Thor's Hero Shrew and masonry design

I have written a few times on this blog about masonry in nature, as evidenced by several species of animals, including seahorses.  A couple years ago a new species of shrew was discovered in the Democratic Republic of Congo, whose spine exhibits incredible strength and toughness.  “Thor’s Hero Shrew” is worth taking a look at from the perspective of interlocking masonry design.

As discussed in this article, “Scientists at Chicago's Field Museum and international collaborators have described a new species of Hero Shrew – the mammal with the most bizarre lower spine on Earth. The interlocking vertebrae of the Hero Shrew render the spine four to five times more robust relative to body mass, a condition not found in any other mammal. The spine has been an enigma to evolutionary biologists, with no known adaptive significance.

This new species of Hero Shrew, named Scutisorex thori, possesses features that may represent intermediate character states between the only other known Hero Shrew species (Scutisorex somereni), and other shrews. In addition, a novel hypothesis for the function of the animal's expanded lower spine has been proposed. The study will be published July 24, 2013 edition of Biology Letters.

First discovered in 1910, the Hero Shrew's most notable feature was not revealed for another seven years, when a specimen was dissected to reveal the most peculiar backbone of any mammal. The remarkable spine of the Hero Shrew is unique among mammals, in that the lower vertebrae have multiple lateral processes that interlock with the processes of neighboring vertebra. The arrangement, along with surrounding musculature, affords the animal extraordinary strength, so much so that the Hero Shrew has traditionally been worn as a talisman.

"This shrew first came to light when explorers came to the eastern part of the Democratic Republic of Congo," said Bill Stanley, Director of Collections and zoologist at the Field Museum. "The explorers watched in amazement as a full-grown man stood on the back of the Hero Shrew, and the animal walked away, unharmed."

Until now, there have been no other species of this bizarre shrew. The new species described in this study represents a possible intermediate between the original Hero Shrew and other shrews, since is possesses an interlocking spine, but with fewer lower vertebrae and lateral processes than the first Hero Shrew species.

"You and I have five lumbar vertebrae," said Stanley. "And so do most other mammals, but the Hero Shrew at least 10. Scutisorex thori has eight vertebrae, and fewer lateral processes than the original species."

The specimen of the new Hero Shrew species was collected in the lowland forest near the Tshuapa River in the Democratic Republic of Congo. Based on the observations of one of the co-authors on the study, the authors present a novel hypothesis for the functional significance of the spine of Scutisorex thori; they suggest that these shrews position themselves between the trunk and leaf bases of Palms, and use their unique spine to exert force and gain access to concentrated sources of beetle larvae that are otherwise protected from predation. The same adaptation may allow these animals to lift logs or rocks to access invertebrates – a food resource that remains unavailable to many other mammals.

The specimen of Scutisorex thori now residing at The Field Museum is a holotype, meaning that it will be the standard for identifying other members of the species. The new species is named in honor of Thorvald "Thor" Holmes, Jr. of the Humboldt State University Vertebrate Museum, at the suggestion of Bill Stanley, who did his graduate work there. The suggested common name is "Thor's Hero Shrew", appropriately invoking Thor, the god of strength in Norse mythology.

"The Age of Discovery is not over," said Stanley. "In fact, discoveries such as these happen in natural history collections, like the ones that we have at The Field Museum. In addition, hypotheses such as the one that we've generated concerning the functional significance of the Hero Shrew's spine fuel the scientific machine. We can't wait to see the results of further scientific studies that test the ideas presented in this article."

As Wikipedia states, “The structure of Thor's hero shrew's cranium and vertebrae suggest that it may be descended from an evolutionary intermediate between the hero shrew and other shrews. Its existence may help explain the evolution of the hero shrew which, Stanley explains, has historically been cited as an excellent example of punctuated equilibrium, a theory that holds that species sometimes evolve very rapidly in short periods of time after long periods of stability. The existence of an intermediate species hints at a more gradual or incremental evolution for the hero shrew's extreme specialization.”

It appears to me that Thor’s hero shrew utilizes an interlocking masonry-like arrangement to achieve the incredibly high spinal strength it possesses.  The ability to sagitally flex its spine would seem to indicate conjugate shearing between lumbar vertebrae, similar to the seahorse tail strength mechanism.

Nature is the Grand Master of design.  Thor’s hero shrew is an exemplar of masonry technique used to achieve high strength and toughness.  Its discovery is perhaps an omen of punctuated equilibrium in the evolution of masonry as used by man.

Thickness of dome walls

Thrust force lines must be kept within wall thickness for a structure to remain standing; this makes wall thickness critically important.  The triangular interlocking masonry system described on this blog can provide different wall thicknesses through different techniques.
 

A spherical dome under gravity is subject to "hoop" stress (like lines of latitude, horizontal) which varies from the crown down to the bottom edge. This stress is completely under compression from the top center down to the haunch (51.82⁰ from vertical).  Below the haunch there are tensile forces pushing out, which grow stronger and tend to introduce cracks at the bottom of the masonry dome.  These tensile hoop forces at the bottom of a masonry dome are typically resolved by either a tension ring, or a massive abutment, or both.

Historically masonry analysis refers to “lunes” which are like the sliced sections of an orange peel.  Each lune is viewed as a discreet arch section for the purpose of stress analysis.  From the top of the lune down to the base of the dome, the stress increases as the weight increases.  The wall thickness increases accordingly to accommodate this increased stress. 

Although vertical cracks are known to develop at the bottom of large domes, this does not necessarily make these domes unsafe.  Many large domes with cracks at their bases are known to have stood for centuries, and are standing still; such as the Hagia Sophia, the Pantheon, and many others. 

A wall in a hemispherical dome is considered "thin shelled" if the wall thickness is 10% (or less) of the radius.  This 10% proportion is considered safe for a full hemisphere which goes 90 degrees from crest to base.  If a spherical dome is a segmental section which only goes 70 degrees from crest to base, then a wall thickness of 4% proportion (thickness to radius) is considered safe.  This shows how dramatically stress increases at the bottom of a spherical dome.

Wall thickness may be increased by simply making the bricks thicker.  That is, the dimension of the block from outside surface of the dome to the inside of the dome can be made larger for a thicker wall.  The center of the wall, at midway between outside and inside, is always an axis of symmetry for the interlocking features of the block.  This means that different thicknesses of block can be used together; they will still interlock and connect to each other.  Thinner blocks can be placed on top of thicker blocks.

Wall thickness can also be adjusted by using a core or depression or cavity within the block.  This technique does not change wall thickness from inside to outside the dome, but within the block itself.   A larger core (or hole) will produce thinner, lighter block.  (It is useful to note that standard rectangular concrete block is also typically hollow, and have cores).  Different core sizes can be used on a given mold.  This technique has been used historically by ancient master masons, including the Pantheon with its “coffers” on the dome interior.   (The first block which I had mass-produced on a block machine all had a hollow triangular core, they work well).

Thickness can also be adjusted by having multiple wythes, or layers of block, like layers of onion skin.  This arrangement is recommended for applications which require a high safety factor, including tornado shelters, hardened structures, blast resistant structures, etc.  This technique of multiple wythes can also be used at the base of very large domes to resolve thrusting forces.  The strength of a structure built with concentric wythes or layers of triangular block can be further increased by weaving the blocks together with a tensile element, like steel cable.