A groundbreaking study by researchers at the University of Pretoria’s (UP) Faculty of Veterinary Science has revealed that the heart operates with declining energy efficiency as body size increases among mammals.

This means that the hearts of larger mammals lose more energy as heat, rather than directing that energy to the blood so that it circulates around the body. A tiny shrew, therefore, has a more efficient heart than that of an elephant. The implication is that eventually an upper limit on body size will be reached, where the heart is so inefficient at pumping blood, that it is no longer viable to do so.

The UP-led study, which was published in the Proceedings of the Royal Society B, was conducted with scientists from the University of Adelaide and Monash University in Australia, and the University of British Columbia in Canada.

The researchers used the relative densities of capillaries and mitochondria in the walls of the heart to determine the metabolic power of the cardiac tissue, and found that these densities decrease in larger mammals. This is because heart rate decreases with body size. “An elephant may have a heart rate of about 30 beats per minute, but a shrew can have a heart rate of over 1 000 beats per minute!” Dr Ned Snelling, experimental physiologist at UP, says

The scientists then compared the metabolic power of the heart against its mechanical power (measured as the energy imparted to the blood as it is pushed through the major vessels) and discovered a mismatch between the two. Because this mismatch increases with body size, it means the heart appears to operate with a declining efficiency in larger mammals. “In terms of energy, larger mammals put more in, but get less out,” Dr Snelling says.

He adds that for centuries, scientists have debated the maximum possible size of animals. The traditional theory has been that 100 tonnes must be close to the maximum size that is physically possible, at least for land animals, because larger weights could not be supported by the strength of the bones and the forces of the muscles.

This new research offers an alternative possibility: that it is declining energy efficiency of the heart that imposes an upper limit on body size.