Why do large animals move slowly? New study points a finger at body heat

According to the study, larger animals must reduce their travel speeds to avoid hyperthermia during extended periods of motion. 

May 23, 2023 04:55 pm | Updated 06:51 pm IST

Image used for representation.

Image used for representation. | Photo Credit: M.A. Sriram/The Hindu

Could the speed at which large animals move be limited by how much heat their bodies are capable of dissipating? A study published on April 18, 2023, suggests so.

A group of German scientists studied the travel speeds of 532 animal species to arrive at the result: that the highest travel speeds are achieved by animals that have intermediate body mass. On the other hand, larger animals must reduce their speeds to avoid hyperthermia during extended periods of motion. Hyperthermia is a condition that causes an abnormally high body temperature.

Also Read | Why bigger animals aren’t always faster

The ability to move is crucial for animals because it allows them to access resources and reproductive opportunities they need to survive in fragmented ecosystems. Animals’ moving speeds usually depend on the mode of locomotion (flying, running, swimming, etc.), body mass, and the experienced temperature.

The study used a hump-shaped relationship between travel speed and body mass, based on previously studied models of maximum speed. The study paper also pointed out that thus far, few other studies have considered how metabolic heat can limit movement speed.

Metabolic heat is the inevitable by-product of muscular contractions, and is directly proportional to the amount of physical work, including locomotion. To keep the core body temperature stable, an animal’s body must sufficiently dissipate ‘excess’ heat to its surroundings. So it is important for animals to decrease their metabolic demands when heat dissipation can’t balance metabolic heat production.

This is why animals must reduce their speeds.

The researchers used three models of how travel speed varies with body mass: a simple metabolic model, a constant heat-dissipation model, and an allometric heat-dissipation model. Comparing between the three showed that the allometric heat-dissipation model best describes the systematic relationship between body mass and realised animal travel speeds across flying, running, and swimming.

This model provided also an important insight: even though the largest animals possess the metabolic potential to sustain higher speeds, the speed that they do realise is limited by the risk of hyperthermia.

The travel speeds of large animals are limited by their heat-dissipation capacities.

The travel speeds of large animals are limited by their heat-dissipation capacities. | Photo Credit: Alexander Dyer, Ulrich Brose, Emilio Berti, Benjamin Rosenbaum, Myriam R. Hirt (2023): The travel speeds of large animals are limited by their heat-dissipation capacities (PLoS Biology)

The study also provided an understanding of animal travel velocities, based on the laws of physics, that can be generalised across species even when biological details of the individual species are unknown. This will help make more realistic predictions of biodiversity dynamics in divided landscapes, as consequences of anthropogenic activities like climate change disrupt the natural movements of animals.

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