Giraffes Evolved Long Legs to Save Energy, Not Just Reach Trees

Giraffes’ long legs raise the heart closer to the head, reducing heart energy use from 21% to 16% of resting metabolism, enabling their exceptional height, researchers say.

• Researchers at the University of Pretoria and the University of Adelaide found giraffes' long legs reduce blood pressure needed to reach the brain, saving a net 5% of energy intake.
• Fossil records show ancient giraffids lengthened legs before necks 16 million years ago, establishing an energy-efficient baseline for later neck elongation, Graham Mitchell explains.
• The model shows a 651 kg adult giraffe's left ventricle uses about 90.5 watts, based on MAP 214 mmHg, cardiac output 41.8 l/min, and cardiac efficiency ~22%, with heart energy at 16%.
• Long legs also create costs: giraffes face a locomotor penalty and must splay forelimbs to drink, lowering the heart ~0.48 meters and risking safety for hydration while saving about 1.5 tonnes of food per year.
• Pulmonary limits suggest the heart cannot sit more than about 35 centimeters higher without risking pulmonary edema, explaining why no taller erect-headed land animal evolved and why sauropod dinosaurs faced biomechanical ceilings.