Shapes and Materials of Armor Protect Against Penetration

Biological Strategy

Three-spined stickleback

Mary Hoff

Image: Gilles San Martin / Flickr / CC BY - Creative Commons Attribution alone

Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Prevent Deformation

When a living system undergoes compression, tension, shear, bending, or twisting, its internal inter-molecular forces can often resist these forces and even change shape temporarily, returning to the original shape when the forces stop. However, if the force is too strong or lasts too long, permanent deformation or structural failure can occur, resulting in death. Therefore, living systems have strategies to resist deformation or help ensure limited deformation. For example, bones have thin crystals and proteinaceous fibers that provide strength and flexibility, protecting them from forces that would otherwise cause deformation on a daily basis.

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Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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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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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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Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

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Sticklebacks use movable spines and fixed plates to provide stability and flexibility in withstanding impacts.

Introduction

It’s not easy being a small fish in a big ocean filled with hungry predators. But the marine threespine stickleback (Gasterosteus aculeatus) has found a way to survive the onslaught of neighbors looking for a seafood dinner.

This 4-inch-long (10-cm) fish is found along coastlines throughout much of the northern hemisphere. It has not only spines but also dozens of overlapping plates of armor along its sides and a strong pelvic girdle to protect itself from harm. Together these structures use not one, but six different features to thwart attackers seeking the juicy meal inside.

Together these structures use not one, but six different features to thwart attackers seeking the juicy meal inside.

The Strategy

First, rather than relying on a single type of protection, the armor uses both movable spines and fixed plates to provide both stability and flexibility to adapt to circumstances.

Each plate also varies in thickness from one end to the other. That means that even though the plates overlap, they produce uniformly thick protection rather than leaving thinner, more vulnerable spots in the sections where just one plate is present

The plates have bumps and indentations that fit together where they overlap. This maintains the overall integrity of the armor while also allowing the plates to shift positions relative to each other as the fish moves.

The bottom-facing plates on the pelvic girdle interlink, creating a structure that’s flexible for easy movement when the fish is not threatened but can lock into a more rigid, protective position during an attack.

The individual lateral plates are layered, with a lightweight porous middle layer sandwiched between two denser surfaces. This helps make them stiff, strong, and lightweight all at the same time.

Finally, the lateral plates have a bumpy outside surface. This not only may make the armor tougher to penetrate than a smooth surface would (perhaps by increasing surface area and causing the force of a predator’s bite to be distributed through the tissue), but also likely reduces drag and shear stress, allowing the fish to swim with minimal energy cost.

Interestingly, in places where sticklebacks have spread inland to inhabit freshwater, the number of plates and density of minerals within them tend to decline. The adaptive value of this is not completely clear, but scientists speculate it may have to do with differences in predation, mineral availability, or other variables between the two types of environments.

The Potential

Each of the six principles offers valuable insights for designing materials and products that resist abrasion, puncture, and other onslaughts. For instance, the texture of the surface of the plates can inform the construction of aircraft, watercraft, and motor vehicles, which need to be both durable and streamlined. Similarly, the sandwich-like configuration can inform the design of underground pipelines, which need to resist penetration by rocks but also be lightweight for transportation and installation. And the durability and flexibility offered by the interlocking plates might inspire new designs for helmets and other protective gear for sports.

Perhaps the most important lessons of all have to do with the value of not putting all of one’s defensive eggs in one basket, and with adapting to circumstances. By combining a variety of strategies and adapting to different environments, the marine threespine stickleback can ward off a range of onslaughts from different predators at the least cost to itself—a valuable lesson in general for coping with an often unpredictable world.