quirks of nature can have implications far outside the animal world, including for the human understanding of body and health. 

The chapter is written by Kristian Sjøgren.