Resisting Shearing Forces

photograph of white aquatic snails attached to a brown rock

Biological Strategy

Limpets

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Image: S. Rae / Flickr / Some rights reserved

Attach Temporarily

Living systems must sometimes, temporarily, stay in one place, climb or otherwise move around, or hold things together. This entails attaching temporarily with the ability to release, which minimizes energy and material use. Some living systems repeatedly attach, detach, and reattach for an extended time, such as over their lifetimes. Despite being temporary, these attachments must withstand physical and other forces until they have achieved their purpose. Therefore, living systems have adapted attachment mechanisms optimized for the amount of time or number of times they must be used. An example is the gecko, which climbs walls by attaching its toes for less than a second. Other examples include insects that attach their eggs to a leaf until they hatch, and insects whose wings temporarily attach during flight but separate after landing.

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Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Manage Shear

The effect of shear stress on a living system is parallel internal surfaces sliding past each other. Slippage occurs in parallel with the force. Think about holding two wooden boards on top of each other and sliding one to the right and the other to the left. This may be easy until you add glue, which increases their shear strength and makes them harder or impossible to slide. Shear can occur in solids, liquids, and gases. Living systems must increase their shear strength to overcome these types of forces. For example, darkling beetles lock their wings together in flight to prevent lateral movement by using many small hairs on each wing. These hairs interlock to provide shear strength, just as two hair brushes put together would be difficult to slide past each other.

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Gastropods

Class Gastropoda (“stomach-foot”): Snails and slugs

There are at least 40,000 known species of gastropod, with many more yet to be discovered, making it the largest class of mollusks. These animals exhibit torsion, meaning their internal organs are twisted 180° during development. The majority of gastropods have shells, and most open up on the right side. The largest gastropods are the size of a small child (3 feet, or 1 meter) while the smallest are barely visible to the naked eye.

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Many organisms, including limpets, resist shearing loads temporarily in part thanks to Stefan adhesion, which occurs when a thin layer of viscous liquid separates two surfaces.

Image: Ken-ichi Ueda /

Image: Richard Tanswell /

“Stefan adhesion. This particular form of adhesion works best against shearing loads…I know of no demonstrated case in which Stefan adhesion is the exclusive mode of attachment, but one suspects its involvement when, as Crisp (1960) showed, a steady force will slowly slide barnacles (some at least) across smooth surfaces. Wigglesworth’s (1987) observations of flies clinging beneath glass sheets raises a similar suspicion. He noticed that the legs slowly slide centripetally across the glass, periodically detaching and swinging outward again. But shear of two surfaces isn’t the only way to load the liquid in shear–a tensile stress between two objects with fluid between will force inward flow between the surfaces, and the no-slip conditions at the liquid-solid interfaces will ensure the occurrence of shear. Thus, Stefan adhesion can play a role in resisting the initial tensile separation of two smooth and well-mated surfaces. It’s of course a player in the mucus locomotion of slugs, snails, and such…” (Vogel 2003:427)

Last Updated September 14, 2016

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

Princeton University Press | 17/06/2013 | Steven Vogel

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