Hero shrews have strong, interlocking spines unlike any other animal, study says

Sep 06, 2023

Hero shrews may be small mammals that look like rodents, but they’re strong, mighty and live up to their name on the inside. Their interlocking spine is unlike anything researchers have ever seen in another mammal.

Exactly why these shrews have such sturdy tricked-out spines is a puzzle researchers are eager to solve.

"Hero shrews have crazy-looking spines – their vertebrae are squished flat like a pancake, and they have a bunch of extra places where they touch the vertebrae next to them. It makes a really long stiff column along their back, and there aren't good field reports as to what this structure might be useful for," said Stephanie Smith, lead study author and a postdoctoral researcher at the Field Museum and the University of Chicago. "So we wanted to look at those vertebrae and figure out how they might be using them."

Native to the Congo Basin of Central Africa, these little gray mammals stay out of sight by snuffling through leaves and fallen material on the ground.

They were first discovered and classified by researchers in 1910, although they were known to the Mangbetu people of the Congo Basin long before then.

The early research about hero shrews included a potentially apocryphal story where the Mangbetu people showed the American and European scientists that a grown man could stand on a hero shrew without harming it. The shrew would pop up and scurry away afterward.

"We don't know how accurate these stories are, and we’ll likely never know," Smith told CNN.

But Smith, who has always been interested in studying small mammals, wanted to investigate their strange spines. And shrews are particularly misunderstood, often mistaken for mice even though they’re more like hedgehogs and moles.

"Shrews are really interesting ecologically, and they’re so small they have almost secret powers," she said. "They’re incredibly diverse, and I think they’re beautiful. They’re dope as hell."

Only two species of hero shrew, including the Thor hero shrew announced in 2013, are known. They’re small and live in an area of political unrest, which makes them difficult to study. So Smith and her fellow researcher Kenneth Angielczyk, a curator of paleontology at the Field Museum, turned to museum specimens there.

"We have specimens of them at the museum, but we can't see them in action. It's almost like studying an animal in the fossil record, where we have specimens that tell us about their anatomy, but we can't bring a live specimen into the lab and observe it," Angielczyk, senior author on the paper, said in a statement.

And without seeing them in action, it's difficult to know why these shrews might need such highly modified spines.

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CT scans of the spines from both known hero shrew species were taken and compared with those of an average shrew.

Typically, a mammal's spine has different recognizable regions including the neck, the thoracic region where ribs are attached and the lumbar region between the ribs and pelvis.

In the hero shrew, both the thoracic and lumbar regions have more individual segments. Rather than five or six lumbar vertebrae, hero shrews have between 10 and 12.

Parts of the spine sticks out like wings, with nodules and tubercles that touch vertebrae in front of and behind them to create an interlocking network along the spine.

"Their spines are arched, and when they contract their muscles to squeeze their vertebrae together, the bones interlock really tightly. When that happens, it becomes one solid block of vertebrae instead of a bunch of bendy pieces," Smith said.

Previously, in the 2013 paper describing the Thor hero shrew, Bill Stanley, the late director of collections at the Field Museum, suggested that the shrews used their strong spines to move like an inchworm, scrunching up and then extending out to peel apart palm stems to reach insects or even shifting logs.

While this doesn't confirm that, the CT scans reveal that hero shrews are better at taking compressive force on their spines, Smith said. And the thick, spongy bone inside their vertebrae makes their spines a lot sturdier. So they could withstand inchworm-like movements without suffering damage.

While animals include some morphology, or structures, that don't have to serve a specific purpose, Smith believed the hero shrew backbone does.

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Such an intricate spine requires a wealth of blood vessels to take care of it, not to mention the vast investment of energy and development in such a small shrew that only weighs a quarter of a pound. And for an animal with a high metabolism that must eat all the time, it's an intense energy investment as well, Smith said.

They also discovered the direction of the force that hero shrews experience.

"These guys can take a lot of force compressing their spine from head to tail," Smith said. "And that's the main direction that force is applied to the spine in an animal that has four legs. It doesn't necessarily solve the question of what are they doing, but it does give us an indicator that they’re habitually experiencing strong forces in that direction."

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Smith also discovered that the original hero shrew's spine is more complex than the 2013 species, although both still have more elaborate spines compared to regular shrews. This will allow them to trace the evolution of the hero shrew.

Going forward, the researchers want to see how these hero shrews develop as juveniles and see how the internal structure of shrews has evolved. They also want to compare hero shrews with Xenarthrans, the group including sloths, anteaters and armadillos. They also have extra articulations on their lumbar vertebrae.

And frankly, it's about being small in a big world.

"Small mammals experience the world differently than we do," Angielczyk said. "Part of what we’re interested in is the question of how you can be a small mammal – what do you need to be effective at that and resist forces that are being applied to your body in different functional contexts. Better knowledge of what it takes to be a small mammal is important for understanding a lot of weird things about mammals in general, including us."