Quite a tail: a mouse has been hiding its armor all along

At first it looks like a slightly hairier rodent. But the spiny mouse’s body is full of secrets. Found in rock outcrops all over Africa and Europe, its back is full of porcupine-like quills made of stiffened fur. It has soft, easily torn skin and a remarkable ability to regenerate, like a type of desert gecko. Now researchers have revealed another surprise in iScience magazine on Wednesday: Their tails are lined with osteoderms, or bony plates, making them the second group of living mammals known to be equipped with subcutaneous armor like an armadillo.

“Although spiny mice are well known and commonly used in all sorts of lab experiments, no one had ever noticed they had them,” said Edward Stanley, a biologist at the Florida Museum of Natural History and an author of the study.

The discovery came when he was scanning CT samples for the openVertebrate project, an effort to build a public online database of 20,000 vertebrate specimens from museum collections across the United States. X-rays of the mouse’s tail made him think: they reminded him of the lizards he had been working on for his doctorate. But the only living mammals with known osteoderms were armadillos.

“I know enough about osteoderms that it’s pretty unknown for rodents to have them,” said Dr. Stanley.

The discovery was accidental, said Malcolm Maden, a biologist at the University of Florida and an author of the study. Dr. Maden already had a long-running research project on spiny mice, centered on their remarkable ability to regenerate skin, muscles, nerves and parts of their spinal cord. Joining forces, the researchers studied how the osteoderms developed over a mouse’s lifespan and sequenced the species’ RNA in an attempt to identify the genetic switches responsible for bone armor development.

Dr. Stanley also scanned specimens of the spiny mouse’s closest relatives – the link rat, bristle mouse and Rudd’s mouse. He found that all three also had armored tails, while more distant relatives did not. The discovery suggested that a common ancestor of all four species possessed the trait.

The purpose of the osteoderms is not clear. Spiny mice can use them to protect themselves from predators while burrowing into crevices, said Dr. Stanley. Another possibility: While the mice’s skin tears easily, the armor may help protect the inner tail structure, such as wearing chainmail under an easily removable glove.

Osteoderms have evolved at least 19 times in different lineages of animals, said Dr. Maggots. They are commonly found in reptiles such as lizards, crocodiles and non-avian dinosaurs. They’ve also been found in a few extinct mammal groups, such as immense armadillo relatives called glyptodonts and giant ground sloths — whose skin armor closely resembles that of the spiny mouse.

Finding osteoderms in a fast-growing, easy-care animal like a mouse could help unravel how and why the forces of evolution have continually spawned subcutaneous bone armor, said Dr. Maggots. Now that they’ve narrowed down a list of genes that may be responsible for this trait, they can try to produce osteoderms in lab studies.

“I want to figure out which genes are responsible for making osteoderms and then make a lab mouse with armor,” said Dr. Maggots.

The building blocks for osteoderms may be in vertebrate heads, said Dr. Stanley. The vertebrate skeleton is largely made up of cartilage that gets bonker over time — but the skull bones form from hardening collagen, which the team suggests may have been reused from the armored heads in early fishing lines.

“If you can grow a skull, you have the genetic architecture to grow bones in your skin,” said Dr. Stanley. The trick will be to use genomics to find out if the osteoderms of the mice’s tails form like their skulls. “That would lend credence to the idea that osteoderms went from armor to skulls and back to armor.”

It’s also possible that osteoderms, which are generally tucked discreetly under the fur and skin, are significantly more common in mammals than commonly believed: No one has actively looked for them, said Dr. Stanley. It took exploratory science, like the openVertebrate Project, to find them, he noted. Dr. Stanley hopes the data from the project will lead to similar discoveries.

“Building that kind of accessibility to museum samples and the digital data extracted from them is going to benefit all sorts of areas,” said Dr. Stanley. “After all, we didn’t know what we were going to find.”