Giraffes are famous for their extraordinarily long necks (which allow them to browse on the leaves of trees) and similarly elongated forelegs (which are much longer than the hind legs). The bony structure of the neck is essentially unchanged from that of other mammals: there are no extra vertabrae, but each of the 7 bones is greatly enlarged. Bone constitutes the bud-like horns called ossicorns, which are covered with the giraffe's skin like the rest of the skull.
Many more subtle modifications to the giraffe's structure have evolved, however, particularly to the circulatory system. A giraffe's heart has to generate around double the normal blood pressure for a large mammal in order to maintain blood flow to the brain against gravity. In the upper neck, a complex pressure-regulation system called the rete mirabile prevents excess blood flow to the brain when the giraffe lowers its head to drink. Conversely, the blood vessels in the lower legs are under great pressure (because of the weight of fluid pressing down on them). In other animals. such pressure would force the blood out through the capillary walls: giraffes, however, have a very tight sheath of thick skin over their lower limbs which maintains high extravascular pressure in exactly the same way as a pilot's g-suit
Giraffe gestation lasts between 14 and 15 months; a single calf is born. The mother gives birth standing up and the embryonic sack actually bursts when the baby falls to the ground. Newborn giraffes are about 1.8 metres tall. Within a few hours of being born, calves can run around and are indistinguishable from a calf that may be a week old already; however, for the first two weeks, they spend most of their time lying down, guarded by the mother. While adult giraffes are too large to be attacked by most predators, the young can fall prey to lions, leopards, hyenas, and wild dogs. Only 25 to 50 percent of giraffe calves reach adulthood; those that do have a life expectancy of between 20 and 25 years.