School of Polymer Science and Polymer Engineering, The University of Akron, Akron, OH 44325, USA.
Integrated Bioscience Program, Department of Biology, The University of Akron, Akron, OH 44325, USA.
J R Soc Interface. 2022 Nov;19(196):20220466. doi: 10.1098/rsif.2022.0466. Epub 2022 Nov 2.
Microscopic papillae on polar bear paw pads are considered adaptations for increased friction on ice/snow, yet this assertion is based on a single study of one species. The lack of comparative data from species that exploit different habitats renders the ecomorphological associations of papillae unclear. Here, we quantify the surface roughness of the paw pads of four species of bear over five orders of magnitude by calculating their surface roughness power spectral density. 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.
北极熊爪垫上的微观乳头被认为是增加在冰/雪上摩擦力的适应特征，但这一说法仅基于对一种物种的单一研究。缺乏来自不同栖息地物种的比较数据，使得乳头的生态形态关联不明确。在这里，我们通过计算表面粗糙度功率谱密度，在五个数量级上定量了四种熊的爪垫的表面粗糙度。我们发现，乳头基底直径的种间变异可以用爪垫宽度来解释，但与美洲黑熊和棕熊的爪垫相比，北极熊的爪垫上的乳头高 1.5 倍，真实表面积多 1.3 倍。基于对三维打印模型表面和雪的摩擦实验，我们得出结论，与其他物种相比，这些因素使北极熊的爪垫在雪上的摩擦剪切应力增加了 1.3-1.5 倍。然而，一旦考虑到爪垫面积，绝对摩擦力估计在物种之间相似，这表明较高的乳头可能补偿了与近亲相比，北极熊相对较小的爪垫所导致的摩擦损失。
