Quills Penetrate Easily

Biological Strategy

North American porcupine

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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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Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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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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Quills of the North American porcupine penetrate tissue easily due to stress concentration at the barbs that likely stretches or tears tissue fibers locally at the interface of the quill.

Watch this video to learn more about porcupine quills.

“North American porcupines are well known for their specialized hairs, or quills that feature microscopic backward-facing deployable barbs that are used in self-defense. Herein we show that the natural quill’s geometry enables easy penetration and high tissue adhesion where the barbs specifically contribute to adhesion and unexpectedly, dramatically reduce the force required to penetrate tissue. Reduced penetration force is achieved by topography that appears to create stress concentrations along regions of the quill where the cross sectional diameter grows rapidly, facilitating cutting of the tissue. Barbs located near the first geometrical transition zone exhibit the most substantial impact on minimizing the force required for penetration. Barbs at the tip of the quill independently exhibit the greatest impact on tissue adhesion force and the cooperation between barbs in the 0–2 mm and 2–4 mm regions appears critical to enhance tissue adhesion force. The dual functions of barbs were reproduced with replica molded synthetic polyurethane quills.” (Cho et al.:21289)

“[T]he quill with barbs required 54% less penetration force compared with the barbless quill…the barbed quill requires significantly less force and work to penetrate into tissue, compared with an 18 gauge hypodermic needle, which has a diameter of 1.161 ± 0.114 mm, similar to the diameter of a porcupine quill (1.262 ± 0.003 mm).” (Cho et al.:21289)

“The analysis revealed that tissue is primarily stretched and deformed by high stress concentrations near the barbs. The local stress concentrations likely reduce the need to deform the entire circumference of tissue surrounding the quill, consequently reducing the penetration force.” (Cho et al.:21290)

Learn more about the porcupine’s strategy in Tom McKeag’s case study, “Sticky Wicket,” on pages 27-30 of Zygote Quarterly 12:

Last Updated November 3, 2020

References

Journal article

Microstructured barbs on the North American porcupine quill enable easy tissue penetration and difficult removal

Proceedings of the National Academy of Sciences | 12/12/2012 | W. K. Cho, J. A. Ankrum, D. Guo, S. A. Chester, S. Y. Yang, A. Kashyap, G. A. Campbell, R. J. Wood, R. K. Rijal, R. Karnik, R. Langer, J. M. Karp

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