Shells Resist External Loading

Biological Strategy

Birds

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Image: Janice Carr (photo credit) and Janice Carr; Sandra L. Westmoreland (content providers) / Centers for Disease Control and Prevention /

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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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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The eggs of birds resist external loading via composite structure.

“The eggshells of birds are mechanically impressive devices, surprisingly resistant to external loading; Vincent (1990), though, complained about how little we understand them, muttering at ‘half-boiled notions’ in the literature. They’re mostly mineral but have a critical 2-4 percent of organic matter, making them into composites. Still, cracks can propagate, of which fact the chick takes advantage to get out–before pushing, it pecks around a circle so it can then break the egg along the dotted line.” (Vogel 2003:340)

Vogel S. Comparative Biomechanics: Life’s Physical World. Princeton: Princeton University Press; 2003. 580 p.

Last Updated August 18, 2016

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

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

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