Two animals could reach the same leaf with very different proportions. One might have a long neck above a relatively low body; another could combine a shorter neck with longer legs. Their mouths would arrive at the same height, but their hearts would face different vertical distances when supplying the head.
Roger S. Seymour and Edward P. Snelling explored that difference in a 2025 study in the Journal of Experimental Biology. Their tool was a numerical model rather than a comparison of newly measured animals. It showed how long legs could reduce one of the continuing energy costs of being a giraffe.
The height that matters to the heart
Blood pressure falls as blood rises above the heart because gravity acts on the column of blood. To maintain adequate pressure at an elevated head, pressure near the heart must be higher. The important distance is therefore the vertical separation between heart and head, not simply the animal’s height above the ground.
The left ventricle supplies the body’s circulation. Its mechanical workload depends on both the pressure it produces and the volume of blood it pumps. Heart muscle also uses more metabolic energy than the mechanical work it delivers, because converting chemical energy into pumping is not perfectly efficient.
Seymour and Snelling combined these relationships with published information about giraffe dimensions, circulation and metabolism. Their standard animal weighed 651 kilograms and stood 3.88 metres tall. The model placed its heart halfway up, 1.94 metres above the ground.
These values describe a deliberately standardised comparison, not a universal adult giraffe. Fixing them made it possible to change the proportions while keeping the total height and mass constant.
A fictional animal makes the comparison clear
For the alternative body plan, the authors imagined an animal with the body and legs of an eland but a neck long enough to reach the standard giraffe’s height. They called it an “elaffe.” Its heart sat about one metre above the ground, leaving a substantially longer blood column between heart and head.
At the model’s intermediate pumping rate, the standard giraffe required mean arterial pressure of 214 millimetres of mercury near the heart. The fictional shorter-legged animal needed about 286 to maintain the same pressure at the head.
The model translated that pressure difference into energy. The giraffe’s left ventricle used approximately 16% of the animal’s estimated resting metabolism; the fictional comparison required about 21%. For context, the authors’ scaling estimate for a conventionally proportioned mammal of the same mass was about 8.9%.
The comparison makes the direction of the benefit tangible. Long legs do not remove the circulatory cost of a high head. They raise the pump closer to its elevated destination, reducing the additional pressure and energy required.
Why the percentages need their assumptions
The exact energy budget is less secure than the underlying geometry. Published estimates of giraffe cardiac output—the volume pumped per minute—vary considerably. Measurements made under anaesthesia may differ from circulation in an awake animal, and the authors discussed disagreement over how to interpret those measurements.
Their principal calculation used 41.8 litres per minute from published measurements of conscious, standing giraffes. They also ran the model with lower and higher literature values. Changing that input changed the estimated energy burden.
Raising the heart still reduced the pressure required at the head across those calculations. Thus, the qualitative conclusion is more robust than presenting 16% as an established measurement of every giraffe’s heart metabolism. It is a modelled estimate based on an explicitly chosen body, pumping rate and efficiency.
This distinction is useful alongside our guide to giraffe heart physiology. Direct measurements establish aspects of the circulation; a controlled model asks what would change if anatomy were different. The imaginary animal’s value is precisely that it makes a comparison unavailable in a real giraffe experiment.
Proportions bring several demands together
The authors also considered a smaller adjustment: shorten the legs and lengthen the neck enough to make reaching ground-level water possible without spreading the forelegs. Their estimate suggested a relatively modest increase in the heart’s energy burden, from about 16% to 17% of resting metabolism.
That result helps keep the evolutionary interpretation proportionate. Cardiovascular economy may favour a higher heart, but the model does not demonstrate that it alone explains the exact leg length or drinking posture of living giraffes. Feeding reach, locomotion and development also belong to the broader question of giraffe adaptations.
Nor could the heart simply be moved far up the neck without consequences. The authors point out that the heart and lungs must operate as a connected system. A large vertical separation between them would alter pressures in the lung circulation, where gas exchange requires delicate, thin-walled surfaces.
The model’s achievement is to isolate one benefit within that larger set of demands. It converts the placement of the heart from a background anatomical detail into an explicit energy question.
More reliable measurements of cardiac output and resting oxygen use in awake giraffes would improve the numerical estimate. Comparative evidence from body proportions and the fossil record could then help assess whether changes in leg and neck length are consistent with this proposed benefit. For now, the study shows why reaching high with the whole body can make physiological sense: long legs lift the circulation’s starting point, not only the animal’s view.
Sources
Roger S Seymour, Edward P Snelling (2025). How long limbs reduce the energetic burden on the heart of the giraffe. The Journal of Experimental Biology. DOI: 10.1242/jeb.251092. Read the full paper.
