Footpads Manage Increasing Body Mass

close up of a white dog's foot pads are seen as a dog lays down

Biological Strategy

Mammals

AskNature Team

Image: Engin Akyurt / Pixabay / Free non-commercial use

Manage Impact

An impact is a high force or mechanical shock that happens over a short period of time, such as a hammer hitting a nail rather than a hand pushing slowly against a wall. Because of their speed and force, impacts don’t allow materials to slowly adjust to the force, which can lead to cracks, ruptures, and complete breakage. Therefore, living systems have strategies that can absorb, dissipate, or otherwise survive that force without the need to add large amounts of material. For example, the Toco toucan’s large beak is very lightweight, yet can withstand impacts because it’s made of a composite material with rigid foam inside and layers of a hard, fibrous material outside.

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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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The footpads of mammals maintain functional integrity as body mass increases through changes in geometry and material properties.

Introduction

In mammals that walk on their toes—like cats, dogs, and other carnivores—footpads are the first parts of the foot to touch the ground with every step. These soft, durable pads take on the full weight and impact of each stride, helping animals move silently, swiftly, and with powerful control. These pads are even more essential for larger animals, as their bodies create much greater forces during movement. The design of these footpads is nothing short of remarkable; not only do they vary to accommodate different body sizes, but they also balance functions like cushioning, energy return, and support. This flexibility makes carnivoran footpads both diverse and specialized, enabling a range of animals to thrive in their environments.

The Strategy

Carnivoran footpads are essential to the way these animals move, and they’re adapted in clever ways to keep pace with the demands of different body sizes. As body mass increases from smaller carnivores like house cats to larger ones like wolves, footpads need to balance two conflicting needs: providing enough surface area to distribute weight and being tough enough to resist the increased pressures and forces of movement.

Studies show that while larger animals’ footpads don’t increase in size exactly in proportion to body mass, they become stiffer to handle the extra weight. This means that while a bigger animal has less relative pad surface area than a smaller one, the stiffness of its pads compensates by reducing the amount they compress under pressure. Stiffer pads lower the strain (or stretching force) on the tissues, which helps prevent damage from repeated impacts. The stiffer the pad, the better it holds up under pressure, helping larger animals avoid injury while still allowing them to move quickly and comfortably.

Another fascinating feature of these footpads is their role in energy storage and return. When the pad compresses under an animal’s weight, it temporarily stores energy. This energy is then released when the animal pushes off the ground, adding a spring to each step that helps propel it forward. Interestingly, larger carnivores store slightly more energy in their hindfoot pads than in their forefoot pads, which means they get a boost of extra energy just as they push off. This small difference in energy return is likely an for efficient movement, allowing larger animals to use less energy to move their bigger bodies.

One final benefit of the specific design of footpads is their influence on bone health. The stiffness of the pads and the specific way they absorb forces help to create just the right load conditions on the bones, which actually promotes healthy bone growth and remodeling. This is especially important for larger animals, as their bones bear more weight and need continuous renewal to stay strong.

The Potential

The sophisticated design of carnivoran footpads could inspire a variety of human-made materials and products. For instance, understanding the balance between flexibility and stiffness in footpads could lead to advanced footwear that offers cushioning and energy return while remaining supportive under pressure. These insights could also inspire more efficient and durable designs for prosthetics, which need to withstand impact while enabling smooth movement. In robotics, adapting the stiffness and energy-return properties of footpads might allow machines to move with greater agility and reduced wear, especially over rugged terrain.

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