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Here’s why a hero shrew has the sturdiest spine of any mammal

At first glance,
hero shrews don’t appear to live up to their name. But these fuzzy, molelike
animals are the Clark Kents of the shrew world, with superpowers hidden beneath
their humble exteriors. Their backbones are like nothing else in the animal
kingdom: the vertebrae interlock, making the spine extremely strong and rigid
when compressed. 

Now a 3-D analysis of the bone structure reveals that the vertebrae are exceptionally dense, with neatly reinforcing struts that lend toughness too. That structure may provide insight into how these unique backbones may benefit the animals in nature, researchers report April 28 in Proceedings of the Royal Society B.

Two hero shrew species
(Scutisorex) can be found in the palm forests of the Democratic Republic
of the Congo. Their strength astonished American and European explorers in the
1910s, when Congo’s local Mangbetu people reportedly demonstrated that an adult
man could step on the animal — only the heft of a deck of cards — without causing
any harm. 

“That story may or
may not be apocryphal,” says Stephanie
Smith,
a vertebrate functional morphologist at the Field Museum of Natural History in
Chicago. That particular demonstration “certainly has not been reproduced.”

spines of an African giant shrew and a hero shrew
The spine of a hero shrew (Scutisorex somereni) features interlocking vertebrae (right), unlike the more typical spine of the African giant shrew (Crocidura olivieri) (left).S. Smith

The shrew’s backbone is flat on top and underneath, featuring broad side flanges with lots of fingerlike projections. To probe its strength, Smith and museum colleague Kenneth Angielczyk used a 3-D X-ray technique to scan the internal structure of vertebrae from 20 skeletal museum specimens. The skeletons were from both hero shrews and the goliath shrew (Crocidura goliath), which is similar in size to the hero shrew, but with a more standard backbone. The researchers hoped that studying the density and orientation of holes and struts in the spongey interior of the bones would reveal the magnitude and direction of forces the animals might have experienced during their lives.

Compared with
goliath shrews, hero shrews had more, and wider, vertebrae. Internally, the
hero shrews’ vertebrae also featured many reinforcing rodlike structures that
made the spongey bone very dense. These struts were mostly oriented in the
head-to-tail direction, while the goliath shrews’ struts were less aligned with
any given direction, the team reports.

Smith likens the hero
shrews’ backbone to a line of soda cans positioned end to end, with side projections
interlaced like the teeth of two combs. Muscles can contract the spine and lock
the segments in place.

“It makes the unit
more like a single block of bone than regular vertebrae, which are more like
bendy units,” Smith says. These details suggest hero shrews endure forces
that are stronger and possibly more frequent than those experienced by goliath
shrews.

It’s unclear how
this modified spine might have evolved, partly because the animals are
difficult to find and study in the wild. Their spines are also so unique that there
are no comparisons among other animals. Strong backbones may help hero shrews wedge
themselves into the space between dead leaves attached at the base of standing
palm trunks in search of insects or larvae, Smith suggests, though she points
out that this behavior has never been observed.

A cross section of the sixth lumbar vertebra of a hero shrew taken perpendicularly across the spine reveals the vertebra’s many projections on the left and right sides (shown as an array of small ovals) that can interlace with neighboring vertebrae’s projections upon compression. This turns the spine into a tough, rigid unit rather than a set of bendy segments.S. Smith

“It’s long been
suspected that hero shrews were doing something that puts their backs under
large compressive forces,” says Katrina
Jones,
an evolutionary biologist at Harvard University. “This [study] is an important
step in understanding this adaptation, given that observing their behavior in
the wild has proved so challenging.”

Smith wants to search
for similarities between the hero shrew’s backbone with those of more shrew
species as well as xenarthrans, a superorder of animals that includes armadillos
and sloths. Some xenarthrans have vertebrae that interlock, though by a
different mechanism and not to the degree seen in hero shrews. It’s
possible similar pressures may have been behind the evolution of the
interlocking spines in both xenarthrans and hero shrews.

“Backbones are an
important part of how animals move, yet we understand surprisingly little about
them,” says Jones. “The hero shrew is a great example of how diverse and
specialized backbones can be, but it also illustrates how much we have yet to
learn about the ecology of living animals.”

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