How long limbs reduce the energetic burden on the heart of the giraffe

Adult giraffes have high mean systemic arterial blood pressure (MAP) of 200-250 mmHg at heart level, which is more than twice that of most mammals. The high MAP is associated with their long neck, because gravity creates a hydrostatic pressure gradient along the carotid arteries such that every metre of height requires an additional 77 mmHg of blood pressure at heart level, and the head can be over 2 m above the heart. The giraffe's MAP remains high regardless of posture or level of activity, so it creates a significant and unrelenting energy burden on the heart. This study quantifies that burden. Because of high MAP, the energy expenditure of the left ventricle is approximately 16% of the resting whole-body metabolic rate of an adult giraffe, compared with 9% in a normal mammal of the same body mass and a shorter neck. A numerical model is presented that varies the vertical position of the heart in a giraffe's body of fixed height and recalculates the energy used by the left ventricle. If the giraffe had evolved its height by extending the neck alone without elongating the limbs, the estimated cost would be 21%. However, the long limbs, which predate the long neck in giraffe evolution, have raised the level of the heart, thus reducing the required MAP and saving energy. The vertical distance between the heart and the erect head of adult giraffes appears to be the maximum ever evolved among terrestrial vertebrates.