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. 2022 Aug 2;119(31):e2122563119.
doi: 10.1073/pnas.2122563119. Epub 2022 Jul 18.

Skin wrinkles and folds enable asymmetric stretch in the elephant trunk

Andrew K Schulz  1 , Madeline Boyle  1 , Colin Boyle  2 , Sophia Sordilla  1 , Catalina Rincon  1 , Scott Hooper  3 , Catie Aubuchon  3 , Joy S Reidenberg  4 , Claire Higgins  2 , David L Hu  1   5
Affiliations

Skin wrinkles and folds enable asymmetric stretch in the elephant trunk

Andrew K Schulz et al. Proc Natl Acad Sci U S A. .

Abstract

The elephant's trunk is multifunctional: It must be flexible to wrap around vegetation, but tough to knock down trees and resist attack. How can one appendage satisfy both constraints? In this combined experimental and theoretical study, we challenged African elephants to reach far-away objects with only horizontal extensions of their trunk. Surprisingly, the trunk does not extend uniformly, but instead exhibits a dorsal "joint" that stretches 15% more than the corresponding ventral section. Using material testing with the skin of a deceased elephant, we show that the asymmetry is due in part to patterns of the skin. The dorsal skin is folded and 15% more pliable than the wrinkled ventral skin. Skin folds protect the dorsal section and stretch to facilitate downward wrapping, the most common gripping style when picking up items. The elephant's skin is also sufficiently stiff to influence its mechanics: At the joint, the skin requires 13 times more energy to stretch than the corresponding length of muscle. The use of wrinkles and folds to modulate stiffness may provide a valuable concept for both biology and soft robotics.

Keywords: bioinspired design; biomaterials; biomechanics; hydrostat.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
(A) The experimental setup for studying elephant trunk elongation. Illustration done by B. Seleb. (B) A time series of the male elephant reaching for a food item. Images are separated by 0.33 s.
Fig. 2.
Fig. 2.
The relation between maximum strain and body mass for animals possessing muscular hydrostats as appendages (–25). The tongues are shown by black points, octopus tentacles by the purple point, and the elephant trunks by the blue points. A red dashed line depicts the power-law best fit. Raw data and citations are given in SI Appendix, Table S3. Silhouettes are from Adobe stock photos.
Fig. 3.
Fig. 3.
(A) Time course of the positions of 14 equidistant points along the trunk of the female elephant (n = 1). The arrows indicate the onset of the waves of extension that travel in the proximal direction. (B) Time course of the average strain in the trunk of the female elephant (n = 7). The blue line is the average axial strain, and the black line is the average lateral strain, both measured from experiments. The red line is the predicted lateral strain based on volume conservation. The dotted line at zero strain is for reference. (C) Time series of the average trunk strain for the dorsal and ventral regions of the female elephant (n = 7 trials). The color bar indicates axial strain.
Fig. 4.
Fig. 4.
Elephant trunk skin and cross-section. (A) Photographs of skin from dorsal skin of the frozen elephant trunk, taken from the proximal region, 110 cm from the distal tip. (B) Ventral skin. The longitudinal axis of the trunk is vertical in the images. The dorsal section is characterized by large folds and the ventral section by clusters of wrinkles. (Scale bar, 1 cm, with the bottom of the images toward the distal tip of the trunk.) (C) The stress–strain relationship for the dorsal skin (black) and ventral skin (blue). Inset shows how natural features were tracked using Digital Image Correlation. (D and E) The relationship between the area fraction and distance z from the tip for components of (D) the frozen female elephant trunk and (E) an octopus arm, reprinted from ref. . The area fraction is defined as the ratio of the cross-sectional area of the component to the total cross-sectional area. Insets show the cross-section of the elephant trunk and octopus arm with the different muscle groups labeled: longitudinal muscle (L) in green, radial muscle (RM) in orange, and oblique muscle (O) in blue. The dashed line indicates nonmuscular areas of the cross-section, which consists of skin, nasal passageway, nerves (N), and blood vessels (BV). Trabeculae connective tissue (TR) is also labeled on the octopus cross-sectional image.
See this image and copyright information in PMC

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