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

Shorter legs could make a giraffe's heart work harder. Keeping the animal equally tall would mean a longer neck and a heart closer to the ground. Blood would then have farther to climb before reaching the brain, increasing the pressure—and energy—the heart must supply.

Roger Seymour and Edward Snelling built a numerical model from published giraffe measurements and mammalian estimates of cardiac efficiency. They compared a model giraffe with an imaginary animal of the same mass and height but shorter, eland-like legs. Its extra-long neck compensated for the lower body. This allowed them to vary heart height while holding the animal's overall height and resting energy requirements constant.

The model estimated that the giraffe's main pumping chamber used about 16% of its resting energy expenditure. With eland-like legs and the correspondingly longer neck, that share rose to about 21%. Those percentages depend on uncertain inputs, particularly how much blood the giraffe heart pumps; they were not measured directly in living animals. The geometric advantage is simpler: long legs raise the heart as well as the head. Their contribution to giraffe height therefore reduces the circulatory burden of reaching high foliage, although the calculation does not establish why long legs originally evolved.