The giraffe is one of the most extraordinary animals that inhabit our planet. It can reach six meters in height, with a neck of over two meters and tapered legs of the same length. Such a strange body requires equally unique anatomical devices to function properly. And a new study published on Journal of Experimental Biology reveals one that has been underestimated until now: the importance of long legs for the evolution of these creatures, which allowed them to stand out above the canopy of trees while maintaining blood pressure, and the resulting caloric expenditure, relatively low.
An extra large neck
Obviously, the most peculiar feature of giraffes is the length of their extra-large neck, which has no equal in the animal world. In this case, the evolutionary usefulness of this anatomical structure has been known for some time: thanks to their long neck, giraffes are in fact the only savannah animals that can reach the succulent leaves placed on the tops of acacia trees. And with this food source at their disposal, giraffes are able to reproduce year-round, and survive periods of drought better than many smaller animals.
This evolutionary advantage, however, brings with it a series of problems. One above all, the need to pump blood towards a head that is several meters above the ground, and at least a couple from the heart. To do this, the heart muscle of giraffes must work much harder than ours: the blood pressure of these animals exceeds 200mm Hg on average, almost double that of most other mammal species, and the resulting energy expenditure means that at rest the heart of a giraffe alone consumes more calories than our entire organism does. Without his long legs, however, the situation would be even worse.
The experiment
Intent on verifying the usefulness of such long legs, the authors of the new study carried out an experiment, analyzing the energy consumption of a fictional animal with the stocky body of an antelope, and a giraffe-like neck long enough to still reach the leaves placed on the tops of the trees. With numbers in hand, researchers have shown that such a creature would have to spend 21 percent of its total energy just to pump blood from the heart (located lower than that of a normal giraffe) to the head, compared to 16 percent for giraffes and 6.7 percent for our species.
Thanks to their long legs, which allow them to reach the desired height with a shorter neck, giraffes therefore save 5 percent of the energy they would spend on pumping blood if they had shorter legs. In one year this is an energy saving equivalent to over 1 and a half tons of less food, to maintain the same energy budget. Enough to make the difference between living and dying, in the arid and hostile environment of the African savannah.
A lucky occurrence
From an evolutionary perspective, it is correct to say that it was the long legs of the ancestors of giraffes (which appeared before the elongation of the neck) that made the anatomical adaptations possible that today allow these animals to feed on the first fruits kept in the treetops. Other animals with oversized necks that have appeared in the chorus of evolution, however, have not been so lucky.
Just look at the Giraffatitan, a dinosaur that lived about 150 million years ago, whose neck reached eight and a half meters in length. In his case, the body, although gigantic (it reached 13 meters in height) was much stockier than that of giraffes. And it was calculated that for this reason, he would have needed a blood pressure of 770mm Hg to pump blood up to his head, a feat which in turn would have required more energy than was consumed by the rest of his body. Unlike giraffes, therefore, these dinosaurs could not extend their necks vertically along their entire length except for short periods of time, because without the necessary pressure the blood would soon stop flowing to them, causing them to quickly faint.