The Hero Shrew’s Remarkable Spine

Biological Strategy

Hero shrew

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Image: William Stanley / The Field Museum / Some rights reserved

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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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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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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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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The spine of hero shrews is remarkably strong yet flexible, due to interlocking vertebral spines along the spinal column.

Introduction

In the vast forests of the Congo Basin in west and central Africa, there lives a small, unassuming creature with a very big secret. The hero shrew is a mouse-sized mammal with a spine that is three times as thick as that of a normal shrew. This super-strong backbone has impressed and puzzled humans for centuries (at least). How does it achieve such strength, and what does it use it for?

The Strategy

The hero shrew’s spine consists of more vertebrae than other shrews, and those vertebrae are shorter front-to-back, but much taller bottom-to-top.

The extended side wings of the vertebrae also feature countless horizontal tubercles that allow the separate bones to fit snugly together, allowing the flexed back to take on the properties of a single massive column.

There are also strong attachment points for the muscles that pull the back together, and in particular arch it upwards.

Scutisorex somereni skeleton (FMNH 227556) by scutisorex on Sketchfab

Complementing these features is the makeup of the bone matter itself. Bone is a “cellular solid”, made of a network of matter and spaces that cause it to be both lightweight and strong. Hero shrew vertebrae though have incredibly dense bone––an indicator that the bones experience intense compression front-to-back.

These features have led researchers to hypothesize that the hero shrew’s striking spine has developed in relationship to a foraging strategy that leads the species to scrunch up and push against plant parts, rocks, and logs in order to access hidden worms and insect larvae. While never documented in the wild, this scenario fits with the observation that people in the region often see the shrews while harvesting similar food sources by prying dead palm fronds off trunks.

The Potential

The hero shrew’s unique spine is an that could be emulated by humans in a variety of ways. The interlocking bones could be used as a model for designing stronger, more flexible joints for prosthetic limbs. They could also inspire joining elements for pre-manufactured buildings in earthquake zones or hitching elements in transportation environments that require flexibility as well as strength.

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Last Updated May 19, 2023

References

The collectors indicated that Scutisorex were commonly encountered during this process and showed LWR runways around the base of these trees that they said were used by Scutisorex. Scutisorex can flex their fortified vertebral column considerably, especially sagitally, and the spine provides robust muscle attachments, especially for the epaxial longissimus muscles, which store elastic energy during flexion. We hypothesize that these shrews position themselves between trunk and leaf bases, and use the spine and associated musculature to exert force between the trunk and the base of leaves, granting access to predictable and potentially concentrated sources of invertebrate larvae that are otherwise protected from predation. This same mechanism may be useful in levering heavy objects such as logs to gain access to aggregates of invertebrate prey such as earthworms (Oligochaeta).

Journal article

A new hero emerges: another exceptional mammalian spine and its potential adaptive significance

Biology Letters | October 23, 2013 | William T. Stanley, Lynn W. Robbins, Jean M. Malekani, Sylvestre Gambalemoke Mbalitini, Dudu Akaibe Migurimu, Jean Claude Mukinzi, Jan Hulselmans, Vanya Prévot, Erik Verheyen, Rainer Hutterer, Jeffrey B. Doty, Benjamin P. Monroe, Yoshinori J. Nakazawa, Zachary Braden, Darin Carroll, Julian C. Kerbis Peterhans, John M. Bates and Jacob A. Esselstyn

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Reference

Journal article

Deciphering an extreme morphology: bone microarchitecture of the hero shrew backbone (Soricidae: Scutisorex)

Proceedings of the Royal Society B | April 29, 2020 | Stephanie M. Smith and Kenneth D. Angielczyk

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