Taller Bumps Increase Grip

Close view of the paw pad of a polar bear

Biological Strategy

Polar bear

AskNature Team

Image: Ucumari Photography / Flickr / CC BY ND - Creative Commons Attribution + NoDerivatives

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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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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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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Taller bumps compensate for loss of friction due to polar bears' relatively smaller paw pads.

Introduction

While they are the biggest bears alive, polar bears have relatively smaller paw pads than their ursine cousins. Presumably, that’s to decrease the amount of their skin that comes in contact with the frozen ground while they walk or stand. There’s just one problem: less skin gripping the ground should mean a greater chance of slipping and sliding, wasting energy while walking, and losing precious fractions of a second when lunging to make an attack.

The Strategy

Polar bears avoid these dangers with a relatively simple solution: the microscopic bumps (papillae) on their paw pads are 1.5 times taller than those on other bear species. That gives them about 1.3 times more true surface area, when you factor in all those undulations. That increase in surface area increases friction to 1.3–1.5 times the amount for other species.

The Potential

The research that revealed these details about polar bear paws was driven by the search for a natural model to follow to increase the grip of tires in icy and snowy conditions. The details of the papillae may help there, or in the design of all kinds of better-gripping surfaces, such as in roadways and pedestrian walkway design, flooring (especially for healthcare facilities and homes for seniors), or recreational uses like rock-climbing gear. They show as well how physical design could reduce the need for toxic chemicals used as adhesives.

Last Updated December 20, 2022

References

“We find that interspecific variation in papillae base diameter can be explained by paw pad width, but that polar bear paw pads have 1.5 times taller papillae and 1.3 times more true surface area than paw pads of the American black bear and brown bear. Based on friction experiments with three-dimensional printed model surfaces and snow, we conclude that these factors increase the frictional shear stress of the polar bear paw pad on snow by a factor of 1.3–1.5 compared with the other species. Absolute frictional forces, however, are estimated to be similar among species once paw pad area is accounted for, suggesting that taller papillae may compensate for frictional losses resulting from the relatively smaller paw pads of polar bears compared with their close relatives.”

Journal article

Polar bear paw pad surface roughness and its relevance to contact mechanics on snow

Journal of the Royal Society Interface | November 2, 2022 | Nathaniel Orndorf, Austin M. Garner, and Ali Dhinojwala

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Reference

“Papillae are not unique to polar bears. Previous work [in that area] made the implicit assumption that papillae themselves are adaptations for enhanced traction on snow without studying the paw pads of other bears. It was fascinating for us to discover that the other North American bears have them as well and that the physical characteristics of the papillae are what matters for traction on snow,” said [Austin] Garner.

Report

UA researchers’ focus on tire traction leads to investigation of polar bear paws

University of Akron Communications and Marketing | December 12, 2022 | The University of Akron News

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Reference

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