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.”

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.

“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.”